Stabilization of Electron-Dense Palladium−Hydrido Complexes in Solid-State Hydrides

Inelastic neutron scattering, supported by IR and Raman spectroscopy, has been used to observe a decreasing Pd−H bond strength in a series of four low-valent palladium−hydrogen complexes in the solid-state hydrides: K 2 [Pd(II)H 4 ], Li 2 [Pd(0)H 2 ], Na 2 [Pd(0)H 2 ], NaBa [Pd(0)H 3 ], and Ba 2 [Pd(0)H 4 ] . Further, self-consistent linear muffin-tin orbital calculations of the electronic structures describe the palladium−hydrido complexes in a sd n hybridization with a decreasing Pd−H bond order as an increasing number of antibonding orbitals must be utilized. A strong support to the bonding from the cations via easily polarizable H - is, however, necessary to explain the stability of the systems. The effect of the large polarizability of hydrogen can in this respect be compared with the more conventional “back-bonding” to ligand orbitals for stabilizing a formal low-valent oxidation state by distributing electron density away from the central atom.