Source Function description of metal-metal bonding in d-block organometallic compounds

The Source Function (SF) analysis is applied to a set of saturated [M2(CO)x, M = Mn, Fe, Co, Ni, x = 10, 9, 8, 7] and unsaturated [Co2(CO)x, x = 8-5] binuclear 3d metal carbonyls, and to the M2(formamidinate)4 (M = Nb, Mo, Tc, Ru, Rh, and Pd) binuclear 4d metal complexes, using ab initio electron densities. A description of the metal-metal (M-M) bonding closely related to that provided by the localization/delocalization indices is afforded. The agreement persists even when the M-M bond is lacking and the internuclear M-M midpoint is taken as a reference point for evaluating the SF contributions. However, use of the local form of the SF unveils interesting differences in how the charge density originates at the M-M midpoint when the system is metal-metal bonded or not. Most of the topological indices conventionally adopted to describe M-M bonds fail in reproducing the expected chemical trends for the set of investigated systems, with the adimensional |Vb|/Gb ratio and the lapalcian at the critical point value being particularly inadequate.