Metal–metal bonding in bridged ligand systems: experimental and theoretical charge densities in Co3(μ3-CX)(CO)9 (X = H, Cl)

Experimental charge density studies on Co3(mu3-CX)(CO)9 (1a X = H, 1b X = Cl) have been undertaken at 100 and 115 K respectively, using MoKalpha X-radiation. The nature of the metal-metal and metal-ligand interactions have been studied by means of deformation densities and by topological analysis of the density using the atoms in molecules (AIM) approach. DFT (B3LYP/6-311G**) calculations on 1a and 1b are in excellent agreement on a qualitative and quantitative level with the experimental conclusions. Within the conceptual framework of the AIM methodology, there is no direct bonding between the Co atoms, since no (3, -1) bond critical point in the density rho is observed and hence no bond path exists between the metal atoms. The delocalisation indices delta(Co, Co) obtained from the DFT wave-functions indicate however that there are significant indirect Co...Co interactions mediated through the bridging alkylidyne ligand. The charge concentrations on the alkylidyne C atoms in 1a and 1b are significantly different, and indicate that the bonding of this atom to the Co3 triangle is more localised in the case of 1b.