Hydroxo-complexes

Unlike cadmium and mercury and, in fact, all metals of Group II, zinc dissolves readily in alkalis forming zincates. in which the zinc atom is contained in a complex hydroxo-anion, for example ... 

The higher the charge density or charge-to-radius ratio, the more likely a metal ion undergoes hydrolysis in aqueous solutions to give hydroxo complexes. Hydroxo complexes may abruptly form polynuclear complexes and precipitate even in solutions more acidic than the pA"a for the first hydroxo complex formation. The first five small metal ions in Table 2 hydrolyze even in acidic solutions and form precipitates. In six-coordination, the charge-to-radius ratio for the first five... 

As part of a biomimetic study for the enzyme class II aldolase" the equilibrium was reported for the cyclen complex, hydroxo-Zn(II)-l-(4-bromophenacyl)-l,4,7,10-tetraaza-cyclododecane and the intramolecular enolate 20, formed from Zn(II) and the enolate of l-(4-bromophenacyl)cyclen. This cyclization reaction was shown to be endothermic by 8.7 kJmol. However, with an entropy of 19 JmoU K it proceeds readily enough to show facile H/D exchange from the CH2 group of the exocyclic ligand. We wonder about the enthalpy and entropy changes associated with a different choice of other polyamines and/or central metals. 

Complex hydroxo metal anions are formed in solution via the following equilibrium reaction ... 

Coordination of the water molecules to a metal center is a Lewis acid/base reaction in which charge is transferred from the ligands (water) to the metal. This makes a coordinated water molecule more acidic than a noncoordinated one, i.e., dissociation of a proton from a coordinated water is easier than from a noncoordinated water. Depending on the magnitude of the charge transfer, the equilibrium concentrations of aquo complexes, hydroxo species, and oxo species (Equation (3)) are different. The formation of metal hydroxide species from aquo complexes is called hydrolysis. 

Formation of the Complexes Hydroxo, Oxo, and so Forth from the Hydrolysis of Metallic Ions... 

The complexes hydroxo, 0x0, and so forth result from the hydrolysis of metallic ions, which is ineluctable in aqueous solutions. 

Involvement of the complexed hydroxo-group has been suggested in accord with a mechanism of the type... 

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