Density functional theory has been applied to the investigation of the reductive cleavage mechanism of methylcobalamin (MeCbl). In the reductive cleavage of MeCbl, the Co-C bond is cleaved homolytically, and formation of the anion radical ([MeCbl]*-) reduces the dissociation energy by approximately 50%. Such dissociation energy lowering in [MeCbl]*- arises from the involvement of two electronic states: the initial state, which is formed upon electron addition, has dominant pi*corrin character, but when the Co-C bond is stretched the unpaired electron moves to the sigma*Co-C state, and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. The pi*corrin-sigma*Co-C states crossing does not take place at the equilibrium geometry of [MeCbl]*- but only when the Co-C bond is stretched to 2.3 A. In contrast to the neutral cofactor, the most energetically efficient cleavage of the Co-C bond is from the base-off form. The analysis of thermodynamic and kinetic data provides a rationale as to why Co-C cleavage in reduced form requires prior departure of the axial base. Finally, the possible connection of present work to B12 enzymatic catalysis and the involvement of anion-radical-like [MeCbl]*- species in relevant methyl transfer reactions is discussed.

CH3CoIIIPc (Pc = dianion of phthalocyanine) has been characterized by equilibrium studies of its trans axial ligation and cyclic voltammetry as a relatively "electron poor" model of methylcobalamin, which in noncoordinating solvents persists as a five-coordinate complex. Axial base (N-donors, PBu3, SCN-, weakly binding O-donors) inhibition of methyl transfer from CH3CoIIIPc shows that the reaction proceeds via the reactive five-coordinate species, even in coordinating solvents. The virtual inactivity of six-coordinate CH3CoIIIPc(L) complexes provides a reference point for important biological processes.

Many complexes of cobalt in the + 1 oxidation state are excellent nucleophiles. The complex with phthalocyanine, Pcz-, is an example; the rates of reaction of complex (CdPc)-, 1-, with Me1 and other alkylating agents in dimethylacetamide (DMA) solution are reported. In contrast to previous studies with other Co(1) complexes, the rate of reaction of the methylated product, (MeCoII'Pc, 3) with I-is also readily measured, allowing a kinetic evaluation of the equilibriumSimilarly, B r also reacts with 3 reversibly, and the rate and equilibrium constants are reported. Thus 1-is also a good leaving group. With CN, 3 gives complex MeCo*IIPcCNreversibly and slowly gives MeCN in a reaction zero order in CN-. The fast identity rate constants for I-attack on Me1 as well as that for B r attack on Me1 are given. Two different paths to the identity methyl transfer rate constant, kcdo, for the 1-+ 3 reaction with use of the Marcus equation gave kcdo = 4.4 M-I s-1 from the 3 + I-data but kcdo = 0.088 M-I s-1 from the 3 + B r data. This discrepancy, which is outside of experimental error, constitutes a deviation from the Marcus treatment; it is discussed.

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