Two extended metal chain compounds, Y₄I₅C and Y₆I₇C₂. Synthesis, structure, properties, and bonding

The title phases are obtained in high yield from the reactions of stoichiometric quantities of Y, YI₃, and graphite at 850-950°C in welded Nb containers. The crystal structures of the two phases determined by standard X-ray diffraction means are related to those published for Er₄I₃ and Er₆I₇ (C2/m, Z = 2, a = 18.479 (6) and 21.557 (8) Å, b = 3.947 (1) and 3.909 (1) Å, c = 8.472 (3) and 12.374 (6) Å, β = 103.22 (4) and 123.55 (3)°, R/R_w = 4.6/5.4 and 3.2/3.7% for Y₄I₅C and Y₆I₇C₂, respectively). The two compounds contain infinite single and double chains of carbon-centered yttrium octahedra, respectively, condensed by edge-sharing and sheathed by edge-bridging iodine atoms together with square-planar iodine atoms that bridge between chains. Extended Hückel MO band calculations were carried out for the empty and the carbon-centered compounds in both structures. Metal-based bands near E_F in the empty chains exhibit characteristic strong interactions with carbon valence orbitals that generate low-lying valence bands. E_F in both cases falls within a broad, metal-based conduction band. Resistivity and magnetic susceptibility measurements on Y₆I₇C₂ support the predicted metallic character.