  We study in detail by means of ab-initio pseudopotential calculations the
electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H,
also during their formation and their evolution upon hydrogenation. The
atom-projected densities of states (DOS) and an accurate analysis of the
valence charge distribution clearly indicate the fundamental contribution of
T_5 defects in originating gap states through their nearest neighbors. The
interaction with hydrogen can reduce the DOS in the gap annihilating T_5
defects.
