Five metal complexes of 2-methylisothiazol-3(2H)-one (MIO), [Co-III(NH3)(5)(MIO)](3+), [Ru-II(NH3)(5)(MIO)](2+), [Ru-III(NH3)(5)(MIO)](3+), [(PtCl3)-Cl-II(MIO)](-), and trans-[(UO2)-O-VI(NO3)(2)(MIO)(2)], were synthesized, and their structures were determined by single-crystal X-ray crystallography. MIO is an ambidentate ligand and coordinates to metal centers through its oxygen atom in the cobalt(III), ruthenium(III), and uranium(VI) complexes and through its sulfur atom in the ruthenium(II) and platinum(III) complexes. This result suggests that MIO shows preferential behavior on its donating atoms. We also studied the electron-donor abilities of the oxygen and sulfur atoms of MIO. Various physical measurements on the conjugate acid of MIO and the MIO complexes allowed us to determine an acid dissociation constant (pK(a)) and donor number (DN) for the oxygen atom of MIO and Lever's electrochemical parameter (E-L) and a relative covalency parameter (k(L)) for the sulfur atom.