The recently-developed Green's-function method for defects in semiconductors is used to investigate the electronic properties of deep substitutional sp-bonded impurities in Si. These impurities are grouped according to the value Δz=zimp-zhost where z is the chemical valence. We choose a typical representative impurity from each group: sulfur with Δz=2 (double donor), zinc with Δz=-2 (double acceptor), and hydrogen with Δz=-3 (triple acceptor). In each case we obtain the impurity-induced state-density changes in the valence bands, and the energy positions and wave functions of the bound states in the forbidden gaps. These results are compared with earlier results for the vacancy, which may be viewed as a Δz=-4 nominal quadruple acceptor. We describe a simple physical model which explains the chemical trends as a function of Δz and compare with available experimental data. © 1982 The American Physical Society.