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

Abstract 1 H and 2 H NMR spectra were recorded for the paramagnetic metallocenes Cp 2 M (M = V, Cr, Mn, Co, Ni). For Cp 2 V, Cp 2 Cr, Cp 2 Mn and Cp 2 Ni, a signal narrowing factor of 36–37 is observed on going from Cp 2 M‐ h 10 to Cp 2 M‐ d 10 , which is close to simple theoretical predictions. Additional quadrupolar relaxation leads to a factor of 27 for cobaltocene. The enhanced resolution of the 2 H NMR spectra makes it possible to observe isotopomeric cobaltocenes with signals showing a secondary isotope shift of 0.71 ppm. In Cp 2 Co‐ d 9 and ‐ d 8 , the residual 1 H atoms lead to a splitting of the 2 H 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 Cp 2 Mn‐ d 10 / h 10 , Cp 2 Co‐ d 10 / h 10 and Cp 2 Ni‐ d 10 / h 10 . They reflect a change in the electron spin distribution after complete deuteriation.