The electronic structure of carbones revealed: insights from valence bond theory

Neutral Cyclic Nitreones: Electronic Structure Analysis Reveals an Unorthodox Perspective of Several Zwitterionic Heterocyclic Species

Nitreones are compounds with the general formula L → N+ ← L'. These compounds exhibit medicinal properties and have found applications in phase transfer catalysis. A few nitreones are cyclic; protonated cycloguanil (an antimalarial agent) is the most prominent example. Recently, a few more cyclic compounds were experimentally reported, in which the central N+ was shown to exhibit nitreone character. This led to attention being paid to the chemistry of neutral cyclic nitreones. A thorough literature search led to two sets of cyclic nitreones: C → N ← C type and P → N ← P type. In this work, we report quantum chemical analysis in exploring the electronic structure of neutral cyclic nitreones. Molecular orbital analysis, electron density analysis, charge, electron localization function (ELF), complexation energy values, and Tolman electronic parameter (TEP) all indicate that the studied compounds do carry nitrogen in the N(I) oxidation state and the two lone pairs are at the central nitrogen; thus, they qualify to be considered as cyclic nitreones.

(Imidazol-2-ylidene) → S coordination interactions and its modulation upon S-oxidation

(NHC) → E coordination interactions are being explored in many chemical species, including carbones and nitreones. (NHC) → S interactions are rare, but increasing attention is being paid to the compounds containing such interactions. The electron deficiency at the S centre is responsible for triggering electron donation from the NHC unit in (NHC) → SR(+) systems. It is well known that the positive charge at the sulfur centre increases upon single oxidation and further increases upon double oxidation. This implies that (NHC) → S interactions may become explicit after S-oxidation in the (NHC) → SR(+) systems. To explore this hypothesis, we performed quantum chemical design and synthesis of (NHC) → SR(+), (NHC) → S(O)R(+), and (NHC) → S(O)2R(+) complexes in which the ligands are imidazol-2-ylidene derivatives. Eight derivatives of the (imidazol-2-ylidene) → SR(+) systems were generated, and their sulfoxide and sulfone derivatives were obtained by oxidation using urea-H2O2 and mCPBA, respectively. The crystal structures of three compounds belonging to a series were determined. A comparison of the geometric, energetic and electronic characteristics confirmed the hypothesis that the (NHC) → S coordination interaction becomes comparatively stronger with an increase in oxygen atoms at the sulfur centre.

Geminal bimetallic coordination of a carbone to main-group and transition metals

The non-bonding carbone lone pair in geometrically-constrained antimony and bismuth carbodiphosphorane complexes readily complexed AuCl to afford rare examples of geminal bimetallic carbone coordination featuring a main-group metal.

Revisiting sp2 Dilithio Methandiides: From Geometric Curiosity to Simple Bonding Description

The reported tetracoordinate dilithio methandiide complex from Liddle and co-workers (1) is investigated from a coordination chemistry perspective, to probe the origin of its intriguing geometry. Through the application of a variety of computational techniques, non-covalent (steric, electrostatic) interactions are found to be dominant. Further, we arrive at a bonding description which emphasizes the tricoordinate sp2 -hybridized nature of the central methandiide carbon, differing somewhat from the original proposal. Thus, 1 is distinct from other dilithio methandiides since it contains only one C-Li σ-bond, and is found to be comparable to a simple aryllithium compound, phenyllithium.

Mesoionic and N-Heterocyclic Carbenes Coordinated N+ Center: Experimental and Computational Analysis

N-Heterocyclic carbenes, carbocyclic carbenes, remote N-heterocyclic carbenes and N-heterocyclic silylenes are known to form L→N⁺ coordination bonds. However, mesoionic carbenes (MICs) are not reported to form coordination bonds with cationic nitrogen. Herein, synthesis and quantum chemical studies were performed on 1,2,3-triazol-5-ylidene stabilized N⁺ center. Six compounds with MIC→N⁺ ←NHC were synthesized. Density functional theory calculations and energy decomposition analysis were carried out to explore the bonding situation between MIC and N⁺ center. The C→N⁺ bond lengths were in the range of 1.295-1.342 Å and bond dissociation energies were <400 kcal/mol. Natural bond orbital analysis supported the presence of excess electron density (>3 electrons) at the N⁺ center. The computational and X-ray diffraction analysis results confirmed the presence of divalent N^I character of center nitrogen and MIC→N⁺ ←NHC coordination interactions.

Reactivity of an N-heterocyclic silylene with a 1,1'-ferrocenediyl backbone towards carbonyl compounds, including carbon suboxide

Reactions of a silylene with a ketene and with carbon suboxide are reported, respectively leading to the first silaallene oxide and to a silylketene, whose reaction with water affords the first structurally characterised stable methyleneketene and constitutes a unique type of single-crystal-to-single-crystal transformation of a molecular solid by a stoichiometric gas-solid chemical reaction.

A Critical Look at Linus Pauling's Influence on the Understanding of Chemical Bonding

The influence of Linus Pauling on the understanding of chemical bonding is critically examined. Pauling deserves credit for presenting a connection between the quantum theoretical description of chemical bonding and Gilbert Lewis's classical bonding model of localized electron pair bonds for a wide range of chemistry. Using the concept of resonance that he introduced, he was able to present a consistent description of chemical bonding for molecules, metals, and ionic crystals which was used by many chemists and subsequently found its way into chemistry textbooks. However, his one-sided restriction to the valence bond method and his rejection of the molecular orbital approach hindered further development of chemical bonding theory for a while and his close association of the heuristic Lewis binding model with the quantum chemical VB approach led to misleading ideas until today.