A theoretical study of the complexes MH(x)(2-) and MCl(y)(2-) in the crystalline A(2)MH(x) and A(2)MCl(y) compounds (A = alkali, alkaline earth; M = Ni, Pd, Pt; x = 2, 4, 6; y = 4, 6) has been carried out using a relativistic density-functional method. A cutoff-type Madelung potential (MP) was used to take into account the crystalline environment. Energies, geometries, force constants, and vibrational frequencies have been determined. The relative stability of ML(6)(2-) versus ML(4)(2-) has been evaluated by the decomposition reaction of ML(6)(2-) --> ML(4)(2-) + L(2)(g) (L = H, Cl). The experimental M-H and M-Cl distances and their trends within the group 10 elements are very well reproduced by the calculations in the MP. The long-range electrostatic potential and the short-range repulsion between Na+ and its trans neighboring Na+ ions are responsible for the especially long H-Na bond length in the Na2PdH2 compound. All free ML(6)(2-) complexes are predicted to be rather stable against disproportionation. The crystal field effect strongly shifts the equilibrium to the right. The calculated reaction energies in the MP reveal that, in the solid state, PdCl62- and PtCl62- are stable but NiH62-, PdH62-, and NiCl62- rather unstable. The results are in agreement with the experimental evidence and also confirm the fact that the phenomenon of higher valency in compounds depends on the metal (M) and the choice of ligand CL). The relativistic effects strongly support the higher oxidation state in the metals.