EXCHANGE COUPLING IN METALLOCENE TRIHYDRIDE COMPLEXES

The H-1 NMR spectra of the metallocene trihydride complexes [(eta-C5H5)2MH3]n+ (M = Mo, W, n = 1; M = Nb, Ta, n = 0) have been investigated as a function of temperature. With the exception of M = Mo, all the complexes exhibit hydride resonances at ambient temperature which are consistent with an AB2 spin system, with moderate values of J(A-B) (7-20 Hz). For M = Nb, a novel temperature dependence of J(A-B) was observed. The observed coupling decreases with decreasing temperature, becoming unresolvable at 243 K, and then reappears at 173 K. The observed coupling is attributed to a magnetic component and a quantum mechanical exchange component, which have opposite signs. The sign of the magnetic coupling is negative. For M = Mo, a single resonance was observed in the hydride region of the H-1 NMR spectrum at room temperature, due to thermally activated A/B site exchange. The static spectrum was obtained at 203 K and reveals an AB2 spin system with an extremely large value of J(A-B) (ca. 1000 Hz). This coupling is also temperature dependent, decreasing to 450 Hz at 153 K.