[rockin_chris]

According to my book, for 1st row Transition Metals, the acidity of a aquo-complex is greater for those elements at the right of the periodic table. They justify this by saying that to the RHS, the cation is more polarizing and weakens the O-H bond to a greater extent.


I do get this, but for me, this is contradictory to the fact that towards the RHS, the d-orbital is more contracted and less available for bonding. THEREFORE, the degree of bonding between the metal and O is lower, and it does not "break" the O-H bond as much.


Any ideas?

thank you!

[dipesh747]

have a look into pi back bonding

[rockin_chris]

have a look into pi back bonding

I don't see how... don't think it has anything to do with it.... 

[dipesh747]

Well if there is more electron density in the metal centre, there is more pi back bonding to the OH which in turn decreases the strength of of the OH. This effect will be  larger if there is more electron density on the metal. This can be confirmed by the change in OH streching frequency when it is attached to different metals.

[cheese (MSW)]

Dipesh747:  Would you care to describe the orbital interactions in M-OH (M-OH2?)
to support your claim that π back bonding is responsible for the increase in acidity of
1st Row TM [M(H2O)6]^2+ ions.  In particular, what are the π acceptor AOs on the
O atom?

[dipesh747]

Firstly, If you disagree, then why don't you help this person by answering the question?

There are two (2b1 and 4a1) unoccupied anti bonding orbitals. I would have assumed (just as for carbonyls) that the metal centre can back bond into these anti bonding orbitals.

[cheese (MSW)]

The idea behind this forum is not to give answers (I know the accepted explanation) but to improve the critical thinking ability of the student asking the question.  I would also hope the person posting an answer would have thought through their answer. OH and OH2 are not CO.  There are indeed π-acceptor MOs on H2O: the O-H σ* antibonding MOs, but these are way too high in energy to overlap with the filled metal d AOs that we are agreed for the RH TM are contracted and not available for strong π bonding.  And this does indeed account for the observation that Ni(CO)4 is much less stable than Cr(CO)6.

[dipesh747]

I would also hope the person posting an answer would have thought through their answer.

I did think it through....didn't know about the energy gap being too big though?

[cheese (MSW)]

This is a straight-forward polarization problem as rockin-chris has correctly proposed.
[M(H2O)6]n+ +  H2O  ⇋  M((OH)(H2O)5]^n-1 +  H3O^+
Ka increases with n; Al^3+(aq) has a pKa near that of acetic acid that is explained
M^n+ ←:O(H)-Hδ+ by bond weakening.
For [M(H2O)6]2+ cmplxs the smaller the M^2+ ion the weaker the O-H bond the higher
the Ka although they are still weak acids;  TM to the RHS of the PT
are smaller: e⁻s enter the 3d set (~ same distance from +ve nucleus) but Zeff increases
because valence e⁻s do a poor job of screening each other. Mn(II) = 91 pm; Cu(II) = 72 pm.
The d e⁻s play little or no part in the explanation.