A molecular orbital study of ruthenium mono- and di-nuclear complexes of 9-9'-bifluorenylidene (BFD): [(CpRu)(n)(BFD)](x) (n=1, x=1+, 0; n=2, x=2+, 1+, 0)

Extended Huckel and density functional theory calculations have been performed on [(CpRu)(BFD)](x) (x=0, 1+) and [(CpRu)(2)(BFD)](x) (x=0, 1+, 2+) (BFD = 9-9'-bifluorenylidene). These complexes are models for their recently reported methylcyclopentadienyl parents. The calculations provide a clear rationalization of the electrochemical behavior of the 18-electron mono- and di-nuclear species. Whatever is the rotational conformation of the coordinated BFD ligand, the LUMO of the oxidized form of the complexes is in fact the LUMO of the BFD ligand. As a consequence, the reduction of the 18-electron species always corresponds to the reduction of the BFD ligand, while the reduction of the metal center appears energetically disfavored. In the light of these results, a reinterpretation of the EPR data recorded for the monoreduced species is suggested.