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The ambiphilicity of pseudo-halides has been the object of extensive debate. Herein, we use a series of trispyrazolylborato beryllium pseudo-halido complexes [TpBe(X')] with X' = CN-, N3 -, NCO- and NCS- to explore the origins of the preferred isomers. Thus, we have synthesised and characterised through NMR and IR spectroscopy as well as single crystal X-ray diffraction these complexes. A combination with quantum chemical calculations within the DFT framework enabled an in-depth understanding of the bonding modes and preferences of the investigated pseudo-halido ligands. [ABSTRACT FROM AUTHOR]

Vibrational spectroscopy in supersonic jet expansions is a powerful tool to assess molecular aggregates in close to ideal conditions for the benchmarking of quantum chemical approaches. The low temperatures achieved as well as the absence of environment effects allow for a direct comparison between computed and experimental spectra. This provides potential benchmarking data which can be revisited to hone different computational techniques, and it allows for the critical analysis of procedures under the setting of a blind challenge. In the latter case, the final result is unknown to modellers, providing an unbiased testing opportunity for quantum chemical models. In this work, we present the spectroscopic and computational results for the first HyDRA blind challenge. The latter deals with the prediction of water donor stretching vibrations in monohydrates of organic molecules. This edition features a test set of 10 systems. Experimental water donor OH vibrational wavenumbers for the vacuum-isolated monohydrates of formaldehyde, tetrahydrofuran, pyridine, tetrahydrothiophene, trifluoroethanol, methyl lactate, dimethylimidazolidinone, cyclooctanone, trifluoroacetophenone and 1-phenylcyclohexane-cis-1,2-diol are provided. The results of the challenge show promising predictive properties in both purely quantum mechanical approaches as well as regression and other machine learning strategies. [ABSTRACT FROM AUTHOR]

Trimethylsilyl-substituted triazaphospholes were synthesized by a [3+2] cycloaddition reaction between organic azides and (CH3)3Si–C≡P. In an attempt to isolate their N-alkylated products, the formation of BF3 adducts of unprecedented triazaphosphol-5-ylidenes was found. The nature of the carboncarbene–boron bond was investigated within the DFT framework, revealing a strong donation of electrons from the carbene carbon atom to the boron atom combined with weak back-bonding. [ABSTRACT FROM AUTHOR]

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C5R5)2] (Ae = Be–Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal–ligand bonding nature is mainly an electrostatic interaction (65–78%), the covalent character is not negligible (33–22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role. [ABSTRACT FROM AUTHOR]

Abstract: Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds E[L.sub.2] with atoms E = C-Pb and the ligands L = [PPh.sub.3], N-heterocyclic [...]

Heavier tetrylenes attract attention for their potential in synthesis, catalysis and small molecule activation. The coordination by N‐heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) results in substantial structural and electronic differences although typically only one of these yields stable derivatives for one and the same tetrylene. We now report both NHC‐ and CAAC‐coordination to a bridged bis(germylene) motif. The NHC‐coordinated bis(germylene) exhibits pyramidal germanium centers with lone pairs of electrons, while with CAAC an unprecedented stable bis(germene) with two Ge=C bonds is isolated. Spectroscopic and crystallographic evidence as well as DFT calculations confirm the effects of σ,π‐conjugation between the two germanium centers in both cases. The coordination of NHC is reversible as the reaction with BPh3 liberates the transient bis(germylene) and thus provides an alternative low‐temperature route towards polymers with Ge=Ge bonds. [ABSTRACT FROM AUTHOR]

Electrophilic AlIII species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low‐valent AlI aluminyl anions have showcased oxidative additions towards arenes C−C and/or C−H bonds. Herein, we communicate compelling evidence of an AlII radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC8 in benzene furnishes a double addition to the benzene ring instead of a C−H bond activation, producing the corresponding cyclohexa‐1,3(orl,4)‐dienes as Birch‐type reduction product. X‐ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable AlII radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection. [ABSTRACT FROM AUTHOR]

The classic pnictogen dichotomy stands for the great contrast between triply bonding very stable N2 molecules and its heavier congeners, which appear as dimers or oligomers. A banner example involves phosphorus as it occurs in nature as P4 instead of P2, given its weak π‐bonds or strong σ‐bonds. The P2 synthetic value has brought Lewis bases and metal coordination stabilization strategies. Herein, we discuss the unrealized encapsulation alternative using the well‐known fullerenes' capability to form endohedral and stabilize otherwise unstable molecules. We chose the most stable fullerene structures from Cn (n = 50, 60, 70, 80) and experimentally relevant from Cn (n = 90 and 100) to computationally study the thermodynamics and the geometrical consequences of encapsulating P2 inside the fullerene cages. Given the size differences between P2 and P4, we show that the fullerenes C70–C100 are suitable cages to side exclude P4 and host only one molecule of P2 with an intact triple bond. The thermodynamic analysis indicates that the process is favorable, overcoming the dimerization energy. Additionally, we have evaluated the host‐guest interaction to explain the origins of their stability using energy decomposition analysis. [ABSTRACT FROM AUTHOR]

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Herein, the first stable anions K[SIPrBp] (4 a‐K) and K[IPrBp] (4 b‐K) (SIPrBp=BpC{N(Dipp)CH2}2, IPrBp=BpC{N(Dipp)CH}2; Bp=4‐PhC6H4; Dipp=2,6‐iPr2C6H3) derived from classical N‐heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a‐K and 4 b‐K are prepared by KC8 reduction of the neutral radicals [SIPrBp] (3 a) and [IPrBp] (3 b), respectively. The radicals 3 a and 3 b as well as [Me‐IPrBp] 3 c (Me‐IPrBp=BpC{N(Dipp)CMe}2) are accessible as crystalline solids on treatment of the respective 1,3‐imidazoli(ni)um bromides (SIPrBp)Br (2 a), (IPrBp)Br (2 b), and (Me−IPrBp)Br (2 c) with KC8. The cyclic voltammograms of 2 a–2 c exhibit two one‐electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions (4 a–4 c)−. Computational calculations suggest a closed‐shell singlet ground state for (4 a–4 c)− with the singlet‐triplet energy gap of 17–24 kcal mol−1. [ABSTRACT FROM AUTHOR]

Phosphanylgallane with hydrogen and halogen substituents (RXGa−PHR, R=organic substituent, X=halogen/hydrogen) are regarded as putative suitable precursors for accessing Ga=P doubly bonded species. Herein, we report on the synthesis, structure, and bonding analysis of a series of Lewis base‐ and Lewis acid/base‐stabilized phosphanylgallane bearing P−H and Ga−Cl/H substitution. To avoid oligomerization, the treatment of IDip.GaCl3 and (IDip)GaH2Cl (IDip=1,3‐bis(2,6‐diisopropylphenyl) imidazole‐2‐ylidene) with LiPHR or LiPHR(BH3) (R=Ph, Tip, Mes, NiPr2, NCy2) affords the corresponding Lewis base and Lewis acid/base coordinated H,Cl‐functionalized monomeric phosphanylgallane, respectively. The structure of these derivatives were determined by spectroscopic and X‐ray crystallographic analyses. The observed Ga−P bond lengths are comparable to those previously reported phosphanylgallane analogues. The nature of the CIDip‐Ga coordination bond was assessed with Energy Decomposition Analysis, suggesting a relatively stable adduct. Reactions of the phosphanylgallane with Brønsted bases were investigated. [ABSTRACT FROM AUTHOR]

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Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C2As2‐diradicaloid {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2; Dipp = 2,6‐iPr2C6H3). Treatment of (IPr)CH2 (1) with AsCl3 affords the Lewis adduct {(IPr)CH2}AsCl3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl2 (3) and {(IPr)CAsCl}2 (5 a) or [{(IPr)CAs}2Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C2As2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet‐triplet energy gap of 8.7 kcal mol−1. The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe)2 and Fe2(CO)9 give rise to {(IPr)CAs(SePh)}2 (7) and {(IPr)CAs}2Fe(CO)4 (8), respectively. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P‐protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N‐heterocyclic carbene (NHC)‐stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P‐terminal, as a room‐temperature‐stable crystalline solid accessible via facile NHC‐induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P‐centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X‐ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation. [ABSTRACT FROM AUTHOR]

Here, we provide evidence of the path-dependency of the energy components of the energy decomposition analysis scheme, EDA, by studying a set of thirty-one closed-shell model systems with the D2h symmetry point group. For each system, we computed EDA components from nine different pathways and numerically showed that the relative magnitudes of the components differ substantially from one path to the other. Not surprisingly, yet unfortunately, the most significant variations in the relative magnitudes of the EDA components appear in the case of species with bonds within the grey zone of covalency and ionicity. We further discussed that the role of anions and their effect on arbitrary Pauli repulsion energy components affects the nature of bonding defined by EDA. The outcome variation by the selected partitioning scheme of EDA might bring arbitrariness when a careful comparison is overlooked. [ABSTRACT FROM AUTHOR]

Although silylene‐carbonyl complexes are known for decades, only recently isolable examples have been accomplished. In this work, the bonding situation is re‐evaluated to explain the origins of their remarkable stability within the Kohn‐Sham molecular orbital theory framework. It is shown that the chemical bond can be understood as CO interaction with the silylene via a donor‐acceptor interaction: a σ‐donation from the σCO into the empty p‐orbital of silicon, and a π‐back donation from the sp2 lone pair of silicon into the π*CO antibonding orbitals. Notably, it was established that the driving force behind the surprisingly stable Si−CO compounds, however, is another π‐back donation from a perpendicular bonding R−Si σ‐orbital into the π*CO antibonding orbitals. Consequently, the pyramidalization of the central silicon atom cannot be associated with the strength of the π‐back donation, in sharp contrast to the established chemical bonding model. Considering this additional bonding interaction not only shed light on the bonding situation, but is also an indispensable key for broadening the scope of silylene‐carbonyl chemistry. [ABSTRACT FROM AUTHOR]

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1,4‐Diarsinine‐1,4‐diide compound [(ADCPh)As]2 (5) (ADCPh={C(DippN)}2CPh, Dipp=2,6‐iPr2C6H3) with a planar C4As2 ring fused between two 1,3‐imidazole scaffolds has been isolated as a red crystalline solid. Compound 5, formally comprising an 8π‐electron C4As2 ring, is antiaromatic and undergoes 2e‐oxidation with AgOTf to form the 6π‐electron aromatic system [(ADCPh)As]2(OTf)2 (6). [ABSTRACT FROM AUTHOR]

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3‐E)GaCl4 [(3‐E).+ = [{(IPr)C(Ph)E}2Fe(CO)3].+, E = P or As; IPr = C{(NDipp)CH}2, Dipp = 2,6‐iPr2C6H3] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}2 (1‐E) with Fe2(CO)9 affords [{(IPr)C(Ph)E}2Fe(CO)3] (2‐E), in which 1‐E binds to the Fe atom in an allylic (η3‐EECvinyl) fashion and functions as a 4e donor ligand. Complexes 2‐E undergo 1e oxidation with GaCl3 to yield (3‐E)GaCl4. Spin density analysis revealed that the unpaired electron in (3‐E).+ is mainly located on the Fe (52–64 %) and vinylic C (30–36 %) atoms. Further 1e oxidation of (3‐E)GaCl4 leads to unprecedented η3‐EECvinyl to η3‐ECvinylCPh coordination shuttling to form the dications (4‐E)(GaCl4)2. [ABSTRACT FROM AUTHOR]

Herein, we report the first 1,4‐diphosphinine‐1,4‐diide compound [(ADCPh)P]2 (5‐Ph) (ADCPh=PhC{(NDipp)C}2; Dipp=2,6‐iPr2C6H3) derived from an anionic dicarbene (ADCPh) as a red crystalline solid. Compound 5‐Ph containing a 16π‐electron planar fused‐tricyclic ring system was obtained by the 4e reduction of [(ADCPh)PCl2]2 (4‐Ph) with Mg (or KC8) in a quantitative yield. Experimental and computational results imply that the central 8π‐electrons C4P2 ring of 5‐Ph, which is fused between two 6π‐electrons C3N2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5‐Ph has two electron‐lone‐pairs, one in a p‐type orbital is in conjugation with the C=C bonds of the C4P2 ring, while the second resides in a σ‐symmetric orbital. This can be shown with the gold complex [(ADCPh)P(AuCl)2]2 (6‐Ph) obtained by reacting 5‐Ph with (Me2S)AuCl. A mixture of 5‐Ph and 4‐Ph undergoes comproportionation in the presence of MgCl2 to form the intermediate oxidation state compound [(ADCAr)P]2(MgCl4) (7‐Ph), which is an aromatic species. [ABSTRACT FROM AUTHOR]

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Chemical bonds are traditionally assigned as electron‐sharing or donor‐acceptor/dative. External criteria such as the nature of the dissociation process, energy partitioning schemes, or quantum chemical topology are invoked to assess the bonding situation. However, for systems with marked multi‐reference character, this binary categorization might not be precise enough to render the bonding properties. A third scenario can be foreseen: spin polarized bonds. To illustrate this, the case of a NaBH3− cluster is presented. According to the analysis NaBH3− exhibits a strong diradical character and cannot be classified as either electron‐sharing or a dative bond. Elaborated upon are the common problems of popular bonding descriptions. Additionally, a simple model, based on the bond order and local spin indicators, which discriminates between all three bonding situations, is provided. [ABSTRACT FROM AUTHOR]

Here, we discuss that unlike bond dissociation energy (BDE) that is a state function quantity, the energy components of the energy decomposition analysis (EDA), i.e. electrostatic interaction, Pauli repulsion, and orbital interaction, are path (process) function quantities. Being a path function means that EDA energy components are not uniquely defined, i.e. the relative magnitudes of the orbital interaction, Pauli repulsion, and electrostatic components may vary depending on the selected pathway for EDA. Therefore, at best, EDA can define whether closely related chemical bonds are more or less ionic/covalent compared with each other. However, a precise assessment of the nature of a certain type of chemical bond using EDA is a questionable task. Besides, we briefly discuss that the widely used EDA pathway, which is merely an arbitrary choice among infinite possible paths, comes to conclusions not consistent with our widely accepted knowledge of bond formation even for the simplest molecules. [ABSTRACT FROM AUTHOR]

B3LYP calculations are used for studying the reaction mechanism of the aminolysis of Fischer carbene complexes of the type [(CO).sub.5]Cr[double bond]C(XMe)R (X = O and S, R = Me and Ph). The results have shown that the transition state of the nucleophilic attack is more advanced and has a higher barrier for alkoxycarbene than thiocarbene complexes because of the stronger [pi]-donor character of the alkoxy group that has reduced the electrophilicity of the attacked carbene atom making the nucleophilic attack more difficult.

Ref.: Can. J. Chem. 94(12): 1006-1014 (2016) doi:10.1139/cjc-2016-0263. There is an error in part b of Scheme 3. The correct Scheme 3 is shown below. It shows schematically the overall [...]

The first terminal arsinidene complexes [{(NHC)C(Ph)}As]Fe(CO)4 (NHC = IPr = C{(NDipp)CH}25, Me-IPr = C{(NDipp)CMe}26; Dipp = 2,6-iPr2C6H3) with a two-coordinated arsenic atom have been reported as crystalline solids. Calculations suggest that 5 and 6 are electrophilic, which has been demonstrated by the reaction of 5 with an NHC nucleophile (IMe4), affording the Lewis adduct [{(IPr)C(Ph)}As(IMe4)]Fe(CO)4 (7) (IMe4 = C(NMeCMe)2). [ABSTRACT FROM AUTHOR]

A bis(dicarba[2]aluminocenophane) was synthesized by reduction of a corresponding dichloro precursor with 1,3-β-diketiminate magnesium(I). The bis(aluminocenophane) is stable under an inert gas atmosphere and was characterized in solution and in the solid state. The crystal structure reveals an unusually short Al–Al single bond and two η5 coordinated cyclopentadienyl groups on each aluminum atom which are disposed in an orthogonal fashion. The aluminum bond can be oxidized and cleaved by element chlorides and tert-butyl isonitrile. The bonding situation was studied within the DFT framework. [ABSTRACT FROM AUTHOR]

The olefinic C−H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH}21; SIPr=C{(NAr)CH2}22; Ar=2,6‐iPr2C6H3), derived from classical N‐heterocyclic carbenes (NHCs), with PCl3 affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl2 (NHC=IPr 3, SIPr 4). Two‐electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes [{(NHC)C(Ph)}P]2 (NHC=IPr 5, SIPr 6) as crystalline solids. Unlike literature‐known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) Å) are comparable to those of the known diphosphenes (2.02–2.08 Å), the C−P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a Csp2 −P single bond length (1.85 Å), indicating a considerable π‐conjugation between C=C and P=P moieties. The HOMO–LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S)27. Interestingly, reaction of 5 with selenium gives the selenadiphosphirane [{(IPr)C(Ph)}P]2Se 8 with an intact P−P bond. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations using density functional theory BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules. The calculation results reveal that the N-imides R3NNX are always clearly higher in energy than the imine isomers R2NN(X)R. In the case of phosphane imides R3PNX and the isomers R2PN(X)R, the substituent R plays a critical role in determining their relative stabilities. When R is hydrogen or phenyl group, R3PNX are always higher in energy than R2PN(X)R but the former are more stable than the latter when R = Cl. Interestingly, the Me3PNX and Me2PN(X)Me are quite close in energy. The energy decomposition analysis suggests that the P–N bond in the phosphane imides R3PNX (R = H, Cl, Me, Ph; X = H, F, Cl) should be described in terms of an electron-sharing single bond between two charged fragments R3P+-NX− that is supported by (R3P)+←(NX)− π-backdonation. The π-bond contributes 14–21% of the total orbital interactions while the σ-bond provides 60–68% of ΔEorb. [ABSTRACT FROM AUTHOR]

Abstract: The reaction pathway for the rupture of the carbon–carbon double bond of C2F4 has been calculated with ab initio methods at the CASSCF(8,8)+NEVPT2/aug‐cc‐pVTZ and CCSD(T)/aug‐cc‐pVTZ levels and with density functional theory using M06‐L and M06‐2X functionals in conjunction with aug‐cc‐pVTZ basis sets. The calculations suggest that the bond dissociation pathway proceeds by a nonlinear reaction course without an activation barrier yielding the CF2 fragments in the (1A1) ground state. A bonding analysis indicates that there is a continuous change in the electronic structure of the CF2 fragments during the elongation of the C–C distance from a (3B1) excited state at the equilibrium geometry of C2F4 to the (1A1) ground state. EDA‐NOCV calculations suggest that the carbon–carbon interactions in C2F4 at equilibrium distance and longer C–C values up to ≈1.60 Å are best described in terms of electron‐sharing bonding between the CF2 fragments in the (3B1) excited state. At longer distances, the situation changes toward dative bonding between CF2 fragments in the (1A1) ground state. [ABSTRACT FROM AUTHOR]

Abstract: The reaction pathway for the rupture of the carbon–carbon double bond of C2F4 has been calculated with ab initio methods at the CASSCF(8,8)+NEVPT2/aug‐cc‐pVTZ and CCSD(T)/aug‐cc‐pVTZ levels and with density functional theory using M06‐L and M06‐2X functionals in conjunction with aug‐cc‐pVTZ basis sets. The calculations suggest that the bond dissociation pathway proceeds by a nonlinear reaction course without an activation barrier yielding the CF2 fragments in the (1A1) ground state. A bonding analysis indicates that there is a continuous change in the electronic structure of the CF2 fragments during the elongation of the C–C distance from a (3B1) excited state at the equilibrium geometry of C2F4 to the (1A1) ground state. EDA‐NOCV calculations suggest that the carbon–carbon interactions in C2F4 at equilibrium distance and longer C–C values up to ≈1.60 Å are best described in terms of electron‐sharing bonding between the CF2 fragments in the (3B1) excited state. At longer distances, the situation changes toward dative bonding between CF2 fragments in the (1A1) ground state. [ABSTRACT FROM AUTHOR]

Abstract: Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6H4)(IPr)] and [(IPr)(C6H4)2(IPr)] (4 and 5, respectively; IPr=C{N(2,6‐iPr2C6H3)}2CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br2C6H4 and 4,4′‐Br2(C6H4)2 with IPr (1), [(IPr)(C6H4)(IPr)](Br)2 (2) and [(IPr)(C6H4)2(IPr)](Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet−triplet energy gap ΔES−T of 10.7 kcal mol−1, whereas 4 features more quinoidal character with a rather large ΔES−T of 25.6 kcal mol−1. In view of the low ΔES−T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory using BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules. The calculated energies suggest that the N-oxides R3NO with R = F, Cl are lower in energy than the amine isomers R2NOR, but the latter form is more stable than the N-oxides when R = H, Me, Ph. In contrast, the phosphane oxides R3PO are always more stable than the phosphanyl isomers R2NOR except for the parent system with R = H, where the two isomers are close in energy. The energy decomposition analysis suggests that the best description of the N–O bond in N-oxides R3NO depends on the nature of the substituent R. The halogen systems F3NO and Cl3NO and the triphenyl species Ph3NO possess dative bonds R3N→O, which are enhanced by R3N←O π backdonation. The contribution of the π backdonation is only 10% of the total orbital interactions ΔEorb in Ph3NO, but it amounts to ∼22% of ΔEorb in F3NO and Cl3NO. The N–O bonds H3NO and Me3NO are better described in terms of electron-sharing single bonds between charged fragments R3N+–O−, which are supported by modest R3N+←O−π backdonation that comprise 13–16% of ΔEorb. In contrast, all phosphane oxides R3PO are best depicted with electron-sharing single bonds between charged fragments R3P+–O−, which are significantly supported by R3P+←O−π backdonation contributing 22–32% of ΔEorb. [ABSTRACT FROM AUTHOR]

The energy decomposition analysis (EDA) is a powerful method for a quantitative interpretation of chemical bonds in terms of three major components. The instantaneous interaction energy ΔEint between two fragments A and B in a molecule A–B is partitioned in three terms, namely (1) the quasiclassical electrostatic interaction ΔEelstat between the fragments; (2) the repulsive exchange (Pauli) interaction ΔEPauli between electrons of the two fragments having the same spin, and (3) the orbital (covalent) interaction ΔEorb which comes from the orbital relaxation and the orbital mixing between the fragments. The latter term can be decomposed into contributions of orbitals with different symmetry which makes it possible to distinguish between σ, π, and δ bonding. After a short introduction into the theoretical background of the EDA we present illustrative examples of main group and transition metal chemistry. The results show that the EDA terms can be interpreted in chemically meaningful way thus providing a bridge between quantum chemical calculations and heuristic bonding models of traditional chemistry. The extension to the EDA–Natural Orbitals for Chemical Valence (NOCV) method makes it possible to breakdown the orbital term ΔEorb into pairwise orbital contributions of the interacting fragments. The method provides a bridge between MO correlations diagrams and pairwise orbital interactions, which have been shown in the past to correlate with the structures and reactivities of molecules. There is a link between frontier orbital theory and orbital symmetry rules and the quantitative charge‐ and energy partitioning scheme that is provided by the EDA–NOCV terms. The strength of the pairwise orbital interactions can quantitatively be estimated and the associated change in the electronic structure can be visualized by plotting the deformation densities. This article is categorized under: Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Computer and Information Science > Visualization [ABSTRACT FROM AUTHOR]