Pentacoordinate geometry

British Biotech has described co-crystal structures of both BB-3497 and actinonin bound in the active site of E. coli PDF [24]. The metal centre (Ni ) in both complexes adopts a pentacoordinate geometry, bound by the two oxygen atoms of the hydroxamate along with Cys-90, His-132 and His-136. This coordination pattern is consistent with the mechanism of de-formylation proposed by Becker et al. [56] and Jain et al. [67], in which a pentacoordinated metal centre stabilises the transition state during hydrolysis of the formamide bond. When compared to the co-crystal structure of a substrate hydrolysis product, Met-Ala-Ser, it is clear that the side chains of these two inhibitors bind into the active site pockets similarly to the substrate [56]. 

One must, however, take into account that zinc porphyrin complexes have a strong tendency to coordinate axially Lewis bases, thus sometime affording pentacoordinate geometries. Figure 51 shows the example of [Zn(TPP)(H20)], in which the axial coordination of the water molecule causes the zinc atom to move out of the tetrapyrrolic plane.93... 

FIGURE 2. Left deprotonation of pVIg(OH2)6] lowers the coordination number of magnesium. Reprinted with permission from Reference 14. Copyright 2005 American Chemical Society. Right pentacoordinate geometry of Mg + in the solid-state structure of the Mg + —F —Pi complex of yeast enolase... 

Based on these differences, it is possible to represent the VBCMDs for typical nucleophilic substitution reactions on Si versus C as shown in Figs. 6.16a and b. In Fig. 6.16a, the ionic curve X SiL3 + X is very stable in the pentacoordinated geometry due to the electrostatic energy of the triple ion structure, much like the case of the (FIIF) species discussed before. Consequently, the pentacoordinated (XSiL3X) species will generally be a... 

Protonation of (/u.-alkenyl)bis(zirconocene) complexes under nonnucleophilic conditions takes a different regio-chemical course than the B(C6p5)3 addition (equation 27). The X-ray crystal structure of (60) is consistent with distorted square-pyramidal pentacoordinated geometry at the central atom Cl. 

The pentacoordinate geometry is confirmed by the observation of a quintet at t 24.5 (/pH = 26Hz) in the NMR spectrum and a doublet 135 ppm downfield of H3P04(/pjj = 26 Hz) in the P NMR spectrum 137). These complexes were not isolated except for [NiH(diphos)2]X (X = AICI4, BF4, HCI2) 258). These results contrast with the previously reported reactions of Ni(diphos)2 and Ni(PPh3)i in which nickel(II) salts and hydrogen were obtained 61). 

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