Microwave spectra and gas-phase structural parameters of bis(η⁵-cyclopentadienyl)tungsten dihydride

Microwave spectra for 11 isotopomers of bis(η⁵- cyclopentadienyl)tungsten dihydride ((C₅H₅) ₂WH₂) were recorded in the 5-14 GHz region using a Flygare-Balle-type pulsed beam spectrometer. Spectra arising from four tungsten isotopomers of both the (C₅H₅)₂WH₂ and (C₅H₅)₂WHD species and three W isotopomers for the (C₅H₅)₂WD₂ complex have been measured. The ∼250 b-type transition frequencies assigned for these near-prolate asymmetric top molecules were accurately described (σ_fit; = 2-4 kHz) using the rotational parameters A, B, and C and one centrifugal distortion constant, Δ_J. The small value obtained for Δ_J indicates a fairly rigid structure. From a least-squares fit using the resulting 33 rotational constants to obtain the molecular structure, we were able to determine the W-H bond length, r(W-H) = 1.703(2) Å, the H-W-H bond angle, ∠(H-W-H) = 78.0(12)°, the W-Cp centroid distance, r(W-Cp) = 1.940(8) Å, the angle made by the Cp centroids with tungsten, ∠(Cp-W-Cp) = 155(2)°, and the average C-C bond length, r(C-C) = 1.429(8) Å. The hydrogen atom separation is r(H-H) = 2.14(2) Å, indicating that this is clearly a "classical dihydride" rather than an "η²-dihydrogen" complex. The WH₂ moiety parameters determined from Kraitchman's equations (r(W-H) = 1.682(2) Å, ∠(H-W-H) = 78.6(2), r(H-H) = 2.130(2) Å) agree well with the least-squares' results. Furthermore, the r_e parameters obtained from DFT calculations agree well with the experimental r₀ structural parameters. To our knowledge, this work marks the first microwave study of a bent-metallocene complex. The present measurements were made with a pulsed-beam Fourier transform spectrometer employing a homodyne-type detection system, and this configuration is described. This homodyne system greatly simplifies the microwave circuit, with no apparent loss in sensitivity.