[Memy]

hello,
i have a question on acids and metal ions coordination:

the pka of water is around 14.
however, when the water dissociates and the hydroxyde ion complexes with a metal ion, the pka of water lowers since the positive charge of the metal ion stabilizes the negative charge of the hydroxyde ion.

Ca2+ has an electronegativiy of 1 while cu2+ has an electronegativity of 1.9 .
thus Ca2+ should be more electropositive than cu2+

However, if i see the Pka of water in presence of Ca2+ is 13.4 while in presence of cu2+ is 10.7

Shouldn`t the pka of water be lower in presence of Ca2+ since this is more eectropositive than Cu2+ and thus stabilizes better the hydroxide anion?

Is it because the ionic radius diminishes on going from Ca2+ to Cu2+ and thus the positive nuclear charge is more effective in the latter case for stabilization purposes?

if this is the case, should then hydroxyde ions be more stabilized in the presence of Zn2+, followed by Cu2+, Ni2+, Co2+, fe2+ and Ca2+?

thank you very much  

[Borek]

First of all - I don't agree with the idea of water Ka chaninging in the presence of metal ions. Ka is constant, period. What happens is that hydroxide ions are removed from the solution by complex, thus moving equilibrium of water dissociation to the right. Call it Le Chatelier principle in action if you want.

Second, electronegativity is not all. Complexes have some percentage of covalent bonding, their stability is partially due to ligand field effect (no idea how it is properly called in English). Thus you can't explain differences between calcium and copper just by electronegativity - Ca doesn't have d electrons, Cu has them. These are completely different worlds.

[plu]

An excellent page on the acidity of hexaaqua metal complexes:

http://www.chemguide.co.uk/inorganic/complexions/acidity.html#top