ON BONDING, STRUCTURE, AND STABILITY OF TERNARY HYDRIDES A(2)MH(2) (A= LI, NA, M = PD, PT)

Bonding, structure, and stability of solid A(2)MH(2) with A = Li, Na; M = Pd, Pt were investigated with a relativistically corrected density -functional approach, which reliably describes the trends among these four compounds. In order to examine the influence of the ligands (A) a nd of the crystalline environment, calculations were also made for fre e A(2)MH(2) molecules and MH22- ions. The free MH22- complex is held t ogether by strong bonds between formally closed shell atomic units bec ause of strong M-d,s hybridization. The M-H bonds are further stabiliz ed by the alkali metal ion ligands and by the crystal surrounding. The crystal field expands the H-A distance and enhances the H-A polarity. Relativistic effects contribute to M-H bonding in the solid state. Th e experimentally determined bond lengths and their trends are in accor dance 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 io n causes a distortion of the Li2PtH2 crystal resulting in an even shor ter Pt-H bond length. In the gas-phase, A(2)PtH(2) is more stable agai nst dissociation than A(2)PdH(2). The stability of the solid compounds is strongly influenced by the cohesive energy of the metal M, and als o by the nature of the alkali metal. The evaluated enthalpies of forma tion favor increasing stability of solid A(2)MH(2) against disproporti onation into M and AH from Pt to Pd and from Li to Na. This is in agre ement with experimental findings. The assignment of the experimental v ibrational excitations should be reconsidered.