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 860-1000°C 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) Å, b=3.4975 (6) Å, c=11.810 (2) Å, β=99.81 (2)^o; space group C2/m, Z=2, R=0.029, R_w=0.046 for 676 reflections, MoKα, 2<50°). The phase contains double chains of condensed scandium octahedra sharing edges with a carbon approximately centered in each ((Sc-C)=2.31 Å) 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 XPS data 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 comopounds, 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.