Electronic structure calculations show the binding of a single molecule of H2O, NH3, H2S, and PH3 to main group cations to be extremely tight (up to 40 kcal mol(-1) for group 1 and up to 95 kcal mol(-1) for group 2 cations); for the smallest cations the bond lengths are very short. The interactions were examined with geometry-optimized calculations at QCISD, MP2, and RHF levels using TZP and DZP basis Sets; the cations were Li+, Na+, K+, Mg2+, and Ca2+ and, for comparison, Hf, Averaged over all four ligands, binding to Na+ and K+ is 64% and 39% as strong, respectively, as to Li+; Ca2+ binding is 53% its strong as Mg2+ binding. Likewise, binding energies of H2O, H2S, and PH3 are 86%, 68%, and 55% as great as those of NH3, invariably the most strongly bound ligand (means over all five cations). Results taken from a range of internuclear distances were used to characterize the short- and long-range metal-ligand interactions. Covalency effects explain the major departures from electrostatic behavior. They were assessed by calculations utilizing "electrostatic-only" wave functions and yielding covalency contributions of 20-50% of binding energies (Li+, Mg2+) but 10% or less for Na+, K+, and Ca2+. Relaxation of the polar molecules in the field of the cations is a small effect; it contributes from less than or equal to 1% of binding energy (H2O, NH3, H2S) to greater than or equal to 5% (PH3).