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

1305of the solvents. When the solvent forms hydrogen bonds, the splitting constant9 deviate above the linear lines. The slope and intersection of the line can be qualitatively accounted for by the Huckel MO calculation on the radical. solvent into account. When the neutral radical forms no hydrogen bonds with the solvent, the expression gives a good linear relationship between the hyperfine splitting constants and the Block and Walker parameters (eq 17) sgCo chemical shifts, line widths, viscosities, and electronic spectral data are reported for solutions of potassium hexacyanocobaltate(II1) in mixed solvents involving dimethyl sulfoxide, water, formic acid, trifluoroacetic acid, and propionic acid. The chemical shifts cover a range of some 350 ppm, and it in concluded that hydrogen-bonding effects are responsible for the differences. The larger high-field shifts and by inference the stronger hydrogen bonds are associated with the stronger acids. There in evidence for preferential solvation of the hexacyanmbaltate anion in mixtures of dimethyl sulfoxide and acids, but not in waterlacid mixtures. In the dimethyl sulfoxide mixtures the hydrogen bonds become progressively weaker as more acid molecules are incorporated into the second coordination sphere. Interactions with potassium or hydrogen cations do not contribute significantly to the shifts or line widths. The energy of the fmt d-d electronic transition shows parallel changes. The NMR line widths show only modest changes in mixed aqueous solutions, but larger changes (factors of up to several hundred) in dimethyl sulfoxide solutions. Since TI is reduced by a similar factor, "chemical exchange" is not responsible for the broadening. The larger changes in relaxation rate are associated with the formation of the stronger hydrogen bonds. Relaxation rates are not proportional to viscosity, showing that changes in relaxation mechanism rather than simple variations of the correlation time are involved. These results are interpreted in terms of changes in the lifetimes of the hydrogen bonds. The stronger hydrogen bonds (with trifluoroacetic acid) have lifetimes greater than the rotational correlation time of the complex leading to an efficient quadrupolar relaxation mechanism for complexes with unsymmetrically substituted second coordination spheres. Estimates of the quadrupole coupling constants and the rotational correlation times show that this mechanism suffices to account for the observed line widths. The weaker hydrogen bonds (with water) have lifetimes shorter than the rotational correlation times, and other relaxation mechanisms dominate. Formic acid shows intermediate behavior.

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