Formation and Characterization of the Boron Dicarbonyl Complex [B(CO)2]−

Ligand-Induced Tuning of the Electronic Structure of Rhombus Tetraboron Cluster

A neutral boron carbonyl complex B₄ (CO)₃ is generated in the gas phase and is characterized by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy and quantum chemical calculations. The complex is identified to have a planar C_2v structure with three CO ligands terminally coordinated to a rhombus B₄ core. It has a closed-shell singlet ground state that correlates to an excited state of B₄ . Bonding analyses on B₄ (CO)₃ as well as the previously reported B₄ and B₄ (CO)₂ indicate that the electronic structure of rhombus tetraboron cluster changes from a close-shell singlet to an open-shell singlet in B₄ (CO)₂ and to a close-shell singlet in B₄ (CO)₃ , demonstrating that the electronic structures of boron clusters can be effectively tuned via sequential CO ligand coordination.

Linear Group 13 E≡E Triple Bonds in E2 Li6 2

The triply bonded heavier main-group compounds have a textbook trans-bent geometry, in contrast to a familiar linear form found for the lightest analogues. Strikingly, the unexpected linear group 13 E≡E triple bonds were herein found in the D_4h -symmetry E₂ Li₆ ²⁺ clusters, and they possess a large barrier (>18.0 kcal/mol) towards the dissociation of Li⁺ . The perfectly surrounded Li₄ motifs and two linear coordinated Li atoms strongly suppress the increasing nonbonded electron density of heavier E atoms, making two degenerate π bonds and one multi-center σ bond in linear heavier main-group triple bonds. The surrounding Li₆ motifs not only creates an effective electronic structure to form a linear E≡E triple bond, but the resulting electrostatic interactions account for the highly stable global E₂ Li₆ ²⁺ clusters.

Highly Coordinated Heteronuclear Calcium-Iron Carbonyl Cation Complexes [CaFe(CO)n ]+ (n=5-12) with d-d Bonding

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)6 Ca-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.

Chemical Bonding in Homoleptic Carbonyl Cations [M{Fe(CO)5 }2 ]+ (M=Cu, Ag, Au)

Syntheses of the copper and gold complexes [Cu{Fe(CO)5 }2 ][SbF6 ] and [Au{Fe(CO)5 }2 ][HOB{3,5-(CF3 )2 C6 H3 }3 ] containing the homoleptic carbonyl cations [M{Fe(CO)5 }2 ]+ (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu2 Fe, Ag2 Fe and Au2 Fe complexes [Cu{Fe(CO)5 }2 ][SbF6 ], [Ag{Fe(CO)5 }2 ][SbF6 ] and [Au{Fe(CO)5 }2 ][HOB{3,5-(CF3 )2 C6 H3 }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 D2 symmetry for the copper and silver cations and D4d 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 (De =137.2 kcal mol-1 )>Cu (De =109.0 kcal mol-1 )>Ag (De =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.

Formation and Characterization of BeFe(CO)4 - Anion with Beryllium-Iron Bonding

Heteronuclear BeFe(CO)₄ ^- anion complex is generated in the gas phase, which is detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The complex is characterized to have a Be-Fe bonded Be-Fe(CO)₄ ^- structure with C_3v symmetry and all of the four carbonyl ligands bonded on the iron center. Quantum chemical studies indicate that the complex has a quite short Be-Fe bond. Besides one electron-sharing σ bond, there are two additional, albeit weak, Be ← Fe(CO)₄ ^- dative π bonding interactions. The findings imply that metal-metal bonding between s-block and transition metals is viable under suitable coordination environment.

Generation and Characterization of the C3 O2 - Anion with an Unexpected Unsymmetrical Structure

The carbon suboxide anion C₃ O₂ ^- 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^- .

CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron

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 (η2 -N2 )BCO isomer with a more activated side-on bonded dinitrogen ligand upon visible light excitation. (η2 -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.

Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons

Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)0 (2pσ )0 (2pπ )2 and the CO/NO- ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.

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.

The Valence Orbitals of the Alkaline-Earth Atoms

Quantum chemical calculations of the alkaline-earth oxides, imides and dihydrides of the alkaline-earth atoms (Ae=Be, Mg, Ca, Sr, Ba) and the calcium cluster Ca6 H9 [N(SiMe3 )2 ]3 (pmdta)3 (pmdta=N,N,N',N'',N''-pentamethyldiethylenetriamine) have been carried out by using density functional theory. Analysis of the electronic structures by charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and (n-1)d orbitals. The alkaline-earth metals Be and Mg build covalent bonds like typical main-group elements, whereas Ca, Sr and Ba covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline-earth metals, but are also very important for understanding and designing experimental studies.

Side-On Bonded Beryllium Dinitrogen Complexes

The preparation and spectroscopic identification of the complexes NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) and the energetically higher lying isomers Be(NN)2 and Be(NN)3 are reported. NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) are the first examples of covalently side-on bonded N2 adducts of a main-group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) should be classified as π complexes rather than metalladiazirines.

Comment on "Realization of Lewis Basic Sodium Anion in the NaBH3 - Cluster"

We challenge the interpretation of the chemical bond in NaBH₃ ^- proposed by Liu et al. We argue that NaBH₃ ^- has an electron-sharing Na-BH₃ ^- covalent bond rather than a dative bond Na^- →BH₃ .

Transition-Metal Chemistry of Alkaline-Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr, Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low-temperature Ne matrix. Both complexes are characterized by a D3 symmetric structure involving three equivalent η6 -bound benzene ligands and a closed-shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal-ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n-1)d AOs of M and strong backdonation from the occupied (n-1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20-electron complexes have 18 effective valence electrons, and, thus, fulfill the 18-electron rule if only the metal-ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

Octacarbonyl Ion Complexes of Actinides [An(CO)8 ]+/- (An=Th, U) and the Role of f Orbitals in Metal-Ligand Bonding

The octacarbonyl cation and anion complexes of actinide metals [An(CO)₈ ]^+/- (An=Th, U) are prepared in the gas phase and are studied by mass-selected infrared photodissociation spectroscopy. Both the octacarbonyl cations and anions have been characterized to be saturated coordinated complexes. Quantum chemical calculations by using density functional theory show that the [Th(CO)₈ ]⁺ and [Th(CO)₈ ]^- complexes have a distorted octahedral (D_4h ) equilibrium geometry and a doublet electronic ground state. Both the [U(CO)₈ ]⁺ cation and the [U(CO)₈ ]^- anion exhibit cubic structures (O_h ) with a ⁶ A_1g ground state for the cation and a ⁴ A_1g ground state for the anion. The neutral species [Th(CO)₈ ] (O_h ; ¹ A_1g ) and [U(CO)₈ ] (D_4h ; ⁵ B_1u ) have also been calculated. Analysis of their electronic structures with the help on an energy decomposition method reveals that, along with the dominating 6d valence orbitals, there are significant 5f orbital participation in both the [An]←CO σ donation and [An]→CO π back donation interactions in the cations and anions, for which the electronic reference state of An has both occupied and vacant 5f AOs. The trend of the valence orbital contribution to the metal-CO bonds has the order of 6d≫5f>7s≈7p, with the 5f orbitals of uranium being more important than the 5f orbitals of thorium.

Bent Phosphaallenes With "Hidden" Lone Pairs as Ligands

Phosphaheteroallenes R-P=C=L, with L = N-heterocyclic carbenes (NHCs), can be viewed to a certain extent as phosphaisonitriles stabilized with NHCs, R-P=C:←L. The suitability of these molecules as ligands for coinage-metal ions was investigated and coordination through the central carbon center was observed in most cases. A combination of experiments, spectroscopic methods, and DFT calculations indicates the presence of a hidden electron pair at the carbon center of R-P=C:←L. Remarkably, this lone pair also inserts intramolecularly in C-H bonds showing the carbene-type reactivity which is expected for phosphaisonitriles.

Stability and donor-acceptor bond in dinuclear organometallics CpM1-M2Cl3 (M1, M2 = B, Al, Ga, In; Cp = η 5-C5H5)

The geometries and stabilities of the dinuclear organometallics CpM₁-M₂Cl₃ (M₁, M₂ = B, Al, Ga, In; Cp = η ⁵-C₅H₅) have been investigated by density functional theory (DFT) at M06 L/6-311G(d, p) levels. The nature of the donor-acceptor M₁ → M₂ bond was also studied based on the atoms in molecules (AIM) theory, energy decomposition analysis (EDA) and natural bond orbital (NBO) analysis. The results show that the electronegativity of the M atom determines the stability and covalent character of the dinuclear organometallics CpM₁-M₂Cl₃. The compounds in which the M with larger electronegativity acts as the donor are more stable than in those in which it acts as the acceptor in the donor-acceptor bond, and the donor-acceptor bond has more covalent characteristics. The strength and polarity of the M₁ → M₂ donor-acceptor bond is determined by the periodicity of the M atom. When the period number of the M₁ atom is smaller than that of M₂, the strength of the M₁ → M₂ bond is larger than that of the M₂ → M₁ bond. For homonuclear dinuclear organometallics, the polarity of the M-M bond increases with increasing atomic number of the M atom. For heteronuclear complexes, the polarity of the M₁-M₂ bond for a given M₁ also increases in the sequence of M₂ = B, Al, Ga, and In. Graphic abstract Molecular graph and electron location function isosurfaces map of CpM 1 -M 2 Cl 3(small red spheres represent bond critical points).

Reactivity of a Dihydrodiborene with CO: Coordination, Insertion, Cleavage, and Spontaneous Formation of a Cyclic Alkyne

Under a CO atmosphere, the dihydrodiborene [(cAAC)HB=BH(cAAC)] underwent coordination of CO concomitant with reversible hydrogen migration from boron to the carbene carbon atom, as well as reversible CO insertion into the B=B bond. Heating the CO adduct resulted in two unusual cAAC ring-expansion products, one presenting a B=C bond to a six-membered 1,2-azaborinane-3-ylidene, the other an unprecedented nine-membered cyclic alkyne resulting from reductive cleavage of CO and spontaneous formation of a C≡C bond.

The Bonding Situation in Metalated Ylides

Quantum chemical calculations have been carried out to study the electronic structure of metalated ylides particularly in comparison to their neutral analogues, the bisylides. A series of compounds of the general composition Ph3 P-C-L with L being either a neutral or an anionic ligand were analyzed and the impact of the nature of the substituent L and the total charge on the electronics and bonding situation was studied. The charge at the carbon atom as well as the dissociation energies, bond lengths, and Wiberg bond indices strongly depend on the nature of L. Here, not only the charge of the ligand but also the position of the charge within the ligand backbone plays an important role. Independent of the substitution pattern, the NBO analysis reveals the preference of unsymmetrical bonding situations (P=C-L or P-C=L) for almost all compounds. However, Lewis structures with two lone-pair orbitals at the central carbon atom are equally valid for the description of the bonding situation. This is confirmed by the pronounced lone-pair character of the frontier orbitals. Energy decomposition analysis mostly reveals the preference of several bonding situations, mostly with dative and ylidic electron-sharing bonds (e.g., P→C- -L). In general, the anionic systems show a higher preference of the ylidic bonding situations compared to the neutral analogues. However, in most of the cases different resonance structures have to be considered for the description of the "real" bonding situation.

Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ): A Theoretical Study

Quantum chemical calculations of the main-group compounds E[C(NHC^Me )₂ ]₂ (E=Be, B⁺ , C²⁺ , N³⁺ , Mg, Al⁺ , Si²⁺ , P³⁺ ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC^Me )₂ ]₂ as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC^Me )₂ ]₂⁺ and C[C(NHC^Me )₂ ]₂²⁺ . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC^Me )₂ ]₂³⁺ that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC^Me )₂ ]₂ and Mg[C(NHC^Me )₂ ]₂ possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC^Me )₂ serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC^Me )₂ C←E→C(NHC^Me )₂ (E=Be, Mg). Dative bonds in which the carbones C(NHC^Me )₂ are donor ligands are suggested for the cations (NHC^Me )₂ C→E←C(NHC^Me )₂ (E=B⁺ , Al⁺ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC^Me )₂ C]⁺ -E-[C(NHC^Me )₂ ]⁺ (E=C, Si, N⁺ , P⁺ ).

NHC-Stabilised Acetylene-How Far Can the Analogy Be Pushed?

Experimental studies suggest that the compound (NHC^bz )₂ C₂ H₂ can be considered as a complex of a distorted acetylene fragment which is stabilised by benzoannelated N-heterocyclic carbene ligands (NHC^bz )→(C₂ H₂ )←(NHC^bz ). A quantum chemical analysis of the electronic structures shows that the description with dative bonds is more favourable than with electron-sharing double bonds (NHC^bz )=(C₂ H₂ )=(NHC^bz ).

Comparison of Hydrogen and Gold Bonding in [XHX](-) , [XAuX](-) , and Isoelectronic [NgHNg](+) , [NgAuNg](+) (X=Halogen, Ng=Noble Gas)

Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.

Unusually Short Be-Be Distances with and without a Bond in Be2 F2 and in the Molecular Discuses Be2 B8 and Be2 B7 (.)

Quantum-chemical calculations at the CCSD(T)/cc-pVTZ level of theory show that beryllium subfluoride, Be2 F2 , has a bond dissociation energy of De =76.9 kcal mol(-1) , which sets a record for the strongest Be-Be bond. The synthesis of this molecule should thus be possible in a low-temperature matrix. The discus-shaped species Be2 B8 and Be2 B7 (-) possess the shortest Be-Be distance for a molecule in the electronic ground state, but there is no Be-Be bond. The cyclic species Be2 B8 and Be2 B7 (-) exhibit double aromaticity with 6σ and 6π electrons, which strongly bind the Be2 fragment to the boron atoms. The very short interatomic distance between the beryllium atoms is due to the Be-B σ and π bonds, which operate like spokes in a wheel pressing the beryllium atoms together. The formation of the Be-B bonds has effectively removed the electronic charge of the valence space between the beryllium atoms. Along the Be-Be axis, there are two cage critical points adjacent to a ring critical point at the midpoint, but there is no bond critical point and no bond path.