Electronic structure of the nickel tetracyanonickelate Ni(CN) 4 2? and nickel carbonyl Ni(CO)4. An ab-initio LCAO-MO-SCF calculation

Abstract

The electronic structure of the Ni(CN)42−ion in its ground and first excited states and of the Ni(CO)4 molecule in its ground state have been investigated through an LCAO-MO-SCF calculation with a gaussian basis set (12, 8, 5/8, 4) contracted to a [5, 4, 2/3, 2 (C)/2, 1 (N or O)] set. The sequence of ionization potentials (I.P.) computed for Ni(CO)4 from the Koopmans' theorem is in agreement with previous assignments for the experimental spectrum. On the contrary, Koopmans' theorem turns out to be not valid for the Ni(CN)42−ion, that is the sequence of I.P. computed as the difference of the total energy for this ion and the mononegative ion is different from the sequence of orbital energies for Ni(CN)42−. The computed I.P. turn to be rather sensitive to the electrostatic potential of the crystal represented through a set of point charges. The Ni atom in Ni(CO)4 bears a small positive charge of 0.24e, as a result of π back-bonding to the ligands. Charge transfer upon coordination and the nature of the ligand-metal bonding is compared in these two complexes through the results of a population analysis and a description in terms of localized orbitals. Separate SCF calculations have been performed for the lowest excited states of Ni(CN)42−. Although π→π⋆ transitions would be expected as the lowest ones on the basis of orbital energy values, the lowest computed transitions correspond to d→d excitations, in agreement with previous assignments for the experimental spectrum. This is traced to the role of Coulomb and exchange terms in the computed transition energies together with the importance of electronic relaxation upon excitation.