Metal–Metal Bonding in Reduced Scandium Halides. Synthesis and Crystal Structure of Scandium Monochloride

The synthesis of a number of reduced scandium chlorides is achieved by reaction of excess metal with ScCl₃ in sealed Ta tubing above 877 °C for periods of weeks to months. Extreme conditions are necessary because of both the kinetic problems of forming extended metal–metal bonded structures and the fact that ScCl_1.5 effectively blocks further reaction on metal surfaces below this temperature. Single-crystal x-ray diffraction data showed that a gray-black laminar phase was ScCl with a sheet structure consisting of close-packed homoatomic layers sequenced Cl–Sc–Sc–Cl along [001] (R3m, trigonal cell; a = 3.473 (2) Å, c = 26.71 (4) Å, Z = 6; R = 0.088, R_W= 0.101 for 79 independent reflections taken with Mo Kα radiation). The phase is thus polytypic (ignoring the difference in metal atom) with ZrCl and isostructural with ZrBr even though there are only two-thirds as many electrons for bonding the double-metal sheets of scandium. The general occurrence of metal–metal bonding in clusters, ribbons, and sheets in transition metal groups 3–5 is considered.