Substitution at thionyl chloride sulfur What is the correct way of writing the mechanism for substitution at the sulfur atom of thionyl chloride:



Should one write the cleavage of the $\ce{S-O}$ &#x3c0; bond? The argument for the first option is that $\ce{S}$ is already tetrahedral and direct displacement could occur. The argument for the second option is that the &#x3c0; bond will be cleaved first as it is the weakest. Another possibility would be the formation of a tetravalent sulfur species (like the $\mathrm{S_N2}$ substitution at silicon). Is there any experimental or computational evidence regarding the matter?
Your second drawing would imply a $\ce{S=O}$ double bond, due to available orbitals it would be a d-p &#x3c0; bond. There is considerable debate as to whether these bonds even exist with most of the evidence presented here at chemistry.stackexchange.com pointing towards a ‘no’. That would rule out your second mechanism.

That said, your first mechanism suffers from comparing sulphur with carbon too much. You assume a rather short-lived transition state and direct displacement. I don’t know for sure, but I don’t think that is the case.

Rather, I would propose a two-step mechanism with a medium-lived tetravalent sulphur. In this mechanism, the attack of the nucleophile would turn pseudotetrahedral (one corner of the tetrahedron being a lone pair) into a pseudo-trigonal bipyramid or a see-saw with $\ce{Nu}$ and $\ce{Cl}$ forming a four-electron-three-centre bond. This would then collapse back into a pseudotetrahedral product liberating the better stabilised anion. Close to your first mechanism, but not identical: