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Quantum chemical calculations are reported for the complexes of alkaline earth metals AeOLi2 (Ae = Be-Ba) at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPPQZVPP levels. The nature of the Ae-OLi2 bond has been analyzed with a variety of methods. The AeOLi2 molecules exhibit an unprecedented σ donor bond AeâOLi2 where the (n)s2 lone-pair electrons of the Ae atom are donated to vacant O-Li2 antibonding orbitals having the largest coefficient at lithium. This is a covalent bond where the accumulation of the associated electronic charge is located at two positions above and below the Ae-OLi2 axis. The bifurcated component of orbital interactions is structurally related to the recently proposed collective bonding model, but exhibits a completely different type of bonding. The most stable isomer of AeOLi2 has a C 2v geometry and a singlet (1A1) electronic ground state. The bond dissociation energy (BDE) of the Ae-OLi2 bonds exhibits a zig-zag trend from BeOLi2 to BaOLi2, with BeOLi2 having the largest BDE (D e = 73.0 kcal mol-1) and MgOLi2 possessing the lowest BDE (D e = 42.3 kcal mol-1) at the CCSD(T) level. The calculation of the atomic partial charges by the Hirshfeld and Voronoi methods suggests that Be and Mg carry small negative charges in the lighter molecules whereas the heavier atoms Ca-Ba have small positive charges. In contrast, the NBO and QTAIM methods give positive charges for all Ae atoms that are larger for Ca-Ba than that calculated by the Hirshfeld and Voronoi approaches. The molecules AeOLi2 have large dipole moments where the negative end is at the Ae atom with the polarity AeâOLi2. The largest dipole moments are predicted for the lighter species BeOLi2 and MgOLi2 and the smallest value is calculated for BaOLi2. The calculation of the vibrational spectra shows a significant red-shift toward lower wave numbers for the Ae-OLi2 stretching mode in comparison to diatomic AeO. Besides the AeâOLi2 σ-donor bonds there are also three dative bonds due to AeâOLi2 backdonation which consist of one σ bond and two π bonds. The appearance of strong AeâOLi2 σ donation leads to quadruple bonds AeOLi2 in all systems AeOLi2, even for the lightest species with Ae = Be, Mg. The valence orbitals of Ca, Sr, and Ba, which are involved in the dative interactions, are the (n)s and (n-1)d AOs whereas Be and Mg use their (n)s and (n)p AOs. The EDA-NOCV results are supported by the AdNDP calculations which give four 2c-2e bonding orbitals. Three bonding orbitals have occupation numbers â¼2. One σ orbital has smaller occupation numbers between 1.32 and 1.73 due to the delocalization to the lithium atoms. The analysis of the electronic structure with the ELF method suggests multicenter bonds with mainly trisynaptic and tetrasynaptic basins, which also support the results of the EDA-NOCV calculations.
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The unprecedented silylene-supported dibenzodiboraoxepin 2 and 9,10-diboraphenanthrene complexes 6 and 8 were synthesized. The (NHSi)2B2(xanthene) [NHSi = PhC(NtBu)2(Me2N)Si:] 2 results from debromination of the bis(NHSi)-stabilized bis(dibromoboryl)xanthene 1 with potassium graphite (KC8); 2 is capable of activating white phosphorus and ammonia to form the B2P4 cage compound 3 and H2N-B-B-H diborane species 4, respectively. The thermal rearrangement of 2 affords the 9,10-dihydro-9,10-diboraphenanthrene 5 through a bis(NHSi)-assisted intramolecular reductive C-O-C deoxygenation process. Notably, the 9,10-diboraphenanthrene derivatives 6 and 8 could be generated by deoxygenation of 2 with KC8 and 1,3,4,5-tetramethylimidazol-2-ylidene, respectively. The aromaticity of 6 and 8 was confirmed by computational studies. Strikingly, the NHSi ligand in 8 engenders the monodeoxygenation of carbon dioxide in toluene at room temperature to form the CO-stabilized 9,10-diboraphenanthrene derivative 9 via the silaoxadiborinanone intermediate 10.
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[This corrects the article DOI: 10.1039/D4SC00142G.].
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Quantum chemical calculations using ab initio methods at the MRCI+Q(8,9)/def2-QZVPPD and CCSD(T)/def2-QZVPPD levels as well as using density functional theory are reported for the diatomic molecules AeN- (Ae=Ca, Sr, Ba). The anions CaN- and SrN- have electronic triplet (3Π) ground states with nearly identical bond dissociation energies De ~57â kcal/mol calculated at the MRCI+Q(8,9)/def2-QZVPPD level. In contrast, the heavier homologue BaN- has a singlet (1Σ+) ground state, which is only 1.1â kcal/mol below the triplet (3Σ-) state. The computed bond dissociation energy of (1Σ+) BaN- is 68.4â kcal/mol. The calculations at the CCSD(T)-full/def2-QZVPPD and BP86-D3(BJ)/def2-QZVPPD levels are in reasonable agreement with the MRCI+Q(8,9)/def2-QZVPPD data, except for the singlet (1Σ+) state, which has a large multireference character. The calculated atomic partial charges given by the CM5, Voronoi and Hirshfeld methods suggest small to medium-sized AeâN- charge donation for most electronic states. In contrast, the NBO method predicts for all species medium to large AeâN- electronic charge donation, which is due to the neglect of the (n)p AOs of Ae atoms as genuine valence orbitals. Neither the bond orders nor the bond lengths correlate with the bond dissociation energies. The EDA-NOCV calculations show that the heavier alkaline earth atoms Ca, Sr, Ba use their (n)s and (n-1)d orbitals for covalent bonding.
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Recently, planar and neutral tricoordinated oxygen embedded in graphene has been imaged experimentally (Nat. Commun., 2019, 10, 4570-4577). In this work, this unusual chemical species is studied utilizing a variety of state-of-the-art methods and combining periodic calculations with a fragmental approach. Several factors influencing the stability of trivalent oxygen are identified. A σ-donation and a π-backdonation mechanism between graphite and oxygen is established. π-Local aromaticity, with a delocalized 4c-2e bond involving the oxygen atom and the three nearest carbon atoms aids in the stabilization of this system. In addition, the framework in which the oxygen is embedded is crucial too to the stabilization, helping to delocalize the "extra" electron pair in the virtual orbitals. Based on the understanding gathered in this work, a set of organic molecules containing planar and neutral trivalent oxygen is theoretically proposed for the first time.
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The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.
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The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below -30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.
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Quantum chemical calculations of the anions AeF- (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 detailed bonding analyses using different charge- and energy partitioning methods show that the molecules possess three distinctively different dative bonds in the lighter species with Ae=Be, Mg and four dative bonds when Ae=Ca, Sr, Ba. The occupied 2p atomic orbitals (AOs) and to a lesser degree the occupied 2s AO of F- donate electronic charge into the vacant spx(σ) and p(π) orbitals of Be and Mg which leads to a triple bond Ae F-. The heavier Ae atoms Ca, Sr, Ba use their vacant (n-1)d AOs as acceptor orbitals which enables them to form a second σ donor bond with F- that leads to quadruply bonded Ae F- (Ae=Ca-Ba). The presentation of molecular orbitals or charge distribution using only one isodensity value may give misleading information about the overall nature of the orbital or charge distribution. Better insights are given by contour line diagrams. The ELF calculations provide monosynaptic and disynaptic basins of AeF- which nicely agree with the analysis of the occupied molecular orbitals and with the charge density difference maps. A particular feature of the covalent bonds in AeF- concerns the inductive interaction of F- with the soft valence electrons in the (n)s valence orbitals of Ae. The polarization of the (n)s2 electrons induces a (n)spx hybridized lone-pair orbital at atom Ae, which yields a large dipole moment with the negative end at Ae. The concomitant formation of a vacant (n)spx AO of atom Ae, which overlaps with the occupied 2p(σ) AO of F-, leads to a strong covalent σ bond.
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The C60 fullerene molecule has attracted tremendous interest for its distinctive nearly spherical structure. By contrast, all-metal counterparts have been elusive: Fullerene-like clusters composed of noncarbon elements typically suffer from instability, resulting in more compact geometries that require multiple embedded atoms or external ligands for stabilization. In this work, we present the synthesis of an all-metal fullerene cluster, [K@Au12Sb20]5-, using a wet-chemistry method. The cluster's structure was determined by single crystal x-ray diffraction, which revealed a fullerene framework consisting of 20 antimony atoms. Theoretical calculations further indicate that this distinct cluster exhibits aromatic behavior.
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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 â C4 + 4L. However, their stability with respect to (L)4C4 â 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 â 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.
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σ-complexes of homoatomic E-E bonds are key intermediates in catalytically relevant oxidative addition reactions, but are as yet unknown for the group 13 elements. Here, stable species best described as σ-complexes of a 1,2-dichlorodigallane derivative with Ni and Pd are reported. They are readily accessed through the combination of a 1,2-dichlorodigallane derivative, which features chelating phosphine functionalities, with Ni0 and Pd0 synthons. In-depth computational analyses of these complexes importantly reveal considerable Ga-Ga bonding interactions in both Ni and Pd complexes, despite the expected elongation of the Ga-Ga bond upon complexation, suggestive of σ-complex character as opposed to more commonly described bis(gallyl) character. Finally, the well-defined disproportion of the Ni complex is described, leading to a unique GaI-nickel complex, with concomitant expulsion of uncomplexed GaIII species.
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Carbodicarbene (CDC) has become an emerging ligand in many fields due to its strong σ-donating ability.
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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) Å is relatively similar to the corresponding Hg-Fe bonds in literature-known [Hg{Fe(CO)4}2]2- dianions (2.52-2.55 Å), which intrigued us to analyze the bonding situation in both the dications and dianions with the energy decomposition analysis with natural orbitals for chemical valence (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 σ back-donation 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 σ-acceptor properties.
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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.
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Quantum chemical calculations of anions AeF- (Ae = Be-Ba) and isoelectronic group-13 molecules EF (E = B-Tl) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 various basis sets. The equilibrium distances, bond dissociation energies and vibrational frequencies are reported. The alkali earth fluoride anions AeF- exhibit strong bonds between the closed-shell species Ae and F- with bond dissociation energies between 68.8 kcal mol-1 for MgF- and 87.5 kcal mol-1 for BeF- and they show an unusual increasing trend MgF- < CaF- < SrF- < BaF-. This is in contrast to the isoelectronic group-13 fluorides EF where the BDE continuously decreases from BF to TlF. The calculated dipole moments of AeF- are very large between 5.97 D for BeF- and 1.78 D for BaF- with the negative end always at the Ae atom (AeâF-). This is explained by the location of the electronic charge of the lone pair at Ae, which is rather distant from the nucleus. The analysis of the electronic structure of AeF- suggests significant charge donation AeâF- into the vacant valence orbitals of Ae. A bonding analysis with the EDA-NOCV method suggests that the molecules are mainly covalently bonded. The strongest orbital interaction in the anions comes from the inductive polarization of the 2pσ electrons of F-, which leads to a hybridization of the (n)s and (n)pσ AOs at Ae. There are two degenerate π donor interactions AeâF- in all anions AeF-, which provide 25-30% to the covalent bonding. There is another σ orbital interaction in the anions, which is very weak in BeF- and MgF-. In contrast, the second stabilizing σ orbital interaction in CaF-, SrF- and BaF- yields a strongly stabilizing σ orbital, because the Ae atoms use their (n - 1)dσ AOs for bonding. The energy lowering of the second σ interaction in the latter anions is even stronger than the π bonding. The EDA-NOCV results suggest that BeF- and MgF- have three strongly polarized bonds, whereas CaF-, SrF- and BaF- have four bonding orbitals. The quadruple bonds in the heavier alkaline earth species are made possible because they use s/d valence orbitals like transition metals for covalent bonding. The EDA-NOCV analysis of the group-13 fluorides EF gives a conventional picture with one very strong σ bond and two rather weak π interactions.
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In this work, we report a low-valent Ga(I) complex, [Ga@Bi10(NbMes)2]3-, with a linear Nb-Ga-Nb fragment, representing the first compound with Nb-Ga and Nb-Bi bonds. Quantum-chemical calculations reveal that the complex is an electron-precise cluster. The possible fragmentation pathway of the title cluster was studied by using electrospray ionization mass spectrometry and theoretical calculations.
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This work describes a strategy not only to isolate a dynamically stable radical with physical property tunability, but to efficiently regulate the radical dissociation with reversibility and photo controllability. The addition of Lewis acid B(C6 F5 )3 (BCF) into the solution of a radical σ-dimer (1-1) led to a stable radical (1â -2B), which has been characterized by EPR spectroscopy, UV/Vis spectroscopy and single crystal X-ray diffraction, in conjunction with theoretical calculation. The radical species is stabilized mainly by captodative effect, single electron transfer and steric effect. The absorption maximum of the radical can be tuned by using different Lewis acids. Dimer 1-1 can be achieved back by addition of a stronger base into the solution of 1â -2B, exhibiting a reversible process. By introducing a photo BCF generator, the dissociation of the dimer and the formation of the radical adduct become photocontrollable.
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Quantum chemical calculations using ab initio methods at the MRCI+Q(6,8)/def2-QZVPP and CCSD(T)/def2-QZVPP levels as well as density functional theory are reported for the diatomic molecules AeB- and isoelectronic AeC (Ae=Ca, Sr, Ba). The boride anions AeB- have an electronic triplet (3 Σ- ) ground state. The quintet (5 Σ- ) state is 5.8-12.3â kcal/mol higher in energy and the singlet (1 Δ) state is 13.1-15.3â kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low-lying triplet (3 Σ- ) state but the quintet (5 Σ- ) state is only 2.2â kcal/mol (SrC) and 2.9â kcal/mol (CaC) above the triplet state. The triplet (3 Σ- ) and quintet (5 Σ- ) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet (3 Σ- ) state are between De =38.3-41.7â kcal/mol for AeB- and De =49.4-57.5â kcal/mol for AeC. The barium species have always the strongest bonds whereas the calcium and strontium compounds have similar BDEs. The bonding analysis indicates that there is little charge migration in AeB- in the direction AeâB- where the alkaline earth atoms carry positive charges between 0.09â e-0.22â e. The positive charges at the Ae atoms are much larger in AeC where the charge migration AeâC is between 0.90â e-0.91â e. A detailed analysis of the interatomic interactions with the EDA-NOCV method shows that all diatomic species AeB- and AeC are built from dative interactions between Ae (1 S, ns2 ) and B- or C (3 P, 2â s2 2pπ 1 2pπ' 1 ). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae+ (2 S, ns1 )+C- (4 S, 2â s2 2pπ 1 2pπ' 1 2pσ 1 ). Inspection of the orbital interactions suggests that the alkaline earth atoms Ca, Sr, Ba use mainly their (n-1)d AOs besides the (n)s AOs for the covalent bonds. This creates a second energetically low-lying σ-bonding MO in the molecules, which feature valence orbitals with the order Ï1 (σ-bonding)<Ï2 (σ-bonding)<Ï3 (degenerate π-bonding). All four occupied valence MOs of AeB- and AeC are bonding orbitals. Since the degenerate π orbitals Ï3 are only singly occupied, the formal bond order is three.