AL chem: bacisity of Fe(OH)2

The charge of Fe3+ is larger than that of Fe2+ (obviously). Lone pair of electrons on the O atom ( in OH-) is attracted more tightly to the Fe3+. H+ from acid can easier take away the OH- from a 2+ charge ion than a 3+ charge ion.

illustrated guide:

H+ ------------(OH-)-------(Fe2+)
easier to remove OH-

H+ ----------------(OH-)---(Fe3+)
more difficult to remove OH-

2011-04-10 12:00:17 補充：
OH- in Fe3+ is more polarized than the OH- in Fe2+. It is different so you should also mention that.

plastic:
i think the argument follows your discussion, while the result is not exactly the same.

What is the meaning of the "base" in the question?

base can dissociate to give hydroxide ion; this's definition of Arrhenius base.
yet, in this case it should follow Bronsted base / Lewis base, which is proton-acceptor / electron pair donor. the argument should follow Lewis base for easier discussion.

yes, Fe(III) has higher charge density, which attracts the electrons on O-atom more strongly. this reduces the electron density (and thus lone pair availability) on O-atom in hydroxo-ligand.
proton is less readily attracted by the lone pair, thus the basicity of Fe(III) is weaker.


what plastic suggests also make sense. but notice that the valency on Fe-center has to be fulfilled anyway, either by hydroxo-ligand (:OH) or aqua-ligand (:OH2).
it's not very likely for the complex to break the Fe-O bond and release hydroxide ion (OH-).
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i think where strong-polarization occurs is Fe-O bond, not the O-H bond.