2-Imidazolidinethione is the organosulfur compound with the formula C<sub>2</sub>H<sub>2</sub>(NH)<sub>2</sub>C=S. It is a cyclic unsaturated thiourea with a short C=S bond length of 169 pm. The compound is often referred to as 2-mercaptoimidazole, which is a tautomer that is not observed. The compound forms a variety of metal complexes. In terms of bonding and reactivity, 2-imidazolidinethione is similar to mercaptobenzimidazole.