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Nickel carbenes are key reactive intermediates in the catalytic cyclopropanation of olefins and other reactions, but isolated examples are scarce and generally rely on low coordination numbers (≤3) to stabilize the metal−ligand multiple bond. Here we report the isolation and characterization of a stable tetracoordinated nickel carbene bearing a triphosphine pincer ligand. Its nucleophilic character is evidenced by reaction with acids, and it can transfer the carbene fragment to CO to form a ketene. A computational study of the Ni=C chemical bond sheds light on the role of the third phosphine in the pincer framework to the stabilization of the nickel carbene fragment. [ABSTRACT FROM AUTHOR]

The symmetry‐decomposed Voronoi deformation density (VDD) charge analysis is an insightful and robust computational tool to aid the understanding of chemical bonding throughout all fields of chemistry. This method quantifies the atomic charge flow associated with chemical‐bond formation and enables decomposition of this charge flow into contributions of (1) orbital interaction types, that is, Pauli repulsive or bonding orbital interactions; (2) per irreducible representation (irrep) of any point‐group symmetry of interacting closed‐shell molecular fragments; and now also (3) interacting open‐shell (i.e., radical) molecular fragments. The symmetry‐decomposed VDD charge analysis augments the symmetry‐decomposed energy decomposition analysis (EDA) so that the charge flow associated with Pauli repulsion and orbital interactions can be quantified both per atom and per irrep, for example, for σ, π, and δ electrons. This provides detailed insights into fundamental aspects of chemical bonding that are not accessible from EDA. [ABSTRACT FROM AUTHOR]

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp3 to sp2 to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above‐mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s‐character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp3 to sp2 to sp. [ABSTRACT FROM AUTHOR]

When carbonyl ligands coordinate to transition metals, their bond distance either increases (classical) or decreases (nonclassical) with respect to the bond length in the isolated CO molecule. C−O expansion can easily be understood by π‐back‐donation, which results in a population of the CO's π*‐antibonding orbital and hence a weakening of its bond. Nonclassical carbonyl ligands are less straightforward to explain, and their nature is still subject of an ongoing debate. In this work, we studied five isoelectronic octahedral complexes, namely Fe(CO)62+, Mn(CO)6+, Cr(CO)6, V(CO)6− and Ti(CO)62−, at the ZORA‐BLYP/TZ2P level of theory to explain this nonclassical behavior in the framework of Kohn–Sham molecular orbital theory. We show that there are two competing forces that affect the C−O bond length, namely electrostatic interactions (favoring C−O contraction) and π‐back‐donation (favoring C−O expansion). It is a balance between those two terms that determines whether the carbonyl is classical or nonclassical. By further decomposing the electrostatic interaction ΔVelstat into four fundamental terms, we are able to rationalize why ΔVelstat gives rise to the nonclassical behavior, leading to new insights into the driving forces behind C−O contraction. [ABSTRACT FROM AUTHOR]

DNA quadruplexes have been the subject of investigation because of their biological relevance and because of their potential application in supramolecular chemistry. Similarly, RNA quadruplexes are now gaining increasing attention. Although DNA and RNA quadruplexes are structurally very similar, the latter show higher stability. In this study we report dispersion‐corrected density functional theory (DFT‐D) quantum chemical calculations that were undertaken to understand the difference in stabilities of RNA and DNA quadruplexes. The smallest meaningful model of a stack of quartets, interacting with alkali metal cations, was simulated in an aqueous environment. The energy decomposition analysis allows for in‐depth examination of the interaction energies, emphasising the role of noncovalent interactions and better electrostatics in determining RNA‐GQs higher stabilities, particularly pinpointing the role of the extra 2′‐OH groups. Furthermore, our computations present new insights on why the cation is required for self‐assembly: unexpectedly the cation is not necessary to relieve the repulsion between the oxygen atoms in the central cavity, but it is needed to overcome the entropic penalty. Why so stable? Dispersion‐corrected (DFT‐D) quantum chemical calculations on the stabilities of RNA and DNA quadruplexes show electrostatic interactions and hydrogen bonds make the decisive difference (see figure). [ABSTRACT FROM AUTHOR]

Theoretical and experimental studies have elucidated the bonding mechanism in hydrogen bonds as an electrostatic interaction, which also exhibits considerable stabilization by charge transfer, polarization, and dispersion interactions. Therefore, these components have been used to rationalize the differences in strength of hydrogen-bonded systems. A completely new viewpoint is presented, in which the Pauli (steric) repulsion controls the mechanism of hydrogen bonding. Quantum chemical computations on the mismatched DNA base pairs CC and GG (C=cytosine, G=guanine) show that the enhanced stabilization and shorter distance of GG is determined entirely by the difference in the Pauli repulsion, which is significantly less repulsive for GG than for CC. This is the first time that evidence is presented for the Pauli repulsion as decisive factor in relative hydrogen-bond strengths and lengths. [ABSTRACT FROM AUTHOR]

Reaction of the donor-stabilized silylene 1 (which is three-coordinate in the solid state and four-coordinate in solution) with BEt3 and BPh3 leads to the formation of the Lewis acid/base complexes 2 and 3, respectively, which are the first five-coordinate silicon compounds with an SiB bond. These compounds were structurally characterized by crystal structure analyses and by multinuclear NMR spectroscopic studies in the solid state and in solution. Additionally, the bonding situation in 2 and 3 was analyzed by quantum chemical studies. [ABSTRACT FROM AUTHOR]

Activation strain model, bonding analysis, density functional calculations, hybridization theory, Pauli repulsion Keywords: activation strain model; bonding analysis; density functional calculations; hybridization theory; Pauli repulsion EN activation strain model bonding analysis density functional calculations hybridization theory Pauli repulsion 6997 6997 1 04/30/21 20210426 NES 210426 B In s-p hybridization b , the contraction of C-H and C-C bonds as the carbon atom involved varies from sp3 to sp2 to sp is a fundamental and ubiquitous phenomenon in chemistry. Cover Feature: Not Carbon s-p Hybridization, but Coordination Number Determines C-H and C-C Bond Length (Chem. Eur. J. 24/2021). [Extracted from the article]

G‐Quadrupex‐RNA is structurally similar to its DNA counterpart but shows higher stability. Quantum chemical (DFT‐D) calculations pinpoint the role of noncovalent interactions and better electrostatics in determining RNA‐GQs higher stabilities, focussing on the contribution of the extra ribonucleosidic 2'‐OH groups. New insight on the necessity of cations for the assembly of these structures has been given, emphasizing how they are required to overcome entropic penalty and not, like previously assumed, to minimize internal repulsion forces. More information can be found in the Full Paper by C. Fonseca Guerra and F. Zaccaria on page 16315. [ABSTRACT FROM AUTHOR]

A new viewpoint on hydrogen bonds is presented, with the Pauli (steric) repulsion playing a decisive role. Hydrogen bonds are traditionally described as electrostatic interactions, which also exhibit stabilization by charge transfer, polarization, and dispersion interactions. Quantum chemical computations on the mismatched DNA base pairs cytosine – cytosine (CC) and guanine – guanine (GG) show that the enhanced stabilization and shorter distance of GG is determined entirely by the difference in the Pauli repulsion. More information can be found in the Communication by C. Fonseca Guerra and S. C. C. van der Lubbe on page 10249. [ABSTRACT FROM AUTHOR]