On Bonding, Structure, and Stability of Ternary Hydrides A2MH2 (A = Li, Na; M = Pd, Pt)

Abstract

Bonding, structure, and stability of solid A2MH2 with A = Li, Na; M = Pd, Pt were investigated with a relati- vistically corrected density-functional approach, which reli- ably describes the trends among these four compounds. In order to examine the influence of the ligands (A) and of the crystalline environment, calculations were also made for free A2MH2 molecules and MH2 2- ions. The free MH2 2- complex is held together by strong bonds between formally closed shell atomic units because of strong M-d,s hybridization. The M-H bonds are further stabilized by the alkali metal ion li- gands and by the crystal surrounding. The crystal field ex- pands the H-A distance and enhances the H-A polarity. Re- lativistic effects contribute to M-H bonding in the solid state. The experimentally determined bond lengths and their trends are in accordance with theory. Due to relativistic and lanthanide effects, the Pt-H bond length becomes nearly as short as the Pd-H one. The small Li ion causes a distortion of the Li2PtH2 crystal resulting in an even shorter Pt-H bond length. In the gas-phase, A2PtH2 is more stable against dis- sociation than A2PdH2. The stability of the solid compounds is strongly influenced by the cohesive energy of the metal M, and also by the nature of the alkali metal. The evaluated enthalpies of formation favor increasing stability of solid A2MH2 against disproportionation into M and AH from Pt to Pd and from Li to Na. This is in agreement with experi- mental findings. The assignment of the experimental vibra- tional excitations should be reconsidered.