- Email: [email protected]
Intramolecular isomerization of chromium and Tungsten bis(trifluorophosphine)carbonyls
Polyhedron Vol. 8, No. 17, PP. 2125-2129, Printed in Great Btitain
1989
0
INTRAMOLECULAR ISOMERIZATION OF CHROMIUM TUNGSTEN BIS(TRIFLUOROPHOSPHINE)CARBONYLS LEIGH DENHAM BOTTOMLEY,” and TOMAS
0277-5387/89 $3.00+ .I0 1989 Pcl-gama Press pk
AND
RONALD J. CLARK?
G. BERGER
Department of Chemistry, Florida State University, Tallahassee, FL 32306-3006, U.S.A. (Received 10 September 1988 ; accepted after revision 31 March 1989) Abstract-The
thermally induced isomerization of chromium and tungsten bis(trifluorophosphine) carbonyl compounds was analysed by gas chromatography. These hexacoordinate chromium and tungsten compounds were found to isomerize intramolecularly without bond breakage. Rates of the isomerization were calculated from the Simplex optimized fit of first-order decay to the data. Activation parameters were also calculated. This example of intramolecular isomerization was concluded to be under steric control.
The intramolecular isomerization of hexaccordinate zera-talent d6 metal complexes is aat ccmmcm. Muecterties &cussed t&e pas&&&~ aE ocf~a&&i&%rifis+za?&ii && % %~iX&gi& itp resentations as early as 1968. ’ He concluded that the activation barriers would be too large to accommodate a non-bond-breaking mechanism. However, soon he and co-workers found the first case of intramolecular isomerization.2 That example concerned the isomerization of dihydrido ccom_r&xe2. ‘i?i_,X&+ $mere X5 =Pe of RX., ‘z = phosphine or phosphite. It was concluded that the isomerization occurred via hydrogen transversal of the tetrahedral face of the pbosphorvs atoms for a vau5e~v o? &ran&s. ‘Later. Pomerqv an& G&am’observed the cis-truns isomerization of M(CO), (SiC13)2, where M = Ru or OS. The stereochemical non-rigidity of W(CO),P(OMe), was uplam~rguuus1~ es&cItisshectb,v Darens’aourg and Baldwin.4 They employed a double labelling experiment and incorporated 13Cand 180, and then monitored the 13C NMR shift induced by igO to delirtea’ce s1ereospeciFrc sites. L&and rearrangements were observed to occur with bonds intact. Examples of intramolecular isomerization of substituted six-coordinate carbonyls are on the increase. The first full kinetics study of this phenomenon was by Darensbourg,5 concerning the cistrans isomerization of Mo(CG),(P--n-B&),. * Present address : Department of Chemistry, Agnes Scott College, Decatur, GA 30030, U.S.A. f Author to whom correspondence should be addressed.
Another more recent illustration6 is the study of tlce cistrans isamerizatian 0C &etied @&(Cffj, (J3COJPR,. i?inaX~, recent work by Ua~el<~ an Z~~~~~~~,~~~~~~"asru's~l~~l
Z~OTKp3UIdS
of the type Mo(CO)4L2 illustrates the same process. Related intramolecular processes have been found by Angelici in thiocarbonyl complexes. ’ While there are examples in the literature of hexacoordinate stereochemical non-rigidity, there have been very few kinetic studies of the metal carbonyls sub‘s~zl&~ ~ti@X@%uXs. -si?e uobslJ7X& _&&Ftua,,v with trifluorophosphine ligands to see if the small size of the ligand would affect the rate of the isonX?rizatiQn Qr ti eQ,uilihriUm cQnc~?ntratian Qf the rwo 5$X&es. EXPERIMENTAL
Cis- and truns-bis(trifluorophosphine)metal carbonyls were synthesized by the photochemical techdrques $sretio& p&l&&&? -I?.rey were _&nara&& by preparative gas chromatography and purity was checked by GC prior to thermal induction. Apparatus
Sealed liquid samples were equilibrated at various temperatures in a silicone oil bath that was capable of maintaining temperatures to two-tenths of a degree. lo Analysis was carried out on a Varian Autoprep A-700 gas chromatograph. The column
2125
2126
L. D. BOTTOMLEY
et al.
was a 7 m, l/4 in. copper tube packed with 35% DC 702 on Chromosorb P (30/60 mesh). The GC was equipped with a thermal conductivity detector. Procedure
Pure samples of either cis- or trans-M(PF,), (CO),, (M = Cr or W) were dried by vacuum sublimation through granular P401 ,, (Baker Chemicals). Samples (0.01 cm3) were then sealed in 3 mm i.d. glass tubing of approximately 10 cm length under vacuum, after several freeze-pumpthaw cycles to remove oxygen. Samples were placed in a constant temperature bath and were removed at various time intervals. Samples were kept at - 196°C until they were analysed by GC. The GC conditions were set so that the cis and truns peaks were baseline resolved. The column temperatures used were the lowest temperatures consistent with reasonably rapid analysis. Cis-tram peaks eluted within 10 min. Pure compounds were often trapped and re-analysed to check if decomposition or isomerization occurred due to the separation process, but none was found. The column temperatures used were 80°C for chromium complexes and 100°C for tungsten complexes. The thermal conductivity detector was calibrated with known amounts of all compounds and small correction factors applied to area measurements accordingly. Area measurements were made by cut and weigh. Labelling experiments were carried out on both the chromium and tungsten tram compounds. A several-fold molar excess of 90% 13C0 (Monsanto) was sealed in a 30 cm3 tube with the pure tram compound. After heating, the cis- and trans-M(PF,),(CO), were separated from each other and examined carefully by IR spectroscopy. RESULTS AND DISCUSSION
Upon heating neat cis-Cr(PF,),(CO), at elevated temperatures (10&14O”C), isomerization to a cistrans mixture was observed. No decomposition occurs and 100% of the starting material can be accounted for. Similar behaviour is seen starting with truns-Cr(PF,)Z(C0)4. The samples are in such small volume tubes that the carbonyl should be predominantly in the liquid phase. The phase of the reactant seems to be of little significance to the rates of reaction. Preliminary work done in the gas phase shows similar rates of transformation. The kinetic behaviour of cis-Cr(PF,),(CO), at 100°C is illustrated in Fig. 1. The same cis-tram ratio is reached from either pure compound. No other components are detected by GC, except at the highest temperature and longest times. Scrambling pro-
1
TIME (hrs) Fig. 1. Isomerization of cti-Cr(PF3)2(C0)4 at 100°C. ducts comprise no more than 1% of the total peak area under these conditions. Analysis by GC is capable of detecting all mononuclear six-coordinate species and the isomeric distribution of di- and trisubstituted metal carbonyls. Tungsten exhibits the same kinetic behaviour as chromium, giving a clean cis-trans isomerization but at slower rates. The reactions appear to proceed via a non-bond breaking mechanism because no other scrambling products are detected. Photochemical activation of the same compounds, which is known to be a bondbreaking process, results in a full array of M(PF,),(CO),_, species. The assumption that the thermal isomerization occurs by an intramolecular process is confirmed by 13C studies. The truns-diphosphines of both chromium and tungsten isomerize to the cis isomer in the presence of 90% labelled ’ 3C0, without any incorporation of the label into the product. The size of the 13C0 peaks in the IR spectrum of the isolated isomer did not increase above natural abundance, indicating no bond breakage. The samples for the 13C exchange were placed in containers of larger volume than the regular samples and existed primarily in the gas phase. Therefore, the solubility of CO in the liquid phase is not of concern. However, the same systems in the presence of UV light yield both isomerization and substitution products, all containing extensive 13C0 incorporation. The rates of isomerization for chromium and tungsten were analysed as opposing first-order reactions, as in eq. (l), in which & = k,/k_ 1 = [truns]/ [cis].
cis-M(PF3)2(CO) A+
I
trans-M(PF3)2(C0)+
(1)
The integrated rate equation’ I is : In &c e
= (k,+k_,)t,
(2)
2127
Intramolecular isomerization of Cr and W compounds
I
I
TIME
,
I 200
. 250
(mid
Fig. 2. Natural log of C- C, vs t for truns-Cr(PF,),(CO),. where C, is the concentration of cis or tram at equilibrium and C is the concentration of cis or tram at any time t. A plot of In (C- C,) vs t should be linear, the slope being equal to the sum of the rate constants. Figure 2 illustrates these plots for trans to cis isomerization of Cr(PF,),(CO), at 100°C. This plot is shown to illustrate that the reactions are first-order and reversible. A model, C = C, exp [- (k, f k_ Jt] + C, [exponential form of eq. (211, was fitted to the kinetic data for cis- and trans-M(PF,)z(C0)4 for the chromium and tungsten compounds. The model was fit to the data using a BASIC Simplex iterative routine. Simplex is known to be an excellent method of curve fitting. The BASIC program used was an adaptation of the PASCAL version by Cacerci and Cacheris. l2 The equilibrium concentration, C,, and the sum of the rate constants were the adjustable parameters. The results of the program were used to calculate k, and k_, from the known &. The results are listed in Table 1. Table 1 gives the sum of the rate constants, the equilibrium ratio of cis-trans, and the forward and reverse rate constants for chromium and tungsten disubstituted metal carbonyls at three tempera-
tures. The rates increase smoothly with increasing temperature. The rate for the c&tram isomerization (k ,) is 1.5-2 times faster than the reverse trans-cis isomerization (k_ ,) for a given metal. For example, the ratio of k,/k_ , for tungsten is 2.0, 2.1 and 2.2 at 100,120, 14o”C, respectively. This is also equal to K&. The equilibrium constant increases only slightly with increasing temperature. The equilibrium constant, and therefore the relative cis concentrations, increases from chromium to tungsten reflecting the relative sizes of the metals. The average Keq for chromium is 1.36 and for tungsten is 2.09. Activation parameters of chromium and tungsten were calculated from the plots of Ink vs l/t illustrated in Fig. 3 and are reported in Table 2. The entropies for chromium and tungsten are negative, indicating that the transition state is more ordered than the ground state. These data are similar to those of Darensbourg for the intramolecular isomerization of Mo((PBu),),(CO),. 5 He observed negative entropies and a AHt of N 24 kcal mol- ’ for this complex. The thermodynamic equilibrium concentrations
14
.. WP2
l2-
2.4
0 CrP2
./
I 2.5
I 2.6
I 2.7
*
a
I/T (K-l)
Fig. 3. Natural log of the rate constant vs reciprocal time for transcis isomerization.
Table 1. Rate constants and K,, for isomerization of M(PF,),(CO),, where M = Cr and W Temperature
(k, + k- J f d
I&
(“Cl
W ‘1
c&-tram
CrP,
100 120 140
0.739 f 0.047 3.795f0.064 16.60k0.738
1.29 1.38 1.41
WP2
100 120 140
0.011+0.001 0.08 1 It 0.002 0.569 + 0.020
1.95 2.10 2.23
k-,&a
k,+o
(x 106s-‘) 115f7 610f9 2697+ 120 2fl 15+1 10954
89+6 442+9 1912+85 l&l 7fl 49k2
2128
L. D. BOTTOMLEY et al. Table 2. Activation parameters” for isomerization of M(PF,)3(C0)4, where M = Cr and W cis-tram
trans-cis
Compound
AH3
ASf
AHi
ASf
CrP, WP,
23.3f 0.1 29.9kO.6
- 14.5fO.l -5.Of 1.6
22.6f 0.1 28.6f0.8
- 16.8f0.3
-9.8k2.1
“Enthalpy in kcal mol- ’ ; entropy in eu. of the cis and trans isomers appear to be a combination of steric and electronic contributions. Metal carbonyls disubstituted with phosphine prefer the trans configuration when steric effects alone are considered. Indeed, many of the bulkier phosphine ligands are known to yield complexes that exist only in the trans form7’i3 If the electronic effects are considered alone then the complex prefers the cis configuration due to a trans effect. Rather than one CO competing with another CO for electron density from the metal d orbitals, CO prefers to reside trans to a poorer rc-bonding acceptor making the carbonyls cis to one another. The experimental findings which result from those two effects can be seen in Table 3. The table combines our data with those of Howell. 7 Listed in that table are the Keg values for the cis-trans isomerization of several chromium and tungsten phosphine and phosphite complexes. The smaller the ligand cone angle, the larger the proportion of cis at equilibrium. Further, the larger the metal atom (W > Cr), the larger the concentration of cis. For chromium and a ligand having a cone angle of 130”, there is very little cis isomer present. As the cone angle decreases to 104” for PF,, the cis form becomes the preferred isomer, with a Keg of 1.36 at 100°C. The cis-trans ratios for the larger tungsten
Table 3. KS for cis-trans isomerization for some phosphines and phosphites of M(PF3)2(C0)4, where M = Cr 0rW Keq (cis-truns)
Cr
W
Cone angle”
0.026 0.07” 0.2b 1.36f0.06
O.llb 0.29’ 0.98’ 2.09kO.14
130 121 107 104
Ligand P(~-Bu)~ P(GPh)x P(GCH3)3 PF,
a Data from ref. 10. bData from ref. 7. ‘This work.
are 2.09. Strictly random isomerization would yield a I& of 4 (12 cis combinations, compared to three trans) if no electronic or steric effects were present. This study supports the previous findings of Darensbourg,’ Howell7 and others, proving that intramolecular processes offer the lowest energy pathway for isomerization of chromium and tungsten carbonyls disubstituted with phosphine ligands. This work adds to the small but growing body of literature on intramolecular octahedral isomerizations. It is clear that the coordination sphere of d6 metals is not rigid. The reaction of the bis(trifluorophosphine) molybdenum carbonyls is not as straightforward as the chromium and tungsten compounds. Isomerization and substitutionjigand scrambling reactions occur simultaneously under identical reaction conditions. While the ligand scrambling must occur via an intermolecular route, the isomerization products appear to occur via both inter- and intramolecular routes. The different reaction chemistry for molybdenum bis(trifluorophosphine) carbonyl compounds is being studied. ’ 4 The disubstituted carbonyl complexes are not the only compounds thought to be fluxional. Experiments on scrambling of 13C0 in cis-M(C0)4 (‘3CO)(P(OMe)3) (M = Cr, MO and W) indicate that site exchange occurs for the M(CO)5PR3 species as well. 4*1 5 The exchange was intramolecular for chromium and tungsten and dissociative for molybdenum, similar to our results. Preliminary studies on the meridional and facial isomers of the triphosphines, M(PF3)3(C0)3, show the same type of thermal reactions as found with the diphosphines.14
REFERENCES
1. (a) E. L. Muetterties, J. Am. Chem. Sot. 1968, 90, 5097 ; (b) E. L. Muetterties, Accts Chem. Res. 1970, 3, 266.
2. (a) F. N. Tebbe, P. Meakin, J. P. Jesson and E. L. Muetterties, J. Am. Chem. Sot. 1970, 92, 1068 ; (b) P. Meakin, L. J. Guggenberger, J. P. Jesson, D. H. Gerlach, F. N. Tebbe, W. G. Peet and E. L. Meut-
Intramolecular
3. 4. 5.
6. 7. 8.
isomerization
terties, J. Am. Chem. Sot. 1970, 92, 3482 ; (c) P. Meakin, E. L. Meutterties, F. N. Tebbe and J. P. Jesson, J. Am. Chem. Sot. 1971,93,4701. R. K. Pomeroy and W. A. G. Graham, J. Am. Chem. Sot. 1972,94,274 and other papers. D. J. Darensbourg and B. J. Baldwin, J. Am. Chem. Sot. 1979,101,6447. D. J. Darensbourg, Znorg. Chem. 1979, 18, 14. D. J. Darensbourg and R. L. Gray, Znorg. Chem. 1984,23,2993. D. T. Dixon, J. C. Kola and J. A. S. Howell, J. Chem. SOL, Dalton Trans. 1984, 1307. B. D. Dombek and R. J. Angelici, J. Am. Chem. Sot. 1976,98,4110.
of Cr and W compounds
2129
9. (a) E. L. Denham, PhD Dissertation. Florida State University, Tallahassee (1985); (b) E. L. Denham and R. J. Clark, J. Chromatogr. 1984,301,253. 10. W. D. Scott, Electronics World 1964, Nov., 46. 11. J. W. Moore and R. G. Pearson, Kinetics and Mechanism, 3rd edn, p. 304. Wiley-Interscience, New York (1981). 12. M. S. Caceci and W. P. Cacheris, Byte 1984, May, 340. 13. J. A. S. Howell and P. M. Burkinshaw, Chem. Rev. 1983,83, 557. 14. T. G. Berger, R. J. Clark and L. D. Bottomley, manuscript in preparation. 15. D. J. Darensbourg, B. J. Baldwin and J. A. Froelich, J. Am. Chem. Sot. 1980,102,4688.
1989
0
INTRAMOLECULAR ISOMERIZATION OF CHROMIUM TUNGSTEN BIS(TRIFLUOROPHOSPHINE)CARBONYLS LEIGH DENHAM BOTTOMLEY,” and TOMAS
0277-5387/89 $3.00+ .I0 1989 Pcl-gama Press pk
AND
RONALD J. CLARK?
G. BERGER
Department of Chemistry, Florida State University, Tallahassee, FL 32306-3006, U.S.A. (Received 10 September 1988 ; accepted after revision 31 March 1989) Abstract-The
thermally induced isomerization of chromium and tungsten bis(trifluorophosphine) carbonyl compounds was analysed by gas chromatography. These hexacoordinate chromium and tungsten compounds were found to isomerize intramolecularly without bond breakage. Rates of the isomerization were calculated from the Simplex optimized fit of first-order decay to the data. Activation parameters were also calculated. This example of intramolecular isomerization was concluded to be under steric control.
The intramolecular isomerization of hexaccordinate zera-talent d6 metal complexes is aat ccmmcm. Muecterties &cussed t&e pas&&&~ aE ocf~a&&i&%rifis+za?&ii && % %~iX&gi& itp resentations as early as 1968. ’ He concluded that the activation barriers would be too large to accommodate a non-bond-breaking mechanism. However, soon he and co-workers found the first case of intramolecular isomerization.2 That example concerned the isomerization of dihydrido ccom_r&xe2. ‘i?i_,X&+ $mere X5 =Pe of RX., ‘z = phosphine or phosphite. It was concluded that the isomerization occurred via hydrogen transversal of the tetrahedral face of the pbosphorvs atoms for a vau5e~v o? &ran&s. ‘Later. Pomerqv an& G&am’observed the cis-truns isomerization of M(CO), (SiC13)2, where M = Ru or OS. The stereochemical non-rigidity of W(CO),P(OMe), was uplam~rguuus1~ es&cItisshectb,v Darens’aourg and Baldwin.4 They employed a double labelling experiment and incorporated 13Cand 180, and then monitored the 13C NMR shift induced by igO to delirtea’ce s1ereospeciFrc sites. L&and rearrangements were observed to occur with bonds intact. Examples of intramolecular isomerization of substituted six-coordinate carbonyls are on the increase. The first full kinetics study of this phenomenon was by Darensbourg,5 concerning the cistrans isomerization of Mo(CG),(P--n-B&),. * Present address : Department of Chemistry, Agnes Scott College, Decatur, GA 30030, U.S.A. f Author to whom correspondence should be addressed.
Another more recent illustration6 is the study of tlce cistrans isamerizatian 0C &etied @&(Cffj, (J3COJPR,. i?inaX~, recent work by Ua~el<~ an Z~~~~~~~,~~~~~~"asru's~l~~l
Z~OTKp3UIdS
of the type Mo(CO)4L2 illustrates the same process. Related intramolecular processes have been found by Angelici in thiocarbonyl complexes. ’ While there are examples in the literature of hexacoordinate stereochemical non-rigidity, there have been very few kinetic studies of the metal carbonyls sub‘s~zl&~ ~ti@X@%uXs. -si?e uobslJ7X& _&&Ftua,,v with trifluorophosphine ligands to see if the small size of the ligand would affect the rate of the isonX?rizatiQn Qr ti eQ,uilihriUm cQnc~?ntratian Qf the rwo 5$X&es. EXPERIMENTAL
Cis- and truns-bis(trifluorophosphine)metal carbonyls were synthesized by the photochemical techdrques $sretio& p&l&&&? -I?.rey were _&nara&& by preparative gas chromatography and purity was checked by GC prior to thermal induction. Apparatus
Sealed liquid samples were equilibrated at various temperatures in a silicone oil bath that was capable of maintaining temperatures to two-tenths of a degree. lo Analysis was carried out on a Varian Autoprep A-700 gas chromatograph. The column
2125
2126
L. D. BOTTOMLEY
et al.
was a 7 m, l/4 in. copper tube packed with 35% DC 702 on Chromosorb P (30/60 mesh). The GC was equipped with a thermal conductivity detector. Procedure
Pure samples of either cis- or trans-M(PF,), (CO),, (M = Cr or W) were dried by vacuum sublimation through granular P401 ,, (Baker Chemicals). Samples (0.01 cm3) were then sealed in 3 mm i.d. glass tubing of approximately 10 cm length under vacuum, after several freeze-pumpthaw cycles to remove oxygen. Samples were placed in a constant temperature bath and were removed at various time intervals. Samples were kept at - 196°C until they were analysed by GC. The GC conditions were set so that the cis and truns peaks were baseline resolved. The column temperatures used were the lowest temperatures consistent with reasonably rapid analysis. Cis-tram peaks eluted within 10 min. Pure compounds were often trapped and re-analysed to check if decomposition or isomerization occurred due to the separation process, but none was found. The column temperatures used were 80°C for chromium complexes and 100°C for tungsten complexes. The thermal conductivity detector was calibrated with known amounts of all compounds and small correction factors applied to area measurements accordingly. Area measurements were made by cut and weigh. Labelling experiments were carried out on both the chromium and tungsten tram compounds. A several-fold molar excess of 90% 13C0 (Monsanto) was sealed in a 30 cm3 tube with the pure tram compound. After heating, the cis- and trans-M(PF,),(CO), were separated from each other and examined carefully by IR spectroscopy. RESULTS AND DISCUSSION
Upon heating neat cis-Cr(PF,),(CO), at elevated temperatures (10&14O”C), isomerization to a cistrans mixture was observed. No decomposition occurs and 100% of the starting material can be accounted for. Similar behaviour is seen starting with truns-Cr(PF,)Z(C0)4. The samples are in such small volume tubes that the carbonyl should be predominantly in the liquid phase. The phase of the reactant seems to be of little significance to the rates of reaction. Preliminary work done in the gas phase shows similar rates of transformation. The kinetic behaviour of cis-Cr(PF,),(CO), at 100°C is illustrated in Fig. 1. The same cis-tram ratio is reached from either pure compound. No other components are detected by GC, except at the highest temperature and longest times. Scrambling pro-
1
TIME (hrs) Fig. 1. Isomerization of cti-Cr(PF3)2(C0)4 at 100°C. ducts comprise no more than 1% of the total peak area under these conditions. Analysis by GC is capable of detecting all mononuclear six-coordinate species and the isomeric distribution of di- and trisubstituted metal carbonyls. Tungsten exhibits the same kinetic behaviour as chromium, giving a clean cis-trans isomerization but at slower rates. The reactions appear to proceed via a non-bond breaking mechanism because no other scrambling products are detected. Photochemical activation of the same compounds, which is known to be a bondbreaking process, results in a full array of M(PF,),(CO),_, species. The assumption that the thermal isomerization occurs by an intramolecular process is confirmed by 13C studies. The truns-diphosphines of both chromium and tungsten isomerize to the cis isomer in the presence of 90% labelled ’ 3C0, without any incorporation of the label into the product. The size of the 13C0 peaks in the IR spectrum of the isolated isomer did not increase above natural abundance, indicating no bond breakage. The samples for the 13C exchange were placed in containers of larger volume than the regular samples and existed primarily in the gas phase. Therefore, the solubility of CO in the liquid phase is not of concern. However, the same systems in the presence of UV light yield both isomerization and substitution products, all containing extensive 13C0 incorporation. The rates of isomerization for chromium and tungsten were analysed as opposing first-order reactions, as in eq. (l), in which & = k,/k_ 1 = [truns]/ [cis].
cis-M(PF3)2(CO) A+
I
trans-M(PF3)2(C0)+
(1)
The integrated rate equation’ I is : In &c e
= (k,+k_,)t,
(2)
2127
Intramolecular isomerization of Cr and W compounds
I
I
TIME
,
I 200
. 250
(mid
Fig. 2. Natural log of C- C, vs t for truns-Cr(PF,),(CO),. where C, is the concentration of cis or tram at equilibrium and C is the concentration of cis or tram at any time t. A plot of In (C- C,) vs t should be linear, the slope being equal to the sum of the rate constants. Figure 2 illustrates these plots for trans to cis isomerization of Cr(PF,),(CO), at 100°C. This plot is shown to illustrate that the reactions are first-order and reversible. A model, C = C, exp [- (k, f k_ Jt] + C, [exponential form of eq. (211, was fitted to the kinetic data for cis- and trans-M(PF,)z(C0)4 for the chromium and tungsten compounds. The model was fit to the data using a BASIC Simplex iterative routine. Simplex is known to be an excellent method of curve fitting. The BASIC program used was an adaptation of the PASCAL version by Cacerci and Cacheris. l2 The equilibrium concentration, C,, and the sum of the rate constants were the adjustable parameters. The results of the program were used to calculate k, and k_, from the known &. The results are listed in Table 1. Table 1 gives the sum of the rate constants, the equilibrium ratio of cis-trans, and the forward and reverse rate constants for chromium and tungsten disubstituted metal carbonyls at three tempera-
tures. The rates increase smoothly with increasing temperature. The rate for the c&tram isomerization (k ,) is 1.5-2 times faster than the reverse trans-cis isomerization (k_ ,) for a given metal. For example, the ratio of k,/k_ , for tungsten is 2.0, 2.1 and 2.2 at 100,120, 14o”C, respectively. This is also equal to K&. The equilibrium constant increases only slightly with increasing temperature. The equilibrium constant, and therefore the relative cis concentrations, increases from chromium to tungsten reflecting the relative sizes of the metals. The average Keq for chromium is 1.36 and for tungsten is 2.09. Activation parameters of chromium and tungsten were calculated from the plots of Ink vs l/t illustrated in Fig. 3 and are reported in Table 2. The entropies for chromium and tungsten are negative, indicating that the transition state is more ordered than the ground state. These data are similar to those of Darensbourg for the intramolecular isomerization of Mo((PBu),),(CO),. 5 He observed negative entropies and a AHt of N 24 kcal mol- ’ for this complex. The thermodynamic equilibrium concentrations
14
.. WP2
l2-
2.4
0 CrP2
./
I 2.5
I 2.6
I 2.7
*
a
I/T (K-l)
Fig. 3. Natural log of the rate constant vs reciprocal time for transcis isomerization.
Table 1. Rate constants and K,, for isomerization of M(PF,),(CO),, where M = Cr and W Temperature
(k, + k- J f d
I&
(“Cl
W ‘1
c&-tram
CrP,
100 120 140
0.739 f 0.047 3.795f0.064 16.60k0.738
1.29 1.38 1.41
WP2
100 120 140
0.011+0.001 0.08 1 It 0.002 0.569 + 0.020
1.95 2.10 2.23
k-,&a
k,+o
(x 106s-‘) 115f7 610f9 2697+ 120 2fl 15+1 10954
89+6 442+9 1912+85 l&l 7fl 49k2
2128
L. D. BOTTOMLEY et al. Table 2. Activation parameters” for isomerization of M(PF,)3(C0)4, where M = Cr and W cis-tram
trans-cis
Compound
AH3
ASf
AHi
ASf
CrP, WP,
23.3f 0.1 29.9kO.6
- 14.5fO.l -5.Of 1.6
22.6f 0.1 28.6f0.8
- 16.8f0.3
-9.8k2.1
“Enthalpy in kcal mol- ’ ; entropy in eu. of the cis and trans isomers appear to be a combination of steric and electronic contributions. Metal carbonyls disubstituted with phosphine prefer the trans configuration when steric effects alone are considered. Indeed, many of the bulkier phosphine ligands are known to yield complexes that exist only in the trans form7’i3 If the electronic effects are considered alone then the complex prefers the cis configuration due to a trans effect. Rather than one CO competing with another CO for electron density from the metal d orbitals, CO prefers to reside trans to a poorer rc-bonding acceptor making the carbonyls cis to one another. The experimental findings which result from those two effects can be seen in Table 3. The table combines our data with those of Howell. 7 Listed in that table are the Keg values for the cis-trans isomerization of several chromium and tungsten phosphine and phosphite complexes. The smaller the ligand cone angle, the larger the proportion of cis at equilibrium. Further, the larger the metal atom (W > Cr), the larger the concentration of cis. For chromium and a ligand having a cone angle of 130”, there is very little cis isomer present. As the cone angle decreases to 104” for PF,, the cis form becomes the preferred isomer, with a Keg of 1.36 at 100°C. The cis-trans ratios for the larger tungsten
Table 3. KS for cis-trans isomerization for some phosphines and phosphites of M(PF3)2(C0)4, where M = Cr 0rW Keq (cis-truns)
Cr
W
Cone angle”
0.026 0.07” 0.2b 1.36f0.06
O.llb 0.29’ 0.98’ 2.09kO.14
130 121 107 104
Ligand P(~-Bu)~ P(GPh)x P(GCH3)3 PF,
a Data from ref. 10. bData from ref. 7. ‘This work.
are 2.09. Strictly random isomerization would yield a I& of 4 (12 cis combinations, compared to three trans) if no electronic or steric effects were present. This study supports the previous findings of Darensbourg,’ Howell7 and others, proving that intramolecular processes offer the lowest energy pathway for isomerization of chromium and tungsten carbonyls disubstituted with phosphine ligands. This work adds to the small but growing body of literature on intramolecular octahedral isomerizations. It is clear that the coordination sphere of d6 metals is not rigid. The reaction of the bis(trifluorophosphine) molybdenum carbonyls is not as straightforward as the chromium and tungsten compounds. Isomerization and substitutionjigand scrambling reactions occur simultaneously under identical reaction conditions. While the ligand scrambling must occur via an intermolecular route, the isomerization products appear to occur via both inter- and intramolecular routes. The different reaction chemistry for molybdenum bis(trifluorophosphine) carbonyl compounds is being studied. ’ 4 The disubstituted carbonyl complexes are not the only compounds thought to be fluxional. Experiments on scrambling of 13C0 in cis-M(C0)4 (‘3CO)(P(OMe)3) (M = Cr, MO and W) indicate that site exchange occurs for the M(CO)5PR3 species as well. 4*1 5 The exchange was intramolecular for chromium and tungsten and dissociative for molybdenum, similar to our results. Preliminary studies on the meridional and facial isomers of the triphosphines, M(PF3)3(C0)3, show the same type of thermal reactions as found with the diphosphines.14
REFERENCES
1. (a) E. L. Muetterties, J. Am. Chem. Sot. 1968, 90, 5097 ; (b) E. L. Muetterties, Accts Chem. Res. 1970, 3, 266.
2. (a) F. N. Tebbe, P. Meakin, J. P. Jesson and E. L. Muetterties, J. Am. Chem. Sot. 1970, 92, 1068 ; (b) P. Meakin, L. J. Guggenberger, J. P. Jesson, D. H. Gerlach, F. N. Tebbe, W. G. Peet and E. L. Meut-
Intramolecular
3. 4. 5.
6. 7. 8.
isomerization
terties, J. Am. Chem. Sot. 1970, 92, 3482 ; (c) P. Meakin, E. L. Meutterties, F. N. Tebbe and J. P. Jesson, J. Am. Chem. Sot. 1971,93,4701. R. K. Pomeroy and W. A. G. Graham, J. Am. Chem. Sot. 1972,94,274 and other papers. D. J. Darensbourg and B. J. Baldwin, J. Am. Chem. Sot. 1979,101,6447. D. J. Darensbourg, Znorg. Chem. 1979, 18, 14. D. J. Darensbourg and R. L. Gray, Znorg. Chem. 1984,23,2993. D. T. Dixon, J. C. Kola and J. A. S. Howell, J. Chem. SOL, Dalton Trans. 1984, 1307. B. D. Dombek and R. J. Angelici, J. Am. Chem. Sot. 1976,98,4110.
of Cr and W compounds
2129
9. (a) E. L. Denham, PhD Dissertation. Florida State University, Tallahassee (1985); (b) E. L. Denham and R. J. Clark, J. Chromatogr. 1984,301,253. 10. W. D. Scott, Electronics World 1964, Nov., 46. 11. J. W. Moore and R. G. Pearson, Kinetics and Mechanism, 3rd edn, p. 304. Wiley-Interscience, New York (1981). 12. M. S. Caceci and W. P. Cacheris, Byte 1984, May, 340. 13. J. A. S. Howell and P. M. Burkinshaw, Chem. Rev. 1983,83, 557. 14. T. G. Berger, R. J. Clark and L. D. Bottomley, manuscript in preparation. 15. D. J. Darensbourg, B. J. Baldwin and J. A. Froelich, J. Am. Chem. Sot. 1980,102,4688.