554 GARYC. FABEK AXD GERARD K. DOBSON and follows the order C1 > CH30 > NH, > CH, > COOhIe. Although the total range is very small (0.18 ppm), it appears that the largest effects are now produced by substituent groups possessing available lone pairs. This might suggest that the change in behavior could be due to the strong demand of electrons from the chromium carbonyl moiety. The electrons are coming from the T orbitals of the benzene ring, ultimately from substituent lone pairs, if available. This decrease in electron density in the T orbital of the substituent will then be compensated by an inductive back donation from the u system of the ring to the substituent. It thus leads to positive charges mainly in the rnetn position but should also affect the ortho position. As a matter of fact, an effect of that kind appears and the signal for ortho protons is also shifted by complex formation to negative values for all substituents possessing an available lone pair (Table VI) shift
(=?iconlpieA
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NH2
~
~
~
>
~-0.30 ~ , d
CONTRIBUTION PROM
CHBO
) -0.23
THE
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C1 -0.10
Inorgunic Chemistry This is compared to a positive shift for the other substituents COOCHR(+0.10) and CH3 (f0.04). Although this conclusion seems to be supported by experimental facts, it is surprising that such a redistribution of charge density does not affect the reactivity of the ester group as shown by the very good correlation which exists between the rate of saponification of the complexes and their corresponding parent conipounds. 2b This might however be due, as already noted, to the fact that the meta effects appear to be rather small. Theoretical calculations which take into proper account the effect of the chromium tricarbonyl moiety might clarify these points. Such calculations are now in progress and will be reported in a forthcoming paper. Acknowledgments.-The authors are very grateful to Drs. E. A. C. Lucken and F. Calderazzo for stimulating discussions and helpful comments, and they thank IIG. F. D. Williams and RIiss I. Hoflinger for tkchnical assistance.
DEPARTMENTS O F CHEMISTRY, THEUXIVERSITY O F GEORGIA, ,krHENS, GEORGIA 30601, A S D THEUNIVERSITY O F SOUTH DAKOTA, VERMILLION, SOUTH DAKOTA 57069
Octahedral Metal Carbonyls : Reactions and Bonding. VII. Kinetics of the Substitution Reactions of 2,5-Dithiahexane Complexes of Chromium Hexacarbonyl and Molybdenum Hexacarbonyll BY GARY C. FABER2" A N D GERARD R. DOBSOKzb
Receioed October 1 3 , 1967 The complexes (DTH)Cr(C0)4and (DTH)Mo(CO)4(DTH = 2,5dithiahexane) react with phosphites with the replacenleiit of the bidentate ligand to yield cis- and t ~ a n s(phosphite)2M (CO)1 complexes according to a second-order rate law. Activation parameters and previous results suggest, however, that the reactions of the two complexes occur substantially through different mechanisms, the Cr reaction proceeding through a preequilibrium step involving the dissociation of one end of the bidentate ligand followed by attack on the resulting five-coordinate species, with the Mo reaction proceeding through the formation of a seven-coordinate intermediate. Reaction rates vary according to the steric nature of the various phosphite ligands employed; the results obtained are compared to those previously obtained for similar systems and are discussed in terms of u- and r-bonding properties of the bidentate ligands.
Introduction Recently the emphasis in the study of bonding in octahedral metal carbonyls has shifted from almost exclusive consideration of infrared spectral data to kinetic and mechanistic investigation of their reactions3 Of particular interest in their implications as to the nature of steric and bonding effects have been the investigations of the kinetics of the reaction of (bidentate)XI(CO)4 molecules [bidentate = 2,2'-dipyridyl (dipy),4 (1) P a r t VI: G. R. Dobson and L. W. Houk, I ~ z o ~ EChim. . Acta, 1, 2 8 i (1967); presented in p a r t a t t h e 154th I\-ational Meeting of t h e American Chemical Society, Chicago, Ill., Sept 1967. (2) (a) N S F Student Trainee, 1966-1967; N D E A Predoctoral Fellow, 1967: (b) Department of Chemistrsr, T h e University of South Dakota, \-errnillion, S.D. 57069. (3) For a brief review of work in this field, see LI. Graziani, P. Zingales, and U. Belluco, I m i g . Chem., 6, 1582 (196.i). (4) (a) K. J. Angelici a n d J. I> k,. Thus the concentration of the species in which one end of D T H is dissociated would be small and nmr evidence for its presence is not likely to be observed. A complex pattern for ethylene protons is observed a t - 75 0 ; this pattern coalesces into a single sharp resoriance a t room temperature. The equivalence of the ethylene protons a t room temperature maybe attributed to a time-averaging of ethylenic vibrations. The nmr spectra obtained are consistent with either mechanism. Bonding Implications.-Angelici and Grahan1~9~ have noted a correlation between the “hardness”8 of substituent groups and the labilization of carbonyls in octahedral metal carbonyl complexes, an unexpected trend if metal-carbon bond strengths are to be inferred from carbonyl stretching frequencies or force constants. Although reactions involving carbonyl replacement proceed a t least in part through a dissociative mechanism, complexes containing labilizing “hard” base (8) R. G. Pearson, J . Am Chem. SOL.,85, 3533 (1963). For the purposes of this discussion, a “hard” base will be taken t o be a relatively good u donor and relatively poor T acceptor, a “soft” base a relatively poor c donor and a relatively good w acceptor.
lnorgnnic Chmnistry ligands generally have lower carbonyl stretching frequencies and, by inference, stronger metal-carbon bonds relative to complexes of nonlabilizing “soft” base ligands. Thus it was necessary to attribute labilization of carbonyls by “hard” bases to their stabilization of the transition state. More recently, however, metal-carbon stretching data for (bidentate)M o ( C O ) complexes ~ have suggested that, in fact, metalcarbon bonds are weaker for hard-base than for softbase substituents. Arguments were advanced which suggested a dominant influence of 5 bonding in the kinetics of these systems.l It has recently been noted for (Xphen)M(C0)4 systems (M = &Eo, W) in which X is one or more substituent groups through which the basicity of Xphen can be varied, that with decreased Xphen basicity the rate of carbonyl replacement via a dissociative mechanism decreases, while the rate of replacement nia a displacement mechanism increases.5b Through consideration of ligand basicity primarily in terms of its effect on the Lewis acidity of the metal,’ a strongly basic ligand “neutralizes” a proportionately larger part of the metallic Lewis acidity than a less basic ligand, inhibiting carbonyl-to-metal u bonding (increasing the lability of carbonyls) and also inhibiting nucleophilic attack on the metal by a prospective displacing ligand. Data for (DTH)hlo(C0)4continue this trend. D T H is more weakly basic than n-ere the Xphen ligands investigated, and thus a greater rate by a displacement mechanism and smaller rate by a dissociative mechanism would be predicted. If i t is assumed that the secondorder rate for (DTH)Mo(C0)4 reflects the rate by the displacement mechanism, the expected trend is followed in that the second-order rate is greater than for (Xphen)Mo(COj4 molecules and no first-order dissociative term is observed. The lack of a first-order term and failure to observe replacement of CO for (DTH)Xo(CO)d is consistent with the higher 1,l-C stretching frequencies reported for (DTH)MO(CO)~ than for (phen)Mo(CO)4.1 The superior .rr-accepting ability of the “soft” DTH relative to Xphen, as may be inferred from the carbonyl stretching fequencies for their complexes, also enhances the relative Lewis acidity of the metal, facilitating nucleophilic. attack, as the large second-order rate constant would indicate. Acknowledgment.-Appreciation is expressed to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Drs. Francis J. Johnston, Thomas D. Walsh, and Donald E. Leyden for valuable discussions.
