Hydroxides coordination

The P450 reaction cycle (Scheme 10.4) starts with four stable intermediates that have been characterized by spectroscopic methods. The resting state of the enzyme is a six-coordinate, low-spin ferric state (complex I) with water (or hydroxide) coordinated trans to the cysteinate ligand. The spin state of the iron changes to high-spin upon substrate binding and results in a five-coordinate ferric ion (com-... 

Cr(CO)6 react with phthalonitrile in hot 1-chloronaphthalene to precipitate the ft form of CrnPc which is readily oxidized to CrI11(Pc)OH in air.197,198 The hydroxide coordinates other ligands to form six-coordinate complexes, Crni(Pc)(OH)X (X = ROH, H20, CN, SCN, SeCN). The hydroxy group is substituted by CN in cone. KCN solution and Cri PcXChOj - is formed. Acetic anhydride acetylates Cr(Pc)(OH)(OH2) in a stepwise fashion to Cr(Pc)(OAc)(OH2) and Cr(Pc)(OAc)(HOAc). 

Chloride Coordination. This predominates over hydroxide coordination—i.e., [MCI] > [MOH] only if Equation 9 is valid. 

This is true only for silver(I), mercury(II), gold(I), and possibly cadmium and lead ions. At the pH of the ocean, hydroxide coordination is important for all ions having an oxidation number greater than 2 ( 36, 37). 

Fluoride Rather Than Hydroxide Coordination. This would be expected for magnesium ion since the necessary condition (Equation 10) is fulfilled,... 

Dixon et al. (1980) have proposed a mechanism for the action of urease (Fig. 5-3). The mechanism involves nucleophilic attack by hydroxide coordinated to one nickel, on urea which is coordinated to the other nickel(II) via the carbonyl oxygen. 

The octanuclear lanthanide hydroxide cluster core was first identified in Er8( X4-0)( X3-OH)i2(THD)s (THD = 2,2,6,6-tetramethyUieptane-3,5-dionate) [25, 87]. It is a triangulated dodecahedron with an interstitial oxo group each of its triangular faces is capped by a IX3-OH group. The same cluster core has also been found in [Eu8(p-6-0)((i,3-0H)i2( X2-0Tf)i6(0Tf)2] (Eigure 6.40) [99] and [Eu8( X4-0)((i,3-0H)i2(DME)8(Se3)(Se4)3(Se5)2] [105]. That the same cluster core is obtained with different ancillary ligands suggests the prevalence of this motif in the lanthanide hydroxide coordination chemistry. 

Crystal structures have been reported for the enzyme with several bound drugs. Metyrapone- and 1-, 2- and 4-phenylimidazole-inhibited complexes of cytochrome P-450cam have each been refined to 2.1 A resolution. Except in the 2-phenylimidazole complex, each complex forms an N Fe interaction to the heme iron atom. In the 2-phenylimidazole complex, water or hydroxide coordinates the heme iron atom and the inhibitor binds in the camphor pocket. Details of the inhibitor binding are shown in Figure 17.25. Eukaryotic cytochrome P-450 is membrane bound and has a different structure from the soluble... 

A nucleophilic hydroxide coordinated to one metal ion (terminal coordination) or bridged to both metals (, -hydroxo coordination) is possible both types are present in the X-ray structures. We first consider the possibility that the nucleophile in the reaction is a water molecule coordinated to the Fe + ion of the binuclear metal center. A terminal metal-hydroxo species serving as a nucleophile in an hydrolysis reaction has precedence in model chemistry [62]. The Lewis acidity of the metal, which plays a role by decreasing the pXa of the coordinated water molecule, is more favorable for Fe + than Fe +. The pAaS of water coordinated to aqueous Fe " " and Fe + ate 2.13 and 8.44, respectively, a > 10 -fold difference in acidity [74]. A lower pAa makes it easier to deprotonate to form the hydroxide, the putative nucleophile in the reaction. The loss of activity of Fe -Fe calcineurin upon reduction to the Fe " -Fe + state may therefore reflect the poorer Lewis acidity of Fe + and the requirement for an hydroxide coordinated to the Fe " " ion in the Ml site [35]. 

Figure 3 The two limiting mechanisms for urease (A) attack of a monodentate hydroxide coordinated to one Ni on urea coordinated to the other, and (B) concerted attack of a bridging hydroxide on a bidentate...

In particular, bridging hydroxides have been proposed as the nucleophile for many of the reactions discussed above. At first sight, coordination of hydroxide to two metal ions might well be expected to drastically decrease its nucleophilicity. Formally, a hydroxide coordinated to two Lewis acids is analogous to the H3O+ ion, which is not known for its nucleophilic character. Nonetheless, a great deal of structural and mechanistic data has been accumulated in recent years that supports the use of a bridging hydroxide as the nucleophile in a number of well-characterized enzymatic reactions, such as that catalyzed by urease. The details as to why coordination of hydroxide to two metal ions does not abolish its nucleophilicity will probably have to be... 

Bunsenite [1313-99-1] [Named after the German chemist and spectroscopist Robert Wilhelm Bunsen(1811-1899)] (ICSD 9866 and PDF 4-835) NiO M = 74.6928 78.58 wt.%Ni 21.42 wt.% 0 (Oxides and hydroxides) Coordinence Ni(6) Cubic a = 417.69 pm Bl,cF8 (Z=4) S.G. Fm3m P.G. 432 Rock salt type Periclase group Isotropic = 2.37 5.5 6898 (6806) Habit octahedral crystals. Color dark pistachio green. Luster vitreous. Diaphaneity transparent. Streak brownie black Clivage unknown. Fracture unevea Chemical soluble with difficulty in strong mineral acids. Occurrence found in the oxidized zone of hydrothermal nickel-uranium veins along with nickel and cobalt arsenates. 

The Fourier transform moduli of EXAFS data of three silica samples that sorbed U(VI) appeared to be very similar (Fig. 3-5). The uranyl U=0 distance of 1.78 0.01 A is typical of U(VI) compounds. Shells for hydroxide coordinated in the equatorial plane appeared in the range 2.22 to 2.31 A, while bond distances for coordinated water molecules were observed from 2.43 to 2.51 A. The presence of a U-Si shell (between 3.1 and 3.3 A) in all samples investigated suggests the formation of inner-sphere uranyl surface complexes, and there was no evidence for a U-U shell, which would indicate surface precipitation or polynuclear uranyl species. The short U-Si bond length of about 3.2 A and coordination number of 1 at pH around 3 suggests bidentate complexation to a single silica atom. At pH 5, the coordination number of 2 for the Si shell suggests the coordination of the uranyl complex to two Si atoms (Sylwester et al., 2000). 

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