Photoelectron spectra of the niobium–molybdenum diatomic anion, obtained at 488 and 514 nm, display vibrationally resolved transitions from the ground state and one excited electronic state of the anion to the ground state and one excited electronic state of the neutral molecule. The electron affinity of NbMo is measured to be 1.130 ± 0.005 eV. Its 2Δ3/2 spin–orbit component is observed to lie 870 ± 20 cm–1 above its previously identified 2Δ5/2 ground state. For 93Nb98Mo, vibrational energies measured for levels up to v = 4 for the 2Δ5/2 and 2Δ3/2 states give harmonic frequency and anharmonicity constant values of ωe = 492 ± 12 cm–1 and ωe x e = 8.0 ± 3.2 cm–1, the former value corresponding to a force constant of 6.80 ± 0.35 mdyn/Å. These two vibrational parameters suggest a bond dissociation energy that is too low by at least a factor of 3, indicating that the ground state potential energy curve of NbMo deviates markedly from a Morse potential at higher energies. An excited electronic state of NbMo, assigned as a 2Σ+ state, is observed at 2900 ± 25 cm–1 (T 0). Vibrational energies up to v = 8 in this excited state give values of ωe = 544 ± 8 cm–1 and ωe x e = 1.9 ± 1.2 cm–1 for 93Nb98Mo. The former value corresponds to a high vibrational force constant of 8.30 ± 0.25 mdyn/Å. Both doublet states of the neutral molecule are accessed from the anion ground state, which is assigned as 1Σ+. For the 93Nb98Mo– anion, the fundamental vibrational frequency (ΔG 1/2) is 484 ± 15 cm–1. Electron affinity data indicate that the bond dissociation energy of NbMo– is 0.213 ± 0.005 eV greater than that of neutral NbMo, whose previously reported value then gives D 0 = 4.85 ± 0.27 eV for the anion. An excited state of the anion lying 3050 ± 25 cm–1 (T 0) above its ground state is assigned as 3Δ, and the energies of its spin–orbit components above the 3Δ3 lowest energy level are measured to be 450 ± 20 cm–1 (3Δ2) and 1100 ± 20 cm–1 (3Δ1). Their uneven spacing suggests that the energy of the 3Δ2 level is lowered by interaction with a higher energy Ω = 2 anion state. The vibrational frequency (ΔG 1/2) for the 3Δ1 and 3Δ2 states is measured to be 433 ± 20 cm–1. Bond length differences among the observed states are estimated from Franck–Condon fits to vibrational band intensity profiles. When combined with the previously reported NbMo bond length, these provide bond length estimates for the ground state of the anion (1.940 ± 0.025 Å) and for the observed excited states. These species offer extreme examples of multiple metal–metal bonding, with formal bond orders of 51/2 for the 2Δ ground and 2Σ+ excited doublet states of NbMo, 6 for the singlet ground state of the anion, and 5 for its low-lying triplet state. The relationships among their bonding properties and those of related multiply bonded transition metal dimers are discussed.

Anion photoelectron spectra and density functional calculations are reported for NbCr(CO) − 2 and NbCr(CO) − 3 complexes prepared by addition of Cr(CO) 6 vapor to a flow tube equipped with a niobium cathode discharge source. Electron affinities (± 0.007 eV) are measured to be 1.668 eV for NbCr(CO) 2 and 1.162 eV for NbCr(CO) 3 , values which exceed the 0.793 eV electron affinity previously measured for ligand-free NbCr. The vibrationally-resolved 488 nm photoelectron spectra are compared with Franck-Condon spectra predicted for various possible isomers and spin states of the anionic and neutral metal carbonyl complexes. Results are also compared with photoelectron spectra of the corresponding chromium carbonyl complexes and of NbCr and NbCr − , which have formal bond orders of 5.5 (2 ∆) and 6 (1 Σ +), respectively. These comparisons help to elucidate the effects of sequential carbonylation on this multiple metal-metal bond, and of the formation of this bond on the chromium-carbonyl interactions.

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We report mass spectra, 488 nm anion photoelectron spectra, and density functional theory (DFT) calculations of organometallic complexes produced by flow tube reactions of niobium with butadiene (C 4 H 6 ), and compare these results with those obtained upon reactions with ethylene (C 2 H 4 ). In the C 4 H 6 experiments, NbC 4 H 4 − is the most abundant product anion, indicating loss of H 2 upon reaction with Nb. DFT analysis of the vibrationally-resolved photoelectron spectrum indicates that the 3 A anion incorporates a five-membered Nb-C 4 ring in which the Nb atom lies outside the C 4 plane. The electron affinity of the corresponding neutral molecule ( 2 A ) is measured to be 0.997 ± 0.006 eV. Upon reaction with C 2 H 4 , at least one additional isomer of NbC 4 H 4 − is produced, giving rise to broad spectral features at higher electron binding energies. Reactions with C 4 H 6 also yield relatively small amounts of the NbC 6 H 6 − and NbC 6 H 4 − product anions, indicating C-C bond activation in addition to dehydrogenation. The former anion displays the 3 A 1 , C 6v Nb-benzene πcomplex structure previously observed upon reaction with C 2 H 4 . The NbC 6 H 4 − anion produced upon reaction with C 4 H 6 yields at least two vibrationally-resolved photodetachment transitions. DFT calculations performed to date suggest that the lower electron binding energy transition, which indicates an electron affinity of 1.110 ± 0.008 eV for the corresponding neutral complex, is due to the 4 B 2 ← 3 B 2 detachment from a planar, C 2v Nb-benzyne anion.