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

The reactions of [NEt 4] 2[Ni 6(CO) 12] with miscellaneous carbon halides (e.g. CCl 4, C 4Cl 6) in CH 2Cl 2 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-, [HNi 25(C 2) 4(CO) 32] 3- and [Ni 22(C 2) 4(CO) 28Cl] 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(η 2-C 2) 4 or Ni 2(μ- η 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