Effects of rovibrational averaging on the 93Nb chemical shift in the [Nb(CO)6]- ion, based on NMR and vibrational spectra

The dependence of the 93Nb NMR spectrum of [Nb(CO)6]- in solution upon temperature (203-323 K) and upon isotopic replacement of 12C16O ligands by 13C16O and 12C18O is reported. The IR and Raman spectra of the hexacarbonyl anion are reported for the solid [Et4N]+[Nb(CO)6]-. In a comparative study of [V(CO)6]-, [Nb(CO)6]-, and [Re(CO)6]+ ions, force constants are calculated in a general quadratic valence force field constrained to be consistent with the interaction force constants in the corresponding neutral hexacarbonyl series Cr(CO)6, Mo(CO)6, and W(CO)6. The harmonic force field augmented with estimated anharmonoic force constants is used to calculate mean NbC and CO bond displacements as a function of temperature and masses. The 93Nb chemical shifts upon isotopic CO replacement are interpreted in terms of rovibrational averaging, and the empirical values of the electronic factors so obtained are found to be consistent with the observed temperature dependence of the shift. 25 references, 3 figures, 5 tables