Protic Equilibria Involving Coordinated Ligands

This is indeed observed and, particularly in higher oxidation states, coordinated water molecules are relatively acidic (Table 9-5). Water coordinated to an iron(iii) center is a stronger acid than acetic acid  

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

Vanadium(iv) is even more strongly polarizing. The first deprotonation process is not observable in aqueous solution. The pentaaquahydroxy ion may be present in 

The blue [(H20)5V( = 0)] + ion is the vanadyl ion which is usually depicted as VO. Actually, the vanadium center is still sufficiently polarizing that a third deprotonation equilibrium is established in aqueous media to generate the ion [(H20)4V(=0)(0H)]+, which contains water, hydroxy and oxo ligands (Eq. 9.19). 

The vanadium(iii) ion [V(H20)6] (9.9) exhibits this behaviour, with a log AT of 4 associated with the formation of the hydroxy-bridged dinuclear complex 9.10. This is a general phenomenon. For example, chromium(iii) and iron(iii) form strictly 

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