Single crystals of the phase pentascandium octachloride, Sc5Cl8, are obtained in several weeks by chemical transport from 940 to 960 °C in a sealed tantalum container starting with powdered metal and ScCl3. The role the lower valent gas-phase species ScCl2(g) plays in the formation of Sc5Cl8 at high temperature is discussed in terms of the postulated transport reaction Sc5Cl8(s)+2ScCl3(g)=7ScCl2(g). The structure of the green-hued, black crystals has been determined with full-matrix least-squares refinement of all atoms with anisotropic thermal parameters. This phase crystallizes in the monoclinic space group C2/m with a = 17.78 (2), b = 3.523 (8), and c = 12.04 (1) Å,β = 130.10 (6)°, and Z = 2; final residuals were R= 0.115 and Rw = 0.136 for 486 independent reflections with 2θ☄ 50° and I > 3σ(I). Structurally Sc5Cl8 contains separate infinite chains of metal octahedra which share trans edges as in Gd2Cl3; the common edges contain the shortest Sc-Sc bond yet observed, 3.021 (7) Å. A second, parallel chain consists of edge-shared octahedra of chloride about isolated scandium(III) atoms. The connectivity description [Sc(Cl)4/3(Cl)2/3]∞[Sc4Cl4(Cl)2/3(Cl)4/3]∞ is well approximated by (ScCl2+)∞-(Sc4Cl6-)∞, the latter emphasizing the anionic nature of the metal chain. The structure can be considered as arising from the formal condensation of M6X12 clusters such as are found in Sc7Cl12. The anionic character of the metal chain allows the metal atoms therein to be formally more reduced than in Gd2Cl3, an effect which is reflected in the relative shortening of the bonds in Sc5Cl8. The sums of metal-metal bond orders in four structurally diverse scandium chlorides as well as in Gd2Cl3 are found to be a smooth function of the number of delocalized electrons therein. Important in the variety of scandium chloride structures found with small changes in composition is the varied role halogen atoms serve in connectivity together with the ability of scandium to generate isolated cations and thence strongly metal-metal bonded anionic arrays.
ASJC Scopus subject areas
- Colloid and Surface Chemistry