Ab Initio calculations of [CoY6−nXn]2+ complexes

The complete active space self-consistent field (CASSCF) and multireference second order perturbation theory (CASPT2) calculations of ${\mathrm{[CoF}}_{\mathrm{6}}{\mathrm{]}}^{\mathrm{4-}},$ ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{6}}{\mathrm{]}}^{\mathrm{2+}},$ ${\mathrm{[Co(NH}}_{\mathrm{3}}{\mathrm{)}}_{\mathrm{6}}{\mathrm{]}}^{\mathrm{2+}},$ ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{5}}{\mathrm{X]}}^{\mathrm{2+}},$ and ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{4}}{\mathrm{X}}_{\mathrm{2}}{\mathrm{]}}^{\mathrm{2+}}$ complexes $({\mathrm{X=CH}}_{\mathrm{3}}{\mathrm{OH,\hspace{0.5em}CH}}_{\mathrm{3}}{\mathrm{SH,\hspace{0.5em}CH}}_{\mathrm{3}}{\mathrm{NH}}_{\mathrm{2}})$ are reported. The potential energy surfaces of 10 lowest quartet states of ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{5}}{\mathrm{X]}}^{\mathrm{2+}}$ complexes near the equilibrium geometry are calculated and splitting of triple-degenerate ${}^4{T}_{\text{1g}}\mathrm{(F),}$ ${}^4{T}_{\text{2g}}\mathrm{(F),}$ and ${}^4{T}_{\text{1g}}\mathrm{(P)}$ electronic states of ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{6}}{\mathrm{]}}^{\mathrm{2+}}$ complex induced by the substitution of one or two water ligands is characterized and quantified. It is shown that the energy differences between originally degenerate states are almost invariant to the changes of metal–ligand distances, and despite their proximity, the crossing does not occur. The coefficients of leading configuration of multireference wave functions of ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{5}}{\mathrm{X]}}^{\mathrm{2+}}$ and ${\mathrm{[Co(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{4}}{\mathrm{X}}_{\mathrm{2}}{\mathrm{]}}^{\mathrm{2+}}$ complexes are shown to approach unity and the usage of single-reference methods is justified. As a consequence, interaction energies of the studied functional groups with ${\mathrm{Co}}^{\mathrm{2+}}$ are computed also at the HF (Hartree–Fock), DFT (density functional theory), and MP2 (second-order Moller–Plesset) levels. They are compared to CASSCF calculations and to the equivalent calculations done for ${\mathrm{Zn}}^{\mathrm{2+}}$ and ${\mathrm{Ni}}^{\mathrm{2+}}$ ions. The computational methodology for the accurate calculations of various cobalt (II) ionic complexes is described and the implications for the theoretical investigation of interactions of chemically and biologically important functional groups with ${\mathrm{Co}}^{\mathrm{2+}}$ are discussed.