Model complexes of hydrated zinc(II) and nickel(II) cations with one water substituted for methanol, methanethiol, and ammonia are studied. The accuracy of various computational methods, influence of the basis set, and the role of the coordination geometry are investigated. It is shown that density functional theory calculations (B3LYP functional) yield very accurate results, but the basis set of at least triple-zeta quality with polarization and diffuse functions should be used. The calculated mean absolute difference between DFT and QCISD(T) reaction energies for the substitution reactions on the central metal ion is less than 0.5 kcal mol(-1) It is also observed th:lt the metal-ligand distances and the reaction energies of monosubstituted hydrated complexes of metal ions are strongly dependent on the number of ligands in the first coordination sphere and the coordination geometry. The implications of having a reliable, accurate, and relatively fast method for the calculation of transition metal complexes are discussed. Several applications oriented toward the interactions of transition metals with the amino acid residues are mentioned.