Q:

Why is mercury a "fancy proton"?

I remember that one of my old organic chemistry profs referred to mercury as being a "fancy proton". I think it was in the context of oxymercuration reduction. 
What might have been his rationale for offering this comparison?

A:

It is a large, soft, Lewis acid. One can use protic acid and water to promote the same reaction, but the product distribution can sometimes be different. 
 In the oxymercuration reaction Hg can form a pi complex with alkenes or alkynes, and these "bridged" cations will not undergo rearrangments to the extent that those produced by protic acids might. 
 Bottom line - they're both essentially performing the same function in this reaction - activating the pi bond towards attack - but the oxymercuration is milder. 

