Unraveling the Major Differences between the Trinuclear Cyclopentadienylmetal Carbonyl Chemistry of Cobalt and That of Nickel-A Theoretical Study.

The geometries and energetics of the trinuclear cyclopentadienylmetal carbonyls Cp 3 M 3 (CO) n (Cp = η 5 -C 5 H 5 ); M = Co, Ni; n = 3, 2, 1, 0) have been investigated by density functional theory. The cobalt and nickel systems are found to be rather different owing to the different electronic configurations of the metal atoms. For cobalt, the small calculated energy separation of 5.0 kcal/mol between the two lowest-energy singlet Cp 3 Co 3 (μ 3 -CO)(μ-CO) 2 and Cp 3 Co 3 (μ-CO) 3 tricarbonyl structures accounts for the experimental results of both isomers as stable species that can be isolated and structurally characterized by X-ray crystallography. The corresponding Cp 3 Ni 3 (CO) 3 species in the nickel system are predicted not to be viable owing to exothermic CO dissociation to give the experimentally observed very stable Cp 3 Ni 3 (μ-CO) 2 , which is found to be the lowest-energy isomer by a substantial margin of ∼25 kcal/mol. In all of the low-energy Cp 3 M 3 (CO) n ( n = 2, 1) structures, including that of the experimentally known triplet spin state Cp 3 Co 3 (μ 3 -CO) 2 , all of the carbonyl groups are face-bridging or face-semi-bridging μ 3 -CO groups bonded to all three metal atoms of the M 3 triangle. In the lowest-energy carbonyl-free Cp 3 M 3 (M = Co, Ni) structures, agostic C-H-M interactions are found using hydrogens of the Cp rings. In addition, the lowest-energy Cp 3 Ni 3 is the only structure among all of the low-energy Cp 3 M 3 (CO) n (M = Co, Ni; n = 3, 2, 1, 0) structures in which each Cp ring is a bridging rather than terminal ligand.