Hydrolysis of cations. Formation constants and standard free energies of formation of hydroxy complexes

Abstract

The formation constants and standard free energies of formation of both mononuclear and polynuclear hydroxy complexes of metal ions throughout the periodic table have been examined. Some empirical correlations are presented that make it possible to predict rough values for these constants. For mononuclear complexes the equation is ..delta..G/sub f//sup 0/)M(OH)/sub y/) = ..delta..G/sub f//sup 0/)M) + By +Cy/sup 2/ + D/y, where B, C, and D are empirical parameterrs and y is the number of coordinated hydroxide ions. These parameters are subject to constraints that limit acceptable values. For example, B, C, and electronegativity are interrelated. This equation can be rearranged to define a new function, U, that is linearly related to y. This allows the interpolation of unmeasured free energies and hence formation constants. For mononuclear complexes predicted values of log K/sub 1//sub y/ have an uncertainty of about +/-0.5. For polynuclear hydroxy complexes a relation previously proposed by Baes and Mesmer has been slightly modified and used to predict unknown formation constants with an uncertainty of log K/sub xy/ of about +/-0.5y. Mercury(II) and silver(I), which form especially stable ''linear'' complexes, do not follow the above correlations because of the structure change that occurs upon going frommore » the M(OH)/sub 2/ to the M(OH)/sub 3/ species. The tetrahedrally coordinated Be/sup 2 +/(aq) ion is more acidic than expected when compared with octahedrally coordinated aquo ions of divalent metals. Each hydrogen must carry a somewhat greater portion of the positive charge than it would in an octahedral complex,Rand this makes the removal of a proton easier. Aqueous Sn/sup 2 +/ is also unusually acidic, and it is suggested that it too has fewer water molecules in the primary cordination sphere than one would expect.« less