Interstitial atoms in metal-metal bonded arrays: the synthesis and characterization of heptascandium decachlorodicarbide, Sc7Cl10C2, and comparison with the interstitial-free Sc7Cl10

Reaction of Sc strips, ScCl3, and graphite at 860-10000C gives Sc7Cl10C2 in quantitative yields, with transport occurring at the higher temperatures; adventitious carbon will also produce the phase. The compound has been shown to be isostructural with Er7I10 by single-crystal X-ray diffraction (a = 18.620 (4) A, b = 3.4975 (6) A, c = 11.810 (2) A, β = 99.81 (2)0; space group C2/m, Z = 2, R = 0.029, R/sub w/ = 0.046 for 676 reflections, MoKα, 2 theta < 500). The phase contains double chains of condensed scandium octahedra sharing edges with a carbon approximately centered in each (anti d(Sc-C) = 2.31 A) together with isolated scandium atoms in a parallel chain of chloride octahedra. The arrangement is very similar to that in the previously known Sc7Cl10, from which the heavy atom arrangement can be derived by displacement of all metal atoms by b/2 so as to convert chlorine functions on the metal chain from face-capping to edge-bridging. The driving force for this is thought to be the reduction of carbon-chlorine repulsive interactions. Core and valence x-ray photoelectron spectra (XPS) for Sc7Cl10, Sc7Cl10Cl2, Sc2Cl2C, ScCl3, and Sc are presented to demonstrate the appearance of a carbide-like state for the interstitial, the presence of two different types of scandium in the first two compounds, the oxidation of the chain that accompanies the carbon insertion, and a substantial Sc-C covalency. The latter arises through mixing of the interstitial's 2s and 2p valence orbitals with metal-metal bonding cluster orbitals of the same symmetry. 37 references, 6 figures, 7 tables