Synthesis, Structure, and Bonding of Sc₆MTe₂ (M = Ag, Cu, Cd):  Heterometal-Induced Polymerization of Metal Chains in Sc₂Te

Abstract

Three new compounds, Sc6AgTe2, Sc6Cu0.80(2)Te2.20(2), and Sc6CdTe2, were prepared by high-temperature solid state techniques, and the structures were determined by single-crystal X-ray diffraction to be orthorhombic, Pnma (No. 62, Z = 4) with a = 20.094(9) Å, 19.853(5) Å, 20.08(1) Å, b = 3.913(1) Å, 3.914(1) Å, 3.915(2) Å, and c = 10.688(2) Å, 10.644(2) Å, 10.679(5) Å, respectively, at 23 °C. The compounds are isotypic with Sc6PdTe2 and represent the first ternary metal-rich rare-earth-metal chalcogenides containing group 11 or group 12 elements. The structure can be viewed as heterometal sheets lying parallel to the b−c planes that are separated by isolated tellurium atoms. These sheets can also be viewed as a polymerization of two different types of metal chains in Sc2Te (blades and zigzag chains) by heterometal (M) replacements of some intervening tellurium atoms. Extended Hückel band calculations reveal that the interior atoms in the metal network achieve negative formal Mulliken charges while Sc atoms on the exterior that have tellurium neighbors have positive values. The heterometal−metal bonding enhances the overlap populations of zigzag chains and blades relative to those in Sc2Te. The calculation results also indicate that these compounds are metallic, as usual