Nucleophilicity of bonding vs lone pairs Does the bonding pair in $\ce{FeCl4^-}$ attack a proton or does a lone pair on the chlorine attack the proton? 



My prof has an argument for the first one; the first one involves fewer steps, so it is probably more favorable. My proposed mechanism has three steps of electron pushing versus two, so okay. 

However, I can see an argument for the second mechanism - the first one involves a bonding pair acting as a nucleophile rather than a lone pair on chlorine. The bonding pair is stabilized by the chlorine and the iron atom, while the lone pair is stabilized by only the chlorine. 

So, which one wins? I'm guessing both mechanisms are operative here.
I don't know the answer to your question, but we can speculate.  A lone pair is more sterically exposed than a bonding pair of electrons and therefor more accessible.  Also a lone pair is generally more nucleophilic because the electrons are concentrated in a smaller region since they are not being shared.  These two arguments would favor the second mechanism.  Finally, when we consider the SN2 reaction where, for example, water is the attacking nucleophile; I've never seen the mechanism drawn where the attacking electrons are from the O-H bond, but rather they always seem to show involvement of the lone pair electrons.  All of these arguments favor the second mechanism.