Complete active space self-consistent field (CASSCF) computations followed by the second-order perturbation theory have been applied to investigate the electronic properties of a structural mimic of the reactant complex formed in the catalytic cycle of cobalamin-dependent methionine synthase (MetH). Two different structural models have been employed to analyze the reaction complex between methylcobalamin (MeCbl) and homocysteine (Hcy). The first model, referred to as the small model (SM), is based on a truncated corrin ring with inner conjugated macrocycle only and has symmetry constrain ? with respect to methylthiolate. The second is based on a full corrin ring, i.e., [Co(III)(corrin)]-Me(+)center dot center dot center dot(-)S-CH(3), without the side chains (which were replaced by hydrogen atoms) and referred to as the large model (LM). The active space chosen for both models includes all essential orbitals participating in the methyl transfer reaction. Although the CASSCF calculations are much more demanding for the LM (due to the structural complexity) than the SM, the results are fully consistent. The energetic of ionic and diradical states have been examined as a function of the C center dot center dot center dot S distance between the methyl group of MeCbl and the sulfur of the thiolate substrate for both models. The most important finding of the present work is the energetic variation of ionic and diradical states as a function of C center dot center dot center dot S distance. The two states cross each other at a C center dot center dot center dot S distance of 4.0 angstrom, i.e., for a distance shorter than similar to 4.0 angstrom, the ionic state is energetically the lowest electronic state, while the diradical state becomes the lowest state at longer distances. However, the potential energy surface of the ionic state shows greater sensitivity with respect to the C center dot center dot center dot S distance than that of the diradical one. The former can be associated with S(N)2-type displacement, where the cleavage of the Co-C bond would be heterolytic, while the latter can be associated with an electron transfer (ET), where the cleavage is homolytic. Finally, the importance of this finding is briefly discussed in the context of MeCbl-dependent enzymatic catalysis.