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The Aqueous Chemistry of Oxides$
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Bruce C. Bunker and William H. Casey

Print publication date: 2016

Print ISBN-13: 9780199384259

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199384259.001.0001

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The Hydrolysis Products: Soluble Multi-cation Clusters

The Hydrolysis Products: Soluble Multi-cation Clusters

(p.87) 5 The Hydrolysis Products: Soluble Multi-cation Clusters
The Aqueous Chemistry of Oxides

Bruce C. Bunker

William H. Casey

Oxford University Press

The term hydrolysis describes the acid–base reactions that remove protons from hydrated metal cations. This word derives from two Greek words: hydro, meaning “water,” and lysis, meaning “to loosen.” Hydrolysis provides two mechanisms for converting one soluble metal complex into another. First, hydrolysis converts coordinated water molecules into coordinated hydroxo- and oxo-anions. For isolated monomers, the extent of hydrolysis depends on cation charge, cation coordination number, and pH (see Chapter 4). For example, as the cation charge increases, complexes can be generated that go from water rich to hydroxide rich to oxide rich, as exemplified by the sequence of species [Na(H2 O)6]+, [Mg(H2 O)5 (OH)]+, [Al(H2 O)4 (OH)2 ]+, [Si(OH)4 ]°, and finally [MnO4]-. Second, hydrolysis activates hydrated cations to participate in ligand-exchange reactions. When the entering ligand is not another isolated water molecule, but involves a hydroxo- or oxo-ligand that is bound to another dissolved metal complex, ligand- exchange reactions are called condensation reactions (or sometimes olation reactions) because they condense small oxide species into larger ones. In condensation reactions, hydroxo- or oxo-ligands form bridges between two or more cations. Species created via condensation range from dimers, trimers, and tetramers to larger species such as [Al13 O4 (OH)24 (H2 O)12]7+, enormous clusters such as the 2-nm-diameter Mo72 Fe30 Ox (OH)y species, and eventually to colloidal particles and extended oxide and hydroxide phases. The complete set of soluble metal-ion complexes containing only water molecules, hydroxide ions, and oxo-anions are called the hydrolysis products. The hydrolysis products represent the minimum basis set of complexes that must be taken into account to rationalize the aqueous chemistry of any oxide or hydroxide phase. The acid–base and ligand-exchange chemistry of monomeric hydrolysis products is described in Chapter 4. In this chapter, we emphasize the polymerization and depolymerization reactions that lead to the formation and disappearance of larger multi-cation clusters.

Keywords:   activity coefficients, back bonding, chelate effect, dimer reaction pathways, heteropolyoxoanions, lacunary structures, olation reactions, polyoxometalates, solubility, uranium peroxide clusters

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