Complexation of the Tc(CO)₃(H₂O)₃ ⁺ ion with halide (F^-, Cl^-, Br^-, I^-) and thiocyanate ions in aqueous solutions was studied by ⁹⁹Tc NMR spectroscopy. The stability constants of the mono-, di-, and tri-substituted complexes were calculated. The stability of halide (Cl ^-, Br^-, I^-) complexes grows as the ionic radius of the halogen increases and its electronegativity decreases. For halide complexes of similar composition, the ⁹⁹Tc chemical shift correlates with the stability of the complexes. The fluoride ion, most probably, does not coordinate with the Tc(CO)₃⁺ ion, in contrast to its close analog, hydroxide ion. Among the ligands studied, the pseudohalide anion NCS^- forms the most stable complexes with the Tc(CO)₃ ⁺ ion.