Theoretical electron densities in transition metal dihydrides

Analysis of the DFT calculated electron density distributions of the first-row transition metal (TM) dihydrides are reported. Nickel dihydride was used to assess the effects of basis set and exchange-correlation functional on calculated density properties, leading to the conclusions that diffuse basis functions and gradient-corrected functionals accurately describe the essential features of NiH2. These calculations indicate a remarkably high degree of covalent character in the Ni-H bond, as measured by most density properties, although the Laplacian of the density and the electron localization function (ELF) apparently show 'closed-shell' interaction. Subsequent B3LYP/6-311++G(f,p) studies of the ground states of the 10 first-row TM dihydrides were performed, and similarities and trends across this series examined. The M-H bond becomes less polar and more covalent across the row, as expected on the basis of electronegativity. We go on to show that several simply calculated electron density properties, notably those at the M-H bond critical point, also recover this trend