METAL FRAGMENT ROTATION IN TRIANGULAR METAL-CLUSTERS - H-1, H-2, AND C-13 NMR-STUDIES OF THE FLUXIONAL BEHAVIOR OF THE ANION [RE-3(MU-H)(3)(MU-ETA(2)-NC5H4)(CO)(10)](-)

Variable-temperature C-13 NMR spectroscopy, 1D SPT, and 2D EXSY experi ments have been used to investigate the fluxionality of the triangular cluster anion [Re-3(mu-H)(3)(mu-eta(2)-NC5H4)(CO)(10)](-). The (CO)-C -13 resonances have been assigned through H-1-C-13 2D HMQC and C-13 2D DQF-COSY experiments and through the measurement of the (CO)-C-13 lon gitudinal relaxation times. The exchange pattern observed in the C-13 2D EXSY experiments proved the occurrence of a synchronous axial-axial and equatorial-equatorial carbonyl exchange on the apical metal. The rate constants at 303 and 313 K compare well with those previously mea sured, through H-1 NMR spectroscopy, for the exchange of the two hydri des bridging the lateral edges of the cluster. The rigid rotation of t he whole apical [H2Re(CO)(4)] moiety around an axis passing through th e midpoint of the basal bimetallic fragment is in agreement with these results. A deuterium kinetic isotope effect k(H)/k(D) = 1.45(24) has been measured through H-1 and H-2 2D EXSY experiments, supporting the involvement of the hydrides in the rate-determining step of the exchan ge process.