Revisiting a historical concept by using quantum crystallography: Are phosphate, sulfate and perchlorate anions hypervalent?

Malte Fugel, Lorraine A. Malaspina, Rumpa Pal, Sajesh P. Thomas, Ming W. Shi, Mark A. Spackman, Kunihisa Sugimoto, Simon Grabowsky

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)


There are many examples of atoms in molecules that violate Lewis’ octet rule, because they have more than four electron pairs assigned to their valence. These atoms are referred to as hypervalent. However, hypervalency may be regarded as an artifact arising from Lewis’ description of molecules, which is based on the assumption that electrons are localized in two-center two-electron bonds and lone pairs. In the present paper, the isoelectronic phosphate (PO 4 3− ), sulfate (SO 4 2− ) and perchlorate (ClO 4 ) anions were examined with respect to the concept of hypervalency. Lewis formulas containing a hypervalent central atom exist for all three anions. Based on X-ray wavefunction refinements of high-resolution X-ray diffraction data of representative crystal structures (MgNH 4 PO 4 ⋅6 H 2 O, Li 2 SO 4 ⋅H 2 O, and KClO 4 ), complementary bonding analyses were performed. In this way, experimental information from the new field of quantum crystallography validate long-known facts, or refute long-standing misunderstandings. It is shown that the P−O and S−O bonds are highly polarized covalent bonds and, thus, the increase in the valence population following three-center four-electron bonding is not sufficient to yield hypervalent phosphorus or sulfur atoms, respectively. However, for the highly covalent Cl−O bond, most bonding indicators imply a hypervalent chlorine atom.

Original languageEnglish
Pages (from-to)6523-6532
Number of pages10
JournalChemistry - A European Journal
Issue number26
Early online date13 Feb 2019
Publication statusPublished - 7 May 2019


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