Photoelectron velocity map imaging spectroscopy of group 14 elements and iron tetracarbonyl anionic clusters MFe(CO)4- (M = Si, Ge, Sn).

The heteronuclear group 14 M-iron tetracarbonyl clusters MFe(CO)4- (M = Si, Ge, Sn) anions have been generated in the gas phase by laser ablation of M-Fe alloys and detected by mass and photoelectron spectroscopy. With the support of quantum chemical calculations, the geometric and electronic structures of MFe(CO)4- (M = Si, Ge, Sn) are elucidated, which shows that all the MFe(CO)4- clusters have the M-Fe bonded, iron-centered, and carbonyl-terminal M-Fe(CO)4 structure with the C2v symmetry and a 2B2 ground state. The M-Fe bond can be considered a double bond, which includes one σ electron sharing bond and one π dative bond. The C-O bonds in those anionic clusters are calculated to be elongated to different extents, and in particular, the C-O bonds in SiFe(CO)4- are elongated more. The Si-Fe alloy thus turns out to be a better collocation to activate the C-O bonds in the gas phase among group 14. The present findings have important implications for the rational development of high-performance catalysts with isolated metal atoms/clusters dispersed on supports.

Spectroscopic Identification of the Dinitrogen Fixation and Activation by Metal Carbide Cluster Anions PtCn- (n = 4-6).

Nitrogen fixation is confronted with great challenges in the field of chemistry. Herein, we report that single metal carbides PtCn- and PtCnN2- (n = 4-6) are indispensable intermediates in the process of nitrogen fixation by mass spectrometry coupled with anionic photoelectron spectroscopy, quantum chemical calculations, and simulated density-of-state spectra. The most stable isomers of these cluster anions are characterized to have linear chain structures. The fixation and activation of dinitrogen are facilitated by the charge transfer from Pt and Cn to N2. The significance of π back-donation of the 5d orbital of the Pt atom to the antibonding π orbits of N2 for dinitrogen fixation and activation is discussed in detail. This study not only provides a theoretical basis at the molecular level for the activation of dinitrogen by mononuclear metal carbide clusters but also provides a new paradigm for dinitrogen fixation.

Spectroscopic Signature of the Carbon-Carbon Coupling Reaction between Carbon Monoxide and Nickel Carbide.

Spectroscopic characterization of ketenylidene complexes is of essential importance for understanding the structure-reactivity relationships of the catalytic sites. Here, we report a size-specific photoelectron velocity map imaging spectroscopic study of the reactions of carbon monoxide with nickel carbide. Quantum chemical calculations have been conducted to search for the energetically favorable isomers and to recognize the experimental spectra. The target products with the chemical formula of NiC(CO)n- (n = 3-5) are characterized to have an intriguing ketenylidene CCO unit. The evolution from NiC(CO)3- to NiC(CO)4- involves the breaking and formation of the Ni-C bond and the coordination conversion between the terminal and bridging carbonyls. Experimental and theoretical analyses reveal an efficient C-C bond formation process within the reactions of carbon monoxide and laser-vaporized nickel carbide. This work highlights the pivotal roles played by metal carbides in the C-C bond formation and also proposes new ideas for the design and chemical control of a broad class of complexes with unique physical and chemical properties.

Observation of unsaturated platinum carbenes Pt2C2n - (n = 1-3) clusters: A photoelectron imaging spectroscopic and theoretical study.

The structural and bonding properties of the Pt2C2n - (n = 1-3) complexes have been investigated by mass-selected photoelectron velocity-map imaging spectroscopy with quantum chemical calculations. The adiabatic detachment energies and vertical detachment energies of Pt2C2n - have been obtained from the measured photoelectron imaging spectra. Theoretical results indicate that the lowest-energy isomers of Pt2C2n - (n = 1-3) possess linear chain-shaped configurations. The binding motif in the most stable isomer of Pt2C2 - has a linear cumulenic structure with a Pt=C=C=Pt configuration, and the structural characteristic persists up to all the lowest-energy isomers of the Pt2C4 - and Pt2C6 - anions. The chemical bonding analyses indicate that the Pt2C2n - (n = 1-3) complexes have multicenter two-electron characteristics.

Photoelectron Velocity Map Imaging Spectroscopic and Theoretical Study of Heteronuclear MNi(CO)7- (M = V, Nb, Ta).

A series of heteronuclear group 5 metal-nickel carbonyls MNi(CO)7- (M = V, Nb, Ta) have been generated via a laser ablation ion source and studied by photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations have been performed to probe the electronic and geometric structures and help to assign the spectra. The adiabatic detachment energies (ADEs) and vertical detachment energies (VDEs) are deduced from spectra to be 3.40/3.58, 3.34/3.55, 3.30/3.50 eV, which are consistent with quantum chemical computational results. The MNi(CO)7- (M = V, Nb, Ta) consists of three bridging carbonyls, one carbonyl terminally bonded to the Ni atom and three carbonyls terminally bonded to the M (M = V, Nb, Ta) atom. These geometries are different from homobinuclear Cr2(CO)7+, Ni2(CO)7+, Pd2(CO)7+, and Fe2(CO)7- and heterobinuclear CuFe(CO)7-, CoZn(CO)7+, and CO is largely activated by a bridging coordination mode. The experimental and theoretical results would provide important information to understand the chemisorbed CO molecules on alloy surfaces or interfaces, which is of great significance to elucidate CO molecule activation processes.

Photoelectron velocity map imaging spectroscopic and theoretical study of heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6).

Mass-selected heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6) were investigated by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations to obtain their chemical bonding and intrinsic electronic structure in the gas phase. The calculated energies (adiabatic detachment energies)/vertical detachment energies (VDEs) match well with experimental values: 1.30/1.49, 1.66/1.95, 2.22/2.48, 2.70/2.89, and 2.95/3.15 eV. The VDE value of VNi(CO) n - increases with an increase of cluster size, implying that the negative electron is stabilized upon the bonding of CO molecules. VNi(CO)2 - consists of one bridging carbonyl and one terminal carbonyl, whose feature is different from MNi(CO)2 - (M = Sc, Y, La, and Ce) with the involvement of one side-on-bonded carbonyl and one terminal CO carbonyl. The building block composed of three bridging carbonyls is favored for VNi(CO)3 -, the structure of which persists up to n = 6. The additional CO ligands are preferentially coordinated in the terminal mode to the Ni atom at n = 4 and then to the V atom at n = 5 and 6. The results obtained in this work would provide a molecular-level understanding about chemisorbed CO molecules on alloy surfaces/interfaces, which is important to understand CO molecule activation processes.

Anion photoelectron spectroscopy and chemical bonding of ThO2- and ThO3.

We conducted a study of electronic structures and chemical bonding of gaseous ThO2- and ThO3- using velocity-map imaging and ab initio calculations. The electron affinity of neutral ThO2 molecule is reported for the first time with the value of 1.21(5) eV. We obtained a vibrationally resolved photoelectron spectroscopy of ThO2- and observed the symmetric stretching frequency of 824(40) cm-1 for neutral molecules. One hot band transition is observed in the spectrum of ThO2-, which allows the measurement of symmetric stretching mode for ThO2-. The ground state of ThO2- is 2A1 with C2v symmetry: the detachment of an electron from the singly occupied molecular orbital (SOMO) results in the ground state of ThO2. Kohn-Sham molecular orbital analyses reveal an σ and two weak π bonds for Th-O multiple bonds in ThO2. Global minimum search methodology combined with quantum chemical calculations are used to find the minima of ThO3 and ThO3-, and the adiabatic detachment energy of ThO3- is calculated to be 3.26 eV at the coupled cluster with singles and doubles plus perturbative triples level. Our theoretical calculations suggest that the ground state of ThO3 is 1A' with a symmetry of Cs, while the most stable ThO3- is 2A1 with C2v symmetry; thus, the transition from ThO3- to ThO3 undergoes a significant geometry reorganization. Molecular orbital analyses suggest that the SOMO of ThO3- is mainly participated by O 2p and O to Th back donation was found in HOMO-2 molecular orbital. This investigation will shed some light on the understanding of covalent bonding in Th-contained molecules.

Probing the bonding of CO to heteronuclear group 4 metal-nickel clusters by photoelectron spectroscopy.

A series of heterobinuclear group 4 metal-nickel carbonyls MNi(CO)n- (M = Ti, Zr, Hf; n = 3-7) has been generated via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations have been carried out to elucidate the geometric and electronic structures and support the spectral assignments. The n = 3 cluster is determined to be capable of simultaneously accommodating three different types of CO bonds (i.e., side-on-bonded, bridging, and terminal modes), resulting in a MNi[η2(µ2-C, O)](µ-CO)(CO)- structure, which represents the smallest metal carbonyl with the involvement of all the main modes of metal-CO coordination to date. The building block of three bridging CO molecules is favored at n = 4, the structure of which persists up to n = 7. The additional CO ligands are bonded terminally to the metal atoms. The present findings provide important new insight into the structure and bonding mechanisms of CO molecules with heteronuclear transition metals, which would have important implications for understanding chemisorbed CO molecules on alloy surfaces.

Probing Chemical Bonding and Electronic Structures in ThO- by Anion Photoelectron Imaging and Theoretical Calculations.

Because of renewed research on thorium-based molten salt reactors, there is growing demand and interest in enhancing the knowledge of thorium chemistry both experimentally and theoretically. Compared with uranium, thorium has few chemical studies reported up to the present. Here we report the vibrationally resolved photoelectron imaging of the thorium monoxide anion. The electron affinity of ThO is first reported to be 0.707 ± 0.020 eV. Vibrational frequencies of the ThO molecule and its anion are determined from Franck-Condon simulation. Spectroscopic evidence is obtained for the two-electron transition in ThO-, indicating the strong electron correlation among the (7sσ)2(6dδ)1 electrons in ThO- and the (7sσ)2 electrons in ThO. These findings are explained by using quantum-chemical calculations including spin-orbit coupling, and the chemical bonding of gaseous ThO molecules is analyzed. The present work will enrich our understanding of bonding capacities with the 6d valence shell.

Observation of promoted C-O bond weakening on the heterometallic nickel-silver: Photoelectron velocity-map imaging spectroscopy of AgNi(CO)n.

We report a joint experimental and theoretical study on heterodinuclear silver-nickel carbonyl clusters: AgNi(CO)n- and AgNi(CO)n (n = 2, 3). The photoelectron spectra and photoelectron angular distribution provide information on the electronic structures and geometries of these complexes. Electron affinities of AgNi(CO)2 and AgNi(CO)3 are measured from the photoelectron velocity-map imaging spectra to be 2.29 ± 0.03 and 2.32 ± 0.03 eV, respectively. The complementary theoretical calculations at the B3LYP level and Franck-Condon simulations are performed to establish their geometrical structures. The C-O stretching modes are activated upon photodetachment and determined to be 2024 and 2028 cm-1 for AgNi(CO)2 and AgNi(CO)3, respectively, which are notably red-shifted with respect to those of corresponding unsaturated binary nickel carbonyls. These findings will shed light on the promoted C-O bond weakening by the introduction of a foreign atom to binary unsaturated TM carbonyl complexes.

Probing the microhydration of metal carbonyls: a photoelectron velocity-map imaging spectroscopic and theoretical study of Ni(CO)3(H2O)n.

A series of microhydrated nickel carbonyls, Ni(CO)3(H2O)n- (n = 0-4), are prepared via a laser vaporization supersonic cluster source in the gas phase and identified by mass-selected photoelectron velocity-map imaging spectroscopy and quantum chemical calculations. Vertical detachment energies for the n = 1-4 anions are measured from the photoelectron spectra to be 1.429 ± 0.103, 1.698 ± 0.090, 1.887 ± 0.080, and 2.023 ± 0.074 eV, respectively. The C-O stretching vibrational frequencies in the corresponding neutral clusters are determined to be 1968, 1950, 1945, and 1940 cm-1 for n = 1-4, respectively, which are characteristic of terminal CO. It is determined that the hydrogen atom of the first water molecule is bound to the nickel center. Addition of a second water molecule prefers solvation at the carbonyl terminal. Spectroscopy combined with theory suggests that the solvation of nickel tricarbonyl is dominated by a water-ring network. The present findings would have important implications for the fundamental understanding of the multifaceted mechanisms of the multibody interaction of water and carbon monoxide with transition metals.

Photoelectron Velocity Map Imaging Spectroscopy of Lead Tetracarbonyl-Iron Anion PbFe(CO)4(.).

Joint research of photoelectron velocity map imaging spectroscopy and density functional theory has been performed to probe the geometrical structures and electronic properties for heterodinuclear iron-lead carbonyl cluster PbFe(CO)4(-), which serves as a monomer of the metal-metal bonded oligomer. The photoelectron detachment of PbFe(CO)4(-) is recorded at two different photon energies with rich spectral features. The ground-state transition obtained at 532 nm reveals a broad vibrationally resolved spectral band, which corresponds to the lead-iron stretching, while the 355 nm spectrum displays many more transitions on the higher-energy side, which correspond to the electronic excited states of PbFe(CO)4. Theoretical calculations at the B3LYP level are performed to explore the ground states of both the anionic and neutral PbFe(CO)4 and to support spectral identification of the fine resolved photoelectron spectra. Moreover, the unique chemical bonding between lead and iron in PbFe(CO)4 is discussed with the aid of natural bond orbital analyses.

Sequential bonding of CO molecules to a titanium dimer: A photoelectron velocity-map imaging spectroscopic and theoretical study of Ti2(CO)n- (n = 1-9).

Binuclear titanium carbonyl cluster anions, Ti2(CO)n- (n = 4-6), are produced via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations are carried out for Ti2(CO)n- (n = 1-9) to explore the trend of sequential bonding of CO molecules to a titanium dimer. It has been found that the CO molecules bind to Ti2 in a side-on fashion and form a stable Ti2[η2(µ2-C, O)]3 structure at n = 3, the motif of which retains up to n = 5. Starting at n = 6, a new building block of two CO groups side-on-bonded to Ti2 is favored, the structure of which persists up to n = 9. In the larger clusters (n = 6-9), the side-on-bonded CO molecule can be stabilized via the removal of two electrons from an anionic titanium carbonyl, which is different from the effect of charge on CO binding in rhodium carbonyls where bridge-bonded CO molecules are selectively destabilized by the removal of an electron from a neutral rhodium carbonyl. The present study provides a stepwise picture for molecular-level understanding of CO bonding on transition-metal clusters, which is directly relevant to the elementary processes of CO at metal catalysts.

Photoelectron velocity-map imaging and theoretical studies of heteronuclear metal carbonyls MNi(CO)3(-) (M = Mg, Ca, Al).

The heteronuclear metal carbonyl anions MNi(CO)3(-) (M = Mg, Ca, Al) have been investigated using photoelectron velocity-map imaging spectroscopy. Electron affinities of neutral MNi(CO)3 (M = Mg, Ca, Al) are measured from the photoelectron spectra to be 1.064 ± 0.063, 1.050 ± 0.064, and 1.541 ± 0.040 eV, respectively. The C-O stretching mode in these three clusters is observed and the vibrational frequency is determined to be 2049, 2000, and 2041 cm(-1) for MgNi(CO)3, CaNi(CO)3, and AlNi(CO)3, respectively. Density functional theory calculations are carried out to elucidate the geometric and electronic structures and to aid the experimental assignments. It has been found that three terminal carbonyls are preferentially bonded to the nickel atom in these heterobinuclear nickel carbonyls MNi(CO)3 (-1/0), resulting in the formation of the Ni(CO)3 motif. Ni remains the 18-electron configuration for MgNi(CO)3 and CaNi(CO)3 neutrals, but not for AlNi(CO)3. This is different from the homobinuclear nickel carbonyl Ni-Ni(CO)3 with the involvement of three bridging ligands. Present findings would be helpful for understanding CO adsorption on alloy surfaces.