A density functional theory study of paramagnetic cyclopentadienylcobalt(III) derivatives: fluoride versus cyanide.

The cobalt(III) complexes Cp₂Co₂F₄ and Cp₂Co₂(CN)₄ have been studied by density functional theory methods as representatives of the experimentally known Cp₂Co₂X₄ species with the weak-field fluoride ligand and the strong-field cyanide ligand. Both complexes were found to have relatively complicated energy surfaces with low-energy triplet and quintet spin state structures as well as the expected singlet-state structures for Co(III) complexes. This existence of singlet-, triplet-, and quintet-state structures of similar energies complicates the study of these complexes by density functional theory. The B3LYP* method of Reiher et al. was chosen in an effort to provide the most reliable estimates of the relative energies of the singlet, triplet, and quintet spin states. The lowest-energy Cp₂Co₂F₄ structure was found to be a doubly bridged quintet spin state structure, with similar triplet and singlet structures lying within ∼4 kcal mol⁻¹ of this quintet structure. The lowest-energy Cp₂Co₂(CN)₄ structure was found to be a triplet spin state structure, with a singlet structure lying within ∼1 kcal mol⁻¹ of this triplet structure. Almost all of the Cp₂Co₂X₄ structures were found to have nonbonding Co···Co distances in excess of 2.9 Å, as expected for Co(III) complexes. In general, structures with trans stereochemistry of the Cp and other terminal ligands were found to be of lower energy than the corresponding structures with cis stereochemistry.