Spin Crossover in Fe(II) Complexes: An Ab Initio Study of Ligand σ-Donation

The relative stability of high-spin and low-spin states has been studied in a series of Fe(II) complexes with model ligands of different a donation strength. The natural (delocalized) orbitals are transformed to atomic-like orbitals to analyze the multiconfigurational wave function in a valence bond picture. This analysis reveals a direct relation between the stabilization of the low-spin state and the importance of the ligand-to-metal charge transfer in the sigma orbitals. The multiconfigurational second-order perturbation theory (CASPT2) energy dependence of the low-spin state as function of the Fe-ligand distance deviates significantly from coupled cluster (CCSD(T)) calculations. This is ascribed to the intrinsic multiconfigurational character of the wave function of the low-spin state, invalidating the use of the simple closed-shell reference in the coupled cluster calculations. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 331-337, 2010