First and second proton affinities of carbon bases

N-Heterocyclic Carbenes: A Benchmark Study on their Singlet-Triplet Energy Gap as a Critical Molecular Descriptor

N-heterocyclic carbenes (NHCs) are used extensively in modern chemistry and materials science. The in-depth understanding of their electronic structure and their metal complexes remains an important topic of research and of experimental and theoretical interest. Herein, the adiabatic singlet-triplet gap as a superior, quantifiable critical descriptor, sensitive to the nature and the structural diversity of the NHCs, for a successful rationalization of experimental observations and computationally extracted trends is established. The choice is supported by a benchmark study on the electronic structures of NHCs, using high-level ab initio methods, that is, complete active space self-consistent field, n-electron valence second-order perturbation theory, multireference configuration interaction + singles + doubles, and domain-based local pair natural orbital-coupled cluster method with single-, double-, and perturbative triple excitations along with density functional theory methods such as BP86, M06, and M06-L, B3LYP, PBE0, TPSSh, CAM-B3LYP, and B2PLYP. In contrast to the adiabatic singlet-triplet (S-T) gap preferred as descriptor, the highest occupied molecular orbital-lowest unoccupied molecular orbital gap or the S-T vertical gap that has been used in the past occasionally leads to controversial results; some of these are critically discussed below. Extrapolation of these ideas to a group of copper-NHC complexes is also described.

The strongest dative bond in main-group compounds. Theoretical study of OAeF- (Ae = Be-Ba)

Quantum chemical calculations of the anions OAeF- (Ae = Be-Ba) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 with various basis sets. The equilibrium structures have linear geometries for Ae = Be and Mg but they are strongly bent for Ae = Sr and Ba while the calcium species has a quasi-linear structure with a very low bending potential. The calculated bond dissociation energies suggest a record-high BDE of De = 144.08 kcal mol-1 for OBeF- at the CCSD(T)/def2-TZVPP level, which is the strongest BDE for a dative bond that has been found so far. The BDE of the heavier homologues have a continuously decreasing order for Ae with Be > Mg (113.01 kcal mol-1) > Ca (84.06 kcal mol-1) > Sr (72.06 kcal mol-1) > Ba (60.00 kcal mol-1). The calculation of the charge distribution reveals a significant charge donation OAe ← F- with a declining sequence for the heavier atoms Ae. The oxygen atom in OAeF- carries always a higher partial charge than the fluorine atom, which contradicts the standard electronegativities of the atoms. The surprising partial charges are explained with the bonding situation of the atoms in the actual electronic structure. The bonding analysis of the OAe-F- bonds using the EDA-NOCV method shows that the bonds have much more electrostatic character than the Ae-F- bonds in the diatomic anions. This finding is supported by the results of the LED partitioning approach. The dative interactions have three major and one minor component. The assignment of a quadruple bond for the heavier species with Ae = Ca, Sr, Ba is not reasonable. The driving force for the bent geometries is the accumulation of electronic charge in the lone-pair region at the Ae atoms, which enhances the electrostatic attraction with the other atoms. An adequate description of the bonding situation is given by the formula O--Ae+ ← F-.

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Carbones - A Classification on the Magnetic Criterion

Carbones (carbodiphosphoranes, bent allenes and chalcogen-stabilized carbones) bear the same resonance contributor X+ -C2- -Y+ (X+ , Y+ =PR3 + , CR2 + , SR2 + , SeR2 + , S+ R2 =NR) and exhibit unique bonding and donating properties at the central carbon atom. A classification is given on basis of both the geometry and the magnetic properties (13 C chemical shift of the central carbon atom and the spatial magnetic properties, through-space NMR shieldings (TS NMRSs), actually the anisotropy effect or the ring current effect of aromatic species). TS NMRS values have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The synergy of geometry (linear or bent, orthogonal or twisted structures) and NMR characteristics (extend of the high field shift of the central carbon atom, anisotropy effect of the allene-like C=C double bonds or the ball-like anisotropy effect of carbone-like central carbon atom) provides a comprehensive picture of the dominating resonance contributor.

Diversification of the Carbodicarbene Class by Embedding an Anionic Component in its Scaffold

Carbodicarbene (CDC) has become an emerging ligand in many fields due to its strong σ-donating ability. Collapse

Key Words

• Anionic carbodicarbene
• carbone
• dative bonding model
• geminal bimetallic complexes

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MESH Headings Collapse

Grants

• NSTC 112-2123-M-001-007 National Science Council
• AS-GC-111-M04 Academia Sinica

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Affiliation(s)

Cheng-Han Yu Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201
Ka-Chun Au-Yeung Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201 Corporate R&D Center, LCY Chemical Corporation, Kaohsiung, Taiwan (R.O.C
Ruiqin Liu School of Chemistry and Molecular Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, China
Chao-Hsien Lee Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201
Dandan Jiang School of Chemistry and Molecular Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, China
Bamlaku Semagne Aweke Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201 Department of Applied Chemistry, National Yang Ming Chiao Tung University, Hsinchu, Taiwan (R.O.C Sustainable Chemical Science and Technology, Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan (R.O.C
Chia-Hung Wu Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201 Department of Chemistry, National Taiwan University, Taipei, Taiwan (R.O.C
Yu-Jou Wang Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201 Department of Chemistry, National Taiwan University, Taipei, Taiwan (R.O.C
Ting-Hsuan Wang Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201
Kien Voon Kong Department of Chemistry, National Taiwan University, Taipei, Taiwan (R.O.C
Glenn P A Yap Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States
Wen-Ching Chen Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201
Gernot Frenking School of Chemistry and Molecular Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, China Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35043, Marburg, Germany
Lili Zhao School of Chemistry and Molecular Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, China
Tiow-Gan Ong Institute of chemistry, Academia Sinica, Taipei, Taiwan (R.O.C., 115201 Department of Chemistry, National Taiwan University, Taipei, Taiwan (R.O.C Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan (R.O.C

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Stabilization of Cyclic C4 by Four Donor Ligands: A Theoretical Study of (L)4C4 (L = Carbene)

Quantum chemical studies using density functional theory were carried out for the (L)4C4 complexes with L = cAAC, DAC, NHC, SNHC, MIC1, and MIC2. The results show that the title complexes are highly stable with respect to dissociation, (L)4C4 → C4 + 4L. However, their stability with respect to (L)4C4 → 2(L)2C2 is crucial for the assessment of their experimental viability. The (L)4C4 complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L)2C2 units. In contrast, the other (L)4C4 complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L)4C4 → 2(L)2C2. The computed adiabatic ionization potentials of (L)4C4 complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1)4C4 and (MIC2)4C4 will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L)4C4 complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L)3C4-CL bond can be best described as a typical electron-sharing double bond with a strong σ-bond and a weaker π-bond. Therefore, the core bonding pictures in the title complexes resemble a [4]radialene. Larger substituents at the carbene ligands enhance the stability of the complexes (L)4C4 against dissociation.

Binuclear Macrocyclic Silver(I) Complex of a Bis(carbone) Pincer Ligand: Synthesis and Application as a Carbone-Transfer Agent

A facile synthesis of a binuclear AgI complex 2 of a bis(carbone) ligand L and its application as a carbone-transfer agent for the generation of other transition-metal complexes of AuI (3), NiII (4), and PdII (5) is presented. Complex 2 was synthesized through multiple synthetic routes under mild reaction conditions using the tetracationic [LH4][OTf·Cl]2 precursor salt, the dicationic [LH2][OTf]2 ylide salt, and the free ligand L. The first two synthesis routes require no prior isolation of the air-, moisture-, and temperature-sensitive free ligand L, thus affording complex 2 with high yield and purity. Multinuclear NMR techniques, high-resolution mass spectrometry, and single-crystal X-ray diffraction analysis confirmed the identity of complex 2 as a binuclear AgI complex of L with a molecular formula of [L2Ag2][OTf]2 and a 16-membered-ring metallomacrocyclic structure. During the transmetalation reaction with AuI, the binuclear nature of complex 2 remains intact to give analogous complex 3 ([L2Au2][OTf]2). However, the dimeric structure was disrupted upon the carbone-transfer reaction with NiII and PdII, yielding mononuclear C-N-C pincer-type complexes 4 ([LNiCl][OTf]) and 5 ([LPdCl][OTf]), respectively. These results demonstrated the versatile use of complex 2 as a carbone-transfer agent to other transition metals regardless of the type or size of the metals or the geometry they prefer.

Formation and infrared spectroscopic characterization of carbon suboxide complexes TM-η1 -C3 O2 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 (C₃ O₂ ) are studied by matrix isolation infrared spectroscopy. The weakly bound complexes TM-η¹ -C₃ O₂ (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.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Stability and bonding of carbon(0)-iron-N2 complexes relevant to nitrogenase co-factor: EDA-NOCV analyses

The factors/structural features which are responsible for the binding, activation and reduction of N₂ to NH₃ by FeMoco of nitrogenase have not been completely understood well. Several relevant model complexes by Holland et al. and Peters et al. have been synthesized, characterized and studied by theoretical calculations. For a matter of fact, those complexes are much different than real active N₂ -binding Fe-sites of FeMoco, which possesses a central C(4-) ion having an eight valence electrons as an μ₆ -bridge. Here, a series of [(S₃ C(0))Fe(II/I/0)-N₂ ]^n- complexes in different charged/spin states containing a coordinated σ- and π-donor C(0)-atom which possesses eight outer shell electrons [carbone, (Ph₃ P)₂ C(0); Ph₃ P→C(0)←PPh₃ ] and three S-donor sites (i.e. ^- S-Ar), have been studied by DFT, QTAIM, and EDA-NOCV calculations. The effect of the weak field ligand on Fe-centres and the subsequent N₂ -binding has been studied by EDA-NOCV analysis. The role of the oxidation state of Fe and N₂ -binding in different charged and spin states of the complex have been investigated by EDA-NOCV analyses. The intrinsic interaction energies of the Fe-N₂ bond are in the range from -42/-35 to -67 kcal/mol in their corresponding ground states. The S₃ C(0) donor set is argued here to be closer to the actual coordination environment of one of the six Fe-centres of nitrogenase. In comparison, the captivating model complexes reported by Holland et al. and Peter et al. possess a stronger π-acceptor C-ring (S₂ C_ring donor, π-C donor) and stronger donor set like CP₃ (σ-C donor) ligands, respectively.

Gauging Radical Stabilization with Carbenes

Carbenes, including N-heterocyclic carbene (NHC) ligands, are used extensively to stabilize open-shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical-stabilizing C-donor ligands. With the large variety of C-donor ligands experimentally investigated and their electronic properties established, we report herein their radical-stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π-donation to- and π-donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π-donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group- and transition metal carbene complexes, and the quantification of redox non-innocence.

A Bis-(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi-Metallic Configurations

Carbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis-(carbone) pincer framework with a well-defined solid-state structure. This bis-(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au-Pd-Au, Au-Ni-Au, and Cu-Ni-Cu configurations. Sophisticated quantum-theoretical analyses found that the metal-metal interactions are too weak to play a significant role in upholding these multi-metallic configurations; rather, the four lone pairs of electrons within the bis-(carbone) framework are the main contributors to the stability of the complexes.

A cyclic divalent N(I) species isoelectronic to carbodiphosphoranes

The formation of a rare type of diphosphazenium cation is described. Its synthesis features a unique oxidative dealkylation of an iminophosphorane-phosphole by a silver(I) salt. DFT study of this compound reveals the low valent character of the N(I) center.

Crystal structure of bis[hydrido-hexaphenylcarbodiphosphoran][tetra-trifluoromethyl-(μ-diiodo)-diplatinat]

C86H78F12I2O2P4Pt2, triclinic, P 1 ‾ $P\overline{1}$ (no. 2), a = 11.520(1) Å, b = 12.196(1) Å, c = 15.382(2) Å, α = 73.20(1)°, β = 75.18(1)°, γ = 86.81(1)°, V = 2000.1(4) Å3, Z = 1, R gt (F) = 0.0631, wR ref (F 2) = 0.1759, T = 193(2) K.

Transition Metal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L_nM-CE_x fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C₁ reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Entering new chemical space with isolable complexes of single, zero-valent silicon and germanium atoms

Monatomic zero-valent silicon and germanium complexes (silylones and germylones), stabilised by neutral donating ligands, emerged only recently as a new class of low-valent group 14 element compounds. Featuring four valence electrons in the form of two lone pairs at a single site, silylones and germylones represent a molecular resting state of single Si and Ge atoms, which are typically only observed at high temperature in the gas phase or in interstellar matter. These species are capable of transferring single Si and Ge atoms to unsaturated substrates and acting as building blocks for novel group 14 species. After introducing this type of compound and the examples known to date, this feature article highlights some chelating bis N-heterocyclic carbene (bis(NHC)) and bis N-heterocyclic silylene (bis(NHSi)) supported Si⁰ and Ge⁰ complexes, for which a range of unprecedented reactivity has been discovered. The characteristic behaviour of these silylones and germylones discussed here consists of (i) coordination to Lewis acids, (ii) oxidation with elemental chalcogens, (iii) bond activation of common organic substrates and inert small molecules; and (iv) homocoupling of the Si⁰ and Ge⁰ centres. This wealth of reactivity has opened the door to a series of Si and Ge compounds, which would be otherwise difficult to realise.

Reactivity of cyano- and isothiocyanatoborylenes: metal coordination, one-electron oxidation and boron-centred Brønsted basicity

Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL'BY (L = CAAC = cyclic alkyl(amino)carbene; L' = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL'BY]˙+. Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL'B(CN) with PhSH yielding [LL'BH(CN)+][PhS-] is fully reversible, that of LL'B(NCS) is rendered irreversible by a subsequent B-to-CCAAC hydrogen shift and nucleophilic attack of PhS- at boron.

Nickel-Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic Carbenes (iMICs)

Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMICAr ) 2 a-g (iMICAr =ArC{N(Dipp)}2 CHC; Ar=aryl; Dipp=2,6-iPr2 C6 H3 ) under nickel catalysis to give 1,3-imidazoles (IMDAr ) 3 a-g (IMDAr =ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N-CDipp bond and the subsequent formation of a C-CDipp bond in 2, which is unprecedented in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC=ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6), gold (7), and palladium (8) compounds. The quantification of the stereoelectronic properties reveals the superior σ-donor strength of 5 compared to that of classical NHCs. Remarkably, the percentage buried volume of 5 (%Vbur =45) is the largest known amongst thus far reported iMICs. Catalytic studies show a remarkable activity of 5, which is consistent with their auspicious stereoelectronic features.

Bonding and stability of donor ligand-supported heavier analogues of cyanogen halides (L')PSi(X)(L)

Fluoro- and chloro-phosphasilynes [X-Si[triple bond, length as m-dash]P (X = F, Cl)] belong to a class of illusive chemical species which are expected to have Si[triple bond, length as m-dash]P multiple bonds. Theoretical investigations of the bonding and stability of the corresponding Lewis base-stabilized species (L')PSi(X)(L) [L' = cAACMe (cyclic alkyl(amino) carbene); L = cAACMe, NHCMe (N-heterocyclic carbene), PMe3, aAAC (acyclic alkyl(amino) carbene); X = Cl, F] have been studied using the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) method. The variation of the ligands (L) on the Si-atom leads to different bonding scenarios depending on their σ-donation and π-back acceptance properties. The ligands with higher lying HOMOs prefer profoundly different bonding scenarios than the ligands with lower lying HOMOs. The type of halogen (Cl or F) on the Si-atom was also found to have a significant influence on the overall bonding scenario. The reasonably higher value and endergonic nature of the dissociation energies along with the appreciable HOMO-LUMO energy gap may corroborate to the synthetic viability of the homo and heteroleptic ligand-stabilized elusive PSi(Cl/F) species in the laboratory.

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

In this contribution, we studied the OC-C bond in carbon suboxide and related allene compounds using the valence bond method. The nature of this bond has been the subject of debate, whether it is a regular, electron sharing bond or a dative bond. We compared the nature of this bond in carbon suboxide with the gold-CO bond in Au(CO)₂⁺, which is a typical dative bond, and we studied its charge-shift bond character. We found that the C-CO bond in carbon suboxide is unique in the sense that it cannot be assigned as either a dative or electron sharing bond, but it is an admixture of electron sharing and dative components, together with a high contribution of ionic character. These findings provide a clear basis for distinguishing the commonly found dative bonds between ligands and transition metals and the present case of what may be described as coordinative bonding to carbon.

From carbones to carbenes and ylides in the coordination sphere of iridium

The carbodiphosphorane-based iridium pincer complex (2) is demonstrated to rearrange in chlorinated organic solvents under cleavage of a P-C-bond to give a chelating phosphine ylide ligand. A detailed mechanistic investigation reveals that these types of donor groups are prone for P-C-bond cleavage in the coordination sphere of transition metal hydrido complexes. Finally, complex 2 is demonstrated to be an efficient hydrogen-borrowing catalyst.

Revisiting the Bonding Scenario of Two Donor Ligand Stabilized C2 Species

Quantum chemical calculations using density functional methods were performed for complexes of type L₂C₂ with L = NHC^Me (1), SNHC^Me (2) (S = saturated), cAAC^Me (3), and diamidocarbene (DAC^Me) (4). The equilibrium structures of 1-4 possess almost linear C₄ cores. A high thermochemical stability of the complexes with respect to dissociation, L₂C₂ → C₂ + 2L, is indicated by the large bond dissociation energy following the order 3 > 4 > 2 > 1. The results show that the use of SNHC^Me and DAC^Me as ligands is preferable over NHC^Me. The bonding analysis using charge and energy decomposition methods reveals that (cAAC^Me)₂C₂ and (DAC^Me)₂C₂ possess genuine cumulene C₄ moieties, which results from the electron-sharing bonding between quintet L₂ and quintet C₂ fragments. In contrast, the bonding in (NHC^Me)₂C₂ and (SNHC^Me)₂C₂ comes from a combination of dative and electron-sharing interactions between doublet L₂⁺ and doublet C₂^- fragments.

Stabilization of Linear C3 by Two Donor Ligands: A Theoretical Study of L-C3 -L (L=PPh3 , NHCMe , cAACMe )*

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3 -L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe )2 and C3 in the respective electronic quintet state yielding (cAACMe )=C3 =(cAACMe ). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3 )2 ]+ or [(NHCMe )2 ]+ and negatively charged [C3 ]- fragments in the respective doublet state.

Carbones and Carbon Atom as Ligands in Transition Metal Complexes

This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL₂ as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and π) donation to the metal atom [M]⇇CL₂. The second part of this review reports complexes which have a neutral carbon atom C as ligand. Carbido complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR₃ → [M]-CR₂ → [M]-CR → [M]-C. This review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands.

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent NI compounds and can be represented with the general formula L → N+ ← L, where L is an electron donating ligand. In the recent past, several divalent NI compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N+ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent NI compounds (nitreones) which carry coordination bonds between ligands and N+ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent NI compounds have been explored.

Bent Allenes or Di-1,3-betaines-An Answer Given on the Magnetic Criterion

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of bent allene 1, the corresponding C-extended 1,3-butadiene derivative 2, and a number of related compounds 3-20 have been calculated using the gauge-independent atomic orbital perturbation method, employing the nucleus-independent chemical shift concept and visualized as isochemical shielding surfaces of various sizes and directions. Prior to that, both structures and ¹³C chemical shifts were calculated and compared with available experimental bond lengths and δ(¹³C)/ppm values (also, as a quality criterion for the computed structures). Bond lengths, the δ(¹³C)/ppm, and the TSNMRS values are employed to qualify and quantify the electronic structure of the studied compounds in terms of dative or classical electron-sharing bonds.

Stable Mesoionic N-Heterocyclic Olefins (mNHOs)

We report a new class of stable mesoionic N-heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground-state structure cannot be described through an uncharged mesomeric Lewis-structure, thereby structurally distinguishing them from traditional N-heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N'-diaryl-1,2,3-triazolium and N,N'-diaryl-imidazolium salts, respectively. In their reactivity, they represent strong σ-donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.

The 13 C chemical shift and the anisotropy effect of the carbene electron-deficient centre: Simple means to characterize the electron distribution of carbenes

Both the ¹³ C chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and ¹³ C chemical shifts were calculated and compared with the experimental δ(¹³ C)/ppm values and geometry parameters (as a quality criterion for obtained structures).

Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Chromium and Cobalt Complexes

Carbodicarbenes (CDCs) possess two lone pairs of electrons on their central carbone C atom (C_carbone). Coordination to a transition metal via a σ bond leaves one pair of electrons with appropriate symmetry for π donation to the metal. However, the high energy of the latter also renders the CDC ligand potentially redox-active. Herein, we explore these alternatives in the redox series [Cr(L)₂]ⁿ⁺ and [Co(L)₂]ⁿ⁺ (n = 2-5), where L is a tridentate ligand comprised of a central CDC and two flanking pyridine donors. To this end, all members of both redox series were synthesized and their electronic structures were investigated by using a combination of ¹H NMR, Evans' NMR, IR, UV-vis, and EPR spectroscopies, SQUID magnetometry, X-ray crystallography, and density functional theory studies. Whereas [Co^II(L)₂]²⁺ is a straightforward low-spin (S = ¹/₂) cobalt(II) complex, the corresponding chromium complex was found to feature an electronic structure that is intermediate between the two limiting resonance forms [Cr^III(L^•-)(L)]²⁺ and [Cr^II(L)₂]²⁺. In the case of the tri-, tetra-, and pentacationic complexes, the qualitatively identical electronic structures [M^III(L)₂]³⁺, [M^III(L^•+)(L)]⁴⁺, and [M^III(L^•+)₂]⁵⁺ were observed for both metals. Thus, the metal ions retain a 3+ oxidation state throughout, and the higher redox states contain oxidized ligands. The majority of the unpaired spin on the cation radical ligands was calculated to be localized in π-symmetry orbitals on the coordinated C_carbone atoms. Analogous behavior was previously reported for the corresponding iron redox series and, as such, redox noninnocence in oxidized CDC and, more broadly, carbone complexes is likely widely accessible.

Stable abnormal N-heterocyclic carbenes and their applications

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO₂), nitrous oxide (N₂O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO₂ and the catalytic reduction of carbon dioxide, including atmospheric CO₂, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

Design of non-ionic carbon superbases: second generation carbodiphosphoranes

A new generation of carbodiphosphoranes (CDPs), incorporating pyrrolidine, tetramethylguanidine, or tris(dimethylamino)phosphazene as substituents is introduced as the most powerful class of non-ionic carbon superbases on the basicity scale to date. The synthetic approach as well as NMR spectroscopic and structural characteristics in the free and protonated form are described. Investigation of basicity in solution and in the gas phase by experimental and theoretical means provides the to our knowledge first reported pK BH + values for CDPs in the literature and suggest them as upper tier superbases.

In search of stable singlet metalla-N-heterocyclic carbenes (MNHCs): a contribution from theory

Theoretical studies predict that the stability of the singlet state of metalla-N-heterocyclic carbenes (MNHCs) is strongly influenced not only by the substituents at the α-nitrogen atoms and the nature of ligands at the transition metal center but also by the substituents at the carbenic backbone. All the MNHCs were found to have a stable singlet ground state and the computed ΔE_S-T values for some of the MNHCs were found to be significantly large and lie within the range of experimentally known carbenes (31.0-84.0 kcal mol^-1). Furthermore, they were found to have superior σ-donation ability to conventional NHCs. The calculated proton affinities, gallium pyramidalization, pK_a and nucleophilicity index values for the MNHCs were found to be in good agreement with their calculated electron donation ability.

Observation of Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Iron Complexes

To probe the possibility that carbodicarbenes (CDCs) are redox active ligands, all four members of the redox series [Fe(1)₂ ]ⁿ⁺ (n=2-5) were synthesized, where 1 is a neutral tridentate CDC. Through a combination of spectroscopy and DFT calculations, the electronic structure of the pentacation is shown to be [Fe^III (1^.+ )₂ ]⁵⁺ (S= 1 / 2 ). That of [Fe(1)₂ ]⁴⁺ is more ambiguous, but it has significant contributions from the open-shell singlet [Fe^III (1)(1^.+ )]⁴⁺ (S=0). The observed spin states derive from antiferromagnetic coupling of their constituent low-spin iron(III) centres and cation radical ligands. This marks the first time redox activity has been observed for carbones and expands the diverse chemical behaviour known for these ligands.