Formation of tetra- and tri-nuclear nickel acetylene complexes; crystal and molecular structures of tris-µ3-(η2-hexafluorobut-2-yne)-tetrakis-(carbonylnickel) and tricarbonyl-µ3-(η-cyclo-octatetraene)-µ3-(η2-hexafluorobut-2-yne)-triangulo-trinickel with evidence for a nickel-stabilised cyclo-octatetraenyl dication

Abstract

Reaction of [Ni(CO)₄] with hexafluorobut-2-yne gives initially [Ni(CO)₂(η²-CF₃C₂CF₃)], which decomposes to afford the cluster complex [Ni₄(CO)₄{µ₃-(η²-CF₃C₂CF₃)}₃], the structure of which has been established by single-crystal X-ray diffraction studies. It crystallises in the monoclinic space group P2₁/m, with two molecules in a unit cell of dimensions a= 8.506(2), b= 16.055(12), c= 9.011(4)Å, and β= 100.52(3)°. The structure has been elucidated via analysis of 1 033 significant reflections, and refined to R 0.13, R′ 0.165. The molecule has essentially C_3v symmetry consisting of a trigonal pyramid of nickel atoms [Ni(basal)–Ni(basal) 2.670, Ni(basal)–Ni(apical) 2.382 Å] each carrying a terminally bonded carbonyl group, and three face-bridging, µ₃(η²-⊥), acetylenic ligands. Reactions of the tetranuclear nickel cluster with 1,2-bis(diphenylphosphino)ethane (dppe) or Bu^tNC gives the mononuclear complexes [NiL₂(η²-CF₃C₂CF₃)](L₂= dppe, L = CNBu^t). In contrast, cyclo-octatetraene affords [Ni₃(CO)₃{µ₃-(η²-CF₃C₂CF₃)}{µ₃-(η²-C₈H₈)}], the structure of which has also been determined by X-ray crystallography. Crystals are monoclinic, space group P2₁/n, with four molecules in a unit cell of dimensions a= 9.5915(19), b= 11.994(3), c= 14.876(4)Å, and β= 99.57(2)°. Least-squares refinement using 2 114 reflections has converged at R 0.040, R′ 0.048. An isosceles triangle of nickel atoms [Ni(basal)–Ni(basal) 2.702, Ni(basal)–Ni(apical) 2.457 Å], each carrying a terminal carbonyl ligand, is sandwiched between a planar cyclo-octatetraene ring and a face-bonding acetylene molecule whose orientation with respect to the metal triangle is complementary, µ₃(η²-∥), to that observed in the tetranuclear complex. Evidence is presented for regarding the C₈H₈ ligand as a co-ordinated 6-π-electron [C₈H₈]²⁺ species.