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The vibrational spectra of the copper(i) cation–dihydrogen complexes Cu+(H2)4, Cu+(D2)4 and Cu+(D2)3H2 are studied using cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations.

The boron-dinitrogen cation complexes B(NN)3+ and B2(NN)3,4+ are produced in the gas phase and are studied by infrared photodissociation spectroscopy in the N-N stretching vibrational frequency region. The geometric and electronic structures are determined by comparison of the experimental spectra with density functional theory calculations. The B(NN)3+ cation is characterized to have a closed-shell singlet ground state with planar D3h symmetry. The B2(NN)3+ cation is determined to have a B═B bonded (NN)2BBNN structure with C2v symmetry. Two isomers of the B2(NN)4+ cation contribute to the experimental spectrum. One is a N2-tagged complex involving a B2(NN)3+ core ion. Another one is a B-B bonded B2(NN)4+ complex with a planar D2h structure. Bonding analyses reveal that the B-NN interactions in these complexes come mainly from covalent orbital interactions, with the NN → B σ donation being stronger than the B → NN π back-donation.

A highly excited electronic state of dicopper is observed and characterized for the first time. The [39.6]0

Homoleptic dinuclear beryllium carbonyl cation complexes have been produced and characterized in the gas phase. Infrared photodissociation spectroscopic and theoretical results confirm that Be2 (CO)5 + is a coordination saturated complex that can be assigned to a mixture of two almost isoenergetic structural isomers. Besides a beryllium-beryllium single-bonded (OC)2 Be-Be(CO)3 + isomer, another structure involving an unusual end-on and side-on bridging carbonyl ligand with very low carbonyl stretching frequency is identified.

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.

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 (D4h ) equilibrium geometry and a doublet electronic ground state. Both the [U(CO)8 ]+ cation and the [U(CO)8 ]- anion exhibit cubic structures (Oh ) with a 6 A1g ground state for the cation and a 4 A1g ground state for the anion. The neutral species [Th(CO)8 ] (Oh ; 1 A1g ) and [U(CO)8 ] (D4h ; 5 B1u ) 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.

The lanthanide octacarbonyl anion complexes Ln(CO)8 - (Ln=Tm, Yb, Lu) were produced in the gas phase and detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching-frequency region. By comparison of the experimental CO-stretching frequencies with calculated data, which are strongly red-shifted with respect to free CO, the Yb(CO)8 - and Lu(CO)8 - complexes were determined to possess octahedral (Oh ) symmetry and a doublet X2 A2u (Yb) and singlet X1 A1g (Lu) electronic ground state, whereas Tm(CO)8 - exhibits a D4h equilibrium geometry and a triplet X3 B1g ground state. The analysis of the electronic structures revealed that the metal-CO attractive forces come mainly from covalent orbital interactions, which are dominated by [Ln(d)]→(CO)8 π backdonation and [Ln(d)]←(CO)8 σ donation (contributes ≈77 and 16 % to covalent bonding, respectively). The metal f orbitals play a very minor role in the bonding. The electronic structure of all three lanthanide complexes obeys the 32-electron rule if only those electrons that occupy the valence orbitals of the metal are considered.

The boron-dinitrogen cation complexes B(NN)

The carbon chain cations HnC4O+ (n = 1, 2) were generated via pulsed laser vaporisation of a graphite target in supersonic expansions of helium seeded with carbon monoxide and acetylene. The infrared spectra were measured via mass-selected infrared photodissociation spectroscopy of their CO-tagged complexes [HnC4O·CO]+ (n = 1, 2). Their geometric and electronic structures were determined by gas-phase infrared photodissociation spectra combined with theoretical calculations. The results show that the HnC4O+ (n = 1, 2) core cations are linear carbon chain derivatives which are terminated with hydrogen/dihydrogen and oxygen atoms. The HC4O+ cation has 3Σ− electronic ground state with cumulene-like carbon chain structure, while H2C4O+ cation has 2B1 electronic ground state with polyyne-like carbon chain structure. The CH and CH2 terminal groups have different influence on the type of carbon-carbon bonds in the carbon chain derivatives, due to the sp and sp2 hybridisation property. The carbon chain cations HnC4O+ (n = 1, 2) are studied by gas-phase infrared photodissociation spectra of CO-tagged complexes [HnC4O·CO]+ (n = 1, 2) combined with theoretical calculations. Studies show that the structures of the HnC4O+ (n = 1, 2) core cations are linear carbon chain derivatives which are terminated with hydrogen/dihydrogen and oxygen atoms. The spectrum of [HC4O·CO]+ shows two bands above 3000 cm−1, suggesting possible isomers.

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., Two of five: Two isomers of five‐atom BCNO2 were synthesized and identified by matrix‐isolation IR spectroscopy and quantum chemical calculations. OCBNO, which is produced by the reaction of boron atoms with CO/NO in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that OCBNO is best described by the bonding interactions of triplet‐state B+ and the CO/NO− ligands in the triplet state forming two degenerate electron‐sharing π bonds and two ligand‐to‐boron dative σ bonds.

Gaseous TMO(CO)n+ (TM = Sc, Y, La) complex cations prepared via laser vaporization were mass-selected and studied by infrared photodissociation spectroscopy in the C–O stretching frequency region. The structures and vibrational frequencies were calculated by density functional theory to support and interpret the experimental results. The saturated coordination number of CO ligands for ScO(CO)n+, YO(CO)n+ and LaO(CO)n+ was demonstrated to be six, seven and nine, respectively, namely, the nominal 18-, 20- and 24-electron gaseous cation complexes were synthesized. Based on our analysis of the electronic structure, the YO(CO)7+ complex also obeys the 18-electron rule, since one of the occupied valence molecular orbitals is formed only by ligand orbitals. The contribution of 4f orbitals in LaO(CO)9+ accounts for its high coordination number with a 24-electron valence shell.

We investigate the gaseous ScO(H2O)1–3Ar+ cations prepared by laser vaporization coupled with supersonic molecular beam using infrared photodissociation spectroscopy in the O–H stretching region. The cation structures are characterized by comparing the experimentally observed frequencies with the simulated vibration spectra. We reveal that stoichiometric ScO(H2O)Ar+ is intrinsically the hydrated oxide cation expressed as H2O–ScOAr+ hydrate rather than Sc(OH)2Ar+ dihydroxide, although the former is higher in energy by 29.5 kcal mol−1 than the latter. Interestingly, when more water molecules are introduced to the complex, we find that the stoichiometric ScO(H2O)2–3Ar+ embraces the core subunit of Sc(OH)2+. Theoretical calculations suggest that the energy barrier of hydrogen transfer plays a critical role in the isomerization from hydrated complex to dihydroxide. When more than one water molecule is involved in the complex, the hydrogen transfer becomes nearly barrierless through a six-member cyclic transition state, leading to the reduction in the energy barrier from 21.8 kcal mol−1 to 4.2 kcal mol−1. Altogether, we conclude that the solvent molecules such as water can decrease the energy barrier and thus induce the formation of hydroxy species in the hydrolysis process.

Carbonyls in the s block Conventional wisdom in chemistry distinguishes transition metals from other elements by their use of d orbitals in bonding. Wu et al. now report that alkaline earth metals can slide their electrons from s- to d-orbital bonding motifs as well (see the Perspective by Armentrout). Calcium, strontium, and barium all form coordination complexes with a cubic arrangement of eight carbonyl ligands and an 18-electron valence shell. The compounds were characterized in frozen neon matrices by vibrational spectroscopy and in gas phase by mass spectrometry. Science , this issue p. 912 ; see also p. 849

We report the gas-phase synthesis of stable 20-electron carbonyl anion complexes of group 3 transition metals, TM(CO)8- (TM=Sc, Y, La), which are studied by mass-selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (Oh ) equilibrium geometry and a singlet (1 A1g ) electronic ground state. The 20-electron systems TM(CO)8- are energetically stable toward loss of one CO ligand, yielding the 18-electron complexes TM(CO)7- in the 1 A1 electronic ground state; these exhibit a capped octahedral structure with C3v symmetry. Analysis of the electronic structure of TM(CO)8- reveals that there is one occupied valence molecular orbital with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)8- fulfill the 18-electron rule when only those valence electrons that occupy metal-ligand bonding orbitals are considered.

The boron carbonyl cluster cations in the form of B3(CO)n+ (n = 4–6) are produced and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. Their geometric structures are determined with the aid of density functional theory calculations. The B3(CO)4+ cation is characterized to have a D2d (OC)2B═B═B(CO)2 structure and 1A1 electronic ground state with a linear boron skeleton. The B3(CO)5+ cation is determined to have a chain boron framework with C2v symmetry. The B3(CO)6+ cation is a weakly bound CO-tagged complex involving a B3(CO)5+ ion core. Bonding analysis reveals that B3(CO)4+ has a chemical bonding pattern similar to allene, while bonding in B3(CO)5+ is similar to that in allyl anion.

Recent studies on boron carbonyl complexes show their intriguing structural and bonding properties, enriching our knowledge on main group coordination chemistry. The isolobal relationships between BCO and CH and the more generally applicable CO/H- and B-/C analogies are employed to understand the structure and bonding of boron carbonyl complexes, bridging the boron carbonyl chemistry to the well-known hydrocarbon analogues.

The carbocations, H3C3O+ and D3C3O+ are produced via pulsed laser vaporization of a graphite target in supersonic expansions of helium seeded with carbon monoxide and acetylene or methane. The infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of their CO-tagged [H3C3O·CO]+ and [D3C3O·CO]+ cation complexes. The geometry of the cation complex is determined by the infrared spectra in conjunction with theoretical calculations. The H3C3O+ and D3C3O+ core cations are determined to be acryloyl cations with 1A ¢ electronic ground state and C s symmetry. The AdNDP bonding analysis indicates that besides the localized two-center-two-electron (2c-2e) C–O, C–C, and C–H σ bonds as well as 2c-2e CO π bonds, there is a delocalized three-center-two-electron (3c-2e) CCC π bond.

We investigate the gaseous ScO(H2O)1-3Ar+ cations prepared by laser vaporization coupled with supersonic molecular beam using infrared photodissociation spectroscopy in the O-H stretching region. The cation structures are characterized by comparing the experimentally observed frequencies with the simulated vibration spectra. We reveal that stoichiometric ScO(H2O)Ar+ is intrinsically the hydrated oxide cation expressed as H2O-ScOAr+ hydrate rather than Sc(OH)2Ar+ dihydroxide, although the former is higher in energy by 29.5 kcal mol-1 than the latter. Interestingly, when more water molecules are introduced to the complex, we find that the stoichiometric ScO(H2O)2-3Ar+ embraces the core subunit of Sc(OH)2+. Theoretical calculations suggest that the energy barrier of hydrogen transfer plays a critical role in the isomerization from hydrated complex to dihydroxide. When more than one water molecule is involved in the complex, the hydrogen transfer becomes nearly barrierless through a six-member cyclic transition state, leading to the reduction in the energy barrier from 21.8 kcal mol-1 to 4.2 kcal mol-1. Altogether, we conclude that the solvent molecules such as water can decrease the energy barrier and thus induce the formation of hydroxy species in the hydrolysis process.

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 (Oh) with a 6A1g ground state for the cation and a 4A1g ground state for the anion. The neutral species [Th(CO)8] (Oh; 1A1g) and [U(CO)8] (D 4h; 5B1u) 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.

Gaseous TMO(CO)

The cation complexes [ArB3O4]+, [ArB3O5]+, [ArB4O6]+ and [ArB5O7]+ were prepared via a laser vaporization supersonic ion source in the gas phase. Their vibrational spectra were measured via mass-selected infrared photodissociation spectroscopy. Spectroscopy combined with quantum chemical calculations revealed that the [ArB3O5]+, [ArB4O6]+ and [ArB5O7]+ cation complexes have planar structures each involving an aromatic boroxol ring and an argon–boron covalent bond. In contrast, the [ArB3O4]+ cation is characterized to be a weakly bound complex with a B3O4+ chain structure.

The boron carbonyl cation complexes B(CO)3+, B(CO)4+ and B2(CO)4+ are studied by infrared photodissociation spectroscopy and theoretical calculations. The B(CO)4+ ions are characterized to be very weakly bound complexes involving a B(CO)3+ core ion, which is predicted to have a planar D3h structure with the central boron retaining the most favorable 8-electron configuration. The B2(CO)4+ cation is determined to have a planar D2h structure involving a B−B one and half bond. The analysis of the B-CO interactions with the EDA-NOCV method indicates that the OC→B σ donation is stronger than the B→CO π back donation in both ions.

In order to obtain fundamental insight into the bonding properties of transition metals, the ( v ′ ,0) B-X transitions ( v ′ = 0 - 2 ) of the heteronuclear CuAg dimer are re-investigated by applying non-linear four-wave mixing and laser-induced fluorescence spectroscopy. The CuAg molecules in a molecular beam are produced by laser ablation of a Cu/Ag alloy target. Rotationally resolved spectra of the B-X transition are recorded. The measurements yield accurate molecular constants for four isotopologues and an equilibrium distance r e = 2.4577 ± 0.0003 A for the B state. In particular, the B state is identified as an Ω = 0 + ion-pair state. The results corroborate the important contribution of the charge-transfer state to the metal–metal bond. The results are in good agreement with the recent calculations at the MRCI level of theory (Alizadeh et al., 2014).

The lanthanide octacarbonyl anion complexes Ln(CO)

The carbon chain cations, HC2n+1O+ (n = 2-5), are produced via pulsed laser vaporization of a graphite target in supersonic expansions containing carbon monoxide and hydrogen. The infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of the CO "tagged" [HC2n+1O·CO]+ cation complexes in the 1600-3500 cm-1 region. The geometries and electronic ground states of these cation complexes are determined by their infrared spectra compared to the predications of theoretical calculations. All of the HC2n+1O+ (n = 2-5) core cations are characterized to be linear carbon chain derivatives terminated by hydrogen and oxygen, which have the closed-shell singlet ground states with polyyne-like carbon chain structures.

The carbon suboxide cation C3O2+ and the protonated carbon suboxide HC3O2+/DC3O2+ were produced in the gas phase. The vibrational spectra were measured via infrared photodissociation spectroscopy of their argon- or CO-tagged complexes. Spectroscopic evidence combined with state-of-the-art quantum chemical calculations indicate that both cations have a bent C2v symmetry and can be designated as dicarbonyls of a carbon cation and methylidyne cation, respectively.

A combined experimental and theoretical study on the main-group tricarbonyls [B(CO)3 ] in solid noble-gas matrices and [C(CO)3 ](+) in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η(1) (μ1 -CO)-bonded carbonyl ligand, which serves as an unprecedented one-electron donor ligand. Thus, the central atoms in both complexes still retain an 8-electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two-electron donors that display classical σ-donation and π-back-donation following the Dewar-Chatt-Duncanson model. The tilted CO ligand is a formal one-electron donor that is bonded by σ-donation and π-back-donation that involves the singly occupied orbital of the radical fragments [B(CO)2 ] and [C(CO)2 ](+) .

We report the spectroscopic identification of the [B3 (NN)3](+) and [B3 (CO)3](+) complexes, which feature the smallest π-aromatic system B3 (+). A quantum chemical bonding analysis shows that the adducts are mainly stabilized by L→[B3 L2 ](+) σ-donation.

We present an approach to determine the absolute thickness profile of flat liquid jets, which takes advantage of the information of thin film interference combined with light absorption, both captured in a single microscopic image. The feasibility of the proposed method is demonstrated on our compact experimental setup used to generate micrometer thin, free-flowing liquid jet sheets upon collision of two identical laminar cylindrical jets. Stable operation was achieved over several hours of the flat jet in vacuum (10-4 mbar), making the system ideally suitable for soft x-ray photon spectroscopy of liquid solutions. We characterize the flat jet size and thickness generated with two solvents, water and ethanol, employing different flow rates and nozzles of variable sizes. Our results show that a gradient of thickness ranging from a minimal thickness of 2 µm to over 10 µm can be found within the jet surface area. This enables the tunability of the sample thickness in situ, allowing the optimization of the transmitted photon flux for the chosen photon energy and sample. We demonstrate the feasibility of x-ray absorption spectroscopy experiments in transmission mode by measuring at the oxygen K-edge of ethanol. Our characterization method and the description of the experimental setup and its reported performance are expected to expand the range of applications and facilitate the use of flat liquid jets for spectroscopy experiments.