MEASUREMENTS OF MICROWAVE-SPECTRA AND STRUCTURAL PARAMETERS FOR CYCLOPENTADIENYLCHROMIUM DICARBONYL NITROSYL

Microwave rotational spectra were measured for eight isotopomers of et a(5)-C5H5Cr(CO)(2)NO using a pulsed-beam, Fourier transform spectromet er. 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 ''pia no-stool'' type structure, with one symmetry plane. The accidental, ne ar-symmetric top inertia tenser gives a dear indication of different b onding and structural parameters for the nitrosyl group and the carbon yl groups, since 3-fold symmetry for the nitrosyl and carbonyl groups would result in a much larger value for B - C. The nitrosyl and carbon yl groups form different angles with the z-axis which passes through t he Cr atom and center of C5H5. The C5H5-Cr-NO angle is 128 degrees, an d the C5H5-Cr-CO angle is 120 degrees. The distance from the Cr atom t o the C5H5 ring is z (Cr-C5H5) = 1.86 Angstrom. The a-axis forms an an gle of 4 degrees with the z-axis. Other structural parameters are obta ined and discussed. The C5H5 ring is oriented so that two of the cyclo pentadienyl carbon atoms are staggered with respect to the NO nitrogen atom. The N-14 quadrupole coupling values are eQq(aa) = 0.312(2) and eQq(bb) = -0.448(2) MHz.