Topological analysis of multiple metal-metal bonds in dimers of the M-2(formamidinate)(4) type with M = Nb, Mo, Tc, Ru, Rh, and Pd

The chemical bond in complexes of the M-2(formamidinate)(4) type with diffe rent nominal bond orders has been investigated within the framework of the present topological theories. The atoms-in-molecules (AIM) analysis of the theoretically calculated electron density shows low rho (r) values at the m etal-metal bond critical point (r(c)), which makes difficult a topological description of the interaction using the electron density as the scalar fun ction. When the electron localization function (ELF) is used instead, four disynaptic metal-metal valence basins, V(M,M), are found for the Mo and Nb dimers, one for each the Rn and Rh complexes, while no disynaptic basins ar e obtained for the Tc and Pd systems. The V(M,M) basins are not the dominan t features of the interaction due to their low population values with the m ain contribution arising from the "4d" metal electrons. However, the molecu lar orbitals involving the "4d" function of the metal essentially contribut e to the metal core basins, C(M). The most important characteristic of the metal-metal bond is the abnormally high values for the metal-metal core cov ariance, B(M,M), and the AIM atomic basins covariances, lambda (c)(rho). Th is large electron fluctuation which occurs between the two metallic cores i s interpreted in terms of simple resonance arguments. Except for Rh, there is an excellent correlation between the core covariances, B(M,M), and the m etal-metal distances.