QTAIM View of Metal–Metal Bonding in Di- and Trinuclear Disulfido Carbonyl Clusters

The experimental and theoretical charge densities in the sulfido-bridged cluster compounds Fe2(μ-S2)­(CO)6 (1), Fe3(μ3-S)2(CO)9 (2), Mn2(μ-S2)­(μ-CO)­(CO)6 (3), and Fe2(μ-S2)­(CO)5(PPh3) (4) have been studied using the quantum theory of atoms in molecules (QTAIM) methodology. High-resolution X-ray diffraction data have been measured for compounds 2–4 at 100 K. The topological analyses show that only in compounds 1 and 4 is there any evidence for metal–metal bonding in terms of the presence of a bond path. For compound 1, the topology of the Fe2S2 cage is highly dependent on the Fe–Fe separation, and the deformation along this vector is an extremely soft mode. The experimentally observed topology for compound 4 is the open “butterfly” topology. The orbital decomposition of the delocalization indices associated with the metal–metal interactions, δ­(ΩM–ΩM), implies significant direct Fe–Fe bonding in compounds 1 and 4 and for two of the Fe–Fe vectors in 2 but only a very minor Mn–Mn interaction in compound 3. The crystal structure of 2 shows a small amount (∼1%) of orientational disorder. As a result, a small degradation of the derived topological parameters is detectable, in comparison with the ordered structures of 3 and 4, and this leads us to discourage any quantitative QTAIM studies on disordered systems.