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.
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