Electron correlation and Hartree-Fock instabilities in polynuclear bis(π-polyenyl)nickel clusters

Abstract

The validity of the Hartree-Fock (HF) picture in the polynuclear Ni complexes 2–7 is investigated by means of semiempirical INDO calculations. The theoretical framework of this approach is the Thouless instability conditions; violations of the spatial, the spin and the time reversal (complex one-particle basis) symmetry are analysed. All systems under investigation show non-singlet instabilities indicating the existence of UHF solutions below the spin paired RHF determinant. In the case of the extended systems 5 and 7 a pronounced off-diagonal coupling in the singlet instability problem is encountered; the strong interaction between metal 3d transitions and inter-ligand transitions leads to severe deviations from the independent particle picture.