Nickel poly-acetylide carbonyl clusters: structural features, bonding and electrochemical behaviour

The reactions of [NEt4](2)[Ni-6(CO)(12)] with miscellaneous carbon halides (e.g. CCl4, C4Cl6) in CH2Cl2 have been extensively investigated particularly focusing on the stoichiometric ratio of the reagents and reaction temperature. This allowed the preparation of the previously known acetylide clusters [Ni-16(C-2)(2)(CO)(23)](4-), [HNi25(C-2)(4)(CO)(32)](3-) and [Ni-22(C-2)(4)(CO)(28)Cl](3-), as well as isolation and full characterisation of the closely related [Ni-17(C-2)(2)(CO)(24)](4-) and [Ni-25(C-2)(4)(CO)(32)](4-) tetraanions. From a structural point of view, all these clusters are based on a Ni-16 square orthobicupola which contain interstitial C-2, Ni(eta(2)-C-2)(4) or Ni-2(mu-eta(2)-C-2)(4) moieties, displaying rather short C-C bonds. Electrochemical studies reveal that all these species undergo different redox processes, even if their stability is rather limited. This is corroborated by an extensive analysis of the interaction between interstitial C-2 acetylide units and the metal cluster cage by Extended Huckel Molecular Orbital (EHMO) calculations, which indicates that tightly bonded C-C units are less effective than isolated C-atoms in stabilising the cluster cage.