Metal-to-ligand electron transfer in diiminopyridine complexes of Mn-Zn. A theoretical study.

A series of complexes ML2(x+) (M = Mn-Zn, L = 2,6-bis(iminomethyl)pyridine) was investigated by theoretical methods. Electron transfer from the metal "t(2g)" orbitals to the ligand pi orbitals is reflected in the elongation of ligand C-N bonds and shortening of the C(py)-C(imine) bonds. Using zinc complexes as references, these deformations could be used to quantify the number of electrons transferred. Strong transfer is found in low-spin MnL2(+) (ca. 2 e) and in high-spin MnL2(+) and low-spin MnL2(2+), FeL2(2+), and CoL2(+) (ca. 1 e each). Smaller transfer is found in CoL(2)(2+), and the transfer is insignificant in high-spin MnL2(2+), NiL2(2+), and CuL2(2+). Analysis of the unpaired electron density on the metal (using the Staroverov-Davidson method) shows that the contribution of a biradical description, in which ligand radical anions are antiferromagnetically coupled to the metal center, is significant in most cases. In the case of CoL2(+) and high-spin MnL2(+), where the metal-ligand bond is weakened, it amounts to over 50% of the total transfer.