RHENIUM-PLATINUM MIXED-METAL CLUSTERS - CHARACTERIZATION IN SOLUTION AND DYNAMIC BEHAVIOR OF THE ISOMERS OF [RE3PT(MU-H)3(CO)14] - AN EXAMPLE OF A LABILE METAL FRAGMENT THAT UNDERGOES INTERMOLECULAR EXCHANGE

The cluster (Re3Pt(mu-H)3(CO)14] (1), which contains a [ReH(CO)5] mole cule bound to the Pt atom of a Re2Pt triangular cluster, exists in sol ution at low temperature as two isomers, with a ratio of ca. 19:1. The values of the H-1-Pt-195 coupling constants and the dynamic behavior suggest that in both the species [ReH(CO)5] is bound to Pt, but with d ifferent locations with respect to the H-bridged Re-Pt triangular edge : cis in the major one (1a), as found in the solid state structure, an d trans in the minor isomer (1b). H-1 2D EXSY experiments at 180 K sho wed the occurrence of two dynamic processes: one interconverts the two isomers, while the other one leads to intermolecular exchange of the [ReH(CO)5] fragment. The addition of [ReH-CO)5] caused a dramatic incr ease of the rate of this exchange even at 180 K and showed that this l atter process is operative only in the major species 1a. The isomeriza tion rate, on the contrary, is insensitive to the addition of [ReH(CO) ,5], indicating its intramolecular nature. The rate constants of the t wo processes obtained at 180 K allowed the calculation of the activati on parameters DELTAG(double dagger)180 = 43.0 kJ/mol and DELTAG(double dagger)180 = 41.9 kJ/mol for the isomerization and for [ReH-CO)5] exc hange, respectively. Several indirect thermodynamic and kinetic eviden ces indicate that above 210 K one or more species, in fast exchange wi th the two isomers, are present in solution. At 264 K, a H-1 2D EXSY e xperiment showed the onset of a new exchange process, implying the cle avage of the H-Re bond of the [ReH(CO)5] fragment, likely via oxidativ e addition of [ReH(CO)5] on Pt.