Trispyrazolylborate
In inorganic chemistry, the trispyrazolylborate ligand, abbreviated Tp−, is an anionic tridentate and tripodal ligand. Trispyrazolylborate refers specifically to the anion [HB(C3N2H3)3]−, but the term trispyrazolylborate refers to derivatives substituted at on the pyrazolyl rings. This family of compounds are sometimes called scorpionate ligands.
As suggested by the resonance structures, the nitrogen centers that are not bonded to boron are basic. These centers bind to three adjacent sites of a metal such that the simple adducts have C3vsymmetry. The facial bonding mode is reminiscent of cyclopentadienyl ligands, although the ligand field stabilization energy of Tp− is weaker as indicated by the fact that Fe(Tp)2 is a spin-crossover complex whereas ferrocene is low-spin.
The Tp ligands are usually prepared from the reaction of pyrazole with potassium borohydride:
Intermediates include the monopyrazolylborate ([H3B(C3N2H3)]−) and the bispyrazolylborate ([H2B(C3N2H3)2]−). KTp (m.p. 188-189 °C) is a colourless solid that soluble in polar solvents.
Condensation of 3-substituted pyrazoles with borohydride affords the corresponding substituted Tp derivatives. The substituent forces boron to the less hindered nitrogen center. Thus 3-phenylpyrazole gives HB(C3N2H2Ph)3]−, abbreviated [TpPh]−, wherein the phenyl substituents project away from the metal. Analogously 3-isopropylpyrazole gives HB(C3N2H2iPr)3]-, abbreviated [TpiPr]−. 3,5-Dimethylpyrazole gives the hexamethylated ligand [HB(C3N2HMe2)3]−, sometimes called Tp*−. Because pyrazoles are readily prepared from 1,3-diketones, a large number of substituted Tp complexes are possible. Derivatives are known with perfluorinated, chiral, and functional substituents.
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