Theoretical Photoelectron Spectroscopy of Metal-Metal Quintuple Bonds: Relativity-Driven Reordering of Frontier Orbitals

A recent reinvestigation of the gas-phase photoelectron spectra of Group 6 metal-metal quadruple-bonded complexes with scalar-relativistic DFT calculations showed that common exchange-correlation functionals reproduce the lowest ionization potentials in a semiquantitative manner. The finding encouraged us to undertake a DFT study of metal-metal quintuple bonds in a set of bisamidinato complexes with the formula MI2[HC(NR)2]2 (M = Cr, Mo, W; R = H, Ph, 2,6-iPr2C6H3) and idealized D2h symmetry. Scalar-relativistic OLYP/STO-TZ2P calculations indicated significant shifts in valence orbital energies among the three metals, which translate to lower first ionization potentials, higher electron affinities, and lower HOMO-LUMO gaps for the W complexes, relative to their Cr and Mo counterparts. These differences are largely attributable to substantially larger relativistic effects in the case of tungsten relative to its lighter congeners.