Topological Analysis of the Electron Density in the N-Heterocyclic Carbene Triruthenium Cluster [Ru₃(μ-H)₂(μ₃-MeImCH)(CO)₉] (Me₂Im = 1,3-dimethylimidazol-2-ylidene)

The bonding in the triruthenium dihydrido cluster compound [Ru3(μ-H)2(μ3-κ2-MeImCH)(CO)9] (1), which contains a face-capping N-heterocyclic carbene ligand (MeImCH) derived from the activation of two C−H bonds of 1,3-dimethylimidazol-2-ylidene (Me2Im), has been studied from the perspective of the atoms in molecules (AIM) quantum theory. Although the AIM approach recognizes the existence of a bond path in only one of the Ru−Ru edges of complex 1, i.e., that unbridged by the hydride ligands Ru(1)−Ru(3), the non-negligible values for the delocalization indexes of the hydride-bridged Ru−Ru edges indicate a delocalized kind of metal−metal interaction in these edges. In fact, a multicenter (5c-6e) interaction involving the Ru3H2 core of the molecule can be proposed. The three-atom C−N−C bridge that spans the Ru(1)−Ru(3) edge of 1 does not delocalize the electronic density of the bridged metal atoms as efficiently as bridges comprising just one atom, such as hydride or CH. The topological parameters of the three Ru−C bonds between the metal atoms and the face-capping NHC ligand are very similar, and they confirm that these interactions are pure σ-bonds. The analysis of the topological parameters for the bonds of the NHC ligand confirms the presence of π-electron delocalization within the five-membered ring as well as the existence of some double-bond character in the interaction of the carbene C atom with the adjacent N atoms.