Lan(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La2(benzene) displays a strong origin band, a short metal–ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La2(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is 1A1g (D3d) with (5dxy,x2–y2 + π*)46s2 electron configuration. Ionization removes a La-based 6s electron and yields a 2A1g ion. The spectrum of La(benzene) is similar to the zero-electron kinetic energy spectrum reported previously by our group, although the spectral line width is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal–ligand bonding of La2(benzene) is considerably stronger than that of La(benzene) due to the threefold binding of each La atom in the dilanthanum species and the twofold binding in the monolanthanum complex.

The reaction between La atoms and 1,3-butadiene is carried out in a laser-vaporization molecular beam source. Metal–hydrocarbon species with formulas La(CnHn) (n = 2, 4, and 6) and La(CmHm+2) (m = 4 and 6) are observed with time-of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum–benzene complex [La(C6H6)] is formed by 1,3-butadiene addition to lanthanacyclopropene [La(C2H2)] followed by molecular hydrogen elimination. Lanthanacyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La[η4-(1-buten-3-yne)] [La(C4H4)] and 1-lanthanacyclopent-3-ene [La(C4H6)]. The La(benzene) complex exhibits distinctive metal–ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations.

We report the first example of metal-mediated acetylene bicyclopentamerization to form naphthalene in the gas phase. The bicyclic aromatic compound was observed in a complex with La. The La(naphthalene) complex was formed by the reaction of laser-ablated La atoms with acetylene molecules in a molecular beam source and was characterized by mass-analyzed threshold ionization spectroscopy. The bicyclo-oligomerization reaction occurs through sequential acetylene additions coupled with dehydrogenation. Three intermediates in the reaction have been identified: lanthanacyclopropene [La(C2H2)], La(cyclobut-1-en-3-yne) [La(C4H2)], and La(benzyne) [(La(C6H4)]. The metal–ligand bonding in the three intermediates is considerably different from that in the La(naphthalene) complex, as suggested by accurately measured adiabatic ionization energies.

La(C2H2) and La(C4H6) are observed from the reaction of laser-vaporized La atoms with ethylene molecules by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. La(C2H2) is identified as a metallacyclopropene and La(C4H6) as a metallacyclopentene. The three-membered ring is formed by concerted H2 elimination and the five-membered cycle by dehydrogenation and C–C bond coupling. Both metallacycles prefer a doublet ground state with a La 6s-based unpaired electron. Ionization of the neutral doublet state of either complex produces a singlet ion state by removing the La-based electron. The ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal–ligand and ligand-based vibrational frequencies of the neutral and ionic states. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal–hydrocarbon cycles are lower than that of the neutral La atom. Deuteration has a small effect on the ionization energies of the two cyclic radicals but distinctive effects on their vibrational frequencies.

A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce(C2H2) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativistic correction, dynamic electron correlation, and spin–orbit coupling. The MATI spectrum exhibits two nearly identical band systems separated by 128 cm–1. The separation is not affected by deuteration. The two-band systems are attributed to spin–orbit splitting and the vibrational bands to the symmetric metal–ligand stretching and in-plane carbon–hydrogen bending excitations. The spin–orbit splitting arises from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The organolanthanide complex is a metallacyclopropene in C2v symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f16s1 and Ce 4f1, respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a π* antibonding orbital of acetylene.

η2-Propadienylidenelanthanum [La(η2-CCCH2)] and deprotiolanthanacyclobutadiene [La(HCCCH)] of La(C3H2) are identified from the reaction mixture of neutral La atom activation of propyne in the gas phase. The two isomers are characterized with mass-analyzed threshold ionization spectroscopy combined with electronic structure calculations and spectral simulations. La(η2-CCCH2) and La(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation, respectively. Both isomers prefer a doublet ground state with a La 6s-based unpaired electron, and La(η2-CCCH2) is slightly more stable than La(HCCCH). Ionization of the neutral doublet state of either isomer produces a singlet ion state by removing the La-based electron. The geometry change upon ionization results in the excitation of a symmetric metal–hydrocarbon stretching mode in the ionic state, whereas thermal excitation leads to the observation of the same stretching mode in the neutral state. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal–hydrocarbon radicals are lower than that of the neutral La atom. Deuteration has a very small effect on the ionization energies of the two isomers and the metal–hydrocarbon stretching mode of La(η2-CCCH2), but it reduces considerably the metal–ligand stretching frequencies of La(HCCCH).

Group 6 metal–bis(mesitylene) sandwich complexes are produced by interactions between the laser-vaporized metal atoms and mesitylene vapor in a pulsed molecular beam source, identified by photoionization time-of-flight mass spectrometry, and studied by pulsed-field ionization zero-electron kinetic energy spectroscopy and density functional theory calculations. Although transition metal–bis(arene) sandwich complexes may adopt eclipsed and staggered conformations, the group 6 metal–bis(mesitylene) complexes are determined to be in the eclipsed form. In this form, rotational conformers with methyl group dihedral angles of 0 and 60° are identified for the Cr complex, whereas the 0° rotamer is observed for the Mo and W species. The 0° rotamer is in a C2v symmetry with the neutral ground state of 1A1 and the singly positive charged ion state of 2A1. The 60° rotamer is in a Ci symmetry with the neutral ground state of 1Ag and the ion state of 2Ag. Partial conversion of the 60 to 0° rotamer is observed from He to He/Ar supersonic expansion for Cr–bis(mesitylene). The unsuccessful observation of the 60° rotamer for the Mo and W complexes is the result of its complete conversion to the 0° rotamer in both He and He/Ar expansions. The adiabatic ionization energies of the 0° rotamers of the three complexes are in the order of Cr–bis(mesitylene) < W–bis(mesitylene) < Mo–bis(mesitylene), which is different from that of the metal atoms. These metal–bis(mesitylene) complexes have lower ionization energies than the corresponding metal–bis(benzene) and −bis(toluene) species.

Gadolinium (Gd) complexes of benzene (C6H6) and (1,3,5,7-cyclooctatetraene) (C8H8) were produced in a laser-vaporization supersonic molecular beam source and studied by single-photon pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal–ligand stretching frequencies were measured for the first time from the ZEKE spectra. Metal–ligand bonding and electronic states of the neutral and cationic complexes were analyzed by combining the spectroscopic measurements with ab initio calculations. The ground states of Gd(C6H6) and [Gd(C6H6)]+ were determined as 11A2 and 10A2, respectively, with C6v molecular symmetry. The ground states of Gd(C8H8) and [Gd(C8H8)]+ were identified as 9A2 and 8A2, respectively, with C8v molecular symmetry. Although the metal–ligand bonding in Gd(C6H6) is dominated by the covalent interaction, the bonding in Gd(C8H8) is largely electrostatic. The bonding in the benzene complex is much weaker than that in the cyclooctatetraene species. The strong bonding in Gd(C8H8) arises from two-electron transfer from Gd to C8H8, which creates a strong charge–charge interaction and converts the tub-shaped ligand into a planar form. In both systems, Gd 4f orbitals are localized and play little role in the bonding, but they contribute to the high electron spin multiplicities.

Determination of electron spin multiplicities of transition-metal radicals and ions challenges both experimentalists and theoreticians. In this work, we report preferred electron spin states of M[C6(CH3)6] and M+(C6(CH3)6], where M = Ti, V, and Co. The neutral radicals were formed in a supersonic metal cluster beam source, and their masses were measured with time-of-flight mass spectrometry. Precise ionization energies of the radicals and metal–ligand stretching frequencies of the ions were measured by pulsed field ionization zero electron kinetic energy spectroscopy. C–H stretching frequencies of the methyl group in the radicals were obtained by infrared–ultraviolet two-photon ionization. Electron spin multiplicities of the radicals and ions were investigated by combining the spectroscopic measurements, density functional theory, and Franck–Condon factor calculations. The preferred spin states are quintet, sextet, and quartet for the neutral Ti, V, and Co radicals, respectively; for the corresponding singly charged cations, they are quartet, quintet, and triplet. In these high-spin states, the aromatic ring remains nearly planar. This finding contrasts to the previous study of Sc(hmbz), for which low-spin states are favored, and the aromatic ring is severely bent.

Metal-organic radicals are reactive and transient because of the existence of unpaired valence electrons, and thus the characterization of these open-shell systems is challenging. In our work, the radicals are synthesized by the reaction of bare metal atoms and organic ligands in a laser-vaporization supersonic molecular beam source and characterized with pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. The molecular beam ZEKE technique routinely yields sub-meV spectral resolution and is a powerful means to study the molecular bonding and structures. This account presents several examples of single-photon ZEKE spectroscopic applications in determining metal binding modes and molecular conformations.

High-resolution electron spectroscopy combines pulsed field ionization zero-electron kinetic energy (ZEKE) detection with in situ laser-assisted synthesis and supersonic expansion. The technique offers sub-meV spectral resolution for the electron spectra of metal complexes and is a powerful tool to study their bonding and structures. This Perspective presents recent progress in single-photon ZEKE spectroscopy of metal−aromatic complexes and focuses on the determination of the electron spin multiplicities, metal binding sites and modes, rotational conformers, and conformational changes of these critical species in organometallic chemistry.

Scandium (Sc) complexes of p-xylene, mesitylene, and hexamethylbenzene were produced in a laser-vaporization molecular beam source and studied with pulsed-field-ionization zero-electron-kinetic-energy spectroscopy, and density functional theory. In addition, infrared−ultraviolet resonant two-photon ionization spectra were recorded for Sc(hexamethylbenzene) in the C−H stretching region. Adiabatic ionization energies and several vibrational frequencies of these complexes were obtained from the spectroscopic measurements, and electronic transitions were determined by combining the spectra with the theoretical data. The ionization energies of the three complexes decrease with increasing number of the methyl groups, whereas the metal−ligand stretching frequencies of the p-xylene and mesitylene complexes are essentially the same and slightly smaller than that of the hexamethylbenzene species. Unlike benzene, the arene ring of the methylbenzene molecules is bent and the π-electrons are localized in a 1,4-diene fashion upon Sc coordination. The distortion of the aromatic ring is due to differential metal binding with the ring carbon atoms in the low-spin ground electronic state.

Group 6 metal (Cr, Mo, and W)−bis(toluene) sandwich complexes are synthesized in a laser-vaporization molecular beam source. Conformational isomers and isomerization of these complexes are studied by variable-temperature pulsed-field-ionization zero-electron-kinetic-energy spectroscopy and density functional theory. For Cr−bis(toluene), four rotational conformers are identified with methyl-group dihedral angles of 0, 60, 120, and 180°. The ground electronic states of these conformers are 1A1 (C2v, 0°), 1A (C2, 60 and 120°), and 1Ag (C2h, 180°) in the neutral form and 2A1 (C2v, 0°), 2A (C2, 60 and 120°), and 2Ag (C2h, 180°) in the singly charged cationic form. For Mo- and W−bis(toluene), the four rotamers are resolved into three (0, 60/120, and 180°) and two (0 and 60/120/180°) groups, respectively. For all three metal sandwiches, the most stable conformer is in the complete eclipsed configuration (0°) and has the highest ionization energy. The conversion from 60/120/180° to 0° rotamer is observed from helium to argon supersonic expansions and is more efficient for the heavier Mo and W species.