Microwave Spectra and Gas-Phase Structural Parameters of Bis(η⁵-cyclopentadienyl)tungsten Dihydride

Abstract

Microwave spectra for 11 isotopomers of bis(η 5 -cyclopentadienyl)tungsten dihydride ((C 5 H 5 ) 2 WH 2 ) 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 (C 5 H 5 ) 2 WH 2 and (C 5 H 5 ) 2 WHD species and three W isotopomers for the (C 5 H 5 ) 2 WD 2 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 “η 2 -dihydrogen” complex. The WH 2 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 r 0 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.