In a series of methylcobaloximes (LCo(DH)(2)CH3, where DH = monoanion of dimethylglyoxime), the influence of P-donor ligands (L) on (1)J(CH) values of the Co-CH3 moiety correlated well with the pK(a) of L except when L was bulky (Cy3P and i-Pr3P). 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 pK(a) indicated a steric effect, multiparameter regressions were performed using various combinations of electronic and steric terms. Terms reflecting only sigma-donor ability (e.g., pK(a)) gave better correlations than terms reflecting combinations of sigma-donor and pi-acceptor properties (e.g., Sigma chi) 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-CH3 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.