Bonding in the ground state and excited states of copper-alkene complexes

The ground state and ¹B₂ excited state of Cu(C₂H₄)⁺ and of CuX(C₂H₄) (X  F, Cl) have been investigated by the Hartree-Fock-Slater (HFS) method. The main metal-ligand interactions in the ground state are ethene π → Cu 4s donation and Cu 3d_π → ethene π^* backdonation, which have comparable contributions to the metal-ligand bond strength. The excitation of CuX(C₂H₄) does not involve an alkene π → metal charge transfer (LMCT), but instead is metal 3d → alkene π^* charge transfer (MLCT) in character. The implications for the photochemistry of olefin-copper(I) complexes are discussed.     

Two Push–Pull Channels Enhance the Dinitrogen Activation by Borylene Compounds

Recently, Braunschweig et al. found that borylene (CAAC)DurB, in which CAAC is a cyclic alkyl(amino) carbene and Dur refers to 2,3,5,6-tetramethylphenyl, can bind and activate N₂, and the resulting [(CAAC)DurB]₂N₂ is of a bent BNNB core. The N₂ ligand in transition metal complexes is generally linear, so herein, the bonding nature of both terminal end-on and end-on bridging borylene-N₂ complexes is investigated with valence bond (VB) theory. In the terminal end-on (CAAC)HBN₂ the bonding follows the mechanism in transition metals with a σ donation and a π back-donation, but in the end-on bridging borylene-N₂ complex, the σ donation comes from the π orbitals of N₂, and thus, there are two opposite and perpendicular push–pull channels. It is the push–pull interaction that governs the enhanced activation of N₂ and the BNNB bent geometry. It is expected that the substituents bonded to B can modulate the bent angle and the strength of the push–pull interaction. Indeed, (CAAC)FB exhibits enhanced catalytic capacity for the activation of N₂.     

13C NMR Spectroscopy of N-Heterocyclic Carbenes Can Selectively Probe σ Donation in Gold(I) Complexes

The Dewar–Chatt–Duncanson (DCD) model provides a successful theoretical framework to describe the nature of the chemical bond in transition-metal compounds and is especially useful in structural chemistry and catalysis. However, how to actually measure its constituents (substrate-to-metal donation and metal-to-substrate back-donation) is yet uncertain. Recently, we demonstrated that the DCD components can be neatly disentangled and the π back-donation component put in strict correlation with some experimental observables. In the present work we make a further crucial step forward, showing that, in a large set of charged and neutral N-heterocyclic carbene complexes of gold(I), a specific component of the NMR chemical shift tensor of the carbenic carbon provides a selective measure of the σ donation. This work opens the possibility of 1) to characterize unambiguously the electronic structure of a metal fragment (LAu(I)^n+/0 in this case) by actually measuring its σ-withdrawing ability, 2) to quickly establish a comparative trend for the ligand trans effect, and 3) to achieve a more rigorous control of the ligand electronic effect, which is a key aspect for the design of new catalysts and metal complexes.     

Structure and Reactivity of a Cobalt(I) Phthalaldehyde Complex with Both σ‐ and π‐Bonded Aldehyde Groups

A bidentate phthalaldehyde ligand with both σ and π coordination of the aldehyde groups is found in [(C₅Me₅)Co{(C(O)H)₂C₆H₄}] (structure depicted). This complex is the “resting state” of the catalyst in the ring closure of the dialdehyde to give the lactone. Interchange of coordination modes occurs with a barrier of 70 kJ mol⁻¹ at 35°C. Investigation of other Co^I chelate complexes with a single aldehyde group shows that the coordination mode of the aldehyde is dictated by the nature of the bonding of the other ligating group.     

Stabilities,Electronic Structures,and Bonding Properties of Iron Complexes (E1E2)Fe(CO)2(CNArTripp2)2 (E1E2=BF,CO, N2, CN−, or NO+)**

The coordination of 10-electron diatomic ligands (BF, CO N₂) to iron complexes Fe(CO)₂(CNAr^Tripp2)₂ [Ar^Tripp2=2,6-(2,4,6-(iso-propyl)₃C₆H₂)₂C₆H₃] have been realized in experiments very recently (Science, 2019 , 363, 1203–1205). Herein, the stability, electronic structures, and bonding properties of (E₁E₂)Fe-(CO)₂(CNAr^Tripp2)₂ (E₁E₂=BF, CO, N₂, CN⁻, NO⁺) were studied using density functional (DFT) calculations. The ground state of all those molecules is singlet and the calculated geometries are in excellent agreement with the experimental values. The natural bond orbital analysis revealed that Fe is negatively charged while E₁ possesses positive charges. By employing the energy decomposition analysis, the bonding nature of the E₂E₁–Fe(CO)₂(CNAr^Tripp2)₂ bond was disclosed to be the classic dative bond E₂E₁→Fe(CO)₂(CNAr^Tripp2)₂ rather than the electron-sharing double bond. More interestingly, the bonding strength between BF and Fe(CO)₂(CNAr^Tripp2)₂ is much stronger than that between CO (or N₂) and Fe(CO)₂(CNAr^Tripp2)₂, which is ascribed to the better σ-donation and π back-donations. However, the orbital interactions in CN⁻→Fe(CO)₂(CNAr^Tripp2)₂ and NO⁺→Fe(CO)₂(CNAr^Tripp2)₂ mainly come from σ-donation and π back-donation, respectively. The different contributions from σ donation and π donation for different ligands can be well explained by using the energy levels of E₁E₂ and Fe(CO)₂(CNAr^Tripp2)₂ fragments.     

Study of the nature of the interaction of CH3CN with Lewis acids

The unperturbed ν⁰ (CN) stretching frequencies of CH₃CN in 18 complexes with Lewis acids are correlated with ν (CN) frequencies in cobalticyanides and with cation parameters which measure σ and π interactions. The results indicate that the nature of the interaction is predominantly of the σ donor—acceptor type wib no detectable π back-donation from the cation to the CN ligand.     

Construction and applications of symmetrized valence bond wave functions

A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O₃⁻, O₃, O₃⁺, and C₃⁻ are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O₃⁻, C₃H₅, and C₃⁻ are calculated with an optimized symmetrized valence bond wave function in the σ–π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S₃ are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X ¹A₁ of S₃, while the excited state 1 ¹B₂ is essentially composed of the dipole structures, and the 1 ³B₂ excited state is comprised from a triplet-biradical structure. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 1–7, 1998     

Formation and infrared spectroscopic characterization of carbon suboxide complexes TM-η1-C3O2 and the inserted ketenylidene complexes OCTMCCO (TM=Cu,Ag, Au) in solid neon

The reactions of coinage metal atoms Cu, Ag and Au with carbon suboxide (C₃O₂) are studied by matrix isolation infrared spectroscopy. The weakly bound complexes TM-η¹-C₃O₂ (TM=Cu, Ag, Au), in which the carbon suboxide ligand binds to the metal center in the monohapto fashion are formed as initial reaction products. The complexes subsequently isomerize to the inserted products OCTMCCO upon visible light (λ = 400–500 nm) excitation. The analysis of the electronic structure using modern quantum chemistry methods suggests that the linear OCTMCCO complexes are best described by the bonding interactions between the TM⁺ cation in the electronic singlet ground state and the [OC…CCO]⁻ ligands in the doublet state forming two TM⁺ ← ligands σ donation and two TM⁺ → ligands π backdonation bonding components. In addition, the CuCCO, AgCCO and AuCCO complexes are also formed, which are predicted to be bent.     

Boron-Made N2: Realization of a B≡B Triple Bond in the B2Al3− Cluster

Until now, all B≡B triple bonds have been achieved by adopting two ligands in the L→B≡B←L manner. Herein, we report an alternative route of designing the B≡B bonds based on the assumption that by acquiring two extra electrons, an element with the atomic number Z can have properties similar to those of the element with the atomic number Z+2. Specifically, we show that due to the electron donation from Al to B, the negatively charged B≡B kernel in the B₂Al₃⁻ cluster mimics a triple N≡N bond. Comprehensive computational searches reveal that the global minimum structure of B₂Al₃⁻ exhibits a direct B–B distance of 1.553 Å, and its calculated electron vertical detachment energies are in excellent agreement with the corresponding values of the experimental photoelectron spectrum. Chemical bonding analysis revealed one σ and two π bonds between the two B atoms, thus confirming a classical textbook B≡B triple bond, similar to that of N₂.     

The Reaction of the Betain‐like Compound O2C2(PPh3)2 with [(cod)PtI2] – Crystal Structure of the Salt (HC{PPh3}2)[(η3‐C8H11)PtI2]

The reaction of the betain‐like compound O₂C₂(PPh₃)₂ ( 1 ) with [(cod)PtX₂] in THF solution gives the salt‐like compounds (HC{PPh₃}₂)[(η³‐C₈H₁₁)PtX₂] ( 3 , X = I; 4 , X = Cl) in about quantitative yields. The new η³‐bonded C₈H₁₁ ligand is the result of a proton transfer from the coordinated cod ligand to 1 with subsequent release of CO₂. The X‐ray analysis of 3 shows the presence of two isomers in a 60:40 ratio, which differ in the bonding of the C₈H₁₁ ligand. 3 crystallizes in the triclinic space group with the unit cell dimensions a = 1091.7(1), b = 1141.5(1), c = 1649.4(2) pm; α = 80.34(1)°, β = 83.62(1)°, γ = 89.03(1)°, V = 2013.7(4)·10⁶ pm³, Z = 2.     

Double Concave Cesium Encapsulation by Two Charged Sumanenyl Bowls

The controlled reaction of Na and Cs, two alkali metals of different ionic sizes and binding abilities, with sumanene (C₂₁H₁₂) affords a novel type of organometallic sandwich [Cs(C₂₁H₁₁⁻)₂]⁻, which crystallized as a solvent‐separated ion pair with a [Na(18‐crown‐6)(THF)₂]⁺ cation (where THF=tetrahydrofuran). The unprecedented double concave encapsulation of a metal ion by two bowl‐shaped sumanenyl anions in [Cs(C₂₁H₁₁⁻)₂]⁻ was revealed crystallographically. Evaluation of bonding and energetics of the remarkable product was accomplished computationally (B2PLYP‐D/TZVP/ZORA), providing insights into its formation.     

The Chemical Bond in C2

Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for H_nCCH_n (n=0–3) and N₂. The quadratic force constants and the stretching potentials of H_nCCH_n have been calculated at the CASSCF/cc‐pVTZ level. The bond dissociation energies of the C?C bonds of C₂ and HC≡CH were computed using explicitly correlated CASPT2‐F12/cc‐pVTZ‐F12 wave functions. The bond dissociation energies and the force constants suggest that C₂ has a weaker C?C bond than acetylene. The analysis of the CASSCF wavefunctions in conjunction with the effective bond orders of the multiple bonds shows that there are four bonding components in C₂, while there are only three in acetylene and in N₂. The bonding components in C₂ consist of two weakly bonding σ bonds and two electron‐sharing π bonds. The bonding situation in C₂ can be described with the σ bonds in Be₂ that are enforced by two π bonds. There is no single Lewis structure that adequately depicts the bonding situation in C₂. The assignment of quadruple bonding in C₂ is misleading, because the bond is weaker than the triple bond in HC≡CH.     

Barium as Honorary Transition Metal in Action: Experimental and Theoretical Study of Ba(CO)+ and Ba(CO)−

Ba(CO)⁺ and Ba(CO)^? have been produced and isolated in a low‐temperature neon matrix. The observed C?O stretching wavenumber for Ba(CO)⁺ of 1911.2 cm^?1 is the most red‐shifted value measured for any metal carbonyl cations, indicating strong π backdonation of electron density from Ba⁺ to CO. Quantum chemical calculations indicate that Ba(CO)⁺ has a ²Π reference state, which correlates with the ²D(5d¹) excited state of Ba⁺ that comprises significant Ba⁺(5d_π¹)→CO(π* LUMO) backbonding, letting the Ba(CO)⁺ complex behave like a conventional transition‐metal carbonyl. A bonding analysis shows that the π backdonation in Ba(CO)⁺ is much stronger than the Ba⁺(5d_σ/6s)←CO(HOMO) σ donation. The Ba⁺ cation in the ²D(5d¹) excited state is a donor rather than an acceptor. Covalent bonding in the radical anion Ba(CO)^? takes place mainly through Ba(5d_π)←CO^?(π* SOMO) π donation and Ba(5d_σ/6s)←CO^?(HOMO) σ donation. The most important valence functions at barium in Ba(CO)⁺ cation and Ba(CO)^? anion are the 5d orbitals.     

Multiple Bonding Between Group 3 Metals and Fe(CO)3−

Heteronuclear Group 3 metal/iron carbonyl anion complexes ScFe(CO)₃⁻, YFe(CO)₃⁻, and LaFe(CO)₃⁻ are prepared in the gas phase and studied by mass-selective infrared (IR) photodissociation spectroscopy as well as quantum-chemical calculations. All three anion complexes are characterized to have a metal–metal-bonded C_3v equilibrium geometry with all three carbonyl ligands bonded to the iron center and a closed-shell singlet electronic ground state. Bonding analyses reveal that there are multiple bonding interactions between the bare group-3 elements and the Fe(CO)₃⁻ fragment. Besides one covalent electron-sharing metal–metal σ bond and two dative π bonds from Fe to the Group 3 metal, there is additional multicenter covalent bonding with the Group 3 atom bonded to Fe and the carbon atoms.     

Condensation of Rare Earth Pyrazolates: The Dimeric,Trimeric and Polymeric Units [Gd2(Pz)6(PzH)4], [Nd3(Pz)9(PzH)2] and $^{1}_{\infty}\rm [Eu(Pz)_{2}(PzH)_{2}]$

Solvent free high‐temperature oxidations of rare earth metals with the heterocycle pyrazole as well as in low to non‐coordinating solvents were investigated to isolate intermediate stages between monomeric and polymeric pyrazolates of the lanthanides. Reaction conditions were tuned according to simultaneous DTA/TG and temperature dependent X‐ray powder diffraction experiments on known monomeric and polymeric pyrazolates, that gave rise to the idea that further structure intermediates could be isolated. Reactions in 1,2,3,4‐tetrahydroquinoline gave the dimeric complex [Gd₂(Pz)₆(PzH)₄](PzH)(Tech) ( 1 ) as well as the triangular complex [Nd₃(Pz)₉(PzH)₂](PzH)(Tech)₂ ( 2 ). The solvent free melt synthesis resulted in a new polymeric form of ( 3 ) (pyrazole, PzH = C₃H₃NNH; pyrazolate anion, Pz^? = C₃H₃NN^?; 1,2,3,4‐tetrahydroquinoline, Tech = C₉H₁₃N). All three compounds contain coordinating pyrazolate amide groups and pyrazole molecules the latter decreasing in numbers upon condensation of the building units. According to simultaneous DTA/TG/MS investigations the condensation process can be identified with the release of pyrazole molecules. 1 consists of dimeric molecules containing trivalent gadolinium with a C.N. of eight. The two gadolinium atoms show different coordination polyhedra. Only σ coordination and bridging is found for 1 . 2 consists of trimeric molecules containing trivalent neodymium. The neodymium atoms also exhibit different coordination polyhedra with C.N.s of eight and nine. Both π and σ coordination is found for 2 , the π coordinating pyrazolate ligands acting as lids of the triangular units. Topological analysis of the electron localization function (ELF) for 2 calculated at the scalar‐relativistic DFT level reveals only weakly covalent π donor η⁵‐Pz–Nd interactions compared to the stronger covalent σ donor Pz–Nd interactions. The topological analysis of both, the ELF and the electron density reveals no significant differences of the respective charges of the Nd atoms. 3 exhibits a one‐dimensional chain structure with Eu^II and a C.N of ten. It can thus be addressed the β form of the referring formula with a new arrangement of the coordinating ligands. Like the α form 3 shows σ and π coordination of pyrazole and pyrazolate ligands. Simultaneous DTA/TG analysis reveals that the low‐temperature α form shows a phase transition into the β form between 110 °C and 130 °C. The three compounds were investigated by low‐temperature single crystal X‐ray analysis, Mid IR and Far IR spectroscopy.     

Isolation of Elusive Phosphinidene-Chlorotetrylenes: The Heavier Cyanogen Chloride Analogues

The elusive phosphinidene-chlorotetrylenes, [PGeCl] and [PSiCl] have been stabilized by the hetero-bileptic cyclic alkyl(amino) carbene (cAAC), N-heterocyclic carbene (NHC) ligands, and isolated in the solid state at room temperature as the first neutral monomeric species of this class with the general formulae (L)P-ECl(L′) (E=Ge, 3 a – 3 c ; E=Si, 6 ; L=cAAC; L′=NHC). Compounds 3 a – 3 c have been synthesized by the reaction of cAAC-supported potassium phosphinidenides [cAAC=PK(THF)_x]_n ( 1 a – 1 c ) with the adduct NHC:→GeCl₂ ( 2 ). Similarly, compound 6 has been synthesized via reaction of 1 a with NHC:→SiCl₂ adduct ( 4 ). Compounds 3 a – 3 c , and 6 have been structurally characterized by single-crystal X-ray diffraction, NMR spectroscopy and mass spectrometric analysis. DFT calculations revealed that the heteroatom P in 3 bears two lone pairs; the non-bonding pair with 67.8 % of s- and 32 % of p character, whereas the other lone pair is involved in π backdonation to the C_cAAC-N π* of cAAC. The Ge atom in 3 contains a lone pair with 80 % of s character, and slightly involved in the π backdonation to C_NHC. EDA-NOCV analyses showed that two charged doublet fragments {(cAAC)(NHC)}⁺, and {PGeCl}⁻ prefer to form one covalent electron-sharing σ bond, one dative σ bond, one dative π bond, and a charge polarized weak π bond. The covalent electron-sharing σ bond contributes to the major stabilization energy to the total orbital interaction energy of 3 , enabling the first successful isolations of this class of compounds ( 3 , 6 ) in the laboratory.     

An Isolable Radical Anion Featuring a 2-Center-3-Electron π-Bond without a Clearly Defined σ-Bond

Featuring an extra electron in the π* antibonding orbital, species with a 2-center-3-electron (2c3e) π bond without an underlying σ bond are scarcely known. Herein, we report the synthesis, isolation and characterization of a radical anion salt [K(18-C-6)]⁺{[(HCNDipp)₂Si]₂P₂}⋅⁻ (i.e. [K(18-C-6)]⁺ 3 ⋅⁻) (18-C-6=18-crown-6, Dipp=2,6-diisopropylphenyl), in which 3 ⋅⁻ features a perfectly planar Si₂P₂ four-membered ring. This species represents the first example of a Si- and P-containing analog of a bicyclo[1.1.0]butane radical anion. The unusual bonding motif of 3 ⋅⁻ was thoroughly investigated via X-ray diffraction crystallography, electron paramagnetic resonance spectroscopy (EPR), and calculations by density functional theory (DFT), which collectively unveiled the existence of a 2c3e π bond between the bridgehead P atoms and no clearly defined supporting P−P σ bond.     

Isomorphous crystals of strychninium 4‐chloro­benzoate and strychninium 4‐nitro­benzoate

In strychninium 4‐chloro­benzoate, C₂₁H₂₃N₂O₂⁺·C₇H₄ClO₂⁻, (I), and strychninium 4‐nitro­benzoate, C₂₁H₂₃N₂O₂⁺·C₇H₄NO₄⁻, (II), the strychninium cations form pillars stabilized by C—H⋯O and C—H⋯π hydrogen bonds. Channels between the pillars are occupied by anions linked to one another by C—H⋯π hydrogen bonds. The cations and anions are linked by ionic N—H⁺⋯O⁻ and C—H⋯X hydrogen bonds, where X = O, π and Cl in (I), and O and π in (II).     

The Quadruple Bonding in C2 Reproduces the Properties of the Molecule

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C₂ is bonded by a quadruple bond that can be distinctly characterized by valence‐bond (VB) calculations. We demonstrate that the quadruply‐bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply‐bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ‐type bond, the bond strength of which is estimated as 17–21 kcal mol^?1. Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol^?1, respectively, above the quadruply‐bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply‐bonded model is underscored by “predicting” the properties of the ³ state. C₂’s very high reactivity is rooted in its fourth weak bond. Thus, carbon and first‐row main elements are open to quadruple bonding!