Unsaturation in binuclear cyclopentadienylrhodium carbonyls: Comparison with their cobalt analogs

The Cp₂Rh₂(CO)_n (n = 4, 3, 2, 1) derivatives have been examined by density functional theory using the BP86 and MPW1PW91 functionals. The known tricarbonyl Cp₂Rh₂(CO)₃ is predicted to have a singly bridged structure with a predicted Rh–Rh single bond distance of ～2.70 Å in close agreement with the experimental value of 2.68 Å, determined by X-ray crystallography. In contrast to the cobalt analog, no evidence for a triply bridged Cp₂Rh₂(μ-CO)₃ structure was found. The known dicarbonyl Cp₂Rh₂(CO)₂ is predicted to have a doubly bridged structure with a predicted RhRh double bond distance of 2.58 Å in close agreement with the experimental RhRh double bond distance of 2.564 Å, found by X-ray crystallography for the permethylated derivative (η⁵-Me₅C₅)₂Rh₂(μ-CO)₂. The monocarbonyl Cp₂Rh₂(CO) is predicted to have a four-electron donor bridging carbonyl group with a Rh–O distance of ～2.5 Å and a RhRh double bond distance of ～2.54 Å. This differs from Cp₂Co₂(CO) which was previously predicted to have only a two-electron donor bridging carbonyl group with a long Co?O distance and a short CoCo distance of ～2.0 Å suggesting a formal triple bond. For Cp₂Rh₂(CO)₄ doubly bridged trans and cis isomers were found within ～1.0 kcal/mol in energy with non-bonding Rh?Rh distances of ～3.2 Å. However, these Cp₂Rh₂(CO)₄ isomers are predicted to be unstable with respect both to CO loss to give Cp₂Rh₂(CO)₃ and to fragmentation into two CpRh(CO)₂ units.