The crystal structures of two cobaloximes, [CoMe(Hdmg),(PMe,)](1 ) and [ C O M ~( H ~~~) ~{ P ( C , H , , ) , ) ] (2) (H,dmg = dimethylglyoxime), are reported and discussed. Compound (1) crystallizes in the space group PT with cell parameters a = 15.830(8), b = 12.279(7), c = 12.257 A, a = 94.85(7), /3 = 84.49(8), y = 130.07(9)', and Z = 4; (2) crystallizes in the space group P2,2,2, with a = 18.50(1), b = 16.83(1), c = 9.943(8) A, and Z = 4. Both structures have been solved by Patterson and Fourier methods and refined by block-diagonal anisotropic least-squares methods to final R values of 0.037 (1 ) and 0.038 (2), using 4 81 2(1) and 2 710(2) independent reflections. The Me-Co-P fragment of (1) is characterized by a C-Co-P angle of 178.9(2)' (mean) and Co-P and Co-C bond lengths of 2.293(1) (mean) and 2.015(3) A (mean); the corresponding figures for (2) are 179.1 (2)",2.463(1),and 2.016(5) Arespectively. Thevalue of 2.463(1) Ais the largestso far reportedfora CoI'I-P bond length. The two nearly planar Hdmg units make a mean interplanar angle of 4" in (1) and are bent towards the axial methyl group. A similar but more pronounced bending is observed in (2). The trend in Co-P distances in these and other similar complexes [CoX(Hdmg),(PR,)] (X = CI or Me; R = OMe, Me, Bu9, Ph, or C6H11) is discussed and compared with 3lP and l H chemical shifts. Deformations observed in the geometry of these octahedral cobalt(ll1) complexes are interpreted in terms of ' steric ' and ' electronic ' influences. Finally, kinetic measurements of the conversion of [CoMe(Hdmg),{P(C,H,,),}] to [CoMe(Hdmg),(py)] (py = pyridine) give no indication of a significant contribution from steric effects on the rate of thefive-to six-co-ordinate step. However, the relatively high rate of displacement of the P(C8HI1), ligand compared to other phosphines of smaller cone angle almost certainly arises from the steric interaction of the C,H11 rings with the Co(Hdmg), moiety.WE are attempting to evaluate steric effects in vitamin B,, models in order to probe the feasibility of the mechanochemical mechanism of Co-C bond cleavage during substrate to product catalysis by the B,, coenzyme-enzyme complex.1-5 Such a study also has some bearing on steric effects of P-donor ligands since such ligands are used to generate steric distortions in model compounds.6-8 These distortions are suggested to occur in the coenzyme, caused by enzyme side chains.lb5 In addition to ground-state structural effects (referred to here as ' influences '), steric factors play some role in modifying reaction rates (referred to as ' effects '). 9910 The cobaloximes (cobalt complexes containing two ' coplanar ' dioxime ligands) have proved to be useful in evaluating the influence of bulky groups on ground-state structural parameters. We recently demonstrated that the Co-C bond length in the compound [CoPr' (Hdmg),-(py)] (where Hdmg = monoanion of dimethylglyoxime [1/690
