Metal–metal bond length variations and the electronic structure of dichromium(II) and dimolybdenum(II) complexes

Abstract

The gas-phase core-electron ionization energies of the complexes [M₂L₄][M = Cr, L = O₂CMe or mhp (6-methylpyridin-2-olate); M = Mo, L = O₂CH, O₂CMe, or mhp] are reported. These data, together with the valence d ionization energies of these molecules, show a correlation between metal electron ionization energies and the large variations in the metal–metal separations found for the dichromium(II) species. A model in which these separations are mainly determined by the electrostatic potential at the metal centre is suggested by the experimental observations and supported by ab initio calculations on [M₂L₄][M = Cr or Mo; L =(CH₂)₂PH₂] reported here. This electrostatic potential is a function of the ligand; the lower the potential the shorter is the metal–metal separation, primarily because of the shallowness of the Cr–Cr potential well. The description of the metal–metal interaction, for a given Cr–Cr separation, is found to be insensitive to the nature of the ligand.