In a series of methylcobaloximes (LCo(DH) 2 CH 3, where DH = monoanion of dimethylglyoxime), the influence of P-donor ligands (L) on 1 J CH values of the Co−CH 3 moiety correlated well with the p K a of L except when L was bulky (Cy 3 P and i -Pr 3 P). In contrast, no single steric parameter of those known for the trivalent P donors examined could be used to describe the variance of 1 J CH . The results indicate that the dominant property of L influencing this 1 J CH is electron donation. Although poor, the best correlation with a steric parameter was found with calculated cone angles (CCA) determined by experimental methods on methanol-coordinated cobaloximes. Since the single parameter fits with p K a indicated a steric effect, multiparameter regressions were performed using various combinations of electronic and steric terms. Terms reflecting only σ-donor ability (e.g., p K a ) gave better correlations than terms reflecting combinations of σ-donor and π-acceptor properties (e.g., Σχ). CCA values also resulted in better multiparameter fits of the data than other reported steric terms. Using a multiparameter approach also allowed for the simultaneous fit of data for L = phosphine and phosphite ligands. Correlations of this type were poor with single parameter fits. The sensitivity of 1 J CH to trans ligand electronic and steric effects indicates that small changes in the hybridization of the Co−CH 3 moiety occur. This method allows us to probe the nature of Co−C bonding in the ground state. The other experimental methods for probing this bond (e.g., X-ray crystallography, FT-Raman spectroscopy, etc.) indicate that this bond is relatively insensitive to the trans ligand in the ground state. Thus, 1 J CH values may be a useful general method for examining B 12 systems and also hold promise for other types of organometallic compounds.
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Moore et al. (1998) studied this question.