NMR EXCHANGE STUDIES ON THE COMPLEXES - CIS-[RH(CO)2(PYRIDINE N-OXIDE)(X)] (X = CL, BR)

C-13 nmr spectra of the complexes cis-[Rh(CO))2(pyO)(X)] (X = Cl, Br; pyO = pyridine N-oxide) are discussed. Variable temperature C-13 nmr spectroscopy has been used to monitor the exchange process occurring in acetone-d6 solution. Rate constants for the exchange reaction have been determined by complete band shape analysis. The enthalpy of activation for the chloro- and bromo-complexes was found to be 53.3 +/- 1.0 kJ mol-1 and 48.3 +/- 0.8 kJ mol-1 respectively. The entropy of activation was found to be 5.5 +/- 0.6 J mol-1 K-1 and -11.0 +/- 0.3 J mol-1 K-1, for the chloro- and bromo-complexes, respectively. It is suggested that the fluxionality of the molecules results from an intermolecular exchange of pyridine N-oxide rather than from loss of carbon monoxide. A mechanism for the exchange process is proposed.