NMR spectroscopy of paramagnetic complexes. Part 36-2/1H NMR of paramagnetic metallocenes: Primary and secondary isotope effects and signal narrowing

1H and 2H NMR spectra were recorded for the paramagnetic metallocenes Cp2M (M = V, Cr, Mn, Co, Ni). For Cp2V, Cp2Cr, Cp2Mn and Cp2Ni, a signal narrowing factor of 36–37 is observed on going from Cp2M‐h10 to Cp2M‐d10, which is close to simple theoretical predictions. Additional quadrupolar relaxation leads to a factor of 27 for cobaltocene. The enhanced resolution of the 2H NMR spectra makes it possible to observe isotopomeric cobaltocenes with signals showing a secondary isotope shift of 0.71 ppm. In Cp2 Co‐d9 and ‐d8, the residual 1H atoms lead to a splitting of the 2H NMR signals, which is traced to the lifting of an orbital degeneracy due to the perturbation of the ligand π‐system by the lighter isotope. Primary isotope shifts of 2.33, 1.24, and 1.45 ppm are found for the pairs Cp2Mn‐d10/h10, Cp2Co‐d10/h10 and Cp2Ni‐d10/h10. They reflect a change in the electron spin distribution after complete deuteriation.