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image:ExampleZnTm.jpg, Structure of TmpTolZnSPh, a tetrahedral complex.
image:RutehniumTm.jpg, Structure of TmMeRu(CO)(PPh3)H, an octahedral complex.
coordination chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
, Tm is an abbreviation for anionic tridentate ligand based on three imidazole
Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole Diazole refers ...
-2- thione groups bonded to a borohydride center. They are examples of scorpionate ligand
In coordination chemistry, the term scorpionate ligand refers to a tridentate (three-donor-site) ligand which would bind to a metal in a ''fac'' manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. T ...
s. Various ligands in this family are known, differing in what substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...
s are on the imidazoles. The most common is TmMe, which has a methyl group on the nitrogen. It is easily prepared by the reaction of molten methimazole
Thiamazole, also known as methimazole, is a medication used to treat hyperthyroidism. This includes Graves disease, toxic multinodular goiter, and thyrotoxic crisis. It is taken by mouth. Full effects may take a few weeks to occur.
Common s ...
(1-methylimidazole-2-thione) with sodium borohydride, giving the sodium salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
of the ligand. Salts of the TmMe anion are known also for lithium and potassium. Other alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
- and aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...
-group variations are likewise named TmR according to those groups.
Ligand characteristics, comparison with Tp−
The TmMe anion is a tridentate, tripodal ligand topologically similar to the more common Tp ligands, but the two classes of ligands differ in several ways. TmMe 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 TmR anion simulates the environment provided by three facial thiolate ligands but without the 3- charge of a facial trithiolate. The large 8-membered SCNBNCSM chelate rings in M(TmMe) complexes are more flexible than the 6-membered CNBNCM rings in M(Tp) complexes. This flexibility enables the formation ofboron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
-metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
bonds, after loss of the B-H bond. This degradation of the coordinated TmMe anion gives a dehydrogenated boranamide B(mt)3 where mt = methimazolate."Polyazolyl Chelate Chemistry. 13. An Osmaboratrane" M. R. St. J. Foreman, A. F. Hill, A. J. P. White, and D. J. Williams ''Organometallics
''Organometallics'' is a biweekly journal published by the American Chemical Society. Its area of focus is organometallic and organometalloid chemistry. This peer-reviewed journal has an impact factor of 3.837 as reported by the 2021 Journal Citat ...
'', 2004, volume 23, p. 913. . "The Sting of the Scorpion: A Metallaboratrane" A. F. Hill, G. R. Owen, A. J. P. White, and D. J. Williams '' Angew. Chem. Int. Ed. Engl.'', 1999, volume 38, p. 2759. {{doi, 10.1002/(SICI)1521-3773(19990917)38:18<2759::AID-ANIE2759>3.0.CO;2-P
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