Electron-sink behaviour of the carbonylnickel clusters [Ni32C6(CO)36]6– and [Ni38C6(CO)42]6–: synthesis and characterization of the anions [Ni32C6(CO)36]n– (n = 5–10) and [Ni38C6(CO)42]n– (n = 5–9) and crystal structure of [PPh3Me]6[Ni32C6(CO)36] · 4 MeCN

The hexacarbide clusters [H(6-n)Ni38C6(CO)42](n-) (n = 3, 4, 5, or 6) have been directly obtained from the reaction of [Ni6(CO)12]2- with C3Cl6, whereas the related anions, [H(6-n)-Ni32C6(CO)36](n-) (n = 5 or 6), have been obtained by degradation under carbon monoxide of [Ni38C6(CO)42]6-, or upon thermal treatment at ca. 110 °C of [Ni10C2(CO)16]2- salts. The compound [PPh3Me]6[Ni32C6(CO)36] · 4 MeCN is triclinic, space group P1̄ (No 2), with a = 15.974(3), b = 17.474(3), c = 18.200(4) Å, α = 61.37(2), β = 69.31(2), γ = 72.35(2)°and Z = 1; final R = 0.033. The structure of [Ni32C6(CO)36]6- has an idealised O(h) symmetry and is based on a truncated octahedral Ni32C6 framework, with all edges spanned by bridging carbonyl groups. The six interstitial carbide atoms are lodged in square-antiprismatic cavities. The overall geometry of the Ni32C6 core is very similar to that found previously in [HNi38C6(CO)42]5-, and shows very close interatomic separations. Both [Ni32C6(CO)36]6- and [H(6-n)Ni38C6)(CO)42](n-) (n = 5 or 6) display electron-sink behaviour. Thus, they have been chemically and electrochemically reduced to their corresponding [Ni32C6(CO)36](n-) (n = 7-10), [Ni38C6(CO)42](n-) (n = 7-9) and [HNi38C6(CO)42](n-) (n = 6-8) derivatives, and several of the involved redox changes show features of electrochemical reversibility. In contrast, both [Ni32C6(CO)36]6- and [H(6-n)Ni38C6(CO)42](n-) (n = 5 or 6) support only one partially reversible oxidation step. Their different behaviour upon protonation or oxidation is an indirect, but unambiguous, proof of the hydride nature of [HNi32C6(CO)36]5- and [H(6-n)Ni36C6(CO)42](n-) (n = 3, 4, or 5), which could not be validated by 1H-NMR spectroscopy.