The Microwave Spectrum of Cyclopentadienyl Niobium Tetracarbonyl

Rotational transitions for the symmetric-top transition metal complex C₅H₅Nb(CO)₄ were measured using a Flygare-Balle-type pulsed-beam microwave spectrometer. The spectrum indicates that in the gas phase, this complex is a prolate symmetric top with B=C=558.842(4) MHz. Transitions were measured in the range 4-12 GHz. The observed splittings due to ⁹³Nb quadrupole coupling were smaller than expected, with eqQ(⁹³Nb)=−1.8(6) MHz. The value D_J=0.04(2) kHz. No evidence for fluxional behavior was observed. The A rotational constant, calculated from the X-ray data, is A=670(30) MHz and calculated B and C constants are in agreement with the present microwave values. This appears to be the first measurement of a microwave spectrum and gas-phase quadrupole coupling for a ⁹³Nb organometallic complex.