Theoretical investigations on the electronic structure, chemical bonding, and ground-state and high pressure properties of metal and complex hydrides are reported. Recently a series of metal hydrides with unusually short H-H separations has been synthesized in Norway. The origin of this unexpected short H-H separation has been explained and several compounds with even shorter H-H separations have been predicted. From a systematic study on around 100 ZrNiAl-type compounds we have demonstrated that the electron localization function is a powerful tool to predict hydrogen positions in metal, intermetallic and alloy matrices. Based on these observations we have devised a new empirical rule, termed as "site-preference rule", for prediction of suitable sites for hydrogen accommodation. First principle total-energy calculations have been carried out for AH, EH2, AEH(3), AXH(4), and A(3)XH(6) (A = alkali metal, E = alkaline earth metal, X = B, Al or Ga) compounds to study structural phase stability at ambient and high pressures. We have reproduced crystal structures of known phases and predicted structural parameters for other compounds within these series. We have found that, if the cation (A(+) or E2+) radius is relatively small, one can expect several pressure-induced structural transitions with huge volume collapse at the first phase-transition point. These materials have mixed chemical bonding character which we have characterized by charge density, charge transfer, electron localization function, site- and angular-momentum projected density of states, crystal orbital Hamilton population analysis, Born effective charges, and Mulliken population analyses. (c) 2006 Elsevier B.V. All rights reserved.