NMR-SPECTROSCOPY OF PARAMAGNETIC-COMPLEXES .39. NATURAL ABUNDANCE H-2-NMR OF PARAMAGNETIC SANDWICH COMPOUNDS

The methylated metallocenes (MeCp)2M with M = V, Cr, Mn, Co, Ni (1-5) were investigated by H-1 and H-2 NMR spectroscopy at natural abundance. The H-2 NMR signals are narrower by a factor of up to 30 compared with the corresponding H-1 NMR signals, thus establishing an inexpensive method and a general improvement of the NMR spectra of paramagnetic pi-complexes. This has allowed the resolution of the signal splitting of Cp deuterons of 1 and 5 which could not be observed earlier in the H-1 NMR spectra. The origin of the small (1 and 5) and large (2-4) signal splittings is discussed and related to an extended Huckel calculation. The relative magnitude of the signal splitting is reproduced, but the signal assignment had to be deduced from C-13 NMR results. Primary isotope shifts of up to 4.0 ppm were found for 1, 2, 4 and 5. The much higher values for 3 (up to 16.6 ppm) reflect the combined influence of the intrinsic isotope shift and the isotope effect on the spin crossover.