Polynuclear metal(III) and (IV) complexes with diisopropyl- and di-n-propyl-phosphates: Steric effects of branched alkyl substituents on the ligand

0022-1902/79/1201-1671/$02.0010

J. inorg, nucl. Chem. Vol. 41, gO, 1671-1676 ~) Pergamon Press Ltd., 1979, Printed in Northern Ireland

P O L Y N U C L E A R M E T A L ( I I I ) A N D (IV) C O M P L E X E S W I T H DIISOPROPYL- AND DI-n-PROPYL-PHOSPHATES: STERIC EFFECTS OF B R A N C H E D A L K Y L S U B S T I T U E N T S ON T H E L I G A N D CHESTER M. MIKULSKI,¢ THAO TRAN,t LOUIS L. PYTLEWSKI:~ and NICHOLAS M. KARAYANNIS§ (Received 17 April 1979) Almlraet--A series of M(dipp)3 (M = AI, Ti, V, Cr, Fe, Dy; dipp = diisopropyl phosphate) and M(dipp)4 (M=Th,U) polymeric complexes were prepared by reaction of triisopropyl phosphate with the corresponding metal chlorides at 100-200°C. The spectral and magnetic properties of the new metal complexes were then compared to those of the previously reported di-n-propyl phosphate (dnpp) analogs. The metal-ligand bonds are generally weaker in the dipp metal complexes (relative to the dnpp complexes). The differences in metal-ligand bond strength between dipp and dnpp metal complexes are larger in the cases of metal ions with the ability for metal-to-ligand ~r-bonding (M=Ti3+,V~÷,Cr3%Fe3%U4*). Since the inductive effects of the substituents in the two ligands compared are almost equal (Z~r'~ values are -0.58 for dipp and -0.6,1 for dnpp), the observed weakening of the M---O bonds in the M(dipp). complexes was attributed to the steric effects of the isopropyl sUbstituents of dipp; these branched alkyl substituents seem to introduce sufficient steric hindrance as to force an increase in the distance between the metal ions and the oxygen atoms, on the one hand, and limit the extent of metal-to-ligand ¢r-bonding, on the other. As a result of this, the extent of spin-spin coupling between adjacent paramagnetic metal ions in the, most probably, linear, chain-like triple-bridged polymeric structures (~M~(O..-P(R)2---O)3--M--qO-.-p(R)2--O)3_ sequences) is also moderated when the R substituents of the (RO):POO- ligands introduce significant steric hindrance (branched alkyl or aryl groups).

INTRODUCTION

the past by these ]aboratories [10-12]. As

The combined inductive and steric effects of the substituents on phosphorus are of great importance in determining the properties and behavior of organophosphorus compounds. This has been established by numerous studies, ir~cluding, for instance, over 100 organic reaction sets of organophosphorus(III) and (V) compounds [1, 2], changes in the P ~ Z bond-order (and 1,v,-z frequency) (Z~---O, S, Se) [3-5], dipole moment [6] and extracting ability [7] of R R ' R " t ~ Z compounds with substituent variation, the properties and catalytic activities of organometallic complexes with a series of phosphines [8], and the strength of the ligand field generated by triorganophosphine oxides [9], alkyI alkylphosphonates and dialkyl phosphates [10-13] towards various metal ions. Satisfactory physicochemical correlations of the I R and electronic spectral properties of polynuclear M L . (M = A 1~, Ti 3÷, V 3÷, Cr 3+, Fe 3+, DF 3+, Th *÷, U*~; n = 3 or 4) complexes with ligands of the two latter types (i.e. (I) and (II), respectively) with the substituent 2o," constants [1, 2] of these ligands, were made in -t Department of Chemistry and Physics, Beaver College, Glenside, PA 19038, U.S.A. ;t Department of Chemistry, Drexel University, Philadelphia, PA 19104, U.S.A. § Amoco Chemicals Corporation, Naperville, IL 60540, U.S.A.

(RO)R'POO(I)

(RO)2POO(II3

Y.cr* is a measure of the inductive effects of the substituents on phosphorus [1, 2, 13], our VM--O or Dq vs Zcr* plots showed anomalously low minima for metal complexes with sterically hindered ligands of the above types [10-12]. This was especially pronounced for the 3d metal complexes of the series (M = T i 3+, V 3÷, Cr 3÷, Fe3~), and was interpreted in terms of weaker metal-ligand bonds, arising by moderation of the extent of metal-toligand d,,-p,, back-bonding, owing to the steric hindrance [14] introduced by these ligands [10-12]. It was also apparent that sterically hindered substituents on the ligands influence the magnetic properties of most of the paramagnetic metal ion complexes. In fact, the Ti 3÷, V 3~, Cr 3+, Fe ~÷ and U 4~ complexes with the least bulky or sterically hindered ligands were found to exhibit subnormal ambient temperature magnetic moments for the 3d I configuration, high-spin 3d 2, 3d 3 and 3d s compounds or octacoordinated U4*; as the steric features of the ligands became more severe, the ftc~ showed a tendency to approach or even reach the normal values for these metal ions. The subnormal magnetic moments of most of these complexes were attributed to spin-spin coupling, occurring by

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C. M. MIKULSKI et al.

a superexchange mechanism, operating via the orbitals of the bridging--O--P--O--- groupings in the proposed double-bridged structures (HI) and (IV) (for ML3 and ML4 complexes, respectively; L = - O--P(R')(OR)---O-- or - - O - - P ( O R ) 2 - - O - - ) [1012]. It is worth mentioning at this point that detailed magnetic investigations of analogous dialkyl (or aryl) phosphinato ( L = - - O - - P ( R ) 2 ~ O ~ ) C r 3+ polymeric complexes of type (V), involving triple CrwL3--Cr--L3--bridges, at 300-4.2 K, led to the conclusion that these compounds comprise one-dimensional antiferromagnetic chains [15]. Regardless of whether the phosphonato and phosphato metal complexes reported by these laboratories in the past are indeed highly crosslinked polymers of types (III), and (IV) or are characterized by essentially linear chain-like structures analogous to (V), the fact that the extent of spin-spin coupling is moderated or even completely offset as the steric effects of the substituents on phosphorus become more severe, may be interpreted in terms of weakening .of the metal-ligand bonds with increasing steric hindrance. Although the steric effects of a bridging ligand would not directly affect the extent of antiferromagnetic exchange, they might do so indirectly, by influencing the strength of the metal-ligand bond. In fact, Muto et al. have attributed the opposite effect of that herein discussed (i.e. increased spin-spin interaction with increasing steric hindrance of the ligand) in a series of binuclear N-oxide bridged Cu 2+ halide complexes with aromatic amine Noxides, to the strengthening of the Ct}--O bonds with increasing steric hindrance of the ligand [16].

"~M/L'~M

L

L

/

L ~L

(II13

L

L

L

(rv~

(v)

L

L

L

Despite the fact that the metal complexes of numerous ligands of types (I) and (II) were studied in the past, our overall data did not provide unambiguous evidence in favor of the importance of the steric effects discussed above. The closest comparisons that could be made were those between di-nbutyl phosphate (dnbp; X~* -0.82) and diisobutyl phosphate (dibp; 2~r*-0.60), both of which involve butoxy substituents, but significantly different Xu* values [10-12,17], or between ethyl ethylphosphonate (eep; ~ t r * - 1 . 3 1 ) and isopropyl methylphosphonate (imp; Xu * - 1.25), which show similar X~r* values, but comprise different alkyl groups. In both the preceding cases, the ligands containing branched alkyl groups (i.e. dibp and imp) proved to form weaker metal-ligand bonds, relative to the strictly n-alkyl substituted comparators (dnbp and eep)[10-I2]. We were, therefore, interested in comparing two ligands with alkyl substituents, characterized by the same number of carbon atoms and having approximately equal inductive effects, so that we would be able to attribute any differences in behavior to purely steric effects. Accordingly, we prepared the complete series of diisopropyl phosphate (dipp; 2or ~ -0.58) M 3~"and M "+ complexes, and compared their properties to those of the previously reported [12] di-npropyl phosphate (dnpp; [cr* -0.64) analogs. This comparative study is the subject of the present paper. EXPERIMENTAL Preparation of complexes. The new metal complexes with dipp were prepared by the general synthetic method previously described[10-12, 18, 19], i.e. by suspending the anhydrous or hydrated metal chloride in excess triisopropyl phosphate(tipp; ROC/RIC product) and gradually increasing the temperature of the mixture; the metal salts dissolved in tipp at 30-70°C, and the precipitation of the new M(dipp) 3 (M = Al, Ti, V, Cr, Fe, Dy) and M(dipp)4 (M--Th, U) complexes occurred when the mixrares had reached temperatures in the 100-200°C region. The precipitation of the complexes was accompanied by evolution of a mixture of HCI, propylene and isopropyl chloride, as expected [10-12, 18-21]. The Ti3÷ complex was prepared in the dry-box (N2 atmosphere)[10], while the rest of the complexes can be prepared by heating tipp and metal salt in the atmosphere. The new metal complexes are generaLly stable in the air and insoluble in water and all common organic solvents (including N, N-dimethylformamide, hexamethylphosphoramide and dimethyl sulfoxide), as was also the case with the previously reported analogous compounds[10-12,18-21]; they do not melt or decompose at temperatures of up to 350°C. Analytical data (Schwarzkopf Microanalytical Laboratory, Woodside, N.Y.), given in Table 1, are in good agreement with the calculated values; the new complexes were found to be chlorine-free, in all cases. This is of interest, because during similar reactions of metal chlorides with triisobutyl phosphate, only some M(dibp)~ complexes were precipitated (M = Al3+, V 3+, Cr3÷, Fe3+, Dy3+)[12], while in other cases chlorinated species (Ti(dibp)Cl2 and M(dibp)~C1 (M = Th,U)) were stabilized [17]. Spectral and magnetic measurements. Methods described elsewhere [10-12, 18, 19, 22] were employed for obtaining Nujol mull IR (Table 2) and electronic (Table 3) spectra, and ambient temperature magnetic susceptibility measurements (Table 3). The IR spectra of the new

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Polynuclear metal(HI) and (IV) complexes Table 1. Analytical data for M(dipp), complexes Analysis M "~

Color

C% Cale. Found

AI 3. Ti 3÷ V 3+ Cr 3+ Fe 3" Dy 3+ Th 4÷ U 4÷

White Lavender Light green Green Off-white White White Olive green

37.90 36.56 36.37 36.31 36.08 30.63 30.13 29.95

37.71 36.12 36.40 35.95 36.17 30.46 29.66 29.57

H% Calc. Found 7.42 7.16 7.12 7.11 7.06 6.00 5.90 5.86

7.57 6.88 7.19 7.33 6.78 6.07 5.79 5.72

P% Calc. Found

Metal % Calc. Found

16.29 15.71 15.63 15.61 15.50 13.16 12.95 12.87

4.73 8.10 8.57 8.73 9.32 23.02 24.26 24.73

16.44 15.73 15.34 15.40 15.63 13.22 13.14 12.99

4.94 7.97 8.52 9.09 9.21 22.74 24.68 24.31

Table 2. Pertinent IR data for M(dipp), and M(dnpp), complexes (era -1)

Complex Al(dipp)3 Al(dnpp)3 Ti(dipp) 3 Ti(dnpp) 3 V(dipp) 3 V(dnpp)3 Cr(dipp)3 Cr(dnpp)~ Fc(dipp) 3 Fe(dnpp)3 Dy(dipp)3 Dy(dnpp)3 Th(dipp)4 Th(dnpp)4 U(dipp)4 U(dnpp)4

Vpoo(asym)

Vpoo(Sym)

1228vs, 1161 vs 1230vs, l l 6 0 v s l177vs 1176 s, sh l172vs 1172vs, sh l166vs 1165 s, sh I166 vs 1168 vs 1178 vs 1184 vs l l 5 0 v s , vb l 1 5 9 s , sh 1148 s, vb 1155 s, sh

1080svs 1081 vs 1070vs, sh 1066vs 1079~ 1069vs 1080vs 1070vs 1083 vs 1073 vs 1079 w s 1071 vs 1069s 1071 s 1070s, b 1079s

POO combination

vM._o

1720w 1736w 1725w, b 1727m, b 1718w, b 1720w, b 1.730w, b 1733w 1735w 1735w 1730w 1735 w 1737w 1730w 1725w 1730w

560vs, 469 vs, 368ms 571 vs, 481 vs, 365vs 557vs, 451m, sh, 287m 588 s, sh, 430s, 319m, b 555 vs, sh, 452 vs, 292vs 595 vs, sh, 450s, 321 s 553 vs, 459s, 308s 597 vs, sh, 439vs, 333s 546 vs, b, 4 3 0 m , sh, 297 m , b 563 vs, 440vs, 320m 519s, sh, 435 m, sh, 377 m , b 522 vs, 433 m, 380 m 519s, sh, 436mw, 352mw, sh 528vs, 430m, 3 5 4 m 520vs, 436m, 347m 530vs, 4 3 2 m , 3 6 0 m

Table 3. Solid-state (Nujol mull) electronic spectra and magnetic susceptibility data (298 K) for M(dipp), and M(dnpp), paramagnetic complexes Complex Ti(dipp)3 Ti(dnpp) 3 V(dipp)3 V(dnpp)3 Cr(dipp) 3 Cr(dnpp)3 Fe(dipp) 3 Fe(dnpp) 3 Dy(dipp)3 Dy(dnpp) 3 U(dipp)4 U(dnpp)4

Am,,(nm) (300 vs, 357 s, sh, 572 s, b, 657 sh (300 vs, 542 vs, sh, 643 sh (300 vs, 452 ms, 706 m, b (300 vs, 441 vs, 673 m (300 vs, 458 vs, vb, 640s, 665s, 691s (300 vs, 421 s, 477 s, 622 s, 650 s, 678 s, sh (300 vs, 335 ms, sh (300 vs, 340 ms, sh (300 vs (300 vs 417s, 445s, 493s, 540re, b, 570row, 635 mw, w b , 705 w, b, 771 m, 800 m, 977 m, b, 1075 w, b, 1182 row, 1260w, vb, 1460w, vb, 1595 mw, b 431 s, vb, 490s, 538m, vb, 571 w, vb, 636m, vb, 671 s,b, 763re, b, 961w, 1063s, l l l l s , 1136s, I176m, I265m, 1490m, vb

Dq(em-1)(~l) 106X~" ((cgsu) 1745 1845 1523 (0.71) 1601 (0.707 1504 1538

Iz,n(gB)

1279 1264 3360 2998 6211 5801 16.473 9966 49,417 49,690 3545

1.75 1.74 2.83 2.68 3.85 3.73 6.27 4.89 10.86 10.91 2.91

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C. M. MIKULSKI et al.

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dipp metal complexes are generally devoid of any bands attributable to the presence of water. Tables 2 and 3 include spectral and magnetic data for the previously reported M(dnpp), complexes [12].

basieity (inductive) effect (Gr), the steric effect (s), ligand-to-metal 1r-bonding (~rL--M), and metal-toligand ~r-bonding (~rM_L); These effects can be expressed as follows:

DISCUSSION

Dq = ~r--s--lrL_m + ~rM--L

The trends anticipated from the preceding discussion and our previous studies in the field were observed during the comparison of the spectral and magnetic properties of the M(dipp). and M(dnpp). complexes. Thus, the VPoo (asym) values (which are considerably more metal-sensitive than the UPoo(sym) or POO(combination) bands [5, 10-12, 23, 24]) fit very well to the previously presented Vr~o(asym) vs ~cr * plots 1"10-12]. The Vpoo(asym) bands in the spectra of the M(dipp). complexes occur at about the same'or slightly lower frequencies, relative to the corresponding absorptions of the M(dnpp)~ analogs, as would be expected from the almost equal ~cr ~ values for these two ligands [1, 2, 10-12] (Table 2). The tentative u~__o band assignments for the M(dipp). compounds were based on our previous studies [10---12], as well as consideration of the various ligand bands at 600-250 crn -~ [25]. From the vM--o data of Table 2 it is clear that there is a definite trend for the highest and lowest frequency vM_..o bands to appear at lower wave numbers in the spectra of the dipp metal complexes, relative to those of the dnpp analogs (the uM_..o bands at 481-430crn -~, although apparently metal-sensitive, do not seem to be affected by the steric effects of any of the ligands of types (I) and (II)); these effects are significantly more pronounced in the case of the 3d metal ions (Ti 3÷, V 3÷, Cr 3÷, Fe 3+) and U 4+. The electronic spectra of the new Ti 3., V 3÷ and Cr 3~" complexes (Table 3) are characterized by broad and/or split d - d transition bands, attributable to low-symmetry hexacoordinated configurations[10, 12, 26]. Band assignments, nm: Ti3÷: :T2~---~eE~ main maximum at 572, with a weaker shoulder at 657; V3+: 3Tt~(F)--~3Tt~(P) 452;---~ 3T2~(F) 706; Cr3+: ~A2, (F)---~'T~g (F) 458;--o4T2~(F) 640, 655, 691 [10, 12, 27]. Approximate Dq calculations (for pure O~ symmetry) yielded the following values (cm -a) for dipp towards octahedral M~÷: M = T i 1745; M = V 1523; M = Cr 1504. These values are considerably smaller than those calculated for the corresponding M(dnpp)3 complexes (1845, 1601 and 1538crn -t, respectively)[12]. The electronic spectrum of U(dipp)~ is very similar to those of the previously reported U L complexes with ligands of types (I) and (II), which were considered as bearing greater resemblance to the spectra of known octa- rather than hexa-coordinated U ~÷ compounds [11, 12, 28, 29]. With respect to the considerably lower Dq and vu-.o values observed for the dipp 3d metal complexes, relative to their dnpp analogs (and more generally for ML~ and ML, complexes with ligands of types (I) and (II), characterized by substituents introducing significant steric hindrance [10-12]), the following discussion is in order: As pointed out by Nathan and Ragsdale [30], there are four basic contributions to the ligand-field parameter Dq: The

with the positive terms tending to increase and the negative terms tending to decrease Dq [30]. Since dipp and dnpp have almost equal tr terms (X~r* values), and the ~rL_-~ term is rather unimportant for 3d metal ions [30], the differences in Dq (and consequently VM---O)between these two ligands can be attributed to the obviously larger value of the s term (steric effect) for dipp, as well as the extent of decrease of the 7ru_L term, under the influence of the steric effects [14] of the dipp ligand. Similar trends of significant VM--O frequency decrease are observed in passing from dnpp to dipp in UL,, where f,~-p,, back-donation is possible [31]. With Dy 3+, whose 4f electrons are effectively shielded[31], and with AI 3+ and Th 4., which involve noble gas electronic cores, metal-to-ligand 7r-bonding is not possible, and the vM--o frequencies are essentially determined by the relative steric effects of the substituents of the dnpp and dipp ligands; as a result of this, the ziuM__o (VM_o(dnpp)----vM...o(dipp)) values for these three metal ions, although positive, are the smallest for the series of metal ions studied. Hence, the present study establishes that the major contributor to ligand-field weakening, owing to the effects of sterically hindering substituents on the ligand, is the limitation of the extent of metal-to-ligand ~rbonding, while the effects of steric hindrance per se in determining the strength of the metal--oxyge/a ~r-bonds are relatively smaller. The magnetic moments of the new dipp metal complexes (Table 3) are normal for all the paramagnetic M 3÷ ions studied [32, 33]. This was expected for Ti 3÷, V 3~" and Cr 3÷, since in the previously studied series of complexes with ligands of types (I) and (II), the /~ea showed a tendency to approach the spin-only values, as the steric features of the substituents became increasingly severe [10, 12], and for Dy 3÷, which formed magnetically normal complexes with all the ligands of interest [12, 19]. On the other hand, Fe(dipp)3 is the first ferric complex of the whole series that exhibits a clearly normal /~.n for high-spin Fe 3÷ (6.27 t~B). The moments of all the previously reported analogs, including [Fe(OOPCI2)3],, ranged between 3.90-5.72 p.B, and there was no apparent correlation between their values and the steric effects of the ligands [12, 34]. Thus, no attempt at a systematic correlation of the normal magnetic moment of Fe(dipp)3 with the preceding data can be made at this point; it would appear that the overall effect of the substituents in dipp on the s and 7rM_L (M= Fe 3~) terms of expression (1) is sufficiently severe as to weaken the Fe-ligand bond to an extent not • favoring spin-spin coupling interactions. Regarding comparisons of the magnetic moments of the M(dipp)3 and M(dnpp)3 (M = Ti, V, Cr) complexes, the dipp compounds exhibit higher /~o~ in all three cases. The moments of the V 3+ and Cr 3+ complexes with dipp are very close to the spin-only values for

(1)

Polynuclear metal(III) and (IV) complexes these metal ions, while those of the dnpp analogs are slightly low and indicative of the effects of spin-spin interaction (especially in view of the previously established trends [10, 12]); nevertheless, the Ti 3~ complexes with these two ligands show almost equal magnetic moments, very close to the spin-only value (1.73/zB). The magnetic moments of both U(dippL and U(dnpp)4 are low for octacoordinated U 4~ compounds, which show moments in the 3.40-3.80 gB region [35], but close to the spin-only value of 2.84 ~B, which is characteristic of octahedral compounds of this metal ion [35-37], The polynuclear UL4 complexes with ligands of types (I) and (II) are generally characterized by coordination number eight, and their relatively low magnetic moments, lying with one exception (L = diphenyl phosphate; ~.~= 3.48 ~,B), in the 2.64-2.97 ~,B region, were considered as arising from spin-spin interactions of the same type to those observed with the 3d metal ion analogs. In fact, increasing steric hindrance of the ligands appears to cause significant ta,.~ increases, and a normal value is reached in the case of the complex involving four very bulky diphenyl phosphato ligands [11, 12]. The small difference in p,.n observed between U(dipp), (2,91/zB) and U(dnpp), (2.85 ~B) is, therefore, significant and consistent with these previously established trends. CONCLUSION The present study lends support to our previous interpretation of the trends of variation of the VM_o, Dq and ~ a values for M 3÷ and M4~ complexes with ligands of types (I) and (II) in terms of the combined electronic and steric effects of the substituents on these organophosphoryl ligands [10-12]. Furthermore, in view of the almost equal inductive effects of the n-propyl and isopropyl substituents, the differences of the above variables between M(dnpp), and M(dipp), complexes can be unambiguously attributed to the steric effects of the isopropyl group. The fact that these differences are more pronounced in complexes with metal ions having available electrons for metal-to-ligand wbonding demonstrated that, in addition to purely steric effects, increasing steric hindrance on the ligand causes limitation of the extent of this type of ~r-bonding. Regarding the nature of the whole series of metal complexes with ligands of types (I) and (II) herein discussed, there is little doubt that these compounds are polymeric. This is strongly suggested by their complete insolubility in organic media and water, and by the facts that they can be precipitated in the form of membrane- or rubber-like materials (when the solution of the metal chloride in the neutral ester is heated at a rapid rate) and that crystal structure determinations of their diorganophosphinato analogs established that these complexes are polypuclear [38-40]. In addition we have recently demonstrated that during reaction of SnCL with diisopropyl methylphosphonate, the Sn[OOPCHa(O-i-CaHv)]2CI2 intermediate product is a tetramer, whilst the final Sn(O3PCH3)2 product is a highly cross-linked polymer [21]. In view of this information and the pronounced tendency of

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R2POO- anionic groups to function as bridging rather than chelating ligands[38-41], it is somewhat surprising that several diethyl- and di-n-butylphosphato metal complexes were recently characterized as monomeric metal chelates [42]. Distinction between structural types (III) or (IV) and (V) for the M[OOP(OR)2]. and M[OOPR'(OR)]. (n = 3, 4) complexes prepared at these laboratories during the present and previous [10--12] studies, should be based on crystal structure determinations, as well as studies of magnetic susceptibility variation with temperature for the compounds involving paramagnetic metal ions. Admittedly, structure (V) is favored by the fact that it seems well established that many M[OOPR2], polymeric complexes, including several Cr 3÷ compounds, are characterized by this linear, chain-like type of structure [15, 38--40]. As regards spin--spin coupling via the bridging - - O - P - O - - - ligand groupings, both structural types, i.e. the douple-bridged (III) or (IV), which involve cross-linking of parallel layers of linear - - M ~ L 2 - MmL2m polymeric segments, and the strictly linear triple-bridged (--MmL3--M~L3--) (V) can account for antfferromagnetic exchange phenomena arising by a chain-like polynuclear structure [15, 43, 44]. An additional consideration that makes structural type (V) more probable for the complexes herein discussed is the more severe crowding of the ligands in the triple-bridged polymeric chains, relative to that in the doublebridged structures (HI) or (IV). The steric interference between the three bridging ligands in the M--L~--M--L~-- chain-like structure may very well be responsible for the rather unusual tendency of decrease of the metat-ligand bored strength with increasing steric hindrance of the substituents on phosphorus. It should be mentioned at this point that the opposite effect, i.e. increasing metal-ligand bond strength with increasing steric hindrance of a given type of ligand, has been observed with many series of monomeric or binuclear (but not involving steric interference between the bridging ligands) metal complexes; in these cases, increases in the degree of severity of the steric hindrance of the ligand tend to render the metal ion inaccessible for interactions with additional ligands present in its environment and result in the stabilization of metal-ligand bonds considerably stronger than would be expected from the position of the ligand in the spectrochemical series [16, 30, 45, 46]. However, examination of molecular models of structural type (V) indicated that substituents on phosphorus that can introduce increased steric hindrance (branched alkyl, aryl groups) will, of necessity, cause an elongation of the distance between the metal ion and the ligands in the L~ML3 moieties; for a given sterically hindered ligand, elongation of the metalligand bonds in the double-bridged structures (III) or (IV) would also occur, but it would not be as pronounced as that caused by the crowding of three ligands between two metal ions in the triplebridged structure (V). Hence, the overall evidence available at this point may be considered as favoring the latter structural type for the whole series of ML3 (M = A1, Ga, In, Sc, Ti, V, Cr, Fe, Y, La, Ce,

C. M. MIKULSKI et al.

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Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu; L = ( R O ) 2 P O O - , (RO)R'POO-) complexes prepared by these laboratories in the past [10-12, 18, 19, 47] and during this work. As far. as the ML4 (M = T h , U) analogs are concerned, a quadruplebridged linear structure of the - - M w L 4 - - M - - L 4 - type would probably result in too severe a steric interference between the four bridging ligands; a more likely alternative is a structure involving linear triple-bridged chains and one chelating, terminal ligand per metal ion, viz. - - ,M,--La--M,- - L a - - .

II

L

ii

L

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