Density functional theory was used to study interactions between phosphine and 17 functional groups, revealing that only the functional groups containing metal ions bind phosphine strongly. This led to a detailed examination of the adsorption of the pnictogen hydride gases ammonia, phosphine, and arsine on 33 metal catecholates in order to examine periodic trends in the binding strength for these adsorbates. Phosphine and arsine adsorption is primarily driven by donation of electron density from the adsorbate molecule to the metal atom, while ammonia binding involves both electron donation and a significant Coulomb attraction between the negatively charged N atom and the positive metal atom. This Coulomb effect results in notably different binding behavior for ammonia than for phosphine and arsine, which are quite similar to each other. Generally, for metals on the left side of the periodic table, the Coulomb effect dominates in ammonia adsorption, and metals on the right side of the periodic table have lower positive charge and accept greater amounts of electron density, making the electron-sharing contribution more important to the binding strength. We find that more electronegative metals, particularly Pd, Ir, Pt, and Au, which accept more electron density from the adsorbate, yield the best selectivity for the target molecules over water because water adsorption is mostly due to Coulomb attraction. We also analyze changes in the d orbital occupancy upon ammonia and phosphine adsorption.