A MOLECULAR-ORBITAL STUDY OF RUTHENIUM MONONUCLEAR AND DINUCLEAR 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 p erformed 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 t heir recently reported methylcyclopentadienyl parents. The calculation s provide a clear rationalization of the electrochemical behavior of t he 18-electron mono- and di-nuclear species. Whatever is the rotationa l 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 con sequence, the reduction of the 18-electron species always corresponds to the reduction of the BFD ligand, while the reduction of the metal c enter appears energetically disfavored. In the light of these results, a reinterpretation of the EPR data recorded for the monoreduced speci es is suggested.