A novel tetrahedral formally zerovalent-palladium hydride complex stabilized by divalent alkaline earth counterions

To investigate how formally low-valent transition-metal hydride complexes a re stabilized without the conventional "back-donation mechanism" of electro n density to ligand orbitals, a new tetrahedral formally zerovalent-palladi um hydride complex has been synthesized and structurally characterized in S r2PdH4 and Ba2PdH4. The two isomorphous hydrides were synthesized by hot si ntering of the binary alkaline-earth hydride with palladium powder at tempe ratures close to 750 degrees C. The structures were determined to be of bet a-K2SO4 type by means of X-ray single-crystal diffraction complemented by n eutron powder diffraction from the corresponding deuteride. The structure c an be described as consisting of tetrahedral palladium hydride complexes an d alkaline earth counterions. But the hydrides are also not far from being of an interstitial type, with hydrogen slightly off-center toward palladium from the octahedral interstices coordinating one palladium and five alkali ne-earth atoms. This intermediate character is reflected in long Pd-H dista nces in the complex, averaging 1.80 Angstrom in Ba2PdH4 and 1.78 Angstrom i n Sr2PdH4. It also emphasizes the importance of the counterion contribution to the stabilization of these unusual, formally low oxidation states.