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.