Hydroxoaqua Metal Ions

However the product of reaction (7) is expected to be a short-lived intermediate, the end product being the di-p(OH) ion which is thermodynamically more stable  

Reaction (8) which involves the breaking of metal-oxygen bonds is slow only in inert complexes. Therefore the most common inert complexes, namely those of Cr(III) and Co(III) were chosen. 

The preparation and properties of many hydroxoaqua complexes of chromium(III) and cobalt III) were reported since the beginning of the century, but some of them cannot be used for growing single crystals. Rapid crystallization or recrystallization yields microcrystalline precipitates while slow crystallization is often obstructed by olation (reaction 8), isomerization and other slow substitution reactions. 

Structures of seven compounds of hydroxoaqua complexes were determined so far. Some important structural features of these compounds are presented in Table 3 and Figs. 8 to 12. 

These structures confirm our assumption that many hydroxoaqua metal complexes are not mononuclear as their classical formulation implies (compounds 15 to 19) but binuclear or polynuclear. The OH and H2O ligands in compounds 15 to 20 merge into 

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The existence of hydrogen-oxide bridging in hydroxoaqua ions in the crystalline state, raised the possibility that these hydrogen-oxide bridged species persist in aqueous solution. Primary results obtained by Three Phase Vapor Tensiometry (TPVT), confirm the existence of binuclear hydroxoaqua ions in concentrated solutions. The existence of these ions throws new-light on the mechanism of some substitution and redox reactions of metal ions. 

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