s-Block Metal Complexes with Bis- and Tris(pyrazolyl)methane and -methanide Ligands

Substituted bis- and tris(pyrazolyl)methanes and -methanides are isoelectronic with the corresponding pyrazolyl-substituted boranates, but the use of these Lewis bases in the coordination chemistry of the s-block metals has attracted attention only quite recently. Many lithium and magnesium complexes have been isolated and characterized, but for the heavier alkali and alkaline earth metals much less information is available. Beryllium complexes are unknown, due to the toxicity of this lightest alkaline earth metal. The bis- and tris(pyrazolyl)methanes and -methanides act as multidentate ligands to the metal ions and commonly bind through the pyrazolyl bases. The presence of the very rare N-2,C coordination mode of the tris(pyrazolyl)methanides destabilizes these ligands and leads to their degradation. This microreview presents syntheses, molecular structures, and characteristic spectroscopic properties of s-block metal complexes with bis- and tris(pyrazolyl)methane and -methanide ligands as well as with heteroscorpionate ligands. Furthermore, degradation of tris(pyrazolyl)methanides at heavy alkaline earth metals (especially of calcium) is discussed on the basis of experimental results and quantum chemical calculations.