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 Cp3M3(CO)n (Cp = η5-C5H5); 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 Cp3Co3(μ3-CO)(μ-CO)2 and Cp3Co3(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 Cp3Ni3(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 Cp3Ni3(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 Cp3M3(CO)n (n = 2, 1) structures,
including that of the experimentally known triplet spin state Cp3Co3(μ3-CO)2, all of
the carbonyl groups are face-bridging or face-semi-bridging μ3-CO groups bonded to all three metal atoms of the M3 triangle. In the lowest-energy carbonyl-free Cp3M3 (M = Co, Ni) structures, agostic C–H–M interactions
are found using hydrogens of the Cp rings. In addition, the lowest-energy
Cp3Ni3 is the only structure among all of the
low-energy Cp3M3(CO)n (M = Co, Ni; n = 3, 2, 1, 0) structures in which
each Cp ring is a bridging rather than terminal ligand.