Electronic structure of Cob(I)alamin: the story of an unusual nucleophile.

The electronic structure and the ligand-field spectrum of cobalt(I) corrin is reported using complete active space multiconfigurational perturbation theory (CASPT2) to address some inconsistencies and the nature of the cobalt(I) "supernucleophile", cob(I)alamin. An assignment of six of the seven intense lines in the experimental spectrum is obtained at a root-mean-square accuracy of 0.14 eV and largest error of 0.21 eV. Agreement is significantly better for CASPT2 than density functional theory (DFT), but DFT does surprisingly well. The correlated wave function implies that the ground state of Co(I) corrin is severely multiconfigurational, with only 67% of the d(8) reference configuration and prominent contributions of 20% from open-shell metal-to-ligand charge-transfer configurations. The ground state exhibits a fascinating degree of covalency between cobalt and the nitrogen orbitals, described by the bonding and antibonding orbital pair of a cobalt d-orbital and a delta-orbital linearly combined from nitrogen orbitals. Thus, the standard description of the d(8) supernucleophile is not completely valid. From a biological perspective, the mixing in of Co(II) configurations in cob(I)alamin may be an important reason for the redox accessibility of the formal Co(I) state of the cofactor, which again provides the catalytic power for one half-reaction of enzymes such as cobalamin-dependent methionine synthase.