Transition metal complexes

Why doesnt [Al(H2O)3(OH)3] undergo complete ligand substitution with OH- forming [Al(OH)6]3-?

But instead forms [Al(OH)4]-?

Thanks.

There are probably several reasons, but ultimately it comes down to the fact that it is thermodynamically unfavourable.

It could be that the energy required to bond two negative ions together is too great, or that the spacial characteristics of six hydroxy ligands around the tiny aluminium(III) ion are unfavourable.