The Rydberg bonding in (NH4)(2)(-) and in its neutral and cationic counterparts have been examined using ab initio electronic structure theory. Analogous calculations on Li-2(-), Na-2(-), and K-2(-) and on their neutral molecules and cations have also been performed for comparison with the Rydberg bonding cases. The dissociation energy D-e, for (NH4)(2)(-) -> NH4 + NH4- is found to be 4 kcal mol(-1) at the highest level of theory used, while the corresponding D-e,'s for the cation and the neutral molecule are 20 and 9 kcal mol(-1), respectively. The vertical electron detachment energy of (NH4)2- is predicted to be 0.46 eV, which is nearly the same as that of NH4-. The trends in the interfragment equilibrium distances R-e,, the harmonic vibrational frequencies omega, and the dissociation energies within alkali dimers, NH4 dimers, and their respective anions and cations have been examined and are discussed.