Pyrazolylmethane complexes

Keywords High-pressure studies Magnetic susceptibility Mossbauer and Nuclear Magnetic Resonance spectroscopy Pyrazolylborate complexes Pyrazolylmethane complexes... 

Although the poly(pyrazolyl)borate complexes of iron(II) have been well known for many years, [1] it is only recently that the complexes with the tris(l-pyrazolylmethane ligand, HC(pz)3, [45-48] have been studied in detail. It should be noted that poly(pyrazolyl)methane ligands, such as the tris(l-pyrazolylmethane ligand, are neutral, whereas the poly(pyrazolyl)bo-rate ligands, such as the tris(l-pyrazolyl)borate ligand, HB(pz)3", are monoanions. As a consequence, the metal(II) poly(pyrazolyl)methane complexes are dications and often have quite different properties from those of the analogous metal(II) poly(pyrazolyl)borate molecular complexes. But, in spite of these differences there are often very close structural similarities between the dicationic complexes and the neutral complexes. Therefore the study of the pyrazolylmethane complexes will parallel that of the borate complexes discussed above. 

The spectroscopic properties and the reactivity towards low-valent metal fragments of these cationic alkylidyne salts mirror those of the pyrazolylborate analogues, with one notable exception being that pyrazolylmethane ligands exhibit a (protic) reactivity at the bridgehead carbon, allowing for modification of the ligand within the alkylidyne complex (Section IV.E), a phenomenon which has not been similarly documented for Tp L M( = CR) complexes (hydridic B-H bond). 

Polyhydrides are complexes such as FeH4(PR3)3, with a H M ratio exceeding 3. Hydrogen is not as electronegative as carbon, and so the metal in a polyhydride is not as oxidized as in a polyalkyl. Polyhydrides therefore retain more of the properties of low-valent complexes than do polyalkyls. For example, many of them are 18e, and relatively soft ligands (in the vast majority of cases a phosphine or a cyclopentadienyl) are required to stabilize them. Rare examples of N-donor-stabilized polyhydrides are [TpReH ] and [BpReH7] (Tp = tris-pyrazolylborate (5J7) Bp = bis-pyrazolylmethane). ... 

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