Unusually Large Isotope Effects on NMR Chemical Shifts of Paramagnetic Organometallic Compounds

H/D exchange in the paramagnetic cluster [{Cp″Cr(μ3-H)}4] (1) with [D6] benzene yielded a mixture of isotopomers including [{Cp″Cr(μ3-D)}4] ([D4]-1). In the 13C NMR spectrum of this mixture the chemical shifts displayed unusually large isotope effects (up to Δδ = 3 per H/D exchange!), caused by the paramagnetism of the molecules. The effect was used to study dynamic processes in the hydride cluster.