Synthesis, Redox Chemistry, and Mixed‐Valence Phenomena of Cyanide‐Bridged Dinuclear Organometallic Complexes

21 new organometallic complexes of the type M–Cn–M′ containing the building blocks M, M′ = (CO) 5 Cr, (CO) 5 Mo, (CO) 5 W, Cp(CO) 2 Mn, Cp(CO) 2 Fe, Cp(CO)(CN)Fe, Cp(dppe)Fe, Cp(PPh 3 ) 2 Ru, Cp(PPh 3 )Ni, and (PPh 3 ) 2 Ag 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) 5 Cr–CN–M′ with M′ = Fe(dppe)Cp, Ni(PPh 3 )Cp, Ag(PPh 3 ) 2 and of (CO) 5 Cr–NC‐Fe(dppe)Cp have proved their identity and the linkage isomerism. Systematic variations of the v (CN) and v (CO) IR bands allow as 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 lease 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 PF 6 or BF 4 salts. The molecular structure of [(CO) 5 Cr–CN–Fe(dppe)Cp]BF 4 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 position was found to be characteristically dependent on solvent polarity of [(CO) 5 Cr–CN–Fe(dppe)Cp]BF 4 . 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.