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Harcourt, Richard D.; Klapötke, Thomas M. (2016): Comment on "A quantitative definition of hypervalency" by M. C. Durrant, Chem. Sci., 2015, 6, 6614. In: Chemical Science, Vol. 7, No. 5: pp. 3443-3447
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Consideration is given to (electronically) hypervalent increased-valence structures, which possess 2c-1e bonds, fractional 2c-2e bonds, and usually normal 2c-2e bonds. For singlet-spin electron-rich systems, increased-valence structures, with Heitler-London 2c-2e bond wavefunctions, are equivalent to resonance between non-hypervalent Kekule and Dewar (or singlet diradical) type Lewis structures. Dewar structures are not considered in the Chem. Sci. 2015, 6, 6614 Edge article on hypervalency. Using one-electron delocalizations from lone-pair atomic orbitals into separate bonding molecular orbitals, increased-valence structures for PCl5, O-3, SO42-, NO3-, N2O4 and S(N)2 reactions are derived from the Edge-article's Kekule-type Lewis structures, and compared with the Edge article's hypervalent structures with 2c-2e bonds. It is also shown that Durrant's method to determine the g parameter for XAY-type systems that possess a symmetrical 3c-4e bonding unit is related to the A-atom charge density.