Unusual Bonding in Trisilaallene and Related Heavy Allenes

Inorganometallic allenes [(Mn(η5-C5H5)(CO)2)2(μ-E)] (E = Si-Pb): bis-allylic anionic delocalisation similar to organometallic allene but differential σ-donation and π-backdonation

The chemistry of heavy group-14 tetrel atoms is known to diverge from that of the lighter congener carbon. Here, we report the structure and bonding in inorganometallic allenes [(MnCp(CO)2)2(μ-E)] (2E, E = Si-Pb; Cp = η5-C5H5). These inorganometallic allenes are structurally similar to the lighter organometallic analog [(MnCp(CO)2)2(μ-C)] (2C). The bonding analysis of these compounds at the M06/def2-TZVPP//BP86/def2-SVP level of theory identifies a linear Mn-E-Mn spine with delocalised, mutually orthogonal π-systems across this back-bone. This results in a bis-allylic anionic bonding scenario. However, the strength of the Mn-E bonding is found to be weaker in these inorganometallic allenes. The energy decomposition analysis at the BP86/TZ2P//BP86/def2-SVP level of theory further reveals that the bonding in these compounds cannot be represented by one unique heuristic bonding model, but multiple bonding models. For all 2E (E = C-Pb), the Dewar-Chatt-Duncanson bonding model is one of the best bonding representations, where the central tetrel atom acts as a 4e- σ-donor and 4e- π-acceptor. The bonding analysis indicates that the carbon atom in the organometallic allene acts as a better π-acceptor than σ-donor, while the heavier tetrel atoms in the inorganometallic allenes are better σ-donors than π-acceptors. The npz-orbital is found to be a better σ-donor than the valence ns-orbital. However, when the bonding representation is changed to a traditional electron-sharing model, the contribution from the ns-orbital was found to be the largest in comparison to the interaction from the remaining three valence np-orbitals. It can be suggested that the ns-orbitals contribute more towards chemical bonding when participating via an electron-sharing interaction than a donor-acceptor interaction.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

Low-Valent Germanylidene Anions: Efficient Single-Site Nucleophiles for Activation of Small Molecules

Rare mononuclear and helical chain low-valent germanylidene anions supported by cyclic (alkyl)(amino)carbene and hypermetallyl ligands were synthesised by stepwise reduction from corresponding germylene precursors via stable and isolable germanium radicals. The electronic structures of the anions can be described with ylidene and ylidone resonance forms with the Ge-C π-electrons capable of binding even weak electrophiles. The germanylidene anions reacted with CO2 to give μ-CO2 -κC:κO complexes, a rare coordination mode for low-valent germanium and inaccessible for the related neutral germylones. These results implicate low-valent germanylidene anions as efficient single-site nucleophiles for activation of small molecules.

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.

Conformationally Switchable Silylone: Electron Redistribution Accompanied by Ligand Reorientation around a Monatomic Silicon

Complexes that could be switched between two electronic states by external stimuli have attracted much attention for their potential application in molecular devices. However, a realization of such a phenomenon with low-valent main-group element-centered complexes remains challenging. Herein, we report the synthesis of cyclic (alkyl)(amino)silylene (CAASi)-ligated monatomic silicon(0) complexes (silylones). The bis(CAASi)-ligated silylone adopts a π-localized ylidene structure (greenish-black color) in the solid state and a π-delocalized ylidene structure (dark-purple color) in solution that could be reversibly switched upon phase transfer (ylidene [L: → :Si = L ↔ L = Si: ← :L]). The observed remarkable difference in the physical properties of the two isomers is attributed to the balanced steric demand and redox noninnocent character of the CAASi ligand which are altered by the orientation of the two terminal ligands with respect to the Si-Si-Si plane: twisted structure (π-localized ylidene) and planar structure (π-delocalized ylidene). Conversely, the CAASi/CDASi-ligated heteroleptic silylone (CDASi = cyclic dialkylsilylene) only exhibited the twisted π-localized ylidene structure regardless of the phase. The synthesized silylones also proved themselves as monatomic silicon surrogates. Thermolysis of the silylones in the presence of an ethane-1,2-diimine afforded the corresponding diaminosilylenes. Analyses of the products suggested a stepwise mechanism that proceeds via a disilavinylidene intermediate.

Synthesis of Low-Valent Dinuclear Group 14 Compounds with Element-Element Bonds by Transylidation

Dinuclear low-valent compounds of the heavy main group elements are rare species owing to their intrinsic reactivity. However, they represent desirable target molecules due to their unusual bonding situations as well as applications in bond activations and materials synthesis. The isolation of such compounds usually requires the use of substituents that provide sufficient stability and synthetic access. Herein, we report on the use of strongly donating ylide-substituents to access low-valent dinuclear group 14 compounds. The ylides not only impart steric and electronic stabilization, but also allow facile synthesis via transfer of an ylide from tetrylene precursors of type R Y2 E to ECl2 (E=Ge, Sn; R Y=TolSO2 (PR3 )C with R=Ph, Cy). This method allowed the isolation of dinuclear complexes amongst a germanium analogue of a vinyl cation, [(Ph Y)2 GeGe(Ph Y)]+ with an electronic structure best described as a germylene-stabilized GeII cation and a ylide(chloro)digermene [Cy Y(Cl)GeGe(Cl)Cy Y] with an unusually unsymmetrical structure.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

A stable bis(methylene)-λ4-selane with a >C[double bond, length as m-dash]Se[double bond, length as m-dash]C< bond containing Se(iv)

Bis(methylene)-λ4-selane 1, which represents a new class of heteroallenes, was synthesized and isolated as a stable purple crystalline solid. X-ray crystallographic analysis revealed a bent allene-type structure with pseudo C2v symmetry and a bent C[double bond, length as m-dash]Se[double bond, length as m-dash]C moiety that contains a 3-center-4-electron π-bond. The NMR spectrum of 1 shows compelling evidence for a slow rotation of the Se[double bond, length as m-dash]C moieties at room temperature. The 77Se NMR spectrum of 1 contains one signal in the region characteristic for Se(iv) compounds.

An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity

The first zerovalent germanium complex ("germylone") 3, [Si^II(Xant)Si^II]Ge⁰, stabilized by a chelating bis(N-heterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[Si^II(Xant)Si^II]GeCl}⁺Cl^- complex 2 with KC₈; it was structurally and spectroscopically characterized, and also studied by density functional theory (DFT) calculations. Natural bond orbital (NBO) analysis of 3 unambiguously exhibits two lone pairs of electrons (one σ-type lone-pair and one 3p(Ge) lone-pair) on the zerovalent Ge atom. This is why the Ge atom can form the corresponding mono- and bis-AlBr₃ Ge → Al Lewis adducts [Si^II(Xant)Si^II]Ge(AlBr₃) 4 and [Si^II(Xant)Si^II]Ge(AlBr₃)₂ 5, respectively. Due to the electron-rich character of the Ge⁰ atom, the germylone 3 displayed quite unusual reactivities. Thus, the reaction of 3 with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid furnished the first boryl(silyl)germylene complex 6, possessing a heteroallylic B···Ge···Si π-conjugation. When 3 was allowed to react with Ni(cod)₂ (cod = 1,5-cyclooctadiene), the unique {[Si^II(Xant)Si^II]Ge^I}₂Ni^II complex with a three-membered ring Ge₂Ni-metallacycle was obtained via reductive coupling of two Ge⁰ atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh₃, which was capable of heterolytic H₂ cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.

A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues

Monoatomic zerovalent main-group element complexes emerged very recently and attracted increasing attention of both theoretical and experimental chemists. In particular, zerovalent silicon complexes and their congeners (metallylones) stabilized by neutral Lewis donors are of significant importance not only because of their intriguing electronic structure but also because they can serve as useful building blocks for novel chemical species. Featuring four valence electrons as two lone pairs at the central atoms, such complexes may form donor-acceptor adducts with Lewis acids. More interestingly, with the central atoms in the oxidation state of zero, they could pave a way to new classes of compounds and functional groups that are otherwise difficult to realize. In this Account, we mainly describe our contributions in the chemistry of monatomic zerovalent silicon (silylone) and germanium (germylone) supported by a chelate bis-N-heterocyclic carbene (bis-NHC) ligand in the context of related species developed by other groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene [:SiCl]⁺ and chlorogermyliumylidene [:GeCl]⁺ as suitable starting materials, we successfully isolated silylone (bis-NHC)Si and germylone (bis-NHC)Ge, respectively. The electronic structures of the latter complexes established by theoretical calculations and spectroscopic data revealed that they are genuine metallylone species with electron-rich silicon(0) and germanium(0) centers. Accordingly, they can react with 1 molar equiv of GaCl₃ to form Lewis adducts (bis-NHC)E(GaCl₃) (E = Si, Ge) and with 2 molar equiv of ZnCl₂ to furnish (bis-NHC)Si(ZnCl₂)₂. Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon(II) and germanium(II) monochalcogenides (bis-NHC)EX(GaCl₃) (X = Se, Te), representing molecular heavier congeners of CO. Moreover, their reaction with elemental chalcogens can also yield monomeric silicon(IV) and germanium(IV) dichalcogenides (bis-NHC)EX₂ (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO₂. Moreover, (bis-NHC)Si could even activate CO₂ to afford the monomolecular silicon dicarbonate complex (bis-NHC)Si(CO₃)₂ via the formation of SiO and SiO₂ complexes as intermediates. Furthermore, starting with a chelate bis-N-heterocyclic silylene supported [:GeCl]⁺, we developed two bis-N-heterocyclic silylene stabilized germylone→Fe(CO)₄ complexes. Our achievements in the chemistry of metallylones demonstrate that the characteristic of monatomic zerovalent silicon and its analogues can provide novel reaction patterns for access to unprecedented species and even extends the series of functional groups of these elements. With this, we can envision that more interesting zerovalent complexes of the main-group elements with unprecedented reactivity will follow in the near future.

Bonding analysis of ylidone complexes EL2 (E = C–Pb) with phosphine and carbene ligands L

Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds EL2 with atoms E = C–Pb and the ligands L = PPh3, N-heterocyclic carbene (NHC), bicyclic NHC, and cyclic alkyl-amino carbene (cAAC). The equilibrium structures are reported and the bonding situation was analyzed with a variety of charge- and energy decomposition methods. Some of the molecules are experimentally known, but most molecules have not yet been prepared. The bonding analysis suggests that all but one molecule should be considered as ylidones that possess dative bonds L→E←L. The sole exception is C(cAAC)2, which has a linear structure and classical double bonds (cAAC)=C=(cAAC). The ylidones EL2 have two lone-pair orbitals at atom E with σ and π symmetry. The π lone pair is somewhat delocalized due to L←E→L π-backdonation. The contribution of L←E→L π-backdonation relative to L→E←L σ-donation increases for the heavier elements. The compounds have rather large first and second proton affinities, which is a characteristic feature of ylidones. They are thus double Lewis bases that could be utilized in chemical reactions.

A Bis(silylenyl)pyridine Zero-Valent Germanium Complex and Its Remarkable Reactivity

The synthesis, reactivity, and electronic structure of the unique germylone iron carbonyl complex [SiNSi]Ge⁰ →Fe(CO)₄ is reported. The compound was obtained in 49 % yield from the reaction of the bis(N-heterocyclic silylenyl)pyridine pincer ligand SiNSi (1,6-C₅ NH₃ -[EtNSi(N^t Bu)₂ CPh]₂ ) with GeCl₂ ⋅(dioxane) to give the corresponding chlorogermyliumylidene chloride precursor [SiNSi]Ge^II Cl⁺  Cl^- , which was further reduced with K₂ Fe(CO)₄ . Single-crystal X-ray diffraction analysis of [SiNSi]Ge→Fe(CO)₄ revealed that the Ge⁰ center adopts a trigonal-pyramidal geometry with a Si-Ge-Si angle of 95.66(2)°. Remarkably, one of the Si^II donor atoms in the complex is five-coordinated because of additional (pyridine)N→Si coordination. Unexpectedly, the reaction of [SiNSi]Ge→Fe(CO)₄ with GeCl₂ ⋅(dioxane) (one molar equivalent) yielded the first push-pull germylone-germylene donor-acceptor complex, [SiNSi]Ge→GeCl₂ →Fe(CO)₄ through the insertion of GeCl₂ into the dative Ge⁰ →Fe bond. The electronic features of the new compounds were investigated by DFT calculations.

Reactivity in the periphery of functionalised multiple bonds of heavier group 14 elements

Heavier group 14 multiple bonds have intrigued chemists since more than a century. The synthesis of stable compounds with double and triple bonds with silicon, germanium, tin and lead had considerable impact on modern ideas of chemical bonding. These developments were made possible by the use of bulky substituents that provide kinetic and thermodynamic protection. Since about a decade the compatibility of heavier multiple bonds with various functional groups has moved into focus. This review covers multiply bonded group 14 species with at least one additional reactive site. The vinylic functionalities of groups 1 and 17, resulting in nucleophilic and electrophilic disila vinyl groups, respectively, are the most prevalent and well-studied. They have been employed repeatedly for the transfer of heavier multiple bonds to yield low-valent group 14 compounds with novel structural motifs. Vinylic functionalities of groups 2 to 16 and a few σ-bonded transition metal complexes are experimentally known, but their reactivity has been studied to a lesser extent. Donor-coordinated multiple bonds are a relatively new field of research, but the large degree of unsaturation as isomers of alkynes (as well as residual functionality in some cases) offers considerable possibility for further manipulation, e.g. for the incorporation into more extended systems. Heavier allyl halides constitute the major part of heavier multiple bonds with a functional group in allylic position and some examples of successful transformations are given. At present, remote functionalities are basically limited to para-phenylene functionalised disilenes. The reported use of the latter for further derivatisation might encourage investigations in this direction. In summary, the study of peripherally functionalised multiple bonds with heavier group 14 elements is already well beyond its infancy and may be an instrumental factor in awakening the potential of group 14 chemistry for applications in polymers and other materials.

2-Adamantylidene and its heavier analogues: hyperconjugation versus lone pair stability and electrophilicity versus nucleophilicity

Too heavy to bend: The Cieplak-type hyperconjugative interaction mainly contributes to the stability of 2C and 2Si whereas the inertness of lone pairs is the major factor for 2Ge and 2Sn. These molecules (2X; X = C–Sn) can be classified as a class of ambiphilic compounds.

An N-heterocyclic silylene-stabilized digermanium(0) complex

The synthesis of an N-heterocyclic silylene-stabilized digermanium(0) complex is described. The reaction of the amidinate-stabilized silicon(II) amide [LSiN(SiMe3)2] (1; L=PhC(NtBu)2) with GeCl2⋅dioxane in toluene afforded the Si(II)-Ge(II) adduct [L{(Me3Si)2N}Si→GeCl2] (2). Reaction of the adduct with two equivalents of KC8 in toluene at room temperature afforded the N-heterocyclic carbene silylene-stabilized digermanium(0) complex [L{(Me3Si)2N}Si→Ge=Ge←Si{N(SiMe3)2}L] (3). X-ray crystallography and theoretical studies show conclusively that the N-heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor-acceptor interaction.

A coordination compound of Ge(0) stabilized by a diiminopyridine ligand

Reduction of the cationic Ge(II) complex [dimpyrGeCl][GeCl3] (dimpyr=2,6-(ArN=CMe)2NC5H3, Ar=2,6-iPr2C6H3) with potassium graphite in benzene affords an air sensitive, dark green compound of Ge(0), [dimpyrGe], which is stabilized by a bis(imino)pyridine platform. This compound is the first example of a complex of a zero-valent Group 14 element that does not contain a carbene or carbenoid ligand. This species has a singlet ground state. DFT studies revealed partial delocalization of one of the Ge lone pairs over the π*(C=N) orbitals of the imines. This delocalization results in a partial multiple-bond character between the Ge atom and imine nitrogen atoms, a fact supported by the X-ray crystallography and IR spectroscopy data.

A functionalized Ge3-compound with a dual character of the central germanium atom

A novel Ge₃-compound with dual character of the central Ge atom has been synthesized via direct formation of a GeC bond by using a stable cyclic alkyl(amino) carbene.

Acyclic germylones: congeners of allenes with a central germanium atom

The cyclic alkyl(amino) carbene (cAAC:)-stabilized acyclic germylones (Me2-cAAC:)2Ge (1) and (Cy2-cAAC:)2Ge (2) were prepared utilizing a one-pot synthesis of GeCl2(dioxane), cAAC:, and KC8 in a 1:2:2.1 molar ratio. Dark green crystals of compounds 1 and 2 were produced in 75 and 70% yields, respectively. The reported methods for the preparation of the corresponding silicon compounds turned out to be not applicable in the case of germanium. The single-crystal X-ray structures of 1 and 2 feature the C-Ge-C bent backbone, which possesses a three-center two-electron π-bond system. Compounds 1 and 2 are the first acyclic germylones containing each one germanium atom and two cAAC: molecules. EPR measurements on compounds 1 and 2 confirmed the singlet spin ground state. DFT calculations on 1/2 revealed that the singlet ground state is more stable by ~16 to 18 kcal mol(-1) than that of the triplet state. First and second proton affinity values were theoretically calculated to be of 265.8 (1)/267.1 (2) and 180.4 (1)/183.8 (2) kcal mol(-1), respectively. Further calculations, which were performed at different levels suggest a singlet diradicaloid character of 1 and 2. The TD-DFT calculations exhibit an absorption band at ~655 nm in n-hexane solution that originates from the diradicaloid character of germylones 1 and 2.

A cyclic germadicarbene ("germylone") from germyliumylidene

By employing the chelate dicarbene 1, the new chlorogermyliumylidene complex 2 could be synthesized and isolated in 95% yield. Dechlorination of 2 with sodium naphthalenide furnishes the unique cyclic germadicarbene 3 which could be isolated in 45% yield. Compound 3 is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of 3 with two lone pairs of electrons at the Ge atom.