guy - 3-6-2005 at 20:49
Why does the [Cr(H2O)3(OH)3] go to [Cr(OH)6]-3 but [Al(H2O)3(OH)3] only goes to [Al(H2O)2(OH)4]-?
Nerro - 4-6-2005 at 02:12
I believe it has to do with the ionradius and the electropositivity of the metal ions. They make OH- out of H2O because they pull the O's of the
H2O to them but repell the H's which then eventually become detached from the H2O to yield OH-. Obviously this also has to do with the pH of hte
solution in which the metals are present. At very high pH's this is more likely to occur.
(That's what my highschool teacher told me, next year I will stumble upon this post and smile at my own ignorance 
)
chemoleo - 4-6-2005 at 07:58
It is because Al is much smaller (with only one subshell/octett of electrons), so the tetrahedral form is preferred - similar to NaBH4.
While chromium is a transition metal that has free electron pairs in the subshells which participate in complexation, i.e. the water molecules are
coordinated by the free electron pairs, forming some hydrogen-bond type bonding.
That's why these hyrates can't be dehydrated without loss of the counterion, and that's why the hydrates have much different properties
to the i.e. metal chloride itself.