How relevant are S = O and P = O double bonds for the description of the acid molecules H2SO3, H2SO4, and H3PO4, respectively?

The acid molecules H2SO3, H2SO4, and H3PO4 are usually drawn using "Lewis s tructures" which exhibit the octet extension by 3d-orbitals on sulfur and p hosphorus, respectively. Thus, S=O and P=O double bonds are assumed to be f ormed. The natural d-orbital occupancies on S and P, however, were calculat ed to be as low as 0.1 e, and therefore, an octet extension can hardly be e xpected. After the natural bond orbitals (NBO) search procedure was forced to attempt to form different Lewis structures of bonds and lone pairs, we d efined the optimal Lewis structure, if a dominant structure exists at all, by the maximum electronic charge in Lewis orbitals. Indeed, sulfur obeys th e octet rule in the optimal zwitterionic Lewis structures and does not form S=O double bonds. No dominant resonance structure could be found for H3PO4 where polarized PO pi-bond and zwitterionic PO bond structures exhibit sim ilar weights.