Electronic Structure of the Sulfonyl and Phosphonyl Groups: A Computational and Crystallographic Study

A computational and X-ray crystallographic investigation of the electronic and geometric structures of a range of
sulfonyl (−SO2−) and phosphonyl (−PO2-−) containing species was undertaken to investigate the nature of valency
and bonding in these functional groups. The traditional representation of sulfonyl and phosphonyl species is with
octet-violating Lewis structures, which require d-orbital participation at the central atom. However, computational
studies cast serious doubt upon this bonding model. In this work, we have employed NBO/NRT analysis to investigate
hybridization, atomic formal charges, donor−acceptor interactions, and resonance structure contributions. Our results
predict that within sulfonyl and phosphonyl systems, bonding interactions are highly polarized, of the form X+−Y-
(X = P, S), and possess additional contributions from reciprocal n → σ* interactions where substituents off sulfur
or phosphorus simultaneously act as donors and acceptors. Experimental evidence for the proposed bonding
arrangement is provided for the sulfonyl functional group through a series of low-temperature X-ray structure
correlations for sulfate monoesters, sulfamates, and methanesulfonates. Examination of changes to bond lengths
and geometries upon substituent variation support the computational results. Together, our studies lend support for
a bonding network in sulfonyl and phosphonyl groups composed of polar interactions augmented with reciprocal
hyperconjugative bonding, which does not necessitate significant d-orbital participation nor formal octet violation at
the central sulfur or phosphorus.