Measurements of Microwave Spectra and Structural Parameters for Cyclopentadienylchromium Dicarbonyl Nitrosyl

Microwave rotational spectra were measured for eight isotopomers of ?⁵-C₅H₅Cr(CO)₂NO using a pulsed-beam, Fourier transform spectrometer. The normal isotopomer is a near-symmetric rotor with A = 1001.693(2), B = 797.2724(6), and C = 797.0143(6) MHz. This complex has a “piano-stool” type structure, with one symmetry plane. The accidental, near-symmetric top inertia tensor gives a clear indication of different bonding and structural parameters for the nitrosyl group and the carbonyl groups, since 3-fold symmetry for the nitrosyl and carbonyl groups would result in a much larger value for B - C. The nitrosyl and carbonyl groups form different angles with the z-axis which passes through the Cr atom and center of C5H5. The C₅H₅-Cr-NO angle is 128°, and the C5H5-G-CO angle is 120°. The distance from the Cr atom to the C₅H₅ ring is z (Cr-C₅H₅) = 1.86 Å. The a-axis forms an angle of 4° with the z-axis. Other structural parameters are obtained and discussed. The C₅H₅ ring is oriented so that two of the cyclopentadienyl carbon atoms are staggered with respect to the NO nitrogen atom. The ¹⁴N quadrupole coupling values are eQq_aa = 0.312(2) and eQq_bb = −0.448(2) MHz.