Revisiting a Historical Concept by Using Quantum Crystallography: Are Phosphate, Sulfate and Perchlorate Anions Hypervalent?

Abstract

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 (PO43−), sulfate (SO42−) and perchlorate (ClO4−) 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 (MgNH4PO4⋅6 H2O, Li2SO4⋅H2O, and KClO4), 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.

New Insights into the Catalytic Activity of Cobalt Orthophosphate Co3(PO4)2from Charge Density Analysis

“…For potassium sulfate, the high polarity of the S−O bond was already established in an experimental charge‐density investigation . In this study, we also show for phosphate that the bond between phosphorus and oxygen is a highly polar covalent bond with no double‐bond character . The evidence for this stems from the high ED and a positive Laplacian, the trace of the Hessian matrix of the second derivative of the ED function, at the BCP.…”

“…[17] In this study,w ea lso show for phosphate that the bond between phosphorus and oxygen is ah ighly polar covalentbond with no double-bond character. [18] The evidence for this stems from the high ED and apositive Laplacian, the trace of the Hessian matrix of the second derivative of the ED function, at the BCP.A se xpected, for all four PÀOb onds a high ED was found at the BCP,w hich suggests ac ovalent bond ( Table 1). The values vary from 1.54 to 1.61 e À3 and are in ag ood agreement with the theoretical data.…”

Bis(6‐diphenylphosphino‐acenaphth‐5‐yl)sulfoxide: A New Ligand for Late Transition Metal Complexes

“…These values are typical for highly polar covalent bonds, where the S–O bond was found to exhibit even some charge‐shift character, underlining the importance of the + S–O – resonance form. [ 7a ] The much longer S–C bonds are predominantly covalent with a negative Laplacian and with |H/ρ( r ) bcp | > G/ρ( r ) bcp , being close to homopolar as indicated by RJI values of 49.2 to 55.2 %. Compared to the short and strong S–O bonds, the M–P and M–Cl interactions are longer and weaker with slightly dominating ionic bond contributions (Tables S2 and S3).…”

“…[ 6 ] However, a certain degree of π‐bonding contributions can be present via negative hyperconjugation, but never exceeding total bond orders of 1.0 for S–O bonds. [ 7 ]…”

Complementary bonding analysis of the N–Si interaction in pentacoordinated silicon compounds using quantum crystallography

“…27 Resonance structures with both N -Si and Si -F bonds are not considered, because these hypervalent bonding representations suggest d-orbital participation, which has been shown to be insignificant for bonds involving atoms from the second and third periods. [28][29][30][31][32][33] We have discussed previously that a joint application of bond analysis methods from real space (e.g.…”