A QUANTUM-CHEMICAL AB-INITIO STUDY OF THE SUPEREXCHANGE COUPLING IN BINUCLEAR OXYGEN-BRIDGED NI(II) COMPLEXES

Quantum chemical ab initio calculations have been performed for binuclear Ni(II) complexes of the form L5NiONiL5 and L4NiO2NiL4, with either one linear oxygen bridge or with two equivalent oxygen bridges and NiONi angles between 80-degrees and 110-degrees. He-like model ligands L have been used instead of the macrocyclic ligands which are necessary in the experiments to stabilize the complexes. The spin coupling between the two Ni(II) ions is antiferromagnetic in the linear oxo-bridged complex, with an exchange integral J of about -30 cm-1 at the CASSCF level and about -80 cm-1 at the multi-reference CEPA level. In the bioxygen-bridged complex the coupling is antiferromagnetic for NiONi angles smaller than 85-degrees and larger than 102-degrees and weakly ferromagnetic in between with J almost-equal-to 10 cm-1. Comparison with experiment and an interpretation of the exchange coupling by means of perturbation theory in terms of covalent and ionic molecular orbital configurations is also given.