The chemical bond in complexes of the M-2(formamidinate)(4) type with different 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 metal-metal bond critical point (r(c)), which makes difficult a topological description of the interaction using the electron density as the scalar function. 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 are obtained for the Tc and Pd systems. The V(M,M) basins are not the dominant features of the interaction due to their low population values with the main contribution arising from the "4d" metal electrons. However, the molecular orbitals involving the "4d" function of the metal essentially contribute 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 covariance, B(M,M), and the AIM atomic basins covariances, lambda (c)(rho). This large electron fluctuation which occurs between the two metallic cores is interpreted in terms of simple resonance arguments. Except for Rh, there is an excellent correlation between the core covariances, B(M,M), and the metal-metal distances.