Metallic properties in the series K2Pd(II)H4, Na2Pd(0)H2 and Li2Pd(0)H2 correlated with the stabilization of a formally zero-valent palladium-hydrogen complex

Two new hydrides, Li 2 PdH 2 and K 2 PdH 4 , are compared with Na 2 PdH 2 . Both Li 2 PdH 2 and Na 2 PdH 2 are found to be metals and their structures are characterized by linear, formally zero-valent PdH 2 complexes in an alkali atom framework. The structures are isomorphous with Na 2 HgO 2 . The palladium-hydrogen bond length is 1.68 A. The usual stabilization mechanism for a low formal oxidation state, by back donation to orbitals on the ligand, is inoperative when hydrogen is the only ligand and the metallic properties of Li 2 PdH 2 and Na 2 PdH 2 are attributed to a stabilization mechanism in which electrons are delocalized into a conduction band. In K 2 PdH 4 , with the Na 2 PtH 4 type structure, the larger and more electropositive potassium atom allows a more common four-coordinated d 8 square planar palladium complex to be formed. The palladium-hydrogen bond length is 1.63 A. The electrons are localized and K 2 PdH 4 is a yellow-green non-conducting powder.