Electron transfer activity of nickelacyclic complex analogues of nickelocene: synthesis of (eta(5)-R-cyclopentadienyl){eta(4)-[1-(eta(5)-R-cyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel complexes (R = H, CH(3)) and crystal structures of the redox couples [(eta(5)- methylcyclopentadienyl){eta(4)-[1-(eta(5)-methylcyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel]((0/+)) and [(eta(5)-methylcyclopentadienyl){eta(5)-[1-(eta(5)-methylcyclopentadienyl)-1-nickelafluorenyl}nickel]((0/+)).

Following previous reports on the synthesis and structure of different nickelacyclic complexes analogues of nickelocene, we now deal with the new metallacyclic compounds (eta5-R-cyclopentadienyl){eta4-[1-(eta5-R-cyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel (R = H, CH3). The redox ability of the whole series of nickelacyclic derivatives has been also investigated by electrochemical and spectroelectrochemical techniques, and the nature of the frontier orbitals responsible for the rich electron transfer activity of this class of compounds has been supported by theoretical considerations. On the basis of the redox properties of a few neutral members of the series, their chemical oxidation afforded the corresponding monocations and the crystal structures of the pertinent redox couples were determined by X-ray single-crystal analysis.