We have calculated the electronic states and the total energies of a variety of tetrahedrally co-ordinated atomic clusters using the molecular orbital theory CNDO of quantum chemistry. All models in the calculations are based on the cluster Si46 with a single vacancy in the center. We find that for the cluster of Si46H60H4*, in which all dangling bonds are saturated with hydrogen, the donor or acceptor levels in the gap clearly appear when the phosphorus or the boron is introduced substitutionally into the cluster, whereas no clear impurity levels is found for that without hydrogenation. In addition, the increase in total energy resulting from introducing phosphorus (or boron) into the hydrogenated cluster Si46H60H4* is evidently more than that of the cluster without hydrogenation. Our results have been explain d theoretically by the "8-N" rule. The specific role of hydrogen in doping mechanism and some recent experiments are discussed for a-Si:H films.