Tris hydroborato, structure

M[pzTp](H20) (M = Na, K) have been determined by x-ray diffraction (30). In contrast to the discrete monomeric structures of the tris (pyrazolyl)hydroborato complexes Tl[TpRR ], K[TpBut2], and Cs[TpBut2] described earlier, the hydrated tetrakis(pyrazolyl)borato complexes M[PzTp](H20) (M = Na, K) exhibit an interesting polymeric-type structure, a portion of which is illustrated in Fig. 14. In each case, the cations occupy two different crystallographic sites, and the cation in one of the sites is coordinated to two pyrazolyl groups in a rr-type fashion. 

The value of the tris(pyrazolyl)hydroborato complexes [TpRR ]ZnOH is that they are rare examples of monomeric four-coordinate zinc complexes with a terminal hydroxide funtionality. Indeed, [TpBut]ZnOH is the first structurally characterized monomeric terminal hydroxide complex of zinc (149). As such, the monomeric zinc hydroxide complexes [TpRR ]ZnOH may be expected to play valuable roles as both structural and functional models for the active site of carbonic anhydrase. Although a limitation of the [TpRR ]ZnOH system resides with their poor solubility in water, studies on these complexes in organic solvents... 

Finally, the tris(pyrazolyl)hydroborato ligand system has also provided an interesting example in which a crystallographic site is disordered between a vacancy and a chain of three atoms. Thus, the x-ray structure of the cobalt complex [TpAnt]CoNCS (Ant = 9-anthryl) revealed the presence of the cocrystallized thallium derivative... 

Monomeric tetrahedral compounds can also be achieved by the use of tris-chelating ligands. The crystal structures of several [tns(pyrazolyl)hydroborato]alkylmagnesium have shown that they have shorter Mg—C bond lengths (2.10-2.18 A) and a typical example is given [Compound 30] [30]. 

II. SYNTHESES AND STRUCTURES OF MONOMERIC FOUR-COORDINATE TRIS(PYRAZOLYL)HYDROBORATO MAGNESIUM ALKYL DERIVATIVES... 

The molecular structures of several [Tp ]MgR derivatives have been determined by single-crystal X-ray diffraction, which confirms that in each case the molecules are solvent-free monomers, and that the tris(pyrazolyl)hydroborato ligand is -coordinated to magnesium. The coordination environment about the magnesium may be described as... 

II. SyiithcjkO and Structures of Monomeric Four-Coordinate Tris(pyrazolyl)hydroborato Magnesium Alkyl Derivatives... 

Figure 31. A model system developed by Parkin and co-workers (a), Tris(2-mercapto-l-phenylimidazolyl)hydroborato lead complex, [Tm ]Pb [C104] (197), is strikingly similar in structure to the lead bound in the active site of the target protein ALAD (b), (only lead and coordinated cysteines are shown) (245). In both systems, lead is bound in a trigonal pyramidal geometry by three sulfurs. Metal-binding studies on the model system (198) and inhibition studies on the enzyme reveal that lead has a higher affinity for this type of site than the native zinc ion. [Part (a) reprinted with permission from B. M. Bridgewater and G. Parkin, J. Am. Chem. Soc., 122, 7140-7141 (2000). Copyright 1999 American Chemical Society. Coordinates for figure part b) were downloaded from the Protein Databank (pdb file id IQNV).]...

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