Comment on "A quantitative definition of hypervalency" by M. C. Durrant, Chem. Sci., 2015, , 6614

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Synthesis and Characterization of Hypervalent Pentacoordinate Carbon Compounds Bearing a 7-6-7-Ring Skeleton

To stabilize S_N 2 transition state-like penta-coordinate carbon species, triaryl-substituted cationic carbon compounds bearing a moderately flexible 7-6-7-ring skeleton with sulfur donors were synthesized and characterized. Electronic effects of para substituents (R=Cl, F, H, CH₃ , SMe, OMe) of the two equatorial aryl groups bound to the cationic central carbon were investigated systematically along with a planar bidentate thioxanthene derivative. X-ray analysis on their solid-state structures showed that the parent (R=H), chloro-, fluoro- and methyl-derivatives were tetracoordinate carbon (sulfonium) structures, while the p-MeO and thioxanthenyl system were pentacoordinate carbocation structures. The Hammett substituent constants for the para substituents (σ_p ⁺ ) correlates well with the bonding in these compounds. The methylthio-derivative with intermediate Hammett substituent constants (p-MeS; σ_p ⁺ =-0.60) showed a tetracooridnate solid-state structure, though solution UV-Vis properties suggested the presence of a penta-coordinate structure. These findings amount to the first unambiguous solution evidence of the hypervalent apical 3c-4e interactions in pentacoordinate carbon compounds.

The chemical bond in solids-revisited

This article complements an earlier topical review of the chemical bond (Jones 2018J. Phys.: Condens. Matter30153001), starting in the mid-19th century and seen from the perspective of a condensed matter physicist. The discussion of applications focused on the structure and properties of phase change materials. We review here additional aspects of chemistry, particularly some that have raised interest recently in this context. Concepts such as 'electron-rich', 'electron-deficient (excess orbital)', 'hypervalent', 'three-centre', and 'metavalent' bonds, and 'multicentre hyperbonding' are now found in the condensed matter literature. They are surveyed here, as well as the bond in metals and the 'Peierls' distortion. What are these concepts, are they related, and are they sometimes new labels for established, but unfamiliar ideas? 'Half bonds' and 'fractional valencies' play a central role in this discussion. It is remarkable that they were introduced 100 years ago, but ignored or forgotten, and have needed to be rediscovered more than once.

Sulfur Tetrahydride and Allied Superhydride Clusters: When Resonance Takes Precedence

Sulfur offers a variety of bonding surprises compared to the parent oxygen atom of the chalcogen family. In the present work, we employ standard quantum chemistry methods to characterize formation of previously unrecognized sulfur tetrahydride (C_4v -symmetric SH₄ ) from hydrogen sulfide (H₂ S) and molecular hydrogen (H₂ ) on the ground state potential energy surface. The unusual intramolecular interactions of SH₄ defy Lewis-like bonding conceptions, exhibiting the dominance of resonance-type donor-acceptor delocalizations well beyond those of SF₄ (C_2v sawhorse geometry) and other known tetrahalides. The distressed character of SH₄ bonding also leads to exotic intermolecular structural motifs in clusters of pure (SH₄ )_n and mixed (SH₄ ⋅⋅⋅H₂ S)_n composition. We evaluate structural, spectroscopic, and electronic properties for various 2D/3D coordination patterns and discuss how (SH₄ ⋅⋅⋅H₂ S)_n -type building blocks may relate to recent experimental studies of superconductivity in high-pressure materials of "SH₃ " stoichiometry.

Synthesis and properties of hypervalent electron-rich pentacoordinate nitrogen compounds

Isolation and structural characterization of hypervalent electron-rich pentacoordinate nitrogen species have not been achieved despite continuous attempts for over a century. Herein we report the first synthesis and isolation of air stable hypervalent electron-rich pentacoordinate nitrogen cationic radical (11-N-5) species from oxidation of their corresponding neutral (12-N-5) species. In the cationic radical species, the nitrogen centers adopt a trigonal bipyramidal geometry featuring a 3-center-5-electron hypervalent attractive interaction. The combination of single crystal X-ray diffraction analysis and computational studies revealed weak N-O interactions between the central nitrogen cation and oxygen atoms. This successful design strategy and isolation of air-stable pentacoordinate hypervalent nitrogen species allow further investigations on reactivity and properties resulting from these unusually weakly coordinating interactions in nitrogen compounds.

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

An extensive characterization of Co3 (PO4 )2 was performed by topological analysis according to Bader's Quantum Theory of Atoms in Molecules from the experimentally and theoretically determined electron density. This study sheds light on the reactivity of cobalt orthophosphate as a solid-state heterogeneous oxidative-dehydration and -dehydrogenation catalyst. Various faces of the bulk catalyst were identified as possible reactive sites given their topological properties. The charge accumulations and depletions around the two independent five- and sixfold-coordinated cobalt atoms, found in the topological analysis, are correlated to the orientation and population of the d-orbitals. It is shown that the (011) face has the best structural features for catalysis. Fivefold-coordinated ions in close proximity to advantageously oriented vacant coordination sites and electron depletions suit the oxygen lone pairs of the reactant, mainly for chemisorption. This is confirmed both from the multipole refinement as well as from density functional theory calculations. Nearby basic phosphate ions are readily available for C-H activation.