HIGH-PRESSURE NMR KINETICS .56. HIGH-PRESSURE PROTON NMR-STUDY OF ACETONITRILE EXCHANGE KINETICS ON [RH2(CH3CN)10]4-17- NMR INVESTIGATION OF AQUEOUS-SOLUTIONS OF [RH2(H2O)10]4+( AND O)

The H-1 NMR spectrum of [Rh2(e-CH3CN)8(a-CH3CN)2](BF4)4 supports the p resence in solution of this dinuclear species with a 4/1 equatorial/ax ial ratio. The lability of the axial acetonitrile is high on the NMR t ime scale, suggesting a dissociative mechanism of exchange with bulk a cetonitrile. In contrast the equatorial acetonitrile exchange rate is slow and has been followed by H-1 NMR isotopic labelling: k298 = 3.1 x 10(-5) s-1, DELTAH(double dagger) = 65.6 kJ mol-1, DELTAS(double dagg er) = -111.0 J K-1 mol-1, and DELTAV(double dagger) = -4.9 cm3 mol-1. It is suggested that a concerted migration of the a-CH3CN molecule to the equatorial plane and release of an e-CH3CN molecule to the bulk ta kes place through a contracted transition state. The O-17 NMR spectrum of the dinuclear Rh(II) aqua-ion is consistent with the entitY [Rh2(e -H2O)8(a-H2O)2]4+. This aqua-ion is not very stable and decomposes, wi thin 1 h at 339 K, forming metallic Rh and [Rh(H2O)6]3+. The equatoria l water exchange rate constant is estimated to be 10 < k298 < 50 s-1, whereas the axial water exchange rate is larger than 10(4) s-1 at 298 K. It is believed that the water exchange mechanisms for the dinuclear Rh(II) aqua-ion are similar to those suggested for the acetonitrile a nalogue.