A Study of NbMo and NbMo^– by Anion Photoelectron Spectroscopy

Abstract

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.