Orbital splitting and pairing energy in open-shell organometallics: a study of two families of 16-electron complexes, CpMPH3 (M = Co, Rh, Ir) and Cp2M (M = Cr, Mo, W)

The singlet-triplet gap for two families of 16-electron organometallic complexes has been examined in detail by DFT calculations at the B3LYP level with polarized basis sets on both metal and ligands. For the first family, the group 6 metallocenes (Cp2M with Cp = eta(5)-C5H5 and M = Cr, Mo, W), the singlet-triplet gap (E-S - E-T) is always positive and decreases continuously on going from Cr to Mo to W. For the family of group 9 CpM(PH3), on the other hand, there is a decrease on going from Co to Rh, followed by a slight increase on going further to Ir. These trends have been analyzed in qualitative monoelectronic terms as a function of the competition be-tween the pairing energy and the orbital gap. While the pairing energy decreases as expected in the order 3d >> 4d > 5d, the orbital gap varies in a different way for the two families and, though quantitatively less important, is responsible for the different trends. It is argued that changes in orbital gap are system-dependent for open-shell organometallic systems, thus it is not possible to establish a universal trend of singlet-triplet gaps for a homologous series of complexes with a group of transition metals.