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 Rhdouble bond; length as m-dashRh double bond distance of 2.58 Å in close agreement with the experimental Rhdouble bond; length as m-dashRh 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 Rhdouble bond; length as m-dashRh 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 Cotriple bond; length of mdashCo 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.