Tm is an abbreviation for anionic tridentate ligand based on three imidazole-2-thioketone groups bonded to a borohydride center. They are examples of scorpionate ligands. Various ligands in this family are known, differing in what substituents are on the imidazoles. The most common is Tm, which has a methyl group on the nitrogen. It is easily prepared by the reaction of molten methimazole (1-methylimidazole-2-thione) with sodium borohydride, giving the sodium salt of the ligand. Salts of the Tm anion are known also for lithium and potassium. Other alkyl- and aryl-group variations are likewise named Tm according to those groups.