Photoelectron spectroscopy of the bis(dithiolene) anions [M(mnt)2]n- (M = Fe - Zn; n = 1, 2): changes in electronic structure with variation of metal center and with oxidation

A detailed understanding of the electronic structures of transition metal bis(dithiolene) centers is important in the context of their interesting redox, magnetic, and optical properties. The electronic structures of the series M(mnt)2]n- (M = Fe - Zn; mnt = 1,2-S2C2(CN)2; n = 1, 2) were examined by a combination of photodetachment photoelectron spectroscopy and density functional theory calculations, providing insights into changes in electronic structure with variation of the metal center and with oxidation. Significant changes were observed for the dianions M(mnt)2]2- due to stabilization of the metal 3d levels from Fe to Zn and the transition from square-planar to tetrahedral coordination about the metal center (Fe-Ni, D(2h) --> Cu D2 --> Zn, D(2d). Changes with oxidation from M(mnt)2]2- to M(mnt)2]1- were largely dependent on the nature of the redox-active orbital in the couple M(mnt)2](2-/1-). In particular, the first detachment feature for Fe(mnt)2]2- originated from a metal-based orbital (Fe(II) --> Fe(III)) while that for Fe(mnt)2]1- originated from a ligand-based orbital, a consequence of stabilization of Fe 3d levels in the latter. In contrast, the first detachment feature for both of Ni(mnt)2]2- and Ni(mnt)2]1- originated from the same ligand-based orbital in both cases, a result of occupied Ni 3d levels being stabilized relative those of Fe 3d and occurring below the highest energy occupied ligand-based orbital for both of Ni(mnt)2]2- and Ni(mnt)2]1- . The combined data illustrate the subtle interplay between metal- and ligand-based redox chemistry in these species and demonstrate changes in their electronic structures with variation of metal center, oxidation, and coordination geometry.