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 (<b>1</b>), Fe₃(μ₃-S)₂(CO)₉ (<b>2</b>), Mn₂(μ-S₂)­(μ-CO)­(CO)₆ (<b>3</b>), and Fe₂(μ-S₂)­(CO)₅(PPh₃) (<b>4</b>) have been studied using the quantum theory of atoms in molecules (QTAIM) methodology. High-resolution X-ray diffraction data have been measured for compounds <b>2</b>–<b>4</b> at 100 K. The topological analyses show that only in compounds <b>1</b> and <b>4</b> is there any evidence for metal–metal bonding in terms of the presence of a bond path. For compound <b>1</b>, 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 <b>4</b> is the open “butterfly” topology. The orbital decomposition of the delocalization indices associated with the metal–metal interactions, δ­(Ω_M–Ω_M), implies significant direct Fe–Fe bonding in compounds <b>1</b> and <b>4</b> and for two of the Fe–Fe vectors in <b>2</b> but only a very minor Mn–Mn interaction in compound <b>3</b>. The crystal structure of <b>2</b> 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 <b>3</b> and <b>4</b>, and this leads us to discourage any quantitative QTAIM studies on disordered systems.