Experimental electron density in a transition metal dimer: Metal-metal and metal-ligand bonds

The accurate experimental electron density distribution of Co-2(CO)(6)(AsPh3)(2) has been determined through X-ray diffraction at T = 123 K. Metal-metal and metal-ligand bonds have been investigated by means of deformation densities and the quantum theory of atoms in molecules. The "expected" lack of charge accumulation in the deformation density map is "contradicted" by the presence of a bond critical point and a bond path line linking the two Cobalt atoms, in agreement with theoretical predictions on similar compounds. A careful analysis of the properties of rho(r) at the bond critical points and of the Laplacian distribution along the bond paths has allowed the full characterization of all bonds in the title compound and, in particular, to discard the apparently straightforward classification of Co-Co as a closed-shell interaction. The radial shape of the atomic Laplacian makes (covalent or polar) shared interactions similar to donor-acceptor ones when at least one "heavy atom" is concerned. Thus, even if it is possible to recognize the shared character of Go-Co and As-C bonds by comparison with the donor-acceptor C-Co and As-Co interactions, this distinction is grounded on subtleties which could possibly not suffice without some a priori chemical insight.