Metal-Metal Bonding in Reduced Scandium Halides. Synthesis and Characterization of Heptascandium Decachloride (Sc₇Cl₁₀). A Novel Metal-Chain Structure

The reaction of scandium metal with scandium trihalide in a sealed tantalum container under a temperature gradient 880-900 °C results in transport of monocrystals of heptascandium decachloride (ScCl_1.43) to the hot zone. The compound crystallizes in the monoclinic space group C2/m with a = 18.620 (7) Å, b = 3.5366 (7) Å, c = 12.250 (5) Å, β = 91.98 (4)°, and Z = 2. Full-matrix least-squares refinement of all atoms with anisotropic thermal parameters gave final residuals R = 0.059 and R_w = 0.072 for 705 independent reflections with 2θ ≤ 50° and I > 3σ(I). The unique feature of the structure is the presence of infinite chains composed of two parallel chains of scandium octahedra which share a common edge. Chlorine atoms cap all outward facing metal triangles and also bridge (a) to and (b) between isolated Sc(III) metal ions, giving the connectivity description (ScCl^b_4/2Cl⁸_2/3)_∞(Sc₆Cl₆Cl⁸_4/3)_∞ where the fractional atoms Cl^a and Cl^b provide the bridging functions. The structure is thus intermediate between the single chains of octahedra found in GdCl_1.5 and the infinite two-dimensional sheets of edge-shared octahedra in ScCl. Magnetic susceptibility measurements between 79 and 297 K (Faraday method) indicate a Curie-Weiss behavior above ∼120 K (μ = 1.04 μ_B, θ = -498 K) with evidence of magnetic ordering at T < 79 K. This and a sharp EPR signal at room temperature (g = 1.97) are interpreted in terms of localized, weakly interacting (at 297 K) d¹ configurations for two (of six) particular scandium atoms in the chain. The occurrence of strongly bonded and well-separated metal chains and the physical properties resulting therefrom suggest the phase can be described as a crystalline analogue of a metal fiber composite.