A number of local and integral topological parameters of the electron density for the triosmium triangular carbonyl clusters [Os3(CO)12] (1), [Os3(μ-H)2(CO)10] (2), [Os3(μ-H)(μ-OH)(CO)10] (3), and [Os3(μ-H)(μ-Cl)(CO)10] (4) have been calculated and interpreted under the perspective of the relativistic and non-relativistic Quantum Theory of Atoms in Molecules (QTAIM). These results have allowed a comparison between topological properties of related but different atom-atom interactions, such as different Os-Os bond orders, H-bridged versus X-bridged (X=OH, Cl) and ligand-unbridged Os-Os interactions, and Os-H versus Os-OH and Os-Cl interactions. For compound 1, an interaction of 3c-2e type between the three metal atoms has been found. The local topological parameters of the unbridged Os-Os bonds in compound 2 differ considerably from those of the bridged Os-Os bond, for which a bond path was nevertheless observed. The calculated topological indexes indicate that bridging OH and Cl ligands are more efficient than bridging hydride ligands in delocalizing the electron density of bridged metal atoms; in fact, no direct bond path has been found for the interaction between bridged Os atoms in 3 or 4, although a non-negligible delocalization index δ(Os...Os) has been obtained for these non-bonding interactions. A multicenter 4c-4e interaction is proposed to exist in the bridged parts, Os2(μ-H)(μ-X) (X=H (2), OH (3), Cl (4)), of compounds 2-4. Compared with compound 2, the smaller electron density shared by the bridged Os atoms in compounds 3 and 4 is compensated by a greater electron density shared by the atoms in Os-X (X=O, Cl) bonds. © 2011 Elsevier B.V.

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