Why doesn't Fe(OH)3 dissolve with excess NaOH when Cr(OH)3 does?

Hiya, in a Chemistry module 5 lesson about the reactions of transition metal complexes, we were told that Fe(OH)3 will not undergo further hydrolysis, i.e. you cant get it to dissolve upon addition of excess NaOH.

But as to why it doesn't dissolve confuses me. Now I've been taught that the ability to form acidic solutions depends upon the polarising ability of the transition metal ion (I.e. the metal ion polarises the surrounding water ligands), which in turn is dependant upon the charge to size ratio of the metal ion.

Now I know that Cr(OH)3 will dissolve if you add excess NaOH to form a green solution... or at least that's what I'm taught.

But Fe3+ would surely have a greater charge density than Cr3+ (because they both have the same charge and across a period the radius decreases due to additional protons)... So why doesn't Fe(OH)3 dissolve!?!? :confused:

Any ideas would be greatly appreciated,

Thanks, Alex

Reply 1

Eau

Cr(OH)3(s) + OH-(aq) <==> Cr(OH)4-(aq) (green)

Cr(OH)4- is quite stable, where as iron prefers to be Fe(OH)3.

The explanation is quite difficult as you need to take the whole complex into account, hybrid orbitals, and stereochemistry.