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) and of the crystalline environment, calculations were also made for free A 2 MH 2 molecules and MH 2 2ยฑ ions. The free MH 2 2ยฑ complex is held together by strong bonds between formally closed shell atomic units because of strong M-d,s hybridization. The MยฑH bonds are further stabilized by the alkali metal ion ligands and by the crystal surrounding. The crystal field expands the HยฑA distance and enhances the HยฑA polarity. Relativistic effects contribute to MยฑH 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 PtยฑH bond length becomes nearly as short as the PdยฑH one. The small Li ion causes a distortion of the Li 2 PtH 2 crystal resulting in an even shorter PtยฑH bond length. In the gas-phase, A 2 PtH 2 is more stable against dissociation than A 2 PdH 2 . 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 A 2 MH 2 against disproportionation into M and AH from Pt to Pd and from Li to Na. This is in agreement with experimental findings. The assignment of the experimental vibrational excitations should be reconsidered.