Question

Is `"SO"_2` a Lewis acid?

Answer 1

Yes. You can see a fuller discussion here, but the gist of it is that `"SO"_2` has a `pi^"*"` antibonding molecular orbital formed by the overlap of the oxygen `2p_z` and sulfur `3p_z` atomic orbitals.

![http://image.slidesharecdn.com/]()

That `pi^"*"`, labeled MO 10, acts as the lowest-unoccupied molecular orbital (LUMO), which can accept electrons.

Since it is closest in energy to sulfur's `3p_z` orbital, the nucleophilic attack will effectively (visually, in a drawn-out reaction mechanism) donate two electrons into sulfur's `3p_z` atomic orbital.

Therefore, as an electron pair (`pi`) acceptor, by definition, `"SO"_2` it can be a Lewis acid.