Interstitial atoms in metal-metal bonded arrays: The synthesis and characterization of heptascandium decachlorodicarbide, Sc 7Cl 10C 2, and comparison with the interstitial-free Sc 7Cl 10

Abstract

Reaction of Sc strips, ScCl/sub 3/, and graphite at 860-1000/sup 0/C gives Sc/sub 7/Cl/sub 10/C/sub 2/ 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 Er/sub 7/I/sub 10/ by single-crystal X-ray diffraction (a = 18.620 (4) A, b = 3.4975 (6) A, c = 11.810 (2) A, ..beta.. = 99.81 (2)/sup 0/; space group C2/m, Z = 2, R = 0.029, R/sub w/ = 0.046 for 676 reflections, MoK..cap alpha.., 2 theta < 50/sup 0/). 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 Sc/sub 7/Cl/sub 10/, 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 Sc/sub 7/Cl/sub 10/, Sc/sub 7/Cl/submore » 10/Cl/sub 2/, Sc/sub 2/Cl/sub 2/C, ScCl/sub 3/, 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.« less