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Thermochemistry of M(η5-C5H5)2Ln complexes (M = Ti, Mo, W)
Polyheakm Vol. 7, No. 16117, pp. 1531-1544. Printed in Great Britain
1988 0
THERMOCHEMISTRY OF. M(q=C&),L, (M = Ti, MO, W) ALBERT0
R. DIAS and JOY& A. MARTINHO
0277-5387/88 $3.00+.00 1988 Pergamon Press plc
COMPLEXES
SIMdES
Centro de Quimica Estrutural, Complexo I, Instituto Superior Tecnico, 1096 Lisboa Codex, Portugal Abstract--The available thermochemical data for the title compounds are summarized and discussed. Metal-ligand bond enthalpies are derived for a variety of ligands through a simple method that involves both the experimental standard enthalpies of formation and theoretical calculations. It is suggested that this method may be used to predict the energetics of new complexes with fair accuracy. In addition, the method provides estimates of metal-ligand stepwise bond dissociation enthalpies. Some of these estimates are compared with recent experimental results obtained for bis(pentamethylcyclopentadieny1) titanium complexes.
If the- ability to estimate new data is used as a criterion to assess the development of organometallic thermochemistry, we will be compelled to conclude that our present knowledge in this area is still rather incipient, particularly for molecules containing d and f elements. This situation contrasts with the one observed for organic substances, for which several reliable prediction schemes are available,le3 but is not surprising, given the large number of elements and hence the different chemical bonds that can be involved in organometallic species. Moreover, even if it were assumed that these schemes could be extended to metal compounds, the size of the data bank required to derive a suitable set of “bond terms” or “group contributions” would be several orders of magnitude larger than for organic compounds. Unfortunately, the virtually infinite variety of bonds and groups in organometallic complexes, hindering the existence of traditional prediction schemes, is also the reason why the development of estimation methods becomes so important. Alternative (though eventually less reliable) procedures have thus to be found. The simplest and most commonly used method to estimate the enthalpy change of any chemical reaction, in the absence of enthalpy of formation data, is to consider the bonds cleaved and formed. For example, the enthalpy of reaction (1) will be obtained if the difference between M-L and M-L
bond dissociation enthalpies is available. &ML(g) + L’(g) + XML’(g) + L(g). (1) Devising an estimation method for these quantities is apparently easier than for enthalpies of formation, since AHj’(X”ML,g) or A@&ML’,g) would require energetic information on each chemical bond in the complexes, and it is also less demanding in terms of size of the experimental data bank. The thermochemical studies on bis(cyclopentadienyl) compounds of the type M(Cp),L, (Cp = q5-CZH5 ; L = H, alkyl, aryl, carbonyl, halogen, etc.) carried out in our laboratory enabled us to address some issues that may contribute to improved reliability of estimated metal-ligand bond dissociation enthalpies and standard enthalpies of formation. As the structural features of the fragment M(Cp), are nearly constant in many complexes, indicating that its enthalpy content is not significantly changed, attention can be concentrated on the “reactive” side of the molecules, i.e. the bonds M-L. The dependence of the strengths of these bonds on the metal and its oxidation state, and on the number, types and geometrical constraints of ligands, has been examined in a wide variety of cases. It is the purpose of the present paper to summarize both the method used to derive metalligand bond enthalpies from experimental data and the main conclusions drawn from these studies. Pre-
1531
1532
A. R. DIAS and J. A. MARTINHO SIMdES
liminary values of some recently determined bond enthalpies are also included in the discussion. \
CALCULATION OF BOND ENTHALPIES FROM EXPERIMENTAL DATA
Enthalpies of reaction of M(CI>)~L,, complexes with several substances, typically HCl or I2 solutions, have been determined with a reaction-solution calorimeter described in detail elsewhere.4 These measurements, together with auxiliary enthalpies of solution and literature data, led to the standard enthalpies of formation of the crystalline complexes. Unlike combustion calorimetry, reaction-solution calorimetry is not an “absolute” method, i.e. it normally yields a difference between the enthalpies of formation of two complexes. It is therefore necessary that one of these quantities is available so that the other can be calculated. All the AH~O[M(CP)~L,,C]values listed here have been obtained using the reported standard enthalpies of formation of the related dichloride molecules, M(Cp)&12, which were derived from static (MO, Ti) and rotating (W) bomb combustion experiments.ss6 Although the first of these methods is not considered very reliable for studying such compounds, we have accepted these values as references, in the absence of further information. Incidentally, AHfO[W(Cp)2C12,c]= -71.1 k2.5 kJ mol-’ (staticbomb) is close to the recent rotating-bomb result, - 63.5 + 7.7 kJ mol- ‘. The standard enthalpies of formation of gaseous M(Cp),L, complexes, necessary for evaluating metal-ligand bond enthalpies, were determined by measuring or estimating the standard enthalpies of sublimation of those molecules. Whenever possible, AH: values were obtained from Knudsen cell experiments (the Knudsen cell is a modified version of the one reported in ref. 7), but in most cases they had to be estimated. A thorough discussion of this subject shows that enthalpies of sublimation of analogous complexes usually fall in a narrow range, thus allowing reasonably accurate estimates. * As happens with any molecule containing several types of ligands, the determination of the enthalpy associated with a given bond from the enthalpy of formation in the gas state depends strongly on the enthalpies ascribed to the remaining bonds. In the case of the bis(cyclopentadieny1) complexes, the difficulty of estimating M-Cp bond enthalpies was avoided by considering the “reference” complexes t The assumption that D(M---Cl) in MCI, is identical to &M-Cl) [and not to E(M-Cl)] in M(Cp),C12 has been used in ref. 5.
4 WCp)z(g) + &)
Scheme I.
WCpMg)+ 2cl(g) Scheme 2. M(Cp),C12 and by using two different concepts of bond enthalpies: bond enthalpy terms, E(M-L), and mean bond! dissociation enthalpies, 6(M-L). These quantities are related through Schemes 1 and 2, from which eqs (2) and (3) were derived. E(M-L)
= E(M-Cl) - A@(Cl,g) -
+ ; AH;(L*,g)
{~~;bWpM+ngl
- ~fffOM~p),C1d}I2 + W, - ER J/2 (2) &M-L)
= E(M-Cl)
+ ; AHfo(L,g)
- AH,“(Cl,g) -
W#Wp)Lgl
-
A@DWWlz,gl}/2 + E&/2.
(3)
The asterisks indicate non-reorganized fragments, i.e. fragments retaining the geometry they had in the initial complex. ER,, ER3 and ERL are the energies associated with the relaxation of the fragments to their ground states. Bond enthalpy terms E(M-L) and E(M-Cl) do not contain the reorganization energies of the fragments. They should therefore be a better measure of the M-L “bond strengths”, affording more meaningful correlations with other structural parameters (e.g. bond lengths) than D(M-L). This point is relevant for the evaluation of E(M-L) and 6(M-L) through eqs (2) and (3). An estimate of M-Cl bond enthalpy in M(Cp),C12 can be made by considering the bond enthalpy in the homoleptic molecule MCl,. If M-Cl bond lengths are similar in both molecules, then E(M-Cl) = b(M-Cl) in MCI, can be transferred to the dichloride complex, i.e. is identified with E(M-Cl) in M(Cp),Clz.t This was indeed the procedure used to calculate E(M-L) data collected in the tables: for molybdenum and tungsten (m = 6) E(M-Cl) were taken as 303.8 and 347.3 kJ mol- i, respectively, and for titanium (m = 4) E(M-Cl) = 430.5 kJ mall I.’ If 6(M-Cl) in MCl, was identified with @M-Cl) in the complex,t then ER3 = 0. As indi-
Thermochemistry of M($GH&L, cated by extended Hiickel molecular orbital calculations, this conclusion does not hold for the molybdenum and tungsten dichlorides (ER3 - -82 and - 103 kJ mall ‘, respectively), although it is reasonable for the titanium analogue (ER,- - 11 kJ mol- ‘). Like ER3, the reorganization energies ER, have also been estimated from curves where the total energy of the fragments M(Cp), is plotted as a function of the angle Cp-M-Cp (0). The most stable geometry of the isolated fragment is achieved with 8 = 140 and 180” for metals having two and four d electrons, respectively, As 0 is usually close to 130” in M(Cp),L, complexes, this accounts for the large reorganization energies for molybdenum and tungsten fragments as compared with titanium.’ The dependence of the M(Cp), energy on the MCp distance is less important. Besides, for each metal, these bond lengths are rather constant. The extended Hiickel results have always been handled in a semi-quantitative way. Accordingly, the bond enthalpies collected in the tables are not affected by the “corrections” in eqs (2) and (3). It must be noted, however, that in the case of E(M-L), eq. (2), the reorganization term (ER3 - ER,)/2 is usually small since the angle 8 is fairly similar for most complexes. Examples and exceptions are shown in the next section. A pragmatic approach can be followed to obtain the enthalpies of formation of non-reorganized species, AHfo(L*,g), which are also required to calculate E(M-Cl) by eq. (2). lo For anionic ligands, it is first considered that the geometry of L* is the same in the complex and in the molecule LH, implying that AH,“(L*,g) can be derived from Scheme 3 or eq. (4). Secondly, E(M-L) is identified with the respective Laidler term. ’ AHfo(L*,g) = E(L-H)
+ AH;(LH,g)
- AH;(H,g) = D(L-H)
- ERL
+ AHfo(LH,s) - A@(H,g).
(4)
A simple calculation involving, for instance, the methyl radical, demonstrates that the introduction of Laidler terms in eq. (4) leads to physically meaningless results : ERL, given by the difference between D(CH,-H) (439.7 kJ mol-I)” and E(C-H) (= 6(C-II) = 415.8 kJ mol- ‘), is positive, implying that the CfV configuration of the free radical
L*(g) + H(g) LWg) E’L-H’b W-W
\I
ERL L(g) + H(g)
Scheme 3.
1533
complexes
WWJ&) + L(g) Scheme 4.
should be more stable than D3,,. Despite this lack of physical meaning, the procedure has the advantage of yielding E(M-L) data that are consistent with the Laidler scheme and eventually can be used to make predictions. Moreover, as shown below, estimates of stepwise bond dissociation enthalpies are independent of the value chosen for E(L-H). If necessary, the assumption of identical geometries of L* in LH and in M(Cp),L, can be corrected by computing their energy difference. As illustrated in the next section, this has occasionally been made by using the Htickel, MINDO-3 or MNDO methods. The estimation of metal-ligand partial (or stepwise) bond dissociation enthalpies follows from the above method in a straightforward manner. The first bond dissociation enthalpy, D ,(M-L), is calculated using eq. (5), derived from Scheme 4, where the symbols have the usual meaning. D ,(M-L)
= E(M-L)
+ ER; + ER,_.
(5)
As E(M-L) is transferred from eq. (2) and it relies on eq. (4), D,(M-L) becomes independent of the value ascribed to E(L-H). In other words, ER, is cancelled when eq. (5) is used. The reorganization energy ER’, is also estimated through the extended Hiickel approximation by optimizing the structure of the free fragment M(Cp)*L.12 The second metal-ligand bond dissociation enthalpy, D,(M-L), is simply the difference between 26(M-L) and D1(M-L). The Hiickel calculations for several systems (L = Cl, H, CH,, SCH3; M = MO, Ti) indicate that ligand to metal rc-donation results in larger values of D2 - D, than a-donation (see Table 10). STANDARD ENTHALPIES OF FORMATION AND BOND ENTHALPY DATA As stated previously, the standard enthalpies of formation of all the bis(cyclopentadieny1) complexes reported here rely on combustion results for AH/O[M(Cp)2C12,c].It must be emphasized that a revision of the static-bomb values (MO, Ti) will not affect the metal-ligand bond enthalpy data: AZ$[M(Cp),C12,c] is cancelled in eqs (2) and (3), since the same value was used to derive
1534
A. R. DIAS and J. A. MARTINHO Table
1. Standard
enthalpies of formation (kJ mol- ‘)
SIMaES
of halogen
complexes
Complex
AH;(c)
AH,0
AH;‘(g)
Ti(Cp)KL Ti(Cp)&
-383.2k7.5 - 148.4+ 13.1
118.8k2.1 (120f 10)b
-264.4k7.8 -28.4f 16.5
Mo(Cp)FL Mo(Cp),Br, Mo(Cp),I,
-95.8k2.5 9.7+ 12.7 69.8k7.8
(100.4f4.2) (100.4k4.2) (100.4+4.2)
4.6 +4.9 110.1 f 13.4 170.2k8.9
W(CP),ClZ WCp),Br, WCP),IZ
-63.5 + 7.7 15.7+ 13.9 65.4+ 10.5
(104.6+4.2) (104.6k4.2) (104.6k4.2)
41.1k8.8 120.3 + 14.5 170.0+11.3
“Data from refs 4, 5, 6, 14 and 15. bEstimated values in parentheses.
AH~U[M(CP)~L,,,C]. A different conclusion is drawn for M--Cp bond enthalpy terms, which, by a first approximation, can be obtained from eq. (6). E(M-Cp)
Table
2. Metal-halogen and metal-cyclopentadienyl bond enthalpies (kJ mol- ‘)
Complex
E(M-X)
&M-X)
E(M-Cp)
Ti(Cp)& Ti(Cp)&
430.5& 1.3 430.5& 1.3 298+9 298k9
318rf: 10
Mo(Cp),Cl, Mo(Cp),Br, Mo(Cp),Iz
303.8k7.1 242f 10 207*9
303.8f7.1 242f 10 207f9
404+12
YCP) ,Cl Z YCp),Brz W(CP)2I*
347.3kO.8 298*7 268k5
347.3kO.8 298+7 268f5
443+ 10
= AH;(M,g)/2 + AHfo(Cp,g)
+ AH,O(Cl,g)--~~fo[M(Cp>,CLg1/2
- E(M-Cl).
(6)
As &M-Cl) is constant, because it is transferred from MCl,, an increase in the enthalpy of formation of the complex will imply a decrease of E(M-Cp). Some enthalpies of formation shown in the tables were recalculated by using recent literature data.? This applies particularly to the tungsten complexes, whose values were adjusted to the recently reported AH_#V(Cp)ZClZ,c].6 Enthalpies of formation of ligands or radicals, necessary to derive M-L bond enthalpies, are given in the Appendix. Halogen complexes
Table 1 collects the available standard enthalpies of formation of these bis(cyclopentadieny1) derivatives. Except for the dichlorides, all the values were obtained from reaction-solution calorimetric results.4*‘4*‘5 The bond enthalpy data are shown in Table 2 and include metal-halogen and metalcyclopentadienyl bond enthalpy terms, and metalhalogen mean bond dissociation enthalpies. As stated before, no corrections due to ER,, ER3 and EGP were included in those values. This hardly affects E(M-X), since (ER,-ER,)/2 must be close to zero, i.e. Cp-M-Cp angles are likely to be similar in M(Cp),Cl* and M(Cp)rX2 (X = Br, I). The same applies to D(Ti-X) (x = Cl, Br, I), t AH/O[M(Cp)2Br,,c]were recalculated from new data for AHf(CBr,,c)-AHT(CHBr,,l) in ref. 13.
“Data not including reorganization
energies ER, and
ER,. because ER3 is negligible, but not so for 6(Mo-X) and b(W-X), whose corrections are predicted to be significant (see previous section). As for E(M-Cp), the values displayed are also upper limits, since EGP was not considered. The usual trends E(M-Cl) > E(M-Br) > E(M-I) and E(Ti-X) > E(W-X) > E(Mo-X) are observed in Table 2, but a striking feature is apparent: E(Ti-Cp) < E(Ti-Cl), while an inverse relationship is noted for molybdenum and tungsten. Although the absolute values in the table may be controversial (only differences E(M-X) - E(M-Cl) and 6(M-X) -&M--Cl), for each metal, can be trusted), it seems hard to believe that the transferability produces errors of ca 100 kJ mol- ‘. Interestingly, the trend for titanium is also observed for zirconium and hafnium, ’ 6 in agreement with the known chemistry of these complexes (e.g. the stability of M(Cp)C13 complexes and the lability of M-Cp bonds for group 4d metals). It is finally noted that E(W-Br) = 296+7 kJ mol-’ in WBr, (AHfO(WBr,,g) and AHfo(TiI,,g)
1535
Thermochemistry of M(qS-CSH&Ln complexes Table 3. Standard enthalpies of formation of oxygen and sulphur complexes (kJ mol- ‘)
-
Complex
AH;
MT(c)
AH&)
(97 + 8)b (104+8) (104+8) (104f8)
Ti(Cp) ASPh) 2 Ti(Cp)&SGH,Me)
-393.7f8.0 -448.9k8.2 -408.1 k8.1 -448.7k7.9 -440+22 -789.7f8.1 -1095*19 -2251.2k8.2 -322.6+ 12.7 -211.3f8.2 -267.5k9.7 34.Ok7.9 -30.3k9.5
(9Of 10) (112k8) (130+8) (108&8) (165+8) (106f 10) (108f8) (113f 10) (11Ok 10)
-296.7+ 11.3 -344.9+ 11.5 -304.1 If: 11.4 -344.7& 11.3 -35Ok24 -677.7+ 11.4 -965+20 -2143.2k11.4 - 157.6* 15.0 - 105.3 + 12.9 - 159.5 + 12.5 147.0+ 12.7 79.7* 13.8
Mo(Cp)z(OzCPh)z Mo(QWWCPJ~ Mo(C~)z(0,Cd&) M~(CP),(O&,HH~) M~(CP),(CG,H,) Mo(Cp)XgOJ Mo(Cp) GPr-n) 2 Mo(Cp) GPr-i) 2 Mo(Cp),Wu-n), Mo(Cp),(=u-02 M~(CP)~(SCIOH~I-~)~ Mo(Cp)z(gPh),
-500.7+3.4 - 1984.2k4.2 - 130.6 + 2.9 - 80.3 f 3.2 - 53.3 f 10.4 -650.4f3.4 - 10.7k5.3 -22.7k5.3 -96.Ok5.5 -68.3k4.3 -361.5k4.3 282.0 f 4.8
(94k 10) (9Ok 10) (100+8) (135+8) (145&8) (72 f 10) (90 f 10) (9Ok 10) (92+ 10) (92 + 10) (105& 10) (95 + 10)
-406.7+ 10.6 - 1894.2+ 10.8 -30.6k8.5 54.7 * 8.6 91.7+ 13.1 - 578.4+ 10.6 79.3f11.3 67.3+ 11.3 -4.o+ 11.4 23.7+ 10.9 - 256.5 + 10.9 377.0+ 11.1
WCp) A0 ,CPh) z WCPWZCCPA W(CP),(CGHJ W(Cp) #Et) z WCp)#Pr-n), WCp) ,(SPh) z
-455.8 + 8.2 - 1925.6+ 8.2 - 105.2k7.8 70.3 f 9.0 16.6k9.2 327.5k9.1
(98 + 10) (94f 10) (104&8) (92f.10) (94k 10) (99f 10)
- 357.8 f 12.9 - 1831.6f 12.9 -1.2k11.2 162.3& 13.5 110.6+ 13.6 426.5 f 13.5
TWpMOPh)~
Ti(Cp),(OCsH,Me-2), Ti(Cp)@Cd%Me-3), Ti(Cp)#%H4Me-4)2 Ti(Cp)2(OC~H&l-2)2 Ti(Cp)#WPh), Ti(Cp)KWCClA Ti(Cp)GWCPA Ti(Cp)&C,~HA Ti(Cp)#Et), Ti(Cp),(SPr-n),
’ Data from refs 18-2 1. bEstimated values in parentheses.
quoted from ref. 17) and E(Ti-I) = 294 + 2 kJ mol- ’ in Ti14” are in excellent agreement with ,??(W-Br) and E(Ti-I) in Table 2, which, we recall, rely on E(W-Cl) and E(Ti-Cl) from WC16 and TiCl.,. This seems to indicate that reliable predictions of the enthalpies of formation of the complexes MUFF can easily be made using eq. (2) together with @M-F) values obtained from the available data for AHfO(MF,,g) : ” AHfO[M(Cp),F2,g] -656, -369 and - 367 kJ mol- ‘, respectively for M = Ti, MO and W [E(M-F) = 585,449 and 510 kJ mol- ‘I. Oxygen and sulphur complexes The standard enthalpies of formation of these molecules, obtained by reaction-solution calor-
imetry, ‘*-*’ are collected in Table 3 and were used to derive metal-oxygen and metal-sulphur bond enthalpy terms and bond dissociation enthalpies (Tables 4 and 5). The available molecular structures for some complexes in these tables show that the geometry of M(Cp), fragment is nearly constant for each metal and similar to the one in the respective dichloride. 1*-20~22~23 This constancy is assumed for the remaining molecules and implies that the correction terms (ER, - ER 42 are not significant. For instance, in the case of the molybdenum sulphate molecule, the angle Cp-M-Cp is 134”, corresponding to (ER,- ER,)/2 N - 13 kJ mol- ‘. In the absence of steric effects, metal-oxygen bond enthalpy terms for each metal fall in a narrow range, suggesting that the average values for Ti (452 kJ mol- ‘), MO (333 kJ mol- ‘) and W (365 kJ
1536
A. R. DIAS and J. A. MARTINHO SIMaES
Table 4. Metal-oxygen bond enthalpies (kJ mol- ‘) Complex
E(M-0)
&M-O)
Ti(Cp)Z(OPh)z Ti(Cp),(OCsH,Me-2), Ti(Cp),(OC,H,Me-3)2 Ti(CpWC&Me-4), Ti(Cp),(OC,H,Cl-2)2 Ti(Cp)@,CPh), Ti(Cp)#WCCl,), Ti(Cp)~(02CCPS)2 Ti(Cp)~(O~G&I,)
462+9 454+9 430&-9 457f9 440+ 10 455*9 464+9 451f9 417+9
373fll 365+ 11 341+11 368+11 351k12 440+6 446-1-11 433+11
WCpWzCPh),
327+ 12 334+ 12 299f9 300+9 300+ 10 33859
312f 10 316f 14
365& 10 365f 10 346f5
349*7 347+11
WCPW,CCF,), M~CPMOG&)
M~(CP)@G&) M~(CP)&CUH~) Mo(Cp),(S03 W(Cp)&WPh), YCP)KGCP,), W(CPMWAH.,)
=Data not including reorganization energies ER, and ER3.
mol- ‘) can be used to estimate standard enthalpies of formation of other complexes within ca +20 kJ mol- r. Furthermore, the average for titanium is not far from the mean value of E(Ti-0) in Ti(OR), molecules (R = alkyl), 466 kJ mol-‘.‘g The low values for the bidentate ligands 1,2benzenodiolate, 1,2naphthalenodiolate and 9,10phenanthrenodiolate were not included in the above averages since they are believed to reflect relatively Table 5. Metal-sulphur bond enthalpies &.I mol- ‘) Complex
E&I-S)
&M-S)
326f 10 331+10 358f 10 333* 10
341* 10 347+ 10 344+ 10
Mo(Cp)@‘h),
220& 12 217f. 12 241+12 206f9 243+ 12 251f12
235k 13 233k 13 256k 12 221+9 259+ 12 237k 12
WCp)#Et), WCp)@+n)z WCp),(SPh),
261+10 266+ 10 288+ 10
277k 10 281 f 10 274k 10
WW2CW2 Ti(Cp),(SPr-n), Ti(Cp) *(SPh)z Ti(Cp)&C~H&W Mo(Cp) @Pr-n) Z Mo(Cp)@‘r-i), M~(CPMSB~-~Z
Mo(Cp),(SEu-0, MWPMSGOHW~),
a Data not including reorganization energies ER, and ER3.
high strain energies (40-80 kJ mol- I).*’ An opposite conclusion is drawn for the complex Mo(Cp),(SO,). The “normal” E(Mo-0) value indicates a negligible strain in the sulphate moiety, this being supported by the molecular structure of the complex and by extended Hiickel calculations. ” Recommending “constant” metal-oxygen bond enthalpy terms in M(Cp),L, complexes for each metal is obviously an over-simplification that can only be allowed because our present data are both scarce and not very precise (this being mainly a consequence of estimated enthalpies of sublimation). A somewhat similar situation is found for metal-sulphur bond enthalpy terms. Defining average E(M-S) values is probably the most judicious use for the data in Table 5, particularly when the uncertainty intervals are considered. However, if the individual values are accepted, several trends are apparent. First, there seems to be an increase of E(M-SR) with the length of the n-alkyl chain. The observation of a similar pattern in the case of the above mentioned Ti(OR)4 molecules lends some support to that trend. Secondly, a large value (35 kJ mol- ‘) is noted for the difference E(Mo-SBun) - E(Mo-SBu-t), reflecting expected steric effects in the t-butyl complex. An interesting comparison between oxygen and sulphur bidentate ligands is provided by the complex Ti(Cp),(S2CgH3Me). The value obtained for E(Ti-S) indicates a negligible strain in the metallacycle. The larger size of the sulphur atom, compared with oxygen, causing a relaxation of the fivemembered ring, has been used as a possible explanation for that different behaviour.*’ The same pattern is observed in cyclic organic molecules (CH),_ rX (X = CH2, 0, NH or S), discussed by Cox and Pilcher. ’ For n = 5 the strain energy decreases by about 15 kJ mol- ’ when oxygen is replaced by sulphur. The usefulness of the bond enthalpy term concept is evidenced by the oxygen and sulphur bidentate complexes: E(M-0) and E(M-S)’ provide the only energetic information on these bonds, since the enthalpies of formation of the free biradicals are not available, and therefore mean bond dissociation enthalpies could not be calculated. Nitrogen andphosphorus complexes
The metal-nitrogen bond enthalpy data (Table 7) were derived from the reaction-solution calorimetric standard enthalpies of formation (Table 6) ‘*I4by the common procedure, i.e. eqs (2) and (3). However, Scheme 3 or eq. (4) were not used for the neutral ligand azobenzene. Instead, AH,O(L*,g) relies on a MNDO result for the reorganization
Thermochemistry of M($-CsH&L,
1537
complexes
Table 6. Standard enthalpies of formation of nitrogen and phosphorus complexes (kJ mol- ‘) Complex
AH,0
AH;(c)
Ti(Cp)2(N3)2 Ti(Cp)@GW~ Ti(Cp) JN,Ph J Ti(Cp),(PMe3),
436.4 f 8.4 181.9f 12.1 338.8+_ 16.7 -261.9+ 17.1
Mo(CpL(NzPh3
563.7 f 8.6
(70 f (log+ (go+ (go+
10)b 10) 15) 10) -
(70f 15)
AH/o(g) 506.4k13.1 290.9* 15.7 428.8 + 22.4 171.9f 19.8 633.7f 17.3
“Data from refs 9, 14 and 25.
bEstimated values in parentheses.
energy of azobenzene to the cis-conformation (- 198 kJ mol-‘).‘4 Also, as &M-N) are meaningless parameters in this case, only values of D(M-N,Ph,) = 2D(M-N) are presented in Table 7. Although (ER, - ER,)/2 corrections are seen to be negligible for the titanium complexes with L = N3 and N2Ph2, and assumed to be so for the remaining Ti and MO molecules,9~‘4 the Hi.ickel calculations indicate that a large negative correction [ER3 N - 82 kJ mol-‘; see eq. (3)] should be applied to the value of D(Mo-NzPhz) in Table 7. As discussed before, ER3 is not significant for the titanium analogues. The calculation method was also slightly modified for evaluating E(Ti-N,) because N-N bond lengths are considerably different in the complex and in N3H.’ Extended Hiickel calculations indicate that the fragment N3 is stabilized in the complex by ca 48 kJ mol-‘. This correction, which implies a negative adjustment to the value obtained from eqs (2) and (4), is included in Table 7. The average between E(Ti-N,) and E(Ti-NCBH6), with 339 kJ mol- ‘, can be compared E(H-NR3 - 311 kJ mol- ’ in Ti(NR3, (R = Me, Et). (E(Ti-Nr,) were recalculated in 8 from data in ref. 24.) E(Ti-N) for the azobenzene complex is lower than the corresponding values for the monodentate hgands. The geometry of coordinated N2Ph2, a distorted cis form, does not seem to favour both o- and x-bonding with the metal atom. I4 The standard enthalpy of formation of Ti(Cp),)(PMe& was recently determined by reaction-solution calorimetry, allowing us to obtain E(Ti-PMe,) and b(Ti-pMe,).25 As in the case of azobenzene, eq. (4) was not used to evaluate AHT(L*,g). MNDO calculations give a negligible 7 The structural data of Ti(Cp)#Me,), ref. 22.
is included in
reorganization energy (- 5 kJ mol- ‘) for PMe3.26 This correction is not included in the result shown in Table 7 and the same happens with - -4 kJ mol.7 The thermo(E&-E&)/2 chemistry of other bis(cyclopentadieny1) complexes containing metal-phosphorus bonds are presently being investigated. For the moment it is of interest to stress that Ti-PMe, and T&CO bond strengths are comparable (see below). Complexes containing metal-hydrogen and metalcarbon bonds Table 8 summarizes all the available standard enthalpies of formation of these bis(cyclopentadienyl) derivatives. With the exception of AHfopi(Cp),(CH,Ph),,c], which was determined by static-bomb combustion calorimetry,’ the data were derived from reaction-solution calorimetric measurements.‘4~27 Preliminary results for MO (Cp)2Et2, Mo(Cp)2(n-Bu)2 and Mo(Cp),(C2H4) are Table 7. Metal-nitrogen and metal-phosphorus bond enthalpies (kJ mol- ‘F D or
Complex
@M-L)
Ti(Q%(N& Ti(Q),(NGH& Ti(CP)&Ph3 Ti(CLGZ(PMe&
329f 348+ 290f 162f
Mo(Cp),CNzPhJ
195* 126
&M-L)
10 10 126
385+ 22
12
162+ 12
381f24’ 192*23
a Data not including reorganization energies ER, and ERp. ‘Bond enthalpy term for one M-N bond. Values include the MNDO result for the reorganization energy of azobenzene to the cixonformation (- 198 kJ mol- ‘). ’ M-N,Ph, bond dissociation enthalpy (cis-azobenzene as a product). See also note a.
A. R. DIAS and J. A. MARTINHO
1538
SIMaES
Table 8. Standard enthalpies of formation of complexes containing metal-hydrogen and metal-carbon bonds (kJ mol- ‘) Complex Ti(CpLMe, Ti(Cp),(Me)Cl Ti(Cp),Ph2 Ti(Cp)G’h)Cl Ti(Cp)&&Me-3), Ti(Cp)&H,Me-4), Ti(Cp)&H&F+% Ti(Cp)&H&Me-rl), Ti(CpL(CH,Ph), Ti(Cp),Fczc Ti(Cp) GO) 2
AH;(c)
AH,0
AH;(g)
-26.6k9.6 -234.7k8.1 262.2 f 8.9 -79.2k7.8 166.8k9.4 169.6k9.6 -1142.6k8.3 -91.8k8.5 195.8 f 5.0 488.2+ 12.1 -295.3 + 12.9
(79.5 + 8.4)b (llO& 10) (88&8) (120f 10) (95 f 8) (95 f 8) (110+8) (104k8) (83.7 & 8.4) (15Ok 15) 84.2 + 3.5
52.9 + 12.7 - 124.7+ 12.9 350.2 + 12.0 40.8+ 12.7 261.8+ 12.3 264.6+ 12.5 -1032.6k11.5 12.2* 11.7 279.5 +9.8 638.2 f 19.3 -2ll.lk13.4
Mo(Cp)zH, Mo(Cp),Me, Mo(Cp),Et, Mo(Cp)&-Bu), Mo(Cp),(C,H3
210.3k5.7 262.4 k 4.0 [219.2+3.31d [143.5f4.6] 284.6 f 8.2
81.4f 1.0 70.4k4.2 93.7f1.8 (105f 10) (80 + 10)
291.7IfI5.8 332.8 + 5.8 [312.9+3.8] [248.5 & 11.O] 364.6* 12.9
W(Cp),H, YCP) z(H)1 W(Cp),Me,
222.4& 8.8 176.3k11.3 271.Ok8.1
84.1k1.6 (lOOf5) (74.6f4.2)
306.5 + 8.9 276.3 f 12.4 345.6k9.1
“Data from refs 5, 14, 29 and 30. bEstimated values in parentheses. ’ Fc = ferrocenyl. dPreliminary values in brackets. See footnote t on this page.
also reported,? of the standard
together with recent measurements enthalpies of sublimation of the
M(Cp)*H, (M = MO, W)” and Mo(Cp)2Et2.30 The analysis of M-L bond enthalpy values (Table 9) for the complexes M(Cp),(L)L’ (L # L’) will be made below. The remaining E(M-L) and @M-L) were calculated by the methods already described. In the case of the carbonyl complex, AH,“(L*,g) relies directly on a MIND0/3 calculation which yields a negligible value (-2 kJ mol- ‘)’ 4 for E&,. As the structure of the ethylene complex is not available, the calculation of the reorganization energy of C2H4 was not attempted and thus only the Mo-C2H4 bond dissociation enthalpy is shown. As happens with the bis(cyclopentadieny1) derivatives mentioned previously, the available t A small correction due to the dilution of the HCC ether solution used in the experimental study is not included in the values for the ethyl and butyl complexes. Although this correction will not affect significantly the bond enthalpy dam, it will be included when the results are fully reported. 28 $The angle Cp-Mo-Cp in the complex Mo(Cp),(n-Bu),, 135.2 implies @X2-ER,)/2 N - 16 kJ mol-‘.32
crystal structures for the complexes in Tables 8 or 9 22,3’$together with extended Hiickel calculations, indicate that corrections (ER,-ER,)/2are relatively small,? except for MOTHS, where the angle Cp-Mo-Cp, 145.8”,” is substantially larger than the one in Mo(Cp),Cl,, 130.5”,33leading to (ER,-ER,)/2 N -33 kJ mol- ‘. This implies that E(Mo-H) presented in Table 9 is a high upper limit and it is reasonable to draw a similar conclusion for the tungsten analogue, whose structure is not known. It has been mentioned that ER3 is small for titanium and relatively large for molybdenum and tungsten. Significant corrections (about -42 and - 52 kJ mol- ’ for MO and W, respectively) may therefore alIe& 6(Mo--L) and b(W-L) in Table 9. A broad look at the titanium-carbon and molybdenum-carbon bond enthalpy terms suggests that, for predictive purposes, the averages 290 kJ mol-’ (Ti) and 135 kJ mol- ’ (MO) can be defined, enabling us to estimate new enthalpies of formation within cu f: 20 kJ mol- I. These estimates can, however, be improved by considering some trends evidenced in the table. For instance, E(Ti-methyl) is 15-25 kJ mol- ’ lower than E(Ti-aryl) and E(Mo-C) decreases with the length of the n-alkyl chain.
Thermochemistry Table
9. Metal-hydrogen and metal-carbon enthalpies (kJ mol- ‘) Complex
274f5 293& 11 287k9 290f 13 299k 10 297+ 10 297k 10 305*9 28Ok 12 278fll 174+8
Mo(Cp),H, Mo(Cp),Me, Mo(Cp),Etz Mo(Cp)&-Bu), M~(CP)#X~,)
257+8 142f8 (137fll)d (127+ 12)
YCP)ZH, WCP) z(H)1 WCp),Me,
311f4 .273* 14 197+3
’ Data not including reorganization
bond
298f6 331flO 342 + 10 341*10 340* 10 349& 10 237k9 (33lk 12) 172+8 257k8 166&-8 (147-&9) (134f 12) 57+20 311*4 221&3
energies ER , and
‘Estimate based on D(Fc-H) N D(C!,HrH). dPreliminary values in parentheses. gee footnote t on previous page. ‘Mo-C*H, bond dissociation enthalpy. gee also note a. and stepwise bond dissociation
The calculation of E(Ti-Me) and E(Ti-Ph) in the complexes Ti(Cp),(L)Cl can be made through eq. (7), derived from two schemes similar to 4, one for the complex and the other for M(Cp),C12.27 The reorganization energies associated with the fragments M(Cp)*Cl from the complex and from M(Cp),Cl* are represented by ER’,, and by ER;, respectively (the subscript 13 indicates a mixed complex, i.e. that the initial complex contains both the ligand of interest and a halogen ; 3 means that the initial complex is the dichloride). Equation (8) was also obtained from the two schemes and includes the first M-L bond dissociation enthalpy, D,(M-L).
E(M-L)
= E(M-Cl) - A@(Cl,g) -
+ AHfo(L*,g)
{~ff@WAMWLgl
- ~~/l?Wp),CLgl) + WC - ER; 3)
D ,(M-L)
= E(M-Cl)
+ AH;(L,g)
(~~/olWWzWCkl
-~~j’W(Q),CLgl)
ER3. 'Fc = ferrocenyl.
Mixed complexes enthalpies
1539
complexes
- A@(Cl,g) -
D or b(M-L)
E(M-L)
Ti(Cp),Me, Ti(Cp),(Me)Cl Ti(Cp)zPhz Ti(CpWh)Cl Ti(Cp)&d%Me-3), Ti(Cp)&H,Me-4)~ Ti(Cp)&H.CF+% Ti(Cp),(CsH,0Me-4)2 Ti(Cp)z(CHzPh)z Ti(Cp)zFc,* Ti(Cp)&%
of M($-C,H,),L,
(7)
t See ref. 12. The reorganization energy for the fragment Ti(Cp),Ph was calculated as -41 kJ mol-‘.26
+ER;.
09
E(Ti-Me) and E(Ti-Ph) in Table 9 were obtained by transferring E(Ti-Cl) from TiC14 and by assuming that the geometry of the Ti(Cp),Cl fragment is similar in the mixed complexes and in the dichloride, implying ER’,, N ER;. It is seen that for the complexes Ti(Cp),Ph* and Ti(Cp),(Ph)Cl,. E(Ti-Ph) is remarkably constant, but the same is not observed for Ti(Cp),Me* and Ti(Cp)*(Me)Cl : the titanium-metal bond is strengthened in the mixed complex. Although this strengthening is not unusual,34 it must be stressed that E(Ti-Cl) was transferred from TiC14 and hence from Ti(Cp),C12. If the titanium-chloride bond is also stronger in the mixed complex, then E(Ti-Me) in Table 9 will be an upper limit. The third mixed complex in Table 9 is W(Cp),(H)I, for which E(W-H) = 273 kJ mol- ’ was obtained. The calculation of this bond enthalpy term was made by a slight modification of the method outlined in eq. (7). The reference molecule W(Cp),C12 was replaced by W(Cp)*12 and E(w-I) was taken as 268 kJ mol- ’ (Table 2). Also, in the absence of structural data for the iodohydride complex, it was assumed that ER’, 3 N ER3. The fact that the value of E(W-H) in the mixed complex is substantially lower than E(w--H) in M(Cp)*H2 apparently indicates that this bond enthalpy cannot be transferred between the two molecules. Recall, however, that the value for the dihydride in Table 9 is not corrected by the difference (ER, - ER J/2, which, in the case of tungsten, is estimated to be ca - 37 kJ mol- I. If this correction is applied, the value for the dihydride will match the one for the iodohydride. Although this good agreement may be fortuitous, it illustrates the importance of reorganization energies when transferring bond enthalpy terms. Several reorganization energies of fragments from complexes WCP)~(W WCP)~L ER; ; L=L’=Cl, ER;; (L = L’ # Cl, L # L’ = Cl, ER’,,), together with values of the stepwise bond dissociation enthalpies Dr(M-L) and D,(M-L’), calculated with data in the previous tables and eqs (5) or (8), are collected in Table 10.7 Several qualitative conclusions can be drawn from these values. One, fragments such as M(Cp)*CI and M(Cp),Ph, associated with large reorganization energies, lead to considerable differences D,(M-L) - D,(M-L). Two, the titanium-methyl 6rst bond dissociation energy in Ti(Cp),Me, is predicted to be higher than in Ti(Cp)*(Me)Cl, since the
1540
A. R. DIAS and J. A. MARTINHO
SIMdES
Table 10. Estimated stepwise bond dissociation enthalpies (kJ mol- ‘) M(Cp)z(L)L’
‘WCp)Fl2
Fragment
ER’”
D,(M-L)
D,(M-L’)
Ti(Cp),(Me)Cl Ti(Cp),Me, Ti(Cp)z(Ph)C1 Ti(Cp),Phz Ti(Cp) 2(CO) 2
Ti(Cp),Cl Ti(Cp),Cl Ti(Cp),Me Ti(Cp),Cl Ti(Cp),Ph Ti(Cp),CO
-41 (-41)b -11 (-41)b -41 -2
390 276 287 292 290 170
471 471 309 471 372 174
Mo(Cp),Cl, Mo(Cp),H, Mo(Cp)zMez
Mo(Cp) ,Cl Mo(Cp) zH Mo(Cp),Me
-65 -11 (-17)
238 246 149
369 268 183
a Reorganization energy of the fragment. bIdentical structures of the fragment in the complex and in Ti(Cp),Cl, assumed. ‘The structure of the complex was estimated. See ref. 12.
were
I
I
400
450 D(L-H)/kJmol-’
Fig. 1. Metal-ligand mean bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies for halogen, nitrogen, oxygen and sulphur ligands.
Thermochemistry of M($-&H&L, energy of Ti(Cp),Cl is larger than Ti(Cp),Me. Three, D,(M-L) > D,(M-L), as could be expected, because D,(M-L) refers to the species M(Cp),L, where the metal is in a lower oxidation state and/or is coordinated to a smaller number of ligands. Although we strongly favour a qualitative view of the data in Table 10, recent preliminary thermochemical results involving several titanium bis(pentamethylcyclopentadieny1) complexes, viz. Ti(Cp*),Mez, Ti(Cp*)zMe, Ti(Cp*),Ph, Ti(Cp*),C12 and Ti(Cp*),Cl, showed that those estimates may also have some quantitative value.3s The following D,(Ti-L) were directly obtained from the standard enthalpies of formation of the corresponding Ti(II1) and Ti(IV) complexes (kJ mall ‘) : D,(Ti-Ph) = 269k28, D,(Ti-Me) = 271 f21, D,(Ti-Cl) = 380 + 20. All are in fair agreement with the predictions in the table. reorganization
1541
complexes CONCLUDING
REMARKS
The present paper shows that the energetics of MEL,, complexes can be reasonably predicted by a judicious use of the “bond enthalpy term method”, even when the enthalpies of formation of the radicals L are not available. In addition, the recent comparison with the experimental data for the bis(pentamethylcyclopentadieny1) titanium molecules gives some credit to the estimates of stepwise Ti-L bond dissociation enthalpies. The usefulness of the method to analyse the thermochemical data of other families of compounds has also been tested.36 As mentioned before, the bond enthalpy term method relies on the Laidler scheme. It is thus not surprising that some of the features observed for organic molecules are reflected in metal-ligand terms. For instance, the Laidler scheme recom-
Table 11. Auxiliary thermochemical data (kJ mol- ‘) L H Cl Br I PhO PhCOO cc1,coo CF,COO CsH,Gz CioHsGz C,~HBGZ SO4 RSd PhS Me&I-W N3 CsH,N Me Et DBU Ph’ PhCH, CP CzH4 co cis-N,Ph2 PMe,
Ref.
A$(L,g)
39 39 39 39 11 40 41 42
217.997 121.302 111.86 106.762 48klO -76.3f3.0 -213+ 13 (-815.7*10)
IL43 11,44 45 11 46 11 11 47 48 39 48,49 48
240.2f8.4 461 k21 146.9kO.6 119+4 (74f8) 328.9 + 8.4 200+6 260f9 52.2+ 1.2 -110.53f0.17 455.7+2.5 -101.1+5.3
D(L-H) 436.0 431.6 366.2 298.4 362+ 10 435.8 * 3.4 (433 * 10) (433 * 10)
375.7& 8.4 345.8k8.4 385&21 439.7* 0.8 421 f4 (418+8) 464.Ok8.4 368&d 347f 10
E(L-H)* 436.0 431.6 366.2 298.4 451 451 451 451 451 451 451 451 360 360 360 377 377 415.8 411 411 421 411
D-E
0 0 0 0 -89 -15 -18 -18
16 -14 8 24 10 7 43 -43
“Gas-phase data. Estimated values in parentheses. *Laidler terms from ref. 1 ( f 8 kJ mol- ‘) or calculated from tabulated enthalpies of formation. ‘The same bond dissociation enthalpy was used for L = Me&H,0 and ClC,H,O. dR = alkyl. ‘The same bond dissociation enthalpy was used for L = MeC6H4, CF,C,H, and MeOCsH,.
1542
A. R. DIAS and J. A. MARTINHO
SIMBES
300 ‘; 5 E 3 : d i IO 200
100
400
450
500 D(L-H)/kJmol-’
Fig. 2. Metal-carbon
and metal-hydrogen mean bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies.
a single term for O-H and S-H bonds, ’ also being possible to define average M-O and M-S terms, within, ca f20 kJ mol-‘. As metalligand bond strengths are more sensitive to the chemical environment than the hydrogen-ligand bonds, it is not surprising that the interval is relatively wide. Other examples showing parallel trends between Laidler terms and metal-ligand terms can be found in the tables mentioned above (compare, e.g. Ti-Ph and Ti-Me bond terms, Table 9, with E(Ph-H) and E(Me-H), Table 11). The most common attempt to “reduce” organometallic thermochemistry to organic and inorganic thermochemistry consists of plotting metal-ligand bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies. It is indeed observed that such plots often lead to good linear variations, thus allowing the prediction of new values.37 The same type of correlation for the Ti, MO and W bis(cyclopentadieny1) complexes is shown in Figs 1 and 2. As expected for these metals, the data for Cdonor ligands (Fig. 2) does not fit the lines obtained mends
for x-donor ligands (Fig. 1). It is also noted that b(M-SPr-n) are significantly lower than the values predicted on the basis of the correlations in Fig. 1. Itjs very unlikely that the deviation is due to wrong estimates of the enthalpies of sublimation of the complexes : it would be necessary to decrease these quantities by cu 60 kJ mol- ’ to obtain a good fit of the three points. Instead, the deviation may be attributed to the larger polarizability of the sulphur atom as compared with oxygen. However, an identical discrepancy is not observed in the case of PhS. D(Mo-H), on the other hand, is clearly above the line defined with three Mo-alkyl bonds in Fig. 2, even if the correction ER,/2(N - 42 kJ mol- ‘) is applied. Although the previous correlations deal with mean bond dissociation enthalpies and therefore important features may be hidden, they seem less successful than recently proposed plots between the standard enthalpies of formation of crystalline complexes and the enthalpies of formation LH (for anionic ligands) or L (for neutral ligands) in their
Thermochemistry
of M($-CSH&L,
standard reference state (rs),3” i.e. their stable physical state at 298.15 K and 1 atm. An advantage
of these latter correlations over the ones involving bond dissociation enthalpies is that they allow estimation of the enthalpies of formation of complexes even in the absence of data for D(L-H), this being particularly important in the case of bidentate (e.g. S04, O&H,) and neutral (e.g. C2H4, N2Ph2) ligands. Interestingly, it can be shown that the plots D(M-L) vs D(L-H) and AH/o(complex,c) vs AHfo(ligand, rs) are mathematically equivalent, provided that a cancellation of sublimation and vaporization enthalpies occurs. This matter will be further explored in a future publication. We have deliberately omitted important chemical implications of the thermochemical data surveyed in the tables above, some of which can be found in the papers where those values were originally reported. Although the main purposes of acquiring thermochemical values are understanding reactivity and providing new insights into the nature of chemical bonds, a broad view of organometallic energetics is rather desirable at the present stage. The development of new and more accurate estimation procedures will undoubtedly be fostered both by the massive amount of thermochemical data that is currently being obtained in some laboratories and by the growing efforts of theoretical chemists to produce reliable values of enthalpies of formation and bond dissociation enthalpies.
complexes
1543
10. A. R. Dias and J. A. Martinho &noes, Rev. Port Quim. 1982,24,191. 11. D. F. McMillen and D. M. Golden, Ann. Rev. Phys. Chem. 1982,X$493.
12. M. J. Calhorda, A. R. Dias, A. M. Galvgo and J. A. Martinho SimGes, J. Organomet. Chem. 1986, 307, 167.
13. J. Bickerton, M. E. Minas da Piedade and G. Pilcher, J. Chem. Therm. 1984,16,661. 14. A. R. Dias, P. B. Dias, H. P. Diogo, A. M. Galvao, M. E. Minas da Piedade and J. A. Martinho Sirnoes, Organometallics
1987,6, 1427.
15. J. C. G. Calado, A. R. Dias and J. A. Martinho Sirnoes, J. Organomet. Chem. 1980,1%,203. 16. G. Pilcher and H. A. Skinner, The Chemistry of Metal-Carbon Bond (Edited by F. R. Hartley and S. Patai), Chap. 2. Wiley, New York (1982). 17. D. R. Stull and H. Prophet, JANAF Thermochemical Tables (2nd Edn). National Bureau of Standards, Washington, D. C. (1971). 18. M. J. Calhorda, A. R. Dias, J. A. Martinho Sirnoes and C. Teixeira, J. Chem. Sot., Dalton Trans. 1984, 2659.
19. M. J. Calhorda, M. A. A. F. de C. T. Carrondo, A. R. Dias, A. M. T. S. Domingos, J. A. Martinho Sirnoes and C. Teixeira, Organometallics 1986, 5, 660.
20. M. J. Calhorda, M. A. A. F. de C. T. Carrondo, A. R. Dias, C. F. Frazilo, M. B. Hursthouse, J. A. Martinho Simdes and C. Teixeira, submitted for publication. 21. M. D. Ribeiro da Silva, Tese de Doutoramento, Faculdade de Ciencias do Porto (1985). 22. D. Cozak and M. Mehrik, Coord. Chem. Rev. 1986, 14, 53.
23. M. A. A. F. de C. T. Carrondo, M. J. Calhorda and M. B. Hursthouse, Acta Cryst. 1987, C43, 880. 24. M. F. Lappert, D. S. Patil and J. B. Pedley, J. Chem.
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A. R. DIAS and J. A. MARTINHO
35. A. R. Dias, M. S. Salema, J. A. Martinho Sirnoes, J. W. Pattiasina and J. H. Teuben, J. Organomet. Chem., in press. 36. J. A. Martinho Sirnoes and J. L. Beauchamp, to be submitted to Chem. Rev. 37. H. E. Bryndza, L. K. Fong, R. A. Paciello, W. Tam and J. E. Bercaw, J. Am. Chem. Sot. 1987,1O!J,1444. 38. A. R. Dias, J. A. Martinho SimBes, C. Teixeira, C. Airoldi and A. P. Chagas, J. Organomet. Chem. 1987,335,71. 39. CODATA Recommended Key Values for Thermodynamics, J. Chem. Therm. 1978,10,903. 40. C. E. H. Baum and S. F. Mellish, Trans. Faraday Sot. 1951,47, 1216. 41. A. R. Dias, M. S. Salema and J. A. Martinho Simdes, Rev. Port. Quim. 1984,26,227. 42. J. C. G. Calado, A. R. Dias, M. S. Salema and J. A. Martinho Simbes, J. Chem. Sot., Dalton Trans. 1981, 1174. 43. B. K. Janousek, K. J. Reed and J. I. Brauman, J. Am. Chem. Sot. 1980,102,3125. 44. S. W. Benson, Chem. Rev. 1978,78,23. 45. M. J. Pellerite, R. L. Jackson and J. I. Brauman, J. Phys. Chem. 1981,85, 1624. 46. P. D. Pacey and J. H. Wimalasena, J. Phys. Chem. 1984,88,5657. 47. D. J. DeFrees, R. T. McIver Jr and W. J. Hehre, J. Am. Chem. Sot. 1980,102,3334. 48. J. B. Pedley and J. Rylance, Sussex-N.P.L. Computer Analysed Thermochemical Data : Organic and Organometallic Compounds. University of Sussex,
Brighton, U.K. (1977). 49. F. W. Schulze, H. J. Petrick, H. K. Cammenga and H. Klinge, 2. Physih. Chem. (Munich) 1977, 187, 1.
SIMdES
50. M. W. Chase Jr, J. L. Curnutt, J. R. Downey Jr, R. A. McDonald, A. N. Syverud and E. A. Valenzuela, JANAF Thermochemical Tables, 1982 Supplement, J. Phys. Chem. Ref Data 1982,11,695. 51. V. L. Glushko and L. V. Gurvich, Thermochemical Constants of Individual Compounds, Vol. 1. Nauka, Moscow (1978). 52. G. Pilcher, CATCH Tables, Nitrogen Compounds. University of Sussex, Brighton, U.K. (1972). 53. A. R. Dias, M. S. Salema and J. A. Martinho SimGes, J. Organomet. Chem. 1981,222, 69. 54. G. Pilcher, unpublished results. APPENDIX The auxiliary data used to calculate metal-ligand bond enthalpies are collected in Table 11. ’ ‘,3s49 The standard enthalpies of formation of gaseous LH, molecules (n = 1 or 2) were taken from the following references : L = halogen (ref. 39); L = PhO, MeCsH40, PhCOO, CFSCOO, RS, PhS, C,H,N, Me, Et, n-Bu, Ph, PhCH*, CF3CsH4, MeOCsH4, Cp (ref. 48) ; L = SO4 (average between the values in refs 50 and 51, -737+8 kJ mol-‘); L = Fc (taken as 227.6k4.2 kJ mol-‘, by using the enthalpy of formation of the crystal in ref. 5 and an average value for the enthalpy of sublimation) ; L = N3 (ref. 52) ; L = 2Cl&H40 (estimated as - 145 f 11 kJ mol- ’ in ref. 53) ; L = CC13CO0 (ref. 48, together with an experimental value for the enthalpy of sublimation,54 gives -428.3It8.4 kJ mol-‘); L = C,4Hs02 (estimated as -144f6 in ref. 21); L= CsH402, C10H,02 (-267.8f1.7 and -181.3k4.7 kJmol_‘, respectively; ref. 21); L = Me&H,& (estimated as 108 k 6 kJ mol- ’ in ref. 18).
1988 0
THERMOCHEMISTRY OF. M(q=C&),L, (M = Ti, MO, W) ALBERT0
R. DIAS and JOY& A. MARTINHO
0277-5387/88 $3.00+.00 1988 Pergamon Press plc
COMPLEXES
SIMdES
Centro de Quimica Estrutural, Complexo I, Instituto Superior Tecnico, 1096 Lisboa Codex, Portugal Abstract--The available thermochemical data for the title compounds are summarized and discussed. Metal-ligand bond enthalpies are derived for a variety of ligands through a simple method that involves both the experimental standard enthalpies of formation and theoretical calculations. It is suggested that this method may be used to predict the energetics of new complexes with fair accuracy. In addition, the method provides estimates of metal-ligand stepwise bond dissociation enthalpies. Some of these estimates are compared with recent experimental results obtained for bis(pentamethylcyclopentadieny1) titanium complexes.
If the- ability to estimate new data is used as a criterion to assess the development of organometallic thermochemistry, we will be compelled to conclude that our present knowledge in this area is still rather incipient, particularly for molecules containing d and f elements. This situation contrasts with the one observed for organic substances, for which several reliable prediction schemes are available,le3 but is not surprising, given the large number of elements and hence the different chemical bonds that can be involved in organometallic species. Moreover, even if it were assumed that these schemes could be extended to metal compounds, the size of the data bank required to derive a suitable set of “bond terms” or “group contributions” would be several orders of magnitude larger than for organic compounds. Unfortunately, the virtually infinite variety of bonds and groups in organometallic complexes, hindering the existence of traditional prediction schemes, is also the reason why the development of estimation methods becomes so important. Alternative (though eventually less reliable) procedures have thus to be found. The simplest and most commonly used method to estimate the enthalpy change of any chemical reaction, in the absence of enthalpy of formation data, is to consider the bonds cleaved and formed. For example, the enthalpy of reaction (1) will be obtained if the difference between M-L and M-L
bond dissociation enthalpies is available. &ML(g) + L’(g) + XML’(g) + L(g). (1) Devising an estimation method for these quantities is apparently easier than for enthalpies of formation, since AHj’(X”ML,g) or A@&ML’,g) would require energetic information on each chemical bond in the complexes, and it is also less demanding in terms of size of the experimental data bank. The thermochemical studies on bis(cyclopentadienyl) compounds of the type M(Cp),L, (Cp = q5-CZH5 ; L = H, alkyl, aryl, carbonyl, halogen, etc.) carried out in our laboratory enabled us to address some issues that may contribute to improved reliability of estimated metal-ligand bond dissociation enthalpies and standard enthalpies of formation. As the structural features of the fragment M(Cp), are nearly constant in many complexes, indicating that its enthalpy content is not significantly changed, attention can be concentrated on the “reactive” side of the molecules, i.e. the bonds M-L. The dependence of the strengths of these bonds on the metal and its oxidation state, and on the number, types and geometrical constraints of ligands, has been examined in a wide variety of cases. It is the purpose of the present paper to summarize both the method used to derive metalligand bond enthalpies from experimental data and the main conclusions drawn from these studies. Pre-
1531
1532
A. R. DIAS and J. A. MARTINHO SIMdES
liminary values of some recently determined bond enthalpies are also included in the discussion. \
CALCULATION OF BOND ENTHALPIES FROM EXPERIMENTAL DATA
Enthalpies of reaction of M(CI>)~L,, complexes with several substances, typically HCl or I2 solutions, have been determined with a reaction-solution calorimeter described in detail elsewhere.4 These measurements, together with auxiliary enthalpies of solution and literature data, led to the standard enthalpies of formation of the crystalline complexes. Unlike combustion calorimetry, reaction-solution calorimetry is not an “absolute” method, i.e. it normally yields a difference between the enthalpies of formation of two complexes. It is therefore necessary that one of these quantities is available so that the other can be calculated. All the AH~O[M(CP)~L,,C]values listed here have been obtained using the reported standard enthalpies of formation of the related dichloride molecules, M(Cp)&12, which were derived from static (MO, Ti) and rotating (W) bomb combustion experiments.ss6 Although the first of these methods is not considered very reliable for studying such compounds, we have accepted these values as references, in the absence of further information. Incidentally, AHfO[W(Cp)2C12,c]= -71.1 k2.5 kJ mol-’ (staticbomb) is close to the recent rotating-bomb result, - 63.5 + 7.7 kJ mol- ‘. The standard enthalpies of formation of gaseous M(Cp),L, complexes, necessary for evaluating metal-ligand bond enthalpies, were determined by measuring or estimating the standard enthalpies of sublimation of those molecules. Whenever possible, AH: values were obtained from Knudsen cell experiments (the Knudsen cell is a modified version of the one reported in ref. 7), but in most cases they had to be estimated. A thorough discussion of this subject shows that enthalpies of sublimation of analogous complexes usually fall in a narrow range, thus allowing reasonably accurate estimates. * As happens with any molecule containing several types of ligands, the determination of the enthalpy associated with a given bond from the enthalpy of formation in the gas state depends strongly on the enthalpies ascribed to the remaining bonds. In the case of the bis(cyclopentadieny1) complexes, the difficulty of estimating M-Cp bond enthalpies was avoided by considering the “reference” complexes t The assumption that D(M---Cl) in MCI, is identical to &M-Cl) [and not to E(M-Cl)] in M(Cp),C12 has been used in ref. 5.
4 WCp)z(g) + &)
Scheme I.
WCpMg)+ 2cl(g) Scheme 2. M(Cp),C12 and by using two different concepts of bond enthalpies: bond enthalpy terms, E(M-L), and mean bond! dissociation enthalpies, 6(M-L). These quantities are related through Schemes 1 and 2, from which eqs (2) and (3) were derived. E(M-L)
= E(M-Cl) - A@(Cl,g) -
+ ; AH;(L*,g)
{~~;bWpM+ngl
- ~fffOM~p),C1d}I2 + W, - ER J/2 (2) &M-L)
= E(M-Cl)
+ ; AHfo(L,g)
- AH,“(Cl,g) -
W#Wp)Lgl
-
A@DWWlz,gl}/2 + E&/2.
(3)
The asterisks indicate non-reorganized fragments, i.e. fragments retaining the geometry they had in the initial complex. ER,, ER3 and ERL are the energies associated with the relaxation of the fragments to their ground states. Bond enthalpy terms E(M-L) and E(M-Cl) do not contain the reorganization energies of the fragments. They should therefore be a better measure of the M-L “bond strengths”, affording more meaningful correlations with other structural parameters (e.g. bond lengths) than D(M-L). This point is relevant for the evaluation of E(M-L) and 6(M-L) through eqs (2) and (3). An estimate of M-Cl bond enthalpy in M(Cp),C12 can be made by considering the bond enthalpy in the homoleptic molecule MCl,. If M-Cl bond lengths are similar in both molecules, then E(M-Cl) = b(M-Cl) in MCI, can be transferred to the dichloride complex, i.e. is identified with E(M-Cl) in M(Cp),Clz.t This was indeed the procedure used to calculate E(M-L) data collected in the tables: for molybdenum and tungsten (m = 6) E(M-Cl) were taken as 303.8 and 347.3 kJ mol- i, respectively, and for titanium (m = 4) E(M-Cl) = 430.5 kJ mall I.’ If 6(M-Cl) in MCl, was identified with @M-Cl) in the complex,t then ER3 = 0. As indi-
Thermochemistry of M($GH&L, cated by extended Hiickel molecular orbital calculations, this conclusion does not hold for the molybdenum and tungsten dichlorides (ER3 - -82 and - 103 kJ mall ‘, respectively), although it is reasonable for the titanium analogue (ER,- - 11 kJ mol- ‘). Like ER3, the reorganization energies ER, have also been estimated from curves where the total energy of the fragments M(Cp), is plotted as a function of the angle Cp-M-Cp (0). The most stable geometry of the isolated fragment is achieved with 8 = 140 and 180” for metals having two and four d electrons, respectively, As 0 is usually close to 130” in M(Cp),L, complexes, this accounts for the large reorganization energies for molybdenum and tungsten fragments as compared with titanium.’ The dependence of the M(Cp), energy on the MCp distance is less important. Besides, for each metal, these bond lengths are rather constant. The extended Hiickel results have always been handled in a semi-quantitative way. Accordingly, the bond enthalpies collected in the tables are not affected by the “corrections” in eqs (2) and (3). It must be noted, however, that in the case of E(M-L), eq. (2), the reorganization term (ER3 - ER,)/2 is usually small since the angle 8 is fairly similar for most complexes. Examples and exceptions are shown in the next section. A pragmatic approach can be followed to obtain the enthalpies of formation of non-reorganized species, AHfo(L*,g), which are also required to calculate E(M-Cl) by eq. (2). lo For anionic ligands, it is first considered that the geometry of L* is the same in the complex and in the molecule LH, implying that AH,“(L*,g) can be derived from Scheme 3 or eq. (4). Secondly, E(M-L) is identified with the respective Laidler term. ’ AHfo(L*,g) = E(L-H)
+ AH;(LH,g)
- AH;(H,g) = D(L-H)
- ERL
+ AHfo(LH,s) - A@(H,g).
(4)
A simple calculation involving, for instance, the methyl radical, demonstrates that the introduction of Laidler terms in eq. (4) leads to physically meaningless results : ERL, given by the difference between D(CH,-H) (439.7 kJ mol-I)” and E(C-H) (= 6(C-II) = 415.8 kJ mol- ‘), is positive, implying that the CfV configuration of the free radical
L*(g) + H(g) LWg) E’L-H’b W-W
\I
ERL L(g) + H(g)
Scheme 3.
1533
complexes
WWJ&) + L(g) Scheme 4.
should be more stable than D3,,. Despite this lack of physical meaning, the procedure has the advantage of yielding E(M-L) data that are consistent with the Laidler scheme and eventually can be used to make predictions. Moreover, as shown below, estimates of stepwise bond dissociation enthalpies are independent of the value chosen for E(L-H). If necessary, the assumption of identical geometries of L* in LH and in M(Cp),L, can be corrected by computing their energy difference. As illustrated in the next section, this has occasionally been made by using the Htickel, MINDO-3 or MNDO methods. The estimation of metal-ligand partial (or stepwise) bond dissociation enthalpies follows from the above method in a straightforward manner. The first bond dissociation enthalpy, D ,(M-L), is calculated using eq. (5), derived from Scheme 4, where the symbols have the usual meaning. D ,(M-L)
= E(M-L)
+ ER; + ER,_.
(5)
As E(M-L) is transferred from eq. (2) and it relies on eq. (4), D,(M-L) becomes independent of the value ascribed to E(L-H). In other words, ER, is cancelled when eq. (5) is used. The reorganization energy ER’, is also estimated through the extended Hiickel approximation by optimizing the structure of the free fragment M(Cp)*L.12 The second metal-ligand bond dissociation enthalpy, D,(M-L), is simply the difference between 26(M-L) and D1(M-L). The Hiickel calculations for several systems (L = Cl, H, CH,, SCH3; M = MO, Ti) indicate that ligand to metal rc-donation results in larger values of D2 - D, than a-donation (see Table 10). STANDARD ENTHALPIES OF FORMATION AND BOND ENTHALPY DATA As stated previously, the standard enthalpies of formation of all the bis(cyclopentadieny1) complexes reported here rely on combustion results for AH/O[M(Cp)2C12,c].It must be emphasized that a revision of the static-bomb values (MO, Ti) will not affect the metal-ligand bond enthalpy data: AZ$[M(Cp),C12,c] is cancelled in eqs (2) and (3), since the same value was used to derive
1534
A. R. DIAS and J. A. MARTINHO Table
1. Standard
enthalpies of formation (kJ mol- ‘)
SIMaES
of halogen
complexes
Complex
AH;(c)
AH,0
AH;‘(g)
Ti(Cp)KL Ti(Cp)&
-383.2k7.5 - 148.4+ 13.1
118.8k2.1 (120f 10)b
-264.4k7.8 -28.4f 16.5
Mo(Cp)FL Mo(Cp),Br, Mo(Cp),I,
-95.8k2.5 9.7+ 12.7 69.8k7.8
(100.4f4.2) (100.4k4.2) (100.4+4.2)
4.6 +4.9 110.1 f 13.4 170.2k8.9
W(CP),ClZ WCp),Br, WCP),IZ
-63.5 + 7.7 15.7+ 13.9 65.4+ 10.5
(104.6+4.2) (104.6k4.2) (104.6k4.2)
41.1k8.8 120.3 + 14.5 170.0+11.3
“Data from refs 4, 5, 6, 14 and 15. bEstimated values in parentheses.
AH~U[M(CP)~L,,,C]. A different conclusion is drawn for M--Cp bond enthalpy terms, which, by a first approximation, can be obtained from eq. (6). E(M-Cp)
Table
2. Metal-halogen and metal-cyclopentadienyl bond enthalpies (kJ mol- ‘)
Complex
E(M-X)
&M-X)
E(M-Cp)
Ti(Cp)& Ti(Cp)&
430.5& 1.3 430.5& 1.3 298+9 298k9
318rf: 10
Mo(Cp),Cl, Mo(Cp),Br, Mo(Cp),Iz
303.8k7.1 242f 10 207*9
303.8f7.1 242f 10 207f9
404+12
YCP) ,Cl Z YCp),Brz W(CP)2I*
347.3kO.8 298*7 268k5
347.3kO.8 298+7 268f5
443+ 10
= AH;(M,g)/2 + AHfo(Cp,g)
+ AH,O(Cl,g)--~~fo[M(Cp>,CLg1/2
- E(M-Cl).
(6)
As &M-Cl) is constant, because it is transferred from MCl,, an increase in the enthalpy of formation of the complex will imply a decrease of E(M-Cp). Some enthalpies of formation shown in the tables were recalculated by using recent literature data.? This applies particularly to the tungsten complexes, whose values were adjusted to the recently reported AH_#V(Cp)ZClZ,c].6 Enthalpies of formation of ligands or radicals, necessary to derive M-L bond enthalpies, are given in the Appendix. Halogen complexes
Table 1 collects the available standard enthalpies of formation of these bis(cyclopentadieny1) derivatives. Except for the dichlorides, all the values were obtained from reaction-solution calorimetric results.4*‘4*‘5 The bond enthalpy data are shown in Table 2 and include metal-halogen and metalcyclopentadienyl bond enthalpy terms, and metalhalogen mean bond dissociation enthalpies. As stated before, no corrections due to ER,, ER3 and EGP were included in those values. This hardly affects E(M-X), since (ER,-ER,)/2 must be close to zero, i.e. Cp-M-Cp angles are likely to be similar in M(Cp),Cl* and M(Cp)rX2 (X = Br, I). The same applies to D(Ti-X) (x = Cl, Br, I), t AH/O[M(Cp)2Br,,c]were recalculated from new data for AHf(CBr,,c)-AHT(CHBr,,l) in ref. 13.
“Data not including reorganization
energies ER, and
ER,. because ER3 is negligible, but not so for 6(Mo-X) and b(W-X), whose corrections are predicted to be significant (see previous section). As for E(M-Cp), the values displayed are also upper limits, since EGP was not considered. The usual trends E(M-Cl) > E(M-Br) > E(M-I) and E(Ti-X) > E(W-X) > E(Mo-X) are observed in Table 2, but a striking feature is apparent: E(Ti-Cp) < E(Ti-Cl), while an inverse relationship is noted for molybdenum and tungsten. Although the absolute values in the table may be controversial (only differences E(M-X) - E(M-Cl) and 6(M-X) -&M--Cl), for each metal, can be trusted), it seems hard to believe that the transferability produces errors of ca 100 kJ mol- ‘. Interestingly, the trend for titanium is also observed for zirconium and hafnium, ’ 6 in agreement with the known chemistry of these complexes (e.g. the stability of M(Cp)C13 complexes and the lability of M-Cp bonds for group 4d metals). It is finally noted that E(W-Br) = 296+7 kJ mol-’ in WBr, (AHfO(WBr,,g) and AHfo(TiI,,g)
1535
Thermochemistry of M(qS-CSH&Ln complexes Table 3. Standard enthalpies of formation of oxygen and sulphur complexes (kJ mol- ‘)
-
Complex
AH;
MT(c)
AH&)
(97 + 8)b (104+8) (104+8) (104f8)
Ti(Cp) ASPh) 2 Ti(Cp)&SGH,Me)
-393.7f8.0 -448.9k8.2 -408.1 k8.1 -448.7k7.9 -440+22 -789.7f8.1 -1095*19 -2251.2k8.2 -322.6+ 12.7 -211.3f8.2 -267.5k9.7 34.Ok7.9 -30.3k9.5
(9Of 10) (112k8) (130+8) (108&8) (165+8) (106f 10) (108f8) (113f 10) (11Ok 10)
-296.7+ 11.3 -344.9+ 11.5 -304.1 If: 11.4 -344.7& 11.3 -35Ok24 -677.7+ 11.4 -965+20 -2143.2k11.4 - 157.6* 15.0 - 105.3 + 12.9 - 159.5 + 12.5 147.0+ 12.7 79.7* 13.8
Mo(Cp)z(OzCPh)z Mo(QWWCPJ~ Mo(C~)z(0,Cd&) M~(CP),(O&,HH~) M~(CP),(CG,H,) Mo(Cp)XgOJ Mo(Cp) GPr-n) 2 Mo(Cp) GPr-i) 2 Mo(Cp),Wu-n), Mo(Cp),(=u-02 M~(CP)~(SCIOH~I-~)~ Mo(Cp)z(gPh),
-500.7+3.4 - 1984.2k4.2 - 130.6 + 2.9 - 80.3 f 3.2 - 53.3 f 10.4 -650.4f3.4 - 10.7k5.3 -22.7k5.3 -96.Ok5.5 -68.3k4.3 -361.5k4.3 282.0 f 4.8
(94k 10) (9Ok 10) (100+8) (135+8) (145&8) (72 f 10) (90 f 10) (9Ok 10) (92+ 10) (92 + 10) (105& 10) (95 + 10)
-406.7+ 10.6 - 1894.2+ 10.8 -30.6k8.5 54.7 * 8.6 91.7+ 13.1 - 578.4+ 10.6 79.3f11.3 67.3+ 11.3 -4.o+ 11.4 23.7+ 10.9 - 256.5 + 10.9 377.0+ 11.1
WCp) A0 ,CPh) z WCPWZCCPA W(CP),(CGHJ W(Cp) #Et) z WCp)#Pr-n), WCp) ,(SPh) z
-455.8 + 8.2 - 1925.6+ 8.2 - 105.2k7.8 70.3 f 9.0 16.6k9.2 327.5k9.1
(98 + 10) (94f 10) (104&8) (92f.10) (94k 10) (99f 10)
- 357.8 f 12.9 - 1831.6f 12.9 -1.2k11.2 162.3& 13.5 110.6+ 13.6 426.5 f 13.5
TWpMOPh)~
Ti(Cp),(OCsH,Me-2), Ti(Cp)@Cd%Me-3), Ti(Cp)#%H4Me-4)2 Ti(Cp)2(OC~H&l-2)2 Ti(Cp)#WPh), Ti(Cp)KWCClA Ti(Cp)GWCPA Ti(Cp)&C,~HA Ti(Cp)#Et), Ti(Cp),(SPr-n),
’ Data from refs 18-2 1. bEstimated values in parentheses.
quoted from ref. 17) and E(Ti-I) = 294 + 2 kJ mol- ’ in Ti14” are in excellent agreement with ,??(W-Br) and E(Ti-I) in Table 2, which, we recall, rely on E(W-Cl) and E(Ti-Cl) from WC16 and TiCl.,. This seems to indicate that reliable predictions of the enthalpies of formation of the complexes MUFF can easily be made using eq. (2) together with @M-F) values obtained from the available data for AHfO(MF,,g) : ” AHfO[M(Cp),F2,g] -656, -369 and - 367 kJ mol- ‘, respectively for M = Ti, MO and W [E(M-F) = 585,449 and 510 kJ mol- ‘I. Oxygen and sulphur complexes The standard enthalpies of formation of these molecules, obtained by reaction-solution calor-
imetry, ‘*-*’ are collected in Table 3 and were used to derive metal-oxygen and metal-sulphur bond enthalpy terms and bond dissociation enthalpies (Tables 4 and 5). The available molecular structures for some complexes in these tables show that the geometry of M(Cp), fragment is nearly constant for each metal and similar to the one in the respective dichloride. 1*-20~22~23 This constancy is assumed for the remaining molecules and implies that the correction terms (ER, - ER 42 are not significant. For instance, in the case of the molybdenum sulphate molecule, the angle Cp-M-Cp is 134”, corresponding to (ER,- ER,)/2 N - 13 kJ mol- ‘. In the absence of steric effects, metal-oxygen bond enthalpy terms for each metal fall in a narrow range, suggesting that the average values for Ti (452 kJ mol- ‘), MO (333 kJ mol- ‘) and W (365 kJ
1536
A. R. DIAS and J. A. MARTINHO SIMaES
Table 4. Metal-oxygen bond enthalpies (kJ mol- ‘) Complex
E(M-0)
&M-O)
Ti(Cp)Z(OPh)z Ti(Cp),(OCsH,Me-2), Ti(Cp),(OC,H,Me-3)2 Ti(CpWC&Me-4), Ti(Cp),(OC,H,Cl-2)2 Ti(Cp)@,CPh), Ti(Cp)#WCCl,), Ti(Cp)~(02CCPS)2 Ti(Cp)~(O~G&I,)
462+9 454+9 430&-9 457f9 440+ 10 455*9 464+9 451f9 417+9
373fll 365+ 11 341+11 368+11 351k12 440+6 446-1-11 433+11
WCpWzCPh),
327+ 12 334+ 12 299f9 300+9 300+ 10 33859
312f 10 316f 14
365& 10 365f 10 346f5
349*7 347+11
WCPW,CCF,), M~CPMOG&)
M~(CP)@G&) M~(CP)&CUH~) Mo(Cp),(S03 W(Cp)&WPh), YCP)KGCP,), W(CPMWAH.,)
=Data not including reorganization energies ER, and ER3.
mol- ‘) can be used to estimate standard enthalpies of formation of other complexes within ca +20 kJ mol- r. Furthermore, the average for titanium is not far from the mean value of E(Ti-0) in Ti(OR), molecules (R = alkyl), 466 kJ mol-‘.‘g The low values for the bidentate ligands 1,2benzenodiolate, 1,2naphthalenodiolate and 9,10phenanthrenodiolate were not included in the above averages since they are believed to reflect relatively Table 5. Metal-sulphur bond enthalpies &.I mol- ‘) Complex
E&I-S)
&M-S)
326f 10 331+10 358f 10 333* 10
341* 10 347+ 10 344+ 10
Mo(Cp)@‘h),
220& 12 217f. 12 241+12 206f9 243+ 12 251f12
235k 13 233k 13 256k 12 221+9 259+ 12 237k 12
WCp)#Et), WCp)@+n)z WCp),(SPh),
261+10 266+ 10 288+ 10
277k 10 281 f 10 274k 10
WW2CW2 Ti(Cp),(SPr-n), Ti(Cp) *(SPh)z Ti(Cp)&C~H&W Mo(Cp) @Pr-n) Z Mo(Cp)@‘r-i), M~(CPMSB~-~Z
Mo(Cp),(SEu-0, MWPMSGOHW~),
a Data not including reorganization energies ER, and ER3.
high strain energies (40-80 kJ mol- I).*’ An opposite conclusion is drawn for the complex Mo(Cp),(SO,). The “normal” E(Mo-0) value indicates a negligible strain in the sulphate moiety, this being supported by the molecular structure of the complex and by extended Hiickel calculations. ” Recommending “constant” metal-oxygen bond enthalpy terms in M(Cp),L, complexes for each metal is obviously an over-simplification that can only be allowed because our present data are both scarce and not very precise (this being mainly a consequence of estimated enthalpies of sublimation). A somewhat similar situation is found for metal-sulphur bond enthalpy terms. Defining average E(M-S) values is probably the most judicious use for the data in Table 5, particularly when the uncertainty intervals are considered. However, if the individual values are accepted, several trends are apparent. First, there seems to be an increase of E(M-SR) with the length of the n-alkyl chain. The observation of a similar pattern in the case of the above mentioned Ti(OR)4 molecules lends some support to that trend. Secondly, a large value (35 kJ mol- ‘) is noted for the difference E(Mo-SBun) - E(Mo-SBu-t), reflecting expected steric effects in the t-butyl complex. An interesting comparison between oxygen and sulphur bidentate ligands is provided by the complex Ti(Cp),(S2CgH3Me). The value obtained for E(Ti-S) indicates a negligible strain in the metallacycle. The larger size of the sulphur atom, compared with oxygen, causing a relaxation of the fivemembered ring, has been used as a possible explanation for that different behaviour.*’ The same pattern is observed in cyclic organic molecules (CH),_ rX (X = CH2, 0, NH or S), discussed by Cox and Pilcher. ’ For n = 5 the strain energy decreases by about 15 kJ mol- ’ when oxygen is replaced by sulphur. The usefulness of the bond enthalpy term concept is evidenced by the oxygen and sulphur bidentate complexes: E(M-0) and E(M-S)’ provide the only energetic information on these bonds, since the enthalpies of formation of the free biradicals are not available, and therefore mean bond dissociation enthalpies could not be calculated. Nitrogen andphosphorus complexes
The metal-nitrogen bond enthalpy data (Table 7) were derived from the reaction-solution calorimetric standard enthalpies of formation (Table 6) ‘*I4by the common procedure, i.e. eqs (2) and (3). However, Scheme 3 or eq. (4) were not used for the neutral ligand azobenzene. Instead, AH,O(L*,g) relies on a MNDO result for the reorganization
Thermochemistry of M($-CsH&L,
1537
complexes
Table 6. Standard enthalpies of formation of nitrogen and phosphorus complexes (kJ mol- ‘) Complex
AH,0
AH;(c)
Ti(Cp)2(N3)2 Ti(Cp)@GW~ Ti(Cp) JN,Ph J Ti(Cp),(PMe3),
436.4 f 8.4 181.9f 12.1 338.8+_ 16.7 -261.9+ 17.1
Mo(CpL(NzPh3
563.7 f 8.6
(70 f (log+ (go+ (go+
10)b 10) 15) 10) -
(70f 15)
AH/o(g) 506.4k13.1 290.9* 15.7 428.8 + 22.4 171.9f 19.8 633.7f 17.3
“Data from refs 9, 14 and 25.
bEstimated values in parentheses.
energy of azobenzene to the cis-conformation (- 198 kJ mol-‘).‘4 Also, as &M-N) are meaningless parameters in this case, only values of D(M-N,Ph,) = 2D(M-N) are presented in Table 7. Although (ER, - ER,)/2 corrections are seen to be negligible for the titanium complexes with L = N3 and N2Ph2, and assumed to be so for the remaining Ti and MO molecules,9~‘4 the Hi.ickel calculations indicate that a large negative correction [ER3 N - 82 kJ mol-‘; see eq. (3)] should be applied to the value of D(Mo-NzPhz) in Table 7. As discussed before, ER3 is not significant for the titanium analogues. The calculation method was also slightly modified for evaluating E(Ti-N,) because N-N bond lengths are considerably different in the complex and in N3H.’ Extended Hiickel calculations indicate that the fragment N3 is stabilized in the complex by ca 48 kJ mol-‘. This correction, which implies a negative adjustment to the value obtained from eqs (2) and (4), is included in Table 7. The average between E(Ti-N,) and E(Ti-NCBH6), with 339 kJ mol- ‘, can be compared E(H-NR3 - 311 kJ mol- ’ in Ti(NR3, (R = Me, Et). (E(Ti-Nr,) were recalculated in 8 from data in ref. 24.) E(Ti-N) for the azobenzene complex is lower than the corresponding values for the monodentate hgands. The geometry of coordinated N2Ph2, a distorted cis form, does not seem to favour both o- and x-bonding with the metal atom. I4 The standard enthalpy of formation of Ti(Cp),)(PMe& was recently determined by reaction-solution calorimetry, allowing us to obtain E(Ti-PMe,) and b(Ti-pMe,).25 As in the case of azobenzene, eq. (4) was not used to evaluate AHT(L*,g). MNDO calculations give a negligible 7 The structural data of Ti(Cp)#Me,), ref. 22.
is included in
reorganization energy (- 5 kJ mol- ‘) for PMe3.26 This correction is not included in the result shown in Table 7 and the same happens with - -4 kJ mol.7 The thermo(E&-E&)/2 chemistry of other bis(cyclopentadieny1) complexes containing metal-phosphorus bonds are presently being investigated. For the moment it is of interest to stress that Ti-PMe, and T&CO bond strengths are comparable (see below). Complexes containing metal-hydrogen and metalcarbon bonds Table 8 summarizes all the available standard enthalpies of formation of these bis(cyclopentadienyl) derivatives. With the exception of AHfopi(Cp),(CH,Ph),,c], which was determined by static-bomb combustion calorimetry,’ the data were derived from reaction-solution calorimetric measurements.‘4~27 Preliminary results for MO (Cp)2Et2, Mo(Cp)2(n-Bu)2 and Mo(Cp),(C2H4) are Table 7. Metal-nitrogen and metal-phosphorus bond enthalpies (kJ mol- ‘F D or
Complex
@M-L)
Ti(Q%(N& Ti(Q),(NGH& Ti(CP)&Ph3 Ti(CLGZ(PMe&
329f 348+ 290f 162f
Mo(Cp),CNzPhJ
195* 126
&M-L)
10 10 126
385+ 22
12
162+ 12
381f24’ 192*23
a Data not including reorganization energies ER, and ERp. ‘Bond enthalpy term for one M-N bond. Values include the MNDO result for the reorganization energy of azobenzene to the cixonformation (- 198 kJ mol- ‘). ’ M-N,Ph, bond dissociation enthalpy (cis-azobenzene as a product). See also note a.
A. R. DIAS and J. A. MARTINHO
1538
SIMaES
Table 8. Standard enthalpies of formation of complexes containing metal-hydrogen and metal-carbon bonds (kJ mol- ‘) Complex Ti(CpLMe, Ti(Cp),(Me)Cl Ti(Cp),Ph2 Ti(Cp)G’h)Cl Ti(Cp)&&Me-3), Ti(Cp)&H,Me-4), Ti(Cp)&H&F+% Ti(Cp)&H&Me-rl), Ti(CpL(CH,Ph), Ti(Cp),Fczc Ti(Cp) GO) 2
AH;(c)
AH,0
AH;(g)
-26.6k9.6 -234.7k8.1 262.2 f 8.9 -79.2k7.8 166.8k9.4 169.6k9.6 -1142.6k8.3 -91.8k8.5 195.8 f 5.0 488.2+ 12.1 -295.3 + 12.9
(79.5 + 8.4)b (llO& 10) (88&8) (120f 10) (95 f 8) (95 f 8) (110+8) (104k8) (83.7 & 8.4) (15Ok 15) 84.2 + 3.5
52.9 + 12.7 - 124.7+ 12.9 350.2 + 12.0 40.8+ 12.7 261.8+ 12.3 264.6+ 12.5 -1032.6k11.5 12.2* 11.7 279.5 +9.8 638.2 f 19.3 -2ll.lk13.4
Mo(Cp)zH, Mo(Cp),Me, Mo(Cp),Et, Mo(Cp)&-Bu), Mo(Cp),(C,H3
210.3k5.7 262.4 k 4.0 [219.2+3.31d [143.5f4.6] 284.6 f 8.2
81.4f 1.0 70.4k4.2 93.7f1.8 (105f 10) (80 + 10)
291.7IfI5.8 332.8 + 5.8 [312.9+3.8] [248.5 & 11.O] 364.6* 12.9
W(Cp),H, YCP) z(H)1 W(Cp),Me,
222.4& 8.8 176.3k11.3 271.Ok8.1
84.1k1.6 (lOOf5) (74.6f4.2)
306.5 + 8.9 276.3 f 12.4 345.6k9.1
“Data from refs 5, 14, 29 and 30. bEstimated values in parentheses. ’ Fc = ferrocenyl. dPreliminary values in brackets. See footnote t on this page.
also reported,? of the standard
together with recent measurements enthalpies of sublimation of the
M(Cp)*H, (M = MO, W)” and Mo(Cp)2Et2.30 The analysis of M-L bond enthalpy values (Table 9) for the complexes M(Cp),(L)L’ (L # L’) will be made below. The remaining E(M-L) and @M-L) were calculated by the methods already described. In the case of the carbonyl complex, AH,“(L*,g) relies directly on a MIND0/3 calculation which yields a negligible value (-2 kJ mol- ‘)’ 4 for E&,. As the structure of the ethylene complex is not available, the calculation of the reorganization energy of C2H4 was not attempted and thus only the Mo-C2H4 bond dissociation enthalpy is shown. As happens with the bis(cyclopentadieny1) derivatives mentioned previously, the available t A small correction due to the dilution of the HCC ether solution used in the experimental study is not included in the values for the ethyl and butyl complexes. Although this correction will not affect significantly the bond enthalpy dam, it will be included when the results are fully reported. 28 $The angle Cp-Mo-Cp in the complex Mo(Cp),(n-Bu),, 135.2 implies @X2-ER,)/2 N - 16 kJ mol-‘.32
crystal structures for the complexes in Tables 8 or 9 22,3’$together with extended Hiickel calculations, indicate that corrections (ER,-ER,)/2are relatively small,? except for MOTHS, where the angle Cp-Mo-Cp, 145.8”,” is substantially larger than the one in Mo(Cp),Cl,, 130.5”,33leading to (ER,-ER,)/2 N -33 kJ mol- ‘. This implies that E(Mo-H) presented in Table 9 is a high upper limit and it is reasonable to draw a similar conclusion for the tungsten analogue, whose structure is not known. It has been mentioned that ER3 is small for titanium and relatively large for molybdenum and tungsten. Significant corrections (about -42 and - 52 kJ mol- ’ for MO and W, respectively) may therefore alIe& 6(Mo--L) and b(W-L) in Table 9. A broad look at the titanium-carbon and molybdenum-carbon bond enthalpy terms suggests that, for predictive purposes, the averages 290 kJ mol-’ (Ti) and 135 kJ mol- ’ (MO) can be defined, enabling us to estimate new enthalpies of formation within cu f: 20 kJ mol- I. These estimates can, however, be improved by considering some trends evidenced in the table. For instance, E(Ti-methyl) is 15-25 kJ mol- ’ lower than E(Ti-aryl) and E(Mo-C) decreases with the length of the n-alkyl chain.
Thermochemistry Table
9. Metal-hydrogen and metal-carbon enthalpies (kJ mol- ‘) Complex
274f5 293& 11 287k9 290f 13 299k 10 297+ 10 297k 10 305*9 28Ok 12 278fll 174+8
Mo(Cp),H, Mo(Cp),Me, Mo(Cp),Etz Mo(Cp)&-Bu), M~(CP)#X~,)
257+8 142f8 (137fll)d (127+ 12)
YCP)ZH, WCP) z(H)1 WCp),Me,
311f4 .273* 14 197+3
’ Data not including reorganization
bond
298f6 331flO 342 + 10 341*10 340* 10 349& 10 237k9 (33lk 12) 172+8 257k8 166&-8 (147-&9) (134f 12) 57+20 311*4 221&3
energies ER , and
‘Estimate based on D(Fc-H) N D(C!,HrH). dPreliminary values in parentheses. gee footnote t on previous page. ‘Mo-C*H, bond dissociation enthalpy. gee also note a. and stepwise bond dissociation
The calculation of E(Ti-Me) and E(Ti-Ph) in the complexes Ti(Cp),(L)Cl can be made through eq. (7), derived from two schemes similar to 4, one for the complex and the other for M(Cp),C12.27 The reorganization energies associated with the fragments M(Cp)*Cl from the complex and from M(Cp),Cl* are represented by ER’,, and by ER;, respectively (the subscript 13 indicates a mixed complex, i.e. that the initial complex contains both the ligand of interest and a halogen ; 3 means that the initial complex is the dichloride). Equation (8) was also obtained from the two schemes and includes the first M-L bond dissociation enthalpy, D,(M-L).
E(M-L)
= E(M-Cl) - A@(Cl,g) -
+ AHfo(L*,g)
{~ff@WAMWLgl
- ~~/l?Wp),CLgl) + WC - ER; 3)
D ,(M-L)
= E(M-Cl)
+ AH;(L,g)
(~~/olWWzWCkl
-~~j’W(Q),CLgl)
ER3. 'Fc = ferrocenyl.
Mixed complexes enthalpies
1539
complexes
- A@(Cl,g) -
D or b(M-L)
E(M-L)
Ti(Cp),Me, Ti(Cp),(Me)Cl Ti(Cp)zPhz Ti(CpWh)Cl Ti(Cp)&d%Me-3), Ti(Cp)&H,Me-4)~ Ti(Cp)&H.CF+% Ti(Cp),(CsH,0Me-4)2 Ti(Cp)z(CHzPh)z Ti(Cp)zFc,* Ti(Cp)&%
of M($-C,H,),L,
(7)
t See ref. 12. The reorganization energy for the fragment Ti(Cp),Ph was calculated as -41 kJ mol-‘.26
+ER;.
09
E(Ti-Me) and E(Ti-Ph) in Table 9 were obtained by transferring E(Ti-Cl) from TiC14 and by assuming that the geometry of the Ti(Cp),Cl fragment is similar in the mixed complexes and in the dichloride, implying ER’,, N ER;. It is seen that for the complexes Ti(Cp),Ph* and Ti(Cp),(Ph)Cl,. E(Ti-Ph) is remarkably constant, but the same is not observed for Ti(Cp),Me* and Ti(Cp)*(Me)Cl : the titanium-metal bond is strengthened in the mixed complex. Although this strengthening is not unusual,34 it must be stressed that E(Ti-Cl) was transferred from TiC14 and hence from Ti(Cp),C12. If the titanium-chloride bond is also stronger in the mixed complex, then E(Ti-Me) in Table 9 will be an upper limit. The third mixed complex in Table 9 is W(Cp),(H)I, for which E(W-H) = 273 kJ mol- ’ was obtained. The calculation of this bond enthalpy term was made by a slight modification of the method outlined in eq. (7). The reference molecule W(Cp),C12 was replaced by W(Cp)*12 and E(w-I) was taken as 268 kJ mol- ’ (Table 2). Also, in the absence of structural data for the iodohydride complex, it was assumed that ER’, 3 N ER3. The fact that the value of E(W-H) in the mixed complex is substantially lower than E(w--H) in M(Cp)*H2 apparently indicates that this bond enthalpy cannot be transferred between the two molecules. Recall, however, that the value for the dihydride in Table 9 is not corrected by the difference (ER, - ER J/2, which, in the case of tungsten, is estimated to be ca - 37 kJ mol- I. If this correction is applied, the value for the dihydride will match the one for the iodohydride. Although this good agreement may be fortuitous, it illustrates the importance of reorganization energies when transferring bond enthalpy terms. Several reorganization energies of fragments from complexes WCP)~(W WCP)~L ER; ; L=L’=Cl, ER;; (L = L’ # Cl, L # L’ = Cl, ER’,,), together with values of the stepwise bond dissociation enthalpies Dr(M-L) and D,(M-L’), calculated with data in the previous tables and eqs (5) or (8), are collected in Table 10.7 Several qualitative conclusions can be drawn from these values. One, fragments such as M(Cp)*CI and M(Cp),Ph, associated with large reorganization energies, lead to considerable differences D,(M-L) - D,(M-L). Two, the titanium-methyl 6rst bond dissociation energy in Ti(Cp),Me, is predicted to be higher than in Ti(Cp)*(Me)Cl, since the
1540
A. R. DIAS and J. A. MARTINHO
SIMdES
Table 10. Estimated stepwise bond dissociation enthalpies (kJ mol- ‘) M(Cp)z(L)L’
‘WCp)Fl2
Fragment
ER’”
D,(M-L)
D,(M-L’)
Ti(Cp),(Me)Cl Ti(Cp),Me, Ti(Cp)z(Ph)C1 Ti(Cp),Phz Ti(Cp) 2(CO) 2
Ti(Cp),Cl Ti(Cp),Cl Ti(Cp),Me Ti(Cp),Cl Ti(Cp),Ph Ti(Cp),CO
-41 (-41)b -11 (-41)b -41 -2
390 276 287 292 290 170
471 471 309 471 372 174
Mo(Cp),Cl, Mo(Cp),H, Mo(Cp)zMez
Mo(Cp) ,Cl Mo(Cp) zH Mo(Cp),Me
-65 -11 (-17)
238 246 149
369 268 183
a Reorganization energy of the fragment. bIdentical structures of the fragment in the complex and in Ti(Cp),Cl, assumed. ‘The structure of the complex was estimated. See ref. 12.
were
I
I
400
450 D(L-H)/kJmol-’
Fig. 1. Metal-ligand mean bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies for halogen, nitrogen, oxygen and sulphur ligands.
Thermochemistry of M($-&H&L, energy of Ti(Cp),Cl is larger than Ti(Cp),Me. Three, D,(M-L) > D,(M-L), as could be expected, because D,(M-L) refers to the species M(Cp),L, where the metal is in a lower oxidation state and/or is coordinated to a smaller number of ligands. Although we strongly favour a qualitative view of the data in Table 10, recent preliminary thermochemical results involving several titanium bis(pentamethylcyclopentadieny1) complexes, viz. Ti(Cp*),Mez, Ti(Cp*)zMe, Ti(Cp*),Ph, Ti(Cp*),C12 and Ti(Cp*),Cl, showed that those estimates may also have some quantitative value.3s The following D,(Ti-L) were directly obtained from the standard enthalpies of formation of the corresponding Ti(II1) and Ti(IV) complexes (kJ mall ‘) : D,(Ti-Ph) = 269k28, D,(Ti-Me) = 271 f21, D,(Ti-Cl) = 380 + 20. All are in fair agreement with the predictions in the table. reorganization
1541
complexes CONCLUDING
REMARKS
The present paper shows that the energetics of MEL,, complexes can be reasonably predicted by a judicious use of the “bond enthalpy term method”, even when the enthalpies of formation of the radicals L are not available. In addition, the recent comparison with the experimental data for the bis(pentamethylcyclopentadieny1) titanium molecules gives some credit to the estimates of stepwise Ti-L bond dissociation enthalpies. The usefulness of the method to analyse the thermochemical data of other families of compounds has also been tested.36 As mentioned before, the bond enthalpy term method relies on the Laidler scheme. It is thus not surprising that some of the features observed for organic molecules are reflected in metal-ligand terms. For instance, the Laidler scheme recom-
Table 11. Auxiliary thermochemical data (kJ mol- ‘) L H Cl Br I PhO PhCOO cc1,coo CF,COO CsH,Gz CioHsGz C,~HBGZ SO4 RSd PhS Me&I-W N3 CsH,N Me Et DBU Ph’ PhCH, CP CzH4 co cis-N,Ph2 PMe,
Ref.
A$(L,g)
39 39 39 39 11 40 41 42
217.997 121.302 111.86 106.762 48klO -76.3f3.0 -213+ 13 (-815.7*10)
IL43 11,44 45 11 46 11 11 47 48 39 48,49 48
240.2f8.4 461 k21 146.9kO.6 119+4 (74f8) 328.9 + 8.4 200+6 260f9 52.2+ 1.2 -110.53f0.17 455.7+2.5 -101.1+5.3
D(L-H) 436.0 431.6 366.2 298.4 362+ 10 435.8 * 3.4 (433 * 10) (433 * 10)
375.7& 8.4 345.8k8.4 385&21 439.7* 0.8 421 f4 (418+8) 464.Ok8.4 368&d 347f 10
E(L-H)* 436.0 431.6 366.2 298.4 451 451 451 451 451 451 451 451 360 360 360 377 377 415.8 411 411 421 411
D-E
0 0 0 0 -89 -15 -18 -18
16 -14 8 24 10 7 43 -43
“Gas-phase data. Estimated values in parentheses. *Laidler terms from ref. 1 ( f 8 kJ mol- ‘) or calculated from tabulated enthalpies of formation. ‘The same bond dissociation enthalpy was used for L = Me&H,0 and ClC,H,O. dR = alkyl. ‘The same bond dissociation enthalpy was used for L = MeC6H4, CF,C,H, and MeOCsH,.
1542
A. R. DIAS and J. A. MARTINHO
SIMBES
300 ‘; 5 E 3 : d i IO 200
100
400
450
500 D(L-H)/kJmol-’
Fig. 2. Metal-carbon
and metal-hydrogen mean bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies.
a single term for O-H and S-H bonds, ’ also being possible to define average M-O and M-S terms, within, ca f20 kJ mol-‘. As metalligand bond strengths are more sensitive to the chemical environment than the hydrogen-ligand bonds, it is not surprising that the interval is relatively wide. Other examples showing parallel trends between Laidler terms and metal-ligand terms can be found in the tables mentioned above (compare, e.g. Ti-Ph and Ti-Me bond terms, Table 9, with E(Ph-H) and E(Me-H), Table 11). The most common attempt to “reduce” organometallic thermochemistry to organic and inorganic thermochemistry consists of plotting metal-ligand bond dissociation enthalpies vs ligand-hydrogen bond dissociation enthalpies. It is indeed observed that such plots often lead to good linear variations, thus allowing the prediction of new values.37 The same type of correlation for the Ti, MO and W bis(cyclopentadieny1) complexes is shown in Figs 1 and 2. As expected for these metals, the data for Cdonor ligands (Fig. 2) does not fit the lines obtained mends
for x-donor ligands (Fig. 1). It is also noted that b(M-SPr-n) are significantly lower than the values predicted on the basis of the correlations in Fig. 1. Itjs very unlikely that the deviation is due to wrong estimates of the enthalpies of sublimation of the complexes : it would be necessary to decrease these quantities by cu 60 kJ mol- ’ to obtain a good fit of the three points. Instead, the deviation may be attributed to the larger polarizability of the sulphur atom as compared with oxygen. However, an identical discrepancy is not observed in the case of PhS. D(Mo-H), on the other hand, is clearly above the line defined with three Mo-alkyl bonds in Fig. 2, even if the correction ER,/2(N - 42 kJ mol- ‘) is applied. Although the previous correlations deal with mean bond dissociation enthalpies and therefore important features may be hidden, they seem less successful than recently proposed plots between the standard enthalpies of formation of crystalline complexes and the enthalpies of formation LH (for anionic ligands) or L (for neutral ligands) in their
Thermochemistry
of M($-CSH&L,
standard reference state (rs),3” i.e. their stable physical state at 298.15 K and 1 atm. An advantage
of these latter correlations over the ones involving bond dissociation enthalpies is that they allow estimation of the enthalpies of formation of complexes even in the absence of data for D(L-H), this being particularly important in the case of bidentate (e.g. S04, O&H,) and neutral (e.g. C2H4, N2Ph2) ligands. Interestingly, it can be shown that the plots D(M-L) vs D(L-H) and AH/o(complex,c) vs AHfo(ligand, rs) are mathematically equivalent, provided that a cancellation of sublimation and vaporization enthalpies occurs. This matter will be further explored in a future publication. We have deliberately omitted important chemical implications of the thermochemical data surveyed in the tables above, some of which can be found in the papers where those values were originally reported. Although the main purposes of acquiring thermochemical values are understanding reactivity and providing new insights into the nature of chemical bonds, a broad view of organometallic energetics is rather desirable at the present stage. The development of new and more accurate estimation procedures will undoubtedly be fostered both by the massive amount of thermochemical data that is currently being obtained in some laboratories and by the growing efforts of theoretical chemists to produce reliable values of enthalpies of formation and bond dissociation enthalpies.
complexes
1543
10. A. R. Dias and J. A. Martinho &noes, Rev. Port Quim. 1982,24,191. 11. D. F. McMillen and D. M. Golden, Ann. Rev. Phys. Chem. 1982,X$493.
12. M. J. Calhorda, A. R. Dias, A. M. Galvgo and J. A. Martinho SimGes, J. Organomet. Chem. 1986, 307, 167.
13. J. Bickerton, M. E. Minas da Piedade and G. Pilcher, J. Chem. Therm. 1984,16,661. 14. A. R. Dias, P. B. Dias, H. P. Diogo, A. M. Galvao, M. E. Minas da Piedade and J. A. Martinho Sirnoes, Organometallics
1987,6, 1427.
15. J. C. G. Calado, A. R. Dias and J. A. Martinho Sirnoes, J. Organomet. Chem. 1980,1%,203. 16. G. Pilcher and H. A. Skinner, The Chemistry of Metal-Carbon Bond (Edited by F. R. Hartley and S. Patai), Chap. 2. Wiley, New York (1982). 17. D. R. Stull and H. Prophet, JANAF Thermochemical Tables (2nd Edn). National Bureau of Standards, Washington, D. C. (1971). 18. M. J. Calhorda, A. R. Dias, J. A. Martinho Sirnoes and C. Teixeira, J. Chem. Sot., Dalton Trans. 1984, 2659.
19. M. J. Calhorda, M. A. A. F. de C. T. Carrondo, A. R. Dias, A. M. T. S. Domingos, J. A. Martinho Sirnoes and C. Teixeira, Organometallics 1986, 5, 660.
20. M. J. Calhorda, M. A. A. F. de C. T. Carrondo, A. R. Dias, C. F. Frazilo, M. B. Hursthouse, J. A. Martinho Simdes and C. Teixeira, submitted for publication. 21. M. D. Ribeiro da Silva, Tese de Doutoramento, Faculdade de Ciencias do Porto (1985). 22. D. Cozak and M. Mehrik, Coord. Chem. Rev. 1986, 14, 53.
23. M. A. A. F. de C. T. Carrondo, M. J. Calhorda and M. B. Hursthouse, Acta Cryst. 1987, C43, 880. 24. M. F. Lappert, D. S. Patil and J. B. Pedley, J. Chem.
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4.
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SIMdES
50. M. W. Chase Jr, J. L. Curnutt, J. R. Downey Jr, R. A. McDonald, A. N. Syverud and E. A. Valenzuela, JANAF Thermochemical Tables, 1982 Supplement, J. Phys. Chem. Ref Data 1982,11,695. 51. V. L. Glushko and L. V. Gurvich, Thermochemical Constants of Individual Compounds, Vol. 1. Nauka, Moscow (1978). 52. G. Pilcher, CATCH Tables, Nitrogen Compounds. University of Sussex, Brighton, U.K. (1972). 53. A. R. Dias, M. S. Salema and J. A. Martinho SimGes, J. Organomet. Chem. 1981,222, 69. 54. G. Pilcher, unpublished results. APPENDIX The auxiliary data used to calculate metal-ligand bond enthalpies are collected in Table 11. ’ ‘,3s49 The standard enthalpies of formation of gaseous LH, molecules (n = 1 or 2) were taken from the following references : L = halogen (ref. 39); L = PhO, MeCsH40, PhCOO, CFSCOO, RS, PhS, C,H,N, Me, Et, n-Bu, Ph, PhCH*, CF3CsH4, MeOCsH4, Cp (ref. 48) ; L = SO4 (average between the values in refs 50 and 51, -737+8 kJ mol-‘); L = Fc (taken as 227.6k4.2 kJ mol-‘, by using the enthalpy of formation of the crystal in ref. 5 and an average value for the enthalpy of sublimation) ; L = N3 (ref. 52) ; L = 2Cl&H40 (estimated as - 145 f 11 kJ mol- ’ in ref. 53) ; L = CC13CO0 (ref. 48, together with an experimental value for the enthalpy of sublimation,54 gives -428.3It8.4 kJ mol-‘); L = C,4Hs02 (estimated as -144f6 in ref. 21); L= CsH402, C10H,02 (-267.8f1.7 and -181.3k4.7 kJmol_‘, respectively; ref. 21); L = Me&H,& (estimated as 108 k 6 kJ mol- ’ in ref. 18).