Coordination hydroxide ligands

Thus, most of the recent experimental data have been rationalized in terms of an active site that contains binuclear- or tetranuclear-manganese clusters with coordinated hydroxide ligands. The majority of the mechanisms for dioxygen evolution that are based on such models involve (1) sequential... 

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

LADH contains a tetrahedral zinc which is coordinated by one histidine nitrogen and two cysteine sulfurs. One aim is to make synthetic analogs of this coordination sphere that also feature the catalytically important water or hydroxide ligand in the fourth coordination site. Analogs containing bound substrates such as alcohols are also of interest. 

The rate constant for the redox step, kr, is unlikely to reflect a simple electron transfer from the monodentate diimine ligand to the metal center because replacement of coordinated water with coordinated hydroxide would be expected to decrease the oxidizing power of the metal-(III) center. This is well documented by the standard reduction potentials of the aqua and hydroxo complexes in Table IV, and it would seem... 

Ligands that can coordinate to an active center in an enzyme and prevent coordination by the substrate will tend to inhibit the action of that enzyme. 1 We have seen that azide can occupy the pocket tailored to fit the carbon dioxide molecule. This prevents the latter from approaching the active site. Furthermore, the infrared evidence indicates that the azide ion actually does bind the zinc atom The asymmetric stretching mode of the azide ion is strongly shifted with respect to the free ion absorption. Thus the zinc is inhibited from acting as a Lewis acid towards water with the formation of a coordinated hydroxide ion. Other inhibitors also bind to the metal atom. As little as 4 x I0-6 M cyanide or hydrogen sulfide inhibits the enzymatic activity by 85%. 

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). 

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

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