Here we report the coordination behaviour of an imidazole-based [S 1 ]-donor ligand, 1,3-dimethyl-imidazole-2(3H)-thione (L1), and [S 2 ]-donor ligand, 3,3′-methylenebis(1-methyl-imidazole-2(3H)-thione) (L2) or 4,4′-(3,3′-methylenebis-(2-thioxo-2,3-dihydro-imidazole-3,1-diyl))dibutanoic acid (L3), with HgX 2 (X = Cl, Br or I) in solution and the solid state. NMR, UV-Vis spectroscopic, and single crystal X-ray studies demonstrated that L1 or L2 coordinated rapidly and reversibly to the mercury center of HgX 2 through the thione moiety. Treatment of L2 with HgCl 2 or HgBr 2 afforded 16-membered metallacycle k 1 -(L2) 2 Hg 2 Cl 4 or k 1 -(L2) 2 Hg 2 Br 4 where two Cl or Br atoms are located inside the ring. In contrast, treatment of L2 with HgI 2 afforded a chain-like structure of k 1 -[L2Hgl 2 ] n , possibly due to the large size of the iodine atom. Interestingly, [S 1 ] and [S 2 ]-donor ligands (L1, L2, and L3) showed an excellent efficacy to protect liver cells against HgCl 2 induced toxicity and the strength of their efficacy is in the order of L3 > L2 > L1. 30\% decrease of ROS production was observed when liver cells were co-treated with HgCl 2 and L1 in comparison to those cells treated with HgCl 2 only. In contrast, 45\% and 60\% decrease of ROS production was observed in the case of cells co-treated with HgCl 2 and thiones L2 and L3, respectively, indicating that [S 2 ]-donor ligands L2 and L3 have better cytoprotective effects against oxidative stress induced by HgCl 2 than [S 1 ]-donor ligand L1. Water-soluble ligand L3 with N-(CH 2 ) 3 CO 2 H substituents showed a better cytoprotective effect against HgCl 2 toxicity than L2 in liver cells. © 2019 The Royal Society of Chemistry.