Ternary thallium platinum and thallium palladium chalcogenides Tl(2)M(4)X(6). Syntheses, crystal structures, and bonding relations

The compounds Tl2Pt4S6, Tl2Pt4Se6, Tl2Pt4Te6 and Tl2Pd4Se6 can be synthesized by a melting reaction from the elements or by the reaction of thallium carbonate, transition metal powder and chalcogen powder in the temperature range between 400 degrees C and 950 degrees C. X-Ray investigations on single crystals and powdered samples revealed a new structure type for the compounds, that can be understood as stacking variant of the already known atom arrangement of the alkaline metal platinum chalkogenides A(2)Pt(4)X(6) (A = alkaline metal, X = S, Se). The short distances thallium-platinum and thallium-palladium, respectively, as well as the results of Extended-Huckel-calculations indicate covalent bonds between the main group and transition metal atoms.