Bonding Trends in Molecular Compounds of Lanthanides: The Double‐Bonded Carbene Cations LnCH2+ (Ln=Sc,Y, La–Lu)

Quasi‐relativistic Douglas–Kroll CASPT2 calculations are reported for the title molecules, mainly to provide primary data for a fit of double‐bond covalent radii. Indeed, a well‐developed σ²π² double bond is identified in all cases. For Eu and Yb, however, it is an excited state. The main valence orbitals of all Ln ions are 6s and 5d. In the σ bonds, more 5d than 6s character is found at the Ln. The Ln?C bond lengths show a systematic lanthanide contraction of 13 pm from La to Lu. An agostic symmetry breaking is demonstrated for Ce but its effect on the Ln? C length is small.     

Prediction of Boron–Boron Triple‐Bond Polymers Stabilized by Janus‐Type Bis(N‐heterocyclic) Carbenes

A class of polymeric compounds containing boron–boron triple bonds stabilized by N‐heterocyclic biscarbenes is proposed. Since a triply bonded B₂ is related to its third excited state, the predicted macromolecule would be composed by several units of an electronically excited first‐row homonuclear dimer. Moreover, it is shown that the replacement of biscarbene with N₂ or CO as spacers could change the bonding profile of the boron–boron units to a cumulene‐like structure. Based on these results, different types of diboryne polymers are proposed, which could lead to an unprecedented set of boron materials with distinct physical properties. The novel diboryne macromolecules could be synthesized by the reaction of Janus‐type biscarbenes with tetrabromodiborane, B₂Br₄, and sodium naphthalenide, [Na(C₁₀H₈)], similarly to Braunschweig’s work on the room temperature stable boron–boron triple bond compounds (Science, 2012 , 336, 1420).     

E5M7+ (E=C–Pb,M=Li–Cs): A Source of Viable Star‐Shaped Clusters

Herein we report the systematic exploration of the potential energy surfaces of a series of clusters with formula E₅M₇⁺ (E=C‐Pb and M=Li‐Cs). Fifteen of these combinations adopt a D_5h three‐dimensional seven‐pointed star‐like structure in a singlet state, where M atoms interact electrostatically with the E₅ ring. The determining factors in the relative preference of having the D_5h structure over the most competitive isomer or vice‐versa are analyzed. These star‐shaped systems satisfy the 4n+2 Hückel's rule and exhibit a strong diatropic (σ and π) response to an external magnetic field.     

Versatile stereoselective cycloadditions between heterocumulenes and phosphagermaallene Tip(tBu)Ge=C=PMes*: experimental and theoretical investigations

Phosphagermaallene Tip(tBu)Ge=C=PMes* 1 (Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) reacts with phenyl isocyanate and tert-butyl isocyanate by a [2+2] cycloaddition that involves the Ge=C and C=O double bonds to afford 1-oxa-2-germacyclobutanes 2 and 3. With N,N'-dicyclohexylcarbodiimide, a [2+2] cycloaddition is observed between the Ge=C and C=N unsaturations to lead to 1-aza-2-germacyclobutane 6 with exocyclic P=C and C=N double bonds. In sharp contrast, 1 reacts with phenyl isothiocyanate, ethyl isothiocyanate, and carbon disulfide according to a [3+2] cycloaddition that involves the whole Ge=C=P unit and the C=S double bond to give transient phosphagermacarbenes (PGeHCs) 11, 12, and 13. These new PGeHCs undergo C-H insertions into one o-tBu group of Mes* (in the case of 11 and 12) or one o-iPr group of Tip (in the case of 13) with formation of tricyclic compounds 8, 9, and 10, respectively. The reaction mechanisms that involve 1 and the phenyl isocyanate and the phenyl isothiocyanate are described and their regioselectivity is explained by theoretical calculations.     

A challenge to chemical intuition: donor-acceptor interactions in H3B-L and H2B+-L (L=CO; EC5H5, E=N-Bi)

The equilibrium geometries and bond energies of the complexes H(3)B-L and H(2)B(+)-L (L=CO; EC(5)H(5): E=N, P, As, Sb, Bi) have been calculated at the BP86/TZ2P level of theory. The nature of the donor-acceptor bonds was investigated by energy decomposition analysis (EDA). The bond strengths of H(3)B-L have the order CO>N>P>As>Sb>Bi. The calculated values are between D(e)=37.1 kcal mol(-1) for H(3)B-CO and D(e)=6.9 kcal mol(-1) for H(3)B-BiC(5)H(5). The bond dissociation energies of the cations H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) are larger than for H(3)B--L, particularly for complexes of the heterobenzene ligands. The calculated values are between D(e)=51.9 kcal mol(-1) for H(2)B(+)-CO and D(e)=122.1 kcal mol(-1) for H(2)B(+)-NC(5)H(5). The trend of the BDE of H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) is N>P>As>Sb>Bi>CO. A surprising result is found for H(2)B(+)-CO, which has a significantly stronger and yet substantially longer bond than H(3)B-CO. The reason for the longer but stronger bond in H(2)B(+)-CO compared with that in H(3)B-CO comes mainly from the change in electrostatic attraction and pi bonding at shorter distances, which increases more in the neutral system than in the cation, and to a lesser extent from the deformation energy of the fragments. The H(2)B(+)<--NC(5)H(5) pi( perpendicular) donation plays an important role for the stronger interactions at shorter distances compared with those in H(3)B-NC(5)H(5). The attractive interaction in H(2)B(+)--CO further increases at bond lengths that are shorter than the equilibrium value, but this is compensated by the energy which is necessary to deform BH(2) (+) from its linear equilibrium geometry to the bent form in the complex. The EDA shows that the contributions of the orbital interactions to the donor-acceptor bonds are always larger than the classical electrostatic contributions, but the latter term plays an important role for the trend in bond strength. The largest contributions to the orbital interactions come from the sigma orbitals. The EDA calculations suggest that heterobenzene ligands may become moderately strong pi donors in complexes with strong Lewis acids, while CO is only a weak pi donor. The much stronger interaction energies in H(2)B(+)-EC(5)H(5) compared with those in H(3)B-EC(5)H(5) are caused by the significantly larger contribution of the pi(perpendicular) orbitals in H(2)B(+)-EC(5)H(5) and by the increase of the binding interactions of the sigma+pi( parallel) orbitals.     

Bonding Analysis of N‐Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition‐Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal–ligand bonding in transition‐metal complexes that contain imidazole (IMID), imidazol‐2‐ylidene (nNHC), or imidazol‐4‐ylidene (aNHC). The calculated complexes are [Cl₄TM(L)] (TM=Ti, Zr, Hf), [(CO)₅TM(L)] (TM=Cr, Mo, W), [(CO)₄TM(L)] (TM=Fe, Ru, Os), and [ClTM(L)] (TM=Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID<nNHC<aNHC. The energy levels of the carbon σ lone‐pair orbitals suggest the trend aNHC>nNHC>IMID for the donor strength, which is in agreement with the progression of the metal–ligand bond‐dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand–metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)₅TM(L)] (L=nNHC, aNHC, IMID) with phosphine complexes (L=PMe₃ and PCl₃) shows that the phosphine ligands are weaker σ donors and better π acceptors than the NHC tautomers nNHC, aNHC, and IMID.     

Structural,Electronic, and Bonding Properties of Zeolite Sn‐Beta: A Periodic Density Functional Theory Study

The structural, electronic, and the bonding properties of the zeolite Sn‐beta (Sn‐BEA) have been investigated by using the periodic density functional theory. Each of the nine different T‐sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane‐wave basis set in conjunction with the ultra‐soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T‐site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework.     

Gold Behaves as Hydrogen in the Intermolecular Self‐Interaction of Metal Aurides MAu4 (M=Ti,Zr, and Hf)

We performed density functional calculations to examine the intermolecular self‐interaction of metal tetraauride MAu₄ (M=Ti, Zr, and Hf) clusters. We found that the metal auride clusters have strong dimeric interactions (2.8–3.1 eV) and are similar to the metal hydride analogues with respect to structure and bonding nature. Similarly to (MH₄)₂, the (μ‐Au)₃ C_s structures with three three‐center two‐electron (3c–2e) bonds were found to be the most stable. Natural orbital analysis showed that greater than 96 % of the Au 6s orbital contributes to the 3c–2e bonds, and this predominant s orbital is responsible for the similarity between metal aurides and metal hydrides (>99 % H 1s). The favorable orbital interaction between occupied Au 6s and unoccupied metal d orbitals leads to a stronger dimeric interaction for MAu₄‐MAu₄ than the interaction for MH₄‐MH₄. There is a strong relationship between the dimeric interaction energy and the chemical hardness of its monomer for (MAu₄)₂ and (MH₄)₂.     

Phosphorescent Cationic Heterodinuclear IrIII/MI Complexes (M=CuI,AuI) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge

A novel class of phosphorescent cationic heterobimetallic Ir^III/M^I complexes, where M^I=Cu^I ( 4 ) and Au^I ( 5 ), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an Ir^III and a M^I center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M^I(IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the “Ir(C^N)” moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (³IL/¹MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating Ir^III complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the ³IL and ¹MLCT manifolds, which mix via spin–orbit coupling.     

Uranium–Carbene–Imido Metalla‐Allenes: Ancillary‐Ligand‐Controlled cis‐/trans‐Isomerisation and Assessment of trans Influence in the R2C=UIV=NR′ Unit (R=Ph2PNSiMe3; R′=CPh3)

Uranium(IV)–carbene–imido complexes [U(BIPM^TMS)(NCPh₃)(κ²‐N,N′‐BIPY)] ( 2 ; BIPM^TMS=C(PPh₂NSiMe₃)₂; BIPY=2,2‐bipyridine) and [U(BIPM^TMS)(NCPh₃)(DMAP)₂] ( 3 ; DMAP=4‐dimethylamino‐pyridine) that contain unprecedented, discrete R₂C=U=NR′ units are reported. These complexes complete the family of E=U=E (E=CR₂, NR, O) metalla‐allenes with feasible first‐row hetero‐element combinations. Intriguingly, 2 and 3 contain cis‐ and trans‐C=U=N units, respectively, representing rare examples of controllable cis/trans isomerisation in f‐block chemistry. This work reveals a clear‐cut example of the trans influence in a mid‐valent uranium system, and thus a strong preference for the cis isomer, which is computed in a co‐ligand‐free truncated model—to isolate the electronic trans influence from steric contributions—to be more stable than the trans isomer by approximately 12 kJ mol^?1 with an isomerisation barrier of approximately 14 kJ mol^?1.     

Oligonuclear Molecular Models of Intermetallic Phases: A Case Study on [Pd2Zn6Ga2(Cp*)5(CH3)3]

The synthesis, characterization, and theoretical investigation by means of quantum‐chemical calculations of an oligonuclear metal‐rich compound are presented. The reaction of homoleptic dinuclear palladium compound [Pd₂(μ‐GaCp*)₃(GaCp*)₂] with ZnMe₂ resulted in the formation of unprecedented ternary Pd/Ga/Zn compound [Pd₂Zn₆Ga₂(Cp*)₅(CH₃)₃] ( 1 ), which was analyzed by ¹H and ¹³C NMR spectroscopy, MS, elemental analysis, and single‐crystal X‐ray diffraction. Compound 1 consisted of two C_s‐symmetric molecular isomers, as revealed by NMR spectroscopy, at which distinct site‐preferences related to the Ga and Zn positions were observed by quantum‐chemical calculations. Structural characterization of compound 1 showed significantly different coordination environments for both palladium centers. Whilst one Pd atom sat in the central of a bi‐capped trigonal prism, thereby resulting in a formal 18‐valence electron fragment, {Pd(ZnMe)₂(ZnCp*)₄(GaMe)}, the other Pd atom occupied one capping unit, thereby resulting in a highly unsaturated 12‐valence electron fragment, {Pd(GaCp*)}. The bonding situation, as determined by atoms‐in‐molecules analysis (AIM), NBO partial charges, and molecular orbital (MO) analysis, pointed out that significant Pd? Pd interactions had a large stake in the stabilization of this unusual molecule. The characterization and quantum‐chemical calculations of compound 1 revealed distinct similarities to related M/Zn/Ga Hume–Rothery intermetallic solid‐state compounds, such as Ga/Zn‐exchange reactions, the site‐preferences of the Zn/Ga positions, and direct M? M bonding, which contributes to the overall stability of the metal‐rich compound.     

Synthesis and Bonding in Carbene Complexes of an Unsymmetrical Dilithio Methandiide: A Combined Experimental and Theoretical Study

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)‐substituted dilithio methandiide and its application for the synthesis of zirconium‐ and palladium‐carbene complexes. These complexes were found to exhibit remarkably shielded ¹³C NMR shifts, which are much more highfield‐shifted than those of “normal” carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock‐type complexes. Various methods show that these systems are best described as highly polarized Schrock‐type complexes, in which the metal–carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as “masked” methandiides. As such, geminal dianions represent a kind of “extreme” Schrock‐type ligands favoring the ionic resonance structure M⁺? CR₂^? as often used in textbooks to explain the nucleophilic nature of Schrock complexes.     

Sandwich-like compounds based on the all-metal aromatic unit Al(4)2- and the main-group metals M (M=Li, Na, K, Be, Mg, Ca)

Inspired by the pioneering experimental characterisation of the all-metal aromatic unit Al(4)2- in the bimetallic molecules MAl4- (M=Li, Na, Cu) and by the very recent theoretical design of sandwich-type transition-metal complexes [Al4MAl4]q- (q=0-2; M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W), we used density functional theory (DFT) calculations (B3LYP/6-311+G(d) to design a series of novel non-transition-metal sandwich complexes based on the all-metal aromatic unit Al4(2-) and the main-group metals M (M=Li, Na, K, Be, Mg, Ca). The traditional homo-decked sandwich compounds [Al4MAl4]q- (without counterions) and (nM)q+[Al4MAl4]q- (with counterions M) (q=2-3, M=Li, Na, K, Be, Mg, Ca), although some of them are truly energy minima, have a much higher energy than many fused isomers. We thus concluded that it seems unlikely for Al4(2-) to sandwich the main-group metal atoms in the homo-decked sandwich form. Alternatively, we proposed a new type of sandwich complex, namely hetero-decked sandwich compounds [CpMAl4]q-, that are the ground-state structures for each M both with and without counterions. It was shown that with the rigid Cp- partner, the all-metal aromatic unit Al(4)2- might indeed act as a "superatom". These new types of all-metal aromatic unit-based sandwich complexes await future experimental verification.     

Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHC?Transition‐Metal Bond Properties

Carbenes derived from five‐membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N‐heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug‐cc‐pVTZ level. The examined parameters include the energies of the σ‐lone pair at C_carbene and the π‐HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet–triplet energy gap was used as a measure of the stability of the N‐heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p_π population and the natural charge at C_carbene with NHC structure. Additionally, the transition metal? NHC bond in L‐AuCl and L‐TiCl₄ and the nature of the orbital interactions between the NHC and the transition‐metal fragment were analysed in detail by the extended transition state–natural orbitals for chemical valence (ETS–NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC? gold and the NHC? titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.     

Electronic Structures of LixV2O5 (x=0.5 and 1): A Theoretical Study

The electronic properties of α‐Li_xV₂O₅ (x=0.5 and 1) are investigated using first principle calculations based on density functional theory with local density approximation. Different intercalation sites for Li in the V₂O₅ lattices are considered, showing different influences on the electronic structures of Li_xV₂O₅. The lowest total energy is found when Li is only intercalated along the c axis between two bridging oxygen ions of sequential V₂O₅ layers. The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅, which is an indirect band gap semiconductor, but leads to a reduction of vanadium ions and an increase of the Fermi level of Li_xV₂O₅ arising from the electron transfer from the Li 2 s orbital to the initially empty conduction band of the V₂O₅ host.     

Reaction of an N‐Heterocyclic Carbene‐Stabilized Silicon(II) Monohydride with Alkynes: [2+2+1] Cycloaddition versus Hydrogen Abstraction

An in depth study of the reactivity of an N‐heterocyclic carbene (NHC)‐stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1 , tBu₃Si(H)Si←NHC, with diphenylacetylene afforded silole 2 , tBu₃Si(H)Si(C₄Ph₄). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1‐alkenyl‐1‐alkynylsilane 3 , tBu₃Si(H)Si(CH?CHPh)(C?CPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.     

Synthesis,Characterization, and Theoretical Investigation of Two‐Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π‐Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π‐accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C‐Pd‐C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange‐colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)₂Pd/Pt] complexes can be quasi‐reversibly oxidized to their corresponding [(cAAC)₂Pd/Pt]⁺ cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.