ELECTRONIC-STRUCTURES OF PEROXONICKEL(II) BIS(ISOCYANIDE) COMPLEXES

Several electronic states of the peroxonickel(II) bis(isocyanide) mole cule Ni(O-2)(CNH)(2) have been calculated using various ab initio meth ods in order to determine the ground state and the order of the low-ly ing excited states. Multireference configuration interaction calculati ons using natural orbitals predict a (1)A(1) ground state, in agreemen t with the observed diamagnetism for the analogous tert-butyl-substitu ted molecule. The first excited state was found to be the B-3(1) state , formed by excitation of an electron from a dioxygen pi(g) orbital, w hose nodal plane corresponds to the Ni(O-2)(CNH)(2) molecular plane, t o the virtual nickel 3d orbital which lies in the NiO2 plane. Natural orbital occupancies obtained from the multireference configuration int eraction calculations show the (1)A(1) state to be formally Ni(II)-O-2 (2-), although substantial occupation of the formally unoccupied nicke l 3d orbital suggests that the contribution to the molecular wave func tion from the superoxo-like Ni(I)-O-2(-) description is strong. The su peroxo description dominates the lowest excited state (B-3(1)), but he re the singly occupied O-2 orbital is perpendicular to the NiO2 plane. In addition to these two states, three other low-lying triplet states were calculated, and the charge densities (rho and del(2) rho) of all five states were examined.