Orbital Splitting and Pairing Energy in Open‐Shell Organometallics: A Study of Two Families of 16‐Electron Complexes [Cp₂M] (M = Cr, Mo, W) and [CpM(PH₃)] (M = Co, Rh, Ir)

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 (Cp 2 M with Cp = η 5 ‐C 5 H 5 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(PH 3 ), 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 between 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)