Binuclear cyclopentadienylmetal carbonyl hydrides of iridium, osmium, and rhenium: The effect of electron count on the preferred structures

The Cp₂M₂H₂(CO)_n(M = Ir, Os, Re; n = 3, 2, 1) systems, related to the experimentally known and structurally characterized permethylated derivatives Cp^∗₂M₂H₂(CO)₂(Cp^∗ = η⁵-Me₅C₅; M = Ir, Os), provide examples of binuclear structures having metal–metal bonds of formal orders ranging from zero to four as determined by the 18-electron rule. The low-energy Cp₂Ir₂H₂(CO)₃structures lacking a direct iridium–iridium bond necessarily have a bridging CO group holding together the two iridium atoms. For Cp₂Ir₂H₂(CO)₂, low-energy unbridged structures are found corresponding to the experimental unbridged Cp^∗₂Ir₂H₂(CO)₂structure. However, for Cp₂Os₂H₂(CO)₂the lowest energy structure has two hydrogen atoms bridging the formal [Formula presented] double bond similar to the experimental doubly bridged Cp^∗₂Os₂H₂(CO)₂structure. The lowest energy structures of the monocarbonyls Cp₂M₂H₂(CO) have all three ligands (2H + CO) bridging a central metal–metal bond. The Cp₂Re₂H₂(CO) structures provide examples of both bridged and unbridged structures containing formal [Formula presented] quadruple bonds with highly bridged structures being preferred energetically.