A variable-temperature ESR study of the paramagnetic complex Rh(dppe)2° has been performed. The complex is found to be fluxional on the ESR time scale. At 270 K the ESR spectrum of the complex in toluene consists of a symmetric five-line pattern with M)Pof 52 G. Upon cooling, an alternating line width effect is seen, and the spectrum changes to a pattern showing coupling to two different pairs of P donors with hyperfine couplings of 37.5 and 65 G. Analysis of the dynamic behavior leads to activation parameters AHt and AS* of 3.54 kcal/mol and -4.6 cal/(mol K), respectively, for the exchange process. Possible mechanisms for the exchange are pairwise lengthening and shortening of the Rh-P bonds and angular distortion of the complex to a species of C2symmetry. A bimolecular equilibrium is also observed by a significant decrease in the intensity of the ESR signal as the temperature is lowered. The AH and AS values for this equilibrium are -13.4 kcal/mol and -49.8 cal/(mol K), respectively, consistent with either a dimerization or the formation of an intimate ion pair, [Rh(dppe)2]+[Rh(dppe)2]~, for this sterically crowded system. The complex Rh(dppe)2° is also found to react with H2at room temperature to form the known hydride complex RhH(dppe)2.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry