Density functional theory methods are used to calculate the structures and energies of silicon trihydride (SiH3) and dihydride (SiH2) species on the Si(100)-(2X1) surface. These species are intermediates in the growth of silicon films by chemical vapor deposition of silane and disilane. The lowest-energy trihydride species is metastable with respect to the lowest-energy dihydride species, but two surface dangling bonds must be available to affect the transformation to the dihydride. In the lowest-energy configurations, dimers either have both dangling bonds occupied or both unoccupied. While the energy difference between isomers with fully occupied and partially occupied dimers will strongly favor fully occupied dimers at low temperatures, there will be a distribution of dimer occupations at high temperatures. The structures and energies of some other local minima corresponding to tri- and dihydrides are also described. While these species are energetically unfavorable and should only exist transiently, they illustrate the relative energetics of some alternative bonding behavior of the silicon surface.