HIGH-SPIN LOW-SPIN TRANSITIONS IN FE(II) COMPLEXES BY EFFECTIVE HAMILTONIAN METHOD

The high-spin-low-spin (HS-LS) transition in iron(II) complexes was st udied by the recently developed quantum chemical effective Hamiltonian method. This method uses a trial wave function which is an antisymmet rized product of the fully correlated function of d-electrons and of t he Slater determinant of the ligand MOs instead of the conventional Ha rtree-Fock single determinant trial wave function built of the molecul ar orbitals spread over an entire complex. This approach allowed us to explicitly take into account the d-electron correlations, the weak co valence of the metal-ligand bonds, and the electronic structure of the ligands. The cooperativity effects in the HS-LS transition occurring in the crystals are briefly discussed and the contribution from the Co ulomb forces to the intermolecular interaction responsible for the coo perativity is estimated.