Synthesis, redox chemistry, and mixed-valence phenomena of cyanide-bridged dinuclear organometallic complexes

Article abstract of DOI:10.1002/cber.19971300912

21 new organometallic complexes of the type M-CN-M′ containing the building blocks M, M' = (CO)₅Cr, (CO)₅Mo, (CO)₅W, Cp(CO)₂Mn, Cp(CO)₂Fe, Cp(CO)(CN)Fe, Cp(dppe)Fe, Cp(PPh₃)₂Ru, Cp(PPh₃)Ni, and (PPh₃)₂Ag were obtained from the reagents M-CN and M-X (X = leaving group). Among them are five pairs of linkage isomers M-CN-M′/M-NC-M′. Structure determinations of (CO)sCr-CN-M′ with M′ = Fe(dppe)Cp, Ni(PPh₃)Cp, Ag(PPh₃)₂ and of (CO)₅Cr-NC-Fe(dppe)Cp have proved their identity and the linkage isomerism. Systematic variations of the v(CN) and v(CO) IR bands allow an assessment of the relative electron pair acceptor strengths of the building blocks M and M' and a reliable identification of the individual linkage isomers. All dinuclear complexes are redox-active, showing at least one reversible oxidation. The redox potentials are characteristically dependent upon the nature of the building blocks M and M′ and upon the orientation of the cyanide link (CN vs. NC). 6 oxidized complexes of the type [M-CN-Fe(dppe)Cp]⁺ were prepared chemically and isolated as PF6 or BF4 salts. The molecular structure of [(CO)_sCr-CN-Fe(dppe)Cp]BF₄ is not significantly different from those of the corresponding neutral Cr-CN-Fe or Cr-NC-Fe complexes. Upon oxidation the v(CN) band of the complexes shifts to lower wavenumbers and becomes much more intense. The oxidized complexes show the paramagnetism due to one unpaired electron. They give rise to very intense metal-to-metal charge-transfer bands in the near infrared region whose postion was found to be characteristically dependent on solvent polarity for [{CO)_sCr-CN-Fe(dppe)Cp]BF₄. A semiquantitative treatment of the optical and electrochemical measurements shows that the electron delocalization and metal-metal interaction in the oxidized dinuclear complexes is significant and that they belong to the class-II mixed-valence systems. VCH Verlagsgesellschaft mbH,.