MUTUAL EXCHANGE AND ISOMERIZATION PROCESSES IN THE 3 ISOMERS OF THE MIXED-METAL TRIANGULAR CLUSTER COMPLEX [RE2PT(MU-H)2(CO)8(PPH3)2]

The dynamic processes operative in the three isomers of the triangular cluster [Re2Pt(mu-H)2(CO)8(PPh3)2] (1) have been studied. H-1 and P-31 variable-temperature NMR spectra of the isomer 1a (in which both the phosphines are bound to Pt) indicate the presence of two mutual exchange processes: one, operative at lower temperature, exchanges only the phosphines, while the second one, active above 238 K, exchanges the hydrides. Computer simulations of the spectra allowed the activation parameters to be calculated (E(a) = 49.2 +/- 1.4 and 60.2 +/- 1.3 kJ/mol, respectively). The isomer 1a at T > 273 K irreversibly rearranges, by the exchange of a phosphine and a carbonyl between Pt and Re, giving the equilibrium mixture of the two isomers 1b and 1c (differing in the location of the phosphine bound to Re): this slow first-order reaction has been studied at three temperatures in a conventional kinetic experiment (284-294 K, E(a) = 80 +/- 7 kJ/mol). 2D EXSY experiments on P-31 spectra (295-316 K) and computer simulations of H-1{P-31} spectra (33-363 K) have been employed to measure the rate of interconversion between the isomers 1b and 1c, and the activation parameters (E(a) = 74.0 +/- 1.7 kJ/mol) have been estimated accordingly. C-13 variable-temperature NMR spectra suggest a localized scrambling of the carbonyls in the Re(CO)3PPh3 moiety of 1b.