Cimetidine is an important hydrogen histamine receptor which has the ability to chelate metal ions in blood plasma and in different tissues. This study aimed to determine the stability constants for the cimetidine ligand with Mn II and Ni II metallic ions, synthesizing complexes and characterizing them by infrared spectroscopy, IR, and hydrogen nuclear magnetic resonance, 1 H NMR. Cimetidine protonation constant regarding to the imidazole group was logK 7.05 and the stability constants for Mn II and Ni II complexes, ML 2 species were logK 3.75 and 2.97, respectively, in 0.100 mol L -1 KCI. The interpretation of IR and H 1 NMR spectra for complexes Mn II -cim 2 and Ni II -cim 2 indicated that their formation occurs through the sulfur atoms in the thiol group, nitrogen atoms of imidazole ring, and nitrogen atoms of secondary amine. The nitrile group seems to be involved in the complexation of the Ni II -cim 2 complex.