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 (eta(5)-C(5)H(5))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 Co(2)(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)Co(2)(CO)(6) by approximately 46 kcal mol(-1). The binuclear Co(2)(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 Co(2)C(2) 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)Co(2)(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.