Alternating C—C Bond Lengths in Gas-Phase (Benzene)chromium Tricarbonyl

The microwave rotational spectrum of (1,2-D₂]benzene)chromium tricarbonyl was measured in the 4–12 GHz range using a Flygare—Balle-type spectrometer. This spectrum contains transitions due to two different structural isomers of this complex. These results are interpreted in terms of a reduction of the symmetry of benzene to C_3v due to interactions with the Cr(CO)₃ moiety. One structural isomer (isomer E) occurs when the deuterium atoms are at the ends of a “long” C—C bond, the other (isomer 5), when the deuterium atoms are at the ends of a “short” C—C bond. The data indicate a difference of 0.016 Å in adjacent benzene C—C bond lengths in this complex.