CARBONYL FLUXIONALITY IN THE NIDO CLUSTER RU-3(CO)(9)(MU(3)-CO)(MU(3)-NPH) - NMR EVIDENCE AND MECHANISM FOR THE EXCHANGE OF THE TRIPLY-BRIDGING CO WITH THE TERMINAL CO GROUPS

Variable-temperature C-13 NMR studies on RU(3)(CO)(9)(mu(3)-CO)(mu(3)-NPh) reveal the presence of two distinct CO exchange processes that involve localized equatorial/axial carbonyl scrambling at each ruthenium vertex and exchange of the equatorial and triply-bridging CO groups, The kinetics and activation parameters for these exchange processes have been determined by both band shape analysis and 2D-EXSY experiments. The latter NMR method unequivocally rules out a sequence involving the direct exchange of the axial and triply-bridging CO groups. The somewhat faster of the two processes (Delta G(double dagger) = 54.9 (0.03) kJ/mol, Delta H-double dagger = 41.7 (1.4) kJ/mol, Delta S-double dagger = -50(6) J/mol K) serves to equilibrate the equatorial/axial CO groups at each Ru(CO)(3) center, while the scrambling of the equatorial/triply-bridging CO groups displays comparable activation parameters (Delta G(double dagger) = 58.1(0.14) kJ/mol, Delta H-double dagger = 45.4 (3.4) kJ/mol, Delta S-double dagger = -49(14) J/mol K). Plausible mechanisms invoking a turnstile rotation and mu(3)-CO --> mu(2)-CO interconversions are presented and discussed relative to each CO exchange step.