The electronic structure of (diiminopyridine)cobalt(I) complexes

DFT calculations show that square-planar (LCoR)-R-I complexes of a diiminopyridine ligand are best regarded as containing low-spin Co-II antiferromagnetically coupled to a ligand radical anion. The lowest triplet state, corresponding to a 3d(z)(2)-->pi* excitation, is calculated to be only a few kcal/mol above the ground state, and is thermally accessible. The anomalous H-1 NMR chemical shifts of the LCoR complexes are suggested to be due to thermal population of the triplet state at room temperature. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004