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

A series of complexes ML2x+ (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 MnL22+, FeL22+, and CoL2+ (ca. 1 e each). Smaller transfer is found in CoL22+ and the transfer is insignificant in high, MnL22+,Nil(2)(2+) and CuL22+. Analysis of the impaired 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.