Structural stability and bonding properties of the hydrogen storage material Mg(2)NiH(4) (monoclinic, C2/c, Z = 8) were investigated and compared to those of Ba(2)PdH(4) (orthorhombic, Pnma, Z = 8) using ab initio density functional calculations. Both compounds belong to the family of complex transition metal hydrides. Their crystal structures contain discrete tetrahedral 18 electron complexes T(0)H(4)(4-) (T = Ni, Pd). However, the bonding situation in the two systems was found to be quite different. For Ba(2)PdH(4), the electronic density of states mirrors perfectly the molecular states of the complex PdH(4)(4-), whereas for Mg(2)NiH(4) a clear relation between molecular states of TH(4)(4-) and the density of states of the solid-state compound is missing. Differences in bonding of Ba(2)PdH(4) and Mg(2)NiH(4) originate in the different strength of the T-H interactions (Pd[bond]H interactions are considerably stronger than Ni[bond]H ones) and in the different strength of the interaction between the alkaline-earth metal component and H (Ba[bond]H interactions are substantially weaker than Mg[bond]H ones). To lower the hydrogen desorption temperature of Mg(2)NiH(4), it is suggested to destabilize this compound by introducing defects in the counterion matrix surrounding the tetrahedral Ni(0)H(4)(4-) complexes. This might be achieved by substituting Mg for Al.