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^Me, 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^Me anion are known also for lithium and potassium. Other alkyl- and aryl-group variations are likewise named Tm^R according to those groups.^[1]

Ligand characteristics, comparison with Tp⁻

The Tm^Me anion is a tridentate, tripodal ligand topologically similar to the more common Tp ligands, but the two classes of ligands differ in several ways. Tm^Me has three "soft" sulfur donor atoms, whereas Tp⁻ has three nitrogen donor atoms. The thioamide sulfur is highly basic, as found for other thioureas. The Tm^R anion simulates the environment provided by three facial thiolate ligands but without the 3- charge of a facial trithiolate.^[2]

The large 8-membered SCNBNCSM chelate rings in M(Tm^Me) complexes are more flexible than the 6-membered CNBNCM rings in M(Tp) complexes. This flexibility enables the formation of boron-metal bonds, after loss of the B-H bond. This degradation of the coordinated Tm^Me anion gives a dehydrogenated boranamide B(mt)₃ where mt = methimazolate.^[3]

• Representative Structures (S in yellow, B in dark blue)
• 
  Structure of Tm^pTolZnSPh, a tetrahedral complex.
• 
  Structure of Tm^MeRu(CO)(PPh₃)H, an octahedral complex.

References