Tm is an abbreviation for anionic tridentate ligand based on three imidazole-2-thioketone groups bonded to a borohydride center.
Various ligands in this family are known, differing in what substituents are on the imidazoles.
The most common is TmMe, which has a methyl group on the nitrogen.
Other alkyl- and aryl-group variations are likewise named TmR according to those groups.
The thioamide sulfur is highly basic, as found for other thioureas.