On the importance of orbital relaxation and correlation in the photoelectron spectra of transition metal complexes

Abstract

Orbital reorganisation energies in the photoelectron spectra of 15 transitional metal complexes with Ti, V, Cr, Mn, Fe, Co, Ni and Zn as 3d centres have been determined by means of the Green's function formalism. Computational background for the present investigation is an improved semi-empirical INDO Hamiltonian. Detailed results and comparisons with experimental data are given for four representative 3d complexes with V, Mn, Fe and Ni atoms. A self-energy approximation has been employed in the second order of perturbation that allows the decomposition of the net reorganisation energies into orbital relaxation, the loss of ground-state correlation and the variation of the electronic correlation in the cationic hole state as the many-body response to the relaxation energy. The magnitude of these corrections is studied as a function of the localisation properties of the corresponding orbital wavefunctions. Outer valence ionisation energies for delocalised ligand electrons can be assigned on the basis of the canonical molecular orbital energies of the electronic ground state (Koopmans' theorem). Relaxational procedures (e.g. Delta SCF formalism) are a suitable approximation in the case of MO with 40-60% metal character where correlation effects are negligibly small.