Dimerization of a fluorocarbyne complex to a tetrahedrane derivative: Fluorocarbyne and difluoroacetylene cobalt carbonyl complexes

Recent work has resulted in the discovery of the fluorocarbyne complex (η⁵-C₅H₅)Mo(CO)₂(CF), in which the CF ligand behaves as a formal three-electron donor like the isoelectronic well-known NO ligand. In this connection the related fluorocarbyne cobalt carbonyls Co(CF)(CO)_n (n = 3, 2, 1) and Co₂(CF) ₂(CO)_n (n = 6, 5, 4, 3, 2) have been studied by density functional theory. The lowest energy structures for the mononuclear Co(CF)(CO)_n derivatives parallel those of the isoelectronic Co(NO)(CO)_n and Ni(CO)_n+1 derivatives. The mononuclear fluorocarbyne complex Co(CF)(CO)₃ is predicted to be thermodynamically unstable with respect to dimerization to the difluoroacetylene complex (FCCF)Co₂(CO)₆ by ∼46 kcal mol^-1. The binuclear Co₂(CF)₂(CO)_n structures can broadly be divided into two classes: (1) Structures in which the two CF ligands are coupled to form a Co₂C₂ tetrahedrane derivative containing an FCCF ligand derived from difluoroacetylene; (2) Structures maintaining separate CF ligands. With important exceptions, the structures of the difluoroacetylene complexes (FCCF)Co₂(CO)_n (n = 6, 5, 4) parallel for the most part the structures predicted for the corresponding unsubstituted acetylene complexes. The relative energies of the Co ₂(CF)₂(CO)_n structures with separate CF ligands indicate that the CF ligand is a more favorable bridging ligand than the ubiquitous CO ligand.