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Metal–ligand bonds in M(cAACDip)2 (M = Be, Mg) are best described with dative bonds between neutral ligands L⇄M⇄L but not with charged species L(−1)→E(+2)←L(−1) as previously stated. The oxidation state of the metals is zero but not +2.

A low-valent Ga(i) complex, [Ga@Bi10(NbMes)2]3−, with a linear Nb–Ga–Nb fragment was synthesized by the reaction of K5Ga2Bi4 with NbMes2. This unusual Zintl cluster represents the first compound with Nb–Ga and Nb–Bi bonds.

Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the

Carbynes (R-[Formula: see text]), species that bear a monovalent carbon atom with three non-bonding valence electrons, are important intermediates and potentially useful in organic synthetic chemistry. However, free species of the type R-[Formula: see text] of any group 14 element (E) have eluded isolation in the condensed phase due to their high reactivity. Here we report the isolation, characterization and reactivity of a crystalline germylyne radical by using a sterically hindered hydrindacene ligand. The germylyne radical bears an essentially one-coordinate germanium atom as shown by single-crystal X-ray diffraction analysis. Electron paramagnetic resonance spectroscopic studies and theoretical calculations show that the germylyne radical features a doublet ground state, and the three non-bonding valence electrons at the germanium atom contribute to the lone pair of electrons as the highest occupied molecular orbital-3 and one unpaired electron as the singly occupied molecular orbital.

The uranyl with two U=O double bonds is a well-known and predominant form of uranium in the environment, but the carbon-based analog with two U=C double bonds has rarely been synthesized. Here, we ...

The non-classical carbonyl complex [Hg{Fe(CO)5}2]2+ [SbF6]-2 is prepared by reaction of Hg(SbF6)2 and excess Fe(CO)5 in anhydrous HF. The single-crystal X-ray structure reveals a linear Fe-Hg-Fe moiety as well as an eclipsed conformation of the eight basal CO ligands. Interestingly, the Hg-Fe bond length of 2.5745(7) Angstrom is relatively similar to the corresponding Hg-Fe bonds in literature-known [Hg{Fe(CO)4}2]2- dianions (2.52-2.55 Angstrom) which intrigued us to analyze the bonding situation in both the dications and dianions with the EDA-NOCV method. Both species are best described as Hg(0) compounds which are also confirmed by the shape of the HOMO-4 and HOMO-5 of the dication and dianion, respectively, in which the electron pair is located mainly at the Hg. Furthermore, for the dication and the dianion the sigma-backdonation from Hg into the [Fe(CO)5]22+ or the [Fe(CO)4]22- fragment is the most dominant orbital interaction and surprisingly these interaction energies are also very similar even in absolute values. The fact that both iron-based fragments are missing two electrons explains their prominent sigma-acceptor properties.

The chemical bonds in the group-2 fluoride anions AeF− (Ae = Ca, Sr, Ba) have four polarized bonding orbitals, namely two occupied σ orbitals and a pair of occupied degenerate π orbitals with sd-hybridized atomic orbitals of the atoms Ae.

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ−, π−, δ−, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going.

A series of possible precursors for generating C2 with the general formula Me3E–C2–I(Ph)FBF3 [E = C (1), Si (2), and Ge (3)] has been theoretically investigated using quantum chemical calculations. The equilibrium geometries of all species show a linear E–C2–I+ backbone. The inspection of the electronic structure of the Me3E–C2 bond by energy decomposition analysis coupled with the natural orbital for chemical valence (EDA-NOCV) method suggests a combination of electron sharing C–C σ-bond and v weak π-dative bond between Me3C and C2 fragments in the doublet state for species 1 (E = C). For species 2 (Si) and 3 (Ge), the analysis reveals σ-dative Me3E–C2 bonds (E = Si, Ge; Me3E←C2) resulting from the interaction of singly charged (Me3E)+ and (C2–IPh(BF4))− fragments in their singlet states. The C2–I bond is diagnosed as an electron sharing σ-bond in all three species, 1, 2 and 3.

HNBeCO is generated in a neon matrix and identified via vibrational spectroscopy. It possesses an unprecedented triple bond, consisting of two π-bonds and a dative σ-bond, with the former bonds being much stronger than the latter bond.

We discuss the fundamental aspects of the EDA-NOCV method and address some critical comments that have been made recently. The EDA-NOCV method unlike most other methods focuses on the process of bond formation between the interacting species and not just only on the analysis of the finally formed bond. This is demonstrated using LiF as an example. There is a difference between the interactions between the initial species which form the bond and are also the final product of bond cleavage, and the interactions between the fragments in the eventually formed molecule. The flexibility of the method allows the choice of the interacting fragments which helps to identify the charge and electron configuration of the fragments which describe the bond. This is very helpful in cases where the bond may be described with several Lewis structures. We reject the idea that it would be a disadvantage to have “bond path functions” as the energy components in the EDA, which actually indicate the variability of the method. The bonding analysis in a different sequence of the bond formation gives important results for the various questions that can be asked. This is demonstrated by using CH2, CO2 and the formation of a guanine quartet as examples. The fact that a bond is always defined by the bound molecule, the fragments, and their states is universal and deeply physical, as we show here again for various examples. The results of the EDA-NOCV method are in full accordance with the physical mechanism of the chemical bond as revealed by Ruedenberg.

The molecules [(NHC)–C2O2–(NHC)]q (q = 0, +1, +2) have strong electron-sharing and dative C–C bonds making all systems thermodynamically stable. The stability of the neutral complex is due to the much lower S/T gap of [NHC–CO] than of CO.

The cationic complex BeOCO+ is produced in a solid neon matrix. Infrared absorption spectroscopic study shows that it has a very high antisymmetric OCO stretching vibration of 2418.9 cm-1, which is about 71 cm-1 blue-shifted from that of free CO2. The quantum chemical calculations are in very good agreement with the experimental observation. Depending on the theoretical method, a linear or quasi-linear structure is predicted for the cation. The analysis of the electronic structure shows that the bonding of Be+ to one oxygen atom induces very little charge migration between the two moieties, but it causes a significant change in the σ-charge distribution that strengthens the terminal C-O bond, leading to the observed blue shift. The bonding analysis reveals that the Be+ ← OCO donation results in strong binding due to the interference of the wave function and a charge polarization within the CO2 fragment and hybridization to Be+ but only negligible charge donation.

Heteronuclear calcium–iron carbonyl cation complexes in the form of [CaFe(CO)n]+ (n=5–12) are produced in the gas phase. Infrared photodissociation spectroscopy in conjunction with quantum chemical calculations confirm that the n=10 complex is the coordination saturated ion where a Fe(CO)4 fragment is bonded with a Ca(CO)6 fragment through two side‐on bridging carbonyl ligands. Bonding analysis indicates that it is best described by the bonding interactions between a [Ca(CO)6]2+ dication and an [Fe(CO)4]− anion forming a Fe→Ca d−d dative bond in the [(CO)6Ca–Fe(CO)4]+ structure, which enriches the pool of experimentally observed complexes of calcium that mimic transition metal compounds. The molecule is the first example of a heteronuclear carbonyl complex featuring a d−d bond between calcium and a transition metal., The [CaFe(CO)10]+ cation is characterized to be a coordination saturated ion where a Fe(CO)4 fragment is bonded with a Ca(CO)6 moiety through two side‐on bridging carbonyl ligands. The complex is best described by the bonding interactions between a [Ca(CO)6]2+ dication and an [Fe(CO)4]− anion forming a Fe−Ca d−d dative bond that is enhanced by the attraction between Ca and the bridging CO ligands.

The elusive plumbylone {[SiII(Xant)SiII]Pb0} 3 stabilized by the bis(silylene)xanthene chelating ligand 1, [SiII(Xant)SiII=PhC(NtBu)2Si(Xant)Si(NtBu)2CPh], and its isolable carbonyl iron complex {[SiII(Xant)SiII]Pb0Fe(CO)4} 4 are reported. The compounds 3 and 4 were obtained stepwise via reduction of the lead(II) dibromide complex {[SiII(Xant)SiII]PbBr2} 2, prepared from the bis(silylene)xanthene 1 and PbBr2, employing potassium naphthalenide and K2Fe(CO)4, respectively. While the genuine plumbylone 3 is rather labile even at −60 °C, its Pb0→Fe(CO)4 complex 4 turned out to be relatively stable and bottleable. However, solutions of 4 decompose readily to elemental Pb and {[SiII(Xant)SiII]Fe(CO)3} 5 at 80 °C. Reaction of 4 with [Rh(CO)2Cl]2 leads to the formation of the unusual dimeric [(OC)2RhPb(Cl)Fe(CO)4] complex 6 with trimetallic Rh−Pb−Fe bonds. The molecular and electronic structures of 3 and 4 were established by Density Functional Theory (DFT) calculations.

The CaCO

Syntheses of the copper and gold complexes [Cu{Fe(CO)5}2][SbF6] and [Au{Fe(CO)5}2][HOB{3,5‐(CF3)2C6H3}3] containing the homoleptic carbonyl cations [M{Fe(CO)5}2]+ (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu2Fe, Ag2Fe and Au2Fe complexes [Cu{Fe(CO)5}2][SbF6], [Ag{Fe(CO)5}2][SbF6] and [Au{Fe(CO)5}2][HOB{3,5‐(CF3)2C6H3}3] are also given. The silver and gold cations [M{Fe(CO)5}2]+ (M=Ag, Au) possess a nearly linear Fe‐M‐Fe’ moiety but the Fe‐Cu‐Fe’ in [Cu{Fe(CO)5}2][SbF6] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF6]− anion. The Fe(CO)5 ligands adopt a distorted square‐pyramidal geometry in the cations [M{Fe(CO)5}2]+, with the basal CO groups inclined towards M. The geometry optimization with DFT methods of the cations [M{Fe(CO)5}2]+ (M=Cu, Ag, Au) gives equilibrium structures with linear Fe‐M‐Fe’ fragments and D 2 symmetry for the copper and silver cations and D 4d symmetry for the gold cation. There is nearly free rotation of the Fe(CO)5 ligands around the Fe‐M‐Fe’ axis. The calculated bond dissociation energies for the loss of both Fe(CO)5 ligands from the cations [M{Fe(CO)5}2]+ show the order M=Au (D e=137.2 kcal mol−1)>Cu (D e=109.0 kcal mol−1)>Ag (D e=92.4 kcal mol−1). The QTAIM analysis shows bond paths and bond critical points for the M−Fe linkage but not between M and the CO ligands. The EDA‐NOCV calculations suggest that the [Fe(CO)5]→M+←[Fe(CO)5] donation is significantly stronger than the [Fe(CO)5]←M+→[Fe(CO)5] backdonation. Inspection of the pairwise orbital interactions identifies four contributions for the charge donation of the Fe(CO)5 ligands into the vacant (n)s and (n)p AOs of M+ and five components for the backdonation from the occupied (n‐1)d AOs of M+ into vacant ligand orbitals., Donation of an electron pair from the HOMO of [Fe(CO)5]2 to the 4s LUMO of Cu+ giving the (+,+) component of the Fe‐Cu‐Fe σ bond in [Cu{Fe(CO)5}2]+. The associated charge flow with the direction red→blue is given by the deformation density.

The carbon suboxide anion C3O2 − is generated in solid neon matrix. It is characterized by infrared absorption spectroscopy as well as quantum chemical calculations to have a planar C s structure where two CO groups with significantly different bond lengths and angles are attached in a zigzag fashion to the central carbon atom. Bonding analysis indicates that it is best described by the bonding interactions between a neutral CO in a triplet excited state and a doublet excited state of CCO−., The carbon suboxide anion C3O2 − is generated in a solid neon matrix, and is characterized to have an unsymmetrical structure by the bonding interactions between a neutral CO in a triplet excited state and a doublet excited state of CCO−.

The synthesis of coinage metal aluminyl complexes, featuring M–Al covalent bonds, is reported via a salt metathesis approach employing an anionic Al(i) (‘aluminyl’) nucleophile and group 11 electrophiles. This approach allows access to both bimetallic (1 : 1) systems of the type (tBu3P)MAl(NON) (M = Cu, Ag, Au; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) and a 2 : 1 di(aluminyl)cuprate system, K[Cu{Al(NON)}2]. The bimetallic complexes readily insert heteroallenes (CO2, carbodiimides) into the unsupported M–Al bonds to give systems containing a M(CE2)Al bridging unit (E = O, NR), with the μ-κ1(C):κ2(E,E′) mode of heteroallene binding being demonstrated crystallographically for carbodiimide insertion in the cases of all three metals, Cu, Ag and Au. The regiochemistry of these processes, leading to the formation of M–C bonds, is rationalized computationally, and is consistent with addition of CO2 across the M–Al covalent bond with the group 11 metal acting as the nucleophilic partner and Al as the electrophile. While the products of carbodiimide insertion are stable to further reaction, their CO2 analogues have the potential to react further, depending on the identity of the group 11 metal. (tBu3P)Au(CO)2Al(NON) is inert to further reaction, but its silver counterpart reacts slowly with CO2 to give the corresponding carbonate complex (and CO), and the copper system proceeds rapidly to the carbonate even at low temperatures. Experimental and quantum chemical investigations of the mechanism of the CO2 to CO/carbonate transformation are consistent with rate-determining extrusion of CO from the initially-formed M(CO)2Al fragment to give a bimetallic oxide that rapidly assimilates a second molecule of CO2. The calculated energetic barriers for the most feasible CO extrusion step (ΔG‡ = 26.6, 33.1, 44.5 kcal mol−1 for M = Cu, Ag and Au, respectively) are consistent not only with the observed experimental labilities of the respective M(CO)2Al motifs, but also with the opposing trends in M–C (increasing) and M–O bond strengths (decreasing) on transitioning from Cu to Au., The differential reactivity of copper, silver and gold aluminyl compounds towards CO2 and other heteroallenes are probed by experimental and quantum chemical methods.

Quantum chemical calculations using density functional methods were performed for complexes of type L2C2 with L = NHCMe (1), SNHCMe (2) (S = saturated), cAACMe (3), and diamidocarbene (DACMe) (4). ...

The boron atoms react with carbon monoxide and dinitrogen forming the end‐on bonded NNBCO complex in solid neon or in nitrogen matrices. The NNBCO complex rearranges to the (η 2‐N2)BCO isomer with a more activated side‐on bonded dinitrogen ligand upon visible light excitation. (η 2‐N2)BCO and its weakly CO‐coordinated complexes further isomerize to the NBNCO and B(NCO)2 molecules with N−N bond being completely cleaved under UV light irradiation. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge‐ and energy‐partitioning methods., CO‐induced dinitrogen fixation and cleavage mediated by boron atoms is reported. The experimental and theoretical results show that the N≡N activation processes proceed with the initial formation of an end‐on bonded NNBCO complex, which rearranges to the NBNCO and B(NCO)2 isocyanate complexes via a side‐on bonded (η2‐N2)BCO intermediate. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge‐ and energy‐partitioning methods.

In contrast to naturally occurring F2, O2 and N2, diatomic C2 is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C2←L compounds but the bonding situation of the central C2 in this motif differs remarkably from that of free C2. Here we have prepared and structurally characterized diatomic C2 as a monoligated complex L→C2 using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHCR=N)2(CH3)P, where NHCR is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C2 complex both have carbene character. The complex underwent intermolecular C–H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol. Diatomic C2 is an elusive species that has only been indirectly observed in the gas phase. It had previously been stabilized in the condensed phase using two ligands, but now a monoligated L→C2 complex has been prepared with a bulky phosphine ligand (L) bearing two imidazolidin-2-iminato groups. Reactivity studies and theoretical quantum chemical analysis point to the C2 moiety having a dicarbene character.

The syntheses of the two novel complexes [Ag{Mo/W(CO)6}2]+[F‐{Al(ORF)3}2]− (RF=C(CF3)3) are reported along with their structural and spectroscopic characterization. The X‐ray structure shows that three carbonyl ligands from each M(CO)6 fragment bend towards the silver atom within binding Ag−C distance range. DFT calculations of the free cations [Ag{M(CO)6}2]+ (M=Cr, Mo, W) in the electronic singlet state give equilibrium structures with C 2 symmetry with two bridging carbonyl groups from each hexacarbonyl ligand. Similar structures with C 2 symmetry (M=Nb) and D 2 symmetry (M=V, Ta) are calculated for the isoelectronic group 5 anions [Ag{M(CO)6}2]− (M=V, Nb, Ta). The electronic structure of the cations is analyzed with the QTAIM and EDA‐NOCV methods, which provide detailed information about the nature of the chemical bonds between Ag+ and the {M(CO)6}2 q (q = −2, M = V, Nb, Ta; q = 0, M = Cr, Mo, W) ligands., Hexacarbonyls as ligands: The molecules M(CO)6 (M=Mo, W) act as ligands towards the silver ion, giving the title compounds [Ag{M(CO)6}2]+[F‐{Al(ORF)3}2]− (RF=C(CF3)3). Their structures and bonding have been analyzed and compared with those of the very recently prepared isoelectronic complexes [Ag{M(CO)6}2]− (M=Nb, Ta).

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, C3 stabilization: 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.

Borepin, a 7-membered boron-containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron-deficient borepins are well-established, reduced electron-rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a, 2 b) and anion (3 a, 3 b) complexes, which have been synthesized by potassium graphite (KC8) reduction of cyclic(alkyl)(amino) carbene-dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X-ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.

Following the recent discovery of stable octa‐coordinated alkaline earth metals with N2 and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT‐based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N2 and CO become much more nucleophilic and electrophilic in all complexes, relevant for N2 fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO–LUMO) orbital interactions., Alkaline earth complexes: The viability of alkaline earth metal‐based N2 fixation is investigated. It is shown that the activation of N2 through complexation originates from the d–π back‐donation. Within one row of the periodic table, the alkaline earth metal shows the strongest nucleophilic activation of the N2 ligand, confirming clear trends in proton affinities.

Double bonds have been realized for a wide variety of elements in the p-, d-, and f-blocks. However, no s-block metal complexes with a double bond have been identified. Here we report the synthesis...

Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight‐coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six‐coordinate Ti(CO)6 + and seven‐coordinate Ti(CO)7. Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20‐electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18‐electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7. The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable., Bonding discoveries: The coordinatively saturated mononuclear carbonyl complexes of Group 4 elements, TM(CO)8 (TM=Zr, Hf) and Ti(CO)7, were prepared in the gas phase and/or in solid neon matrix. The analysis of the bonding situation shows that the formal 20‐electron complexes observe the 18‐electron rule and that Ti(CO)8 is unstable for the loss of one CO because of steric repulsion.