The metal-rich palladium chalcogenides Pd 2MCh2 ( M=Fe, Co, Ni; Ch=Se, Te): Crystal structure and topology of the electron density

Abstract

Crystals of Pd 2 MCh 2 ( M=Fe, Co, Ni; Ch=Se, Te) were synthesized by heating the elements at 823–1323 K in silica ampoules under argon atmosphere. Their structures were determined by single-crystal X-ray diffraction at room temperature. The metallic compounds crystallize in a variant of the K 2ZnO 2 type ( Ibam, Z=4, Pd 2CoSe 2: a=5.993(1), b=10.493(2), c=5.003(1) Å; Pd 2FeSe 2: a=5.960(1), b=10.576(2), c=5.078(1) Å; Pd 2CoTe 2: a=6.305(1), b=11.100(2), c=5.234(1) Å; Pd 2NiTe 2: a=6.286(1), b=11.194(2), c=5.157(1) Å). One-dimensional ∞ 1 [ MC h 4 / 2 ] tetrahedra chains with remarkably short M— M bonds run along [001], separated by [Pd 2] dumbbells with palladium in fivefold coordination of selenium or tellurium atoms. The structure may also be described as a filled variant of the SiS 2 type. M atoms occupy 1 4 of the tetrahedral voids and the Pd atoms fill all octahedral voids in a distorted ccp motif of chalcogen atoms. Even though the Pd 2 MCh 2 compounds are isotypic to K 2ZnO 2 from the crystallographic viewpoint, we find a different bonding situation with additional homo- and heteronuclear metal–metal bonds between the Pd and Co atoms. The electronic structures and topologies of the electron densities of Pd 2CoSe 2 and isotypic Na 2CoSe 2 are analyzed and compared by using Bader's AIM theory. Different values of topological charge transfer and electron density flatness indices uncover striking quantitative differences in the nature of chemical bonding between the metallic compound Pd 2CoSe 2 and nonmetallic Na 2CoSe 2.