One of the canonical structures for sulfur dioxide - $\ce{SO2}$ - has sulfur (with a lone electron pair) double bonded to each oxygen atom to form a total of 4 bonds for sulfur - which can be achieved via valence expansion into empty d-orbitals.
What then is the hybridization of the valence-expanded sulfur? It is described as sp². But how can that be? This seems unlikely because d-orbitals are involved since the sulfur underwent valence expansion.
On might imagine a pair of electrons from the 3s/3p oribital(s) being promoted to an empty d-orbital and then having the 3s and 3p orbitals hybridize in to sp². If this is true it would mean that the lone electron pair of the valence-expanded sulfur consists of 2 electrons occupying and unhybridized d-orbital. But is this correct?