The metal-rich palladium chalcogenides Pd2MCh2 (M=Fe, Co, Ni; Ch=Se, Te): Crystal structure and topology of the electron density

Crystals of Pd2MCh2 (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 K2ZnO2 type (Ibam, Z=4, Pd2CoSe2: a=5.993(1), b=10.493(2), c=5.003(1) Å; Pd2FeSe2: a=5.960(1), b=10.576(2), c=5.078(1) Å; Pd2CoTe2: a=6.305(1), b=11.100(2), c=5.234(1) Å; Pd2NiTe2: a=6.286(1), b=11.194(2), c  =5.157(1) Å). One-dimensional ∞1[MCh4/2] tetrahedra chains with remarkably short M—M bonds run along [001], separated by [Pd2] dumbbells with palladium in fivefold coordination of selenium or tellurium atoms. The structure may also be described as a filled variant of the SiS2 type. M   atoms occupy 14 of the tetrahedral voids and the Pd atoms fill all octahedral voids in a distorted ccp motif of chalcogen atoms. Even though the Pd2MCh2 compounds are isotypic to K2ZnO2 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 Pd2CoSe2 and isotypic Na2CoSe2 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 Pd2CoSe2 and nonmetallic Na2CoSe2.

Graphical abstractThe metallic selenide Pd2CoSe2 and homologue Pd2MCh2 compounds (M=Fe, Co, Ni; Ch  =Se, Te) are the first ternary palladium chalcogenides with iron group metals. The crystal structure shows one-dimensional ∞1[CoSe4/2] tetrahedra with short CoCo bonds, which are connected by Pd2 dumbbells. Electronic structure calculations reveal strong metal–metal bonding.Figure optionsDownload full-size imageDownload as PowerPoint slide