DFT calculations with the UX3LYP hybrid functional and a medium-sized 6-311++G(d,p) basis set were performed to examine the gas-phase structure of paramagnetic (S = 1) six-coordinate complexes [Ni(NH3)(n)(H2O)(6-n)](2+) 0 <= 17 <= 6. Significant interligand hydrogen bonding was found in [Ni(H2O)(6)](2+), but this becomes much less significant as NH3 replaces H2O in the coordination sphere of the metal. Bond angles and bond lengths obtained from these calculations compare reasonably well with available crystallographic data. The mean calculated Ni-O bond length in [Ni(H2O)(6)](2+) is 2.093 angstrom, which is 0.038 angstrom longer than the mean of the crystallographically observed values (2.056(22) angstrom, 108 structures) but within 2 sigma of the experimental values. The mean calculated Ni-N bond length in [Ni(NH3)(6)](2+) is 2.205(3) angstrom, also longer (by 0.070 angstrom) than the crystallographically observed mean (2.135(18) angstrom, 7 structures). Valence bond angles are reproduced within 1 degrees. The successive replacement of H2O by NH3 as ligands results in an increase in the stabilization energy by 7 +/-2 kcal mol(-1) per additional NH3 ligand. The experimentally observed increase in the lability of H2O in Ni(II) as NH3 replaces H2O in the coordination sphere is explained by an increase in the Ni-OH, bond length. It was found from a natural population analysis that complexes with the highest stabilization energies are associated with the greatest extent of ligand-to-metal charge transfer, and the transferred electron density is largely accommodated in the metal 4s and 3d orbitals. An examination of the charge density rho(bcp) and the Laplacian of the charge density del(2)rho(bcp) at the metal-ligand bond critical points (bcp) in the series show a linear correlation with the charge transferred to the metal. Values of del(2)rho(bcp) are positive, indicative of a predominantly closed-shell interaction. The charge transferred to the metal increases as it, the number of NH3 ligands in the complex, increases. This lowers the polarizing ability of the metal on the ligand donors and the average metal-ligand bond length increases, resulting in a direct correlation between the dissociation energy of the complexes and the reciprocal of the average metal-ligand bond length. There is a strong correlation between the charge transferred to the metal and experimental Delta H values for successive replacement of H2O by NH3, but a correlation with stability constants (log beta values) breaks when n = 5 and 6, probably because of entropic effects in solution. Nevertheless, DFT calculations may be a useful way of estimating the stability constants of metal-ligand systems.