Polynuclear aqua cations

Numerous hydroxo aqua cations exhibit a strong tendency to polymerize. The chemistry of polynuclear complexes is very varied and largely remains poorly understood. 

Substitution at polynuclear aqua-ions is often much slower than at the respective parent mononuclear aqua-ions, as has been demonstrated for, e.g., aluminum(III), iron(III), zirconium(IV), and hafnium(IV). A further example has been provided by aluminum, where the Al,3 polynuclear oxo-aqua-species takes several months to equilibrate with lactate. Very different rates of complex formation are used to sort out speciation in multicomponent polynuclear/mononuclear aqua-cation systems. The problems involved in this approach are well illustrated by a Al NMR study of the numerous reaction pathways in the Al " /ferron [ferron = 7-iodo-8-hydroxyquinoline-5-sulfonic acid, (18)] reaction. The slowness of reaction of polynuclear species can cause difficulties in the determination of total metal content, e.g., of aluminum by ferron or Alizarin S (19), in natural waters. In certain situations these... 

Metal ions in aqueous solution generally hydrolyze to form a series of mononuclear ind polynuclear hydroxy complexes. Iron(III) for example forms [FeOH] ", [Fe(OH)2], Fe(OH)3(aq), [Fe(OH)4] , [Fe2(OH)2] and probably other polynuclear species. The formation af a monohydroxy complex (equation 1) can be used as a guide to the degree of electron withdrawal by the cation. Typical pJCa values for various cations are listed in Table 2. (The pJCa values of and HjO are -1.7 and 15.7 respectively.) The tetrahedrally coordinated [Be(H20)4] ion is more acidic than expected when compared with octahedrally coordinated aqua ions of divalent metals. Each hydrogen must carry a somewhat greater portion of the positive charge than it would in an octahedral complex, and this makes removal of a proton easier. Similar considerations apply to square planar Pd (aq). Mercury(II) and tin(II) are also more acidic than would be expected. The high acidity of mercury(II) is due to the unusual stability of linear H2O—Hg—OH, while Sn (aq) has between two and three coordinated water molecules in the inner coordination sphere. ... 

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