Theoretical topological analysis of the electron density in a series of triosmium carbonyl clusters: [Os3(CO)12], [Os3(μ-H)2(CO)10], [Os3(μ-H)(μ-OH)(CO)10], and [Os3(μ-H)(μ-Cl)(CO)10]

► In [Os3(CO)12], the interaction between the three metal atoms is of 3c-2e type. ► A 4c-4e interaction is found in the bridged parts of Os2(μ-H)(μ-X) (X = H, OH, Cl). ► OH and Cl ligands are more efficient than H ligands for delocalizing electron density. ► Topology of Os-Os formal single and double bonds is very different from each other. ► Relativistic QTAIM calculations give quantitatively accurate results in these systems.