SYNTHESIS AND DYNAMIC NMR CHARACTERIZATION OF THE MIXED-METAL RHENIUM-PLATINUM ADDUCT [(CO)4RE(MU-H)(MU-CO)PT(PPH3)2] - AN INTERMEDIATE IN THE RHENIUM-PLATINUM CO/PPH3 EXCHANGE

The reaction of [Pt(PPh3)2(C2H4)] with [ReX(CO)5] (X = H, Br) at room temperature results in an instantaneous CO/PPh3 exchange between Re and Pt, affording [ReX(CO)4(PPh3)] quantitatively, the main Pt product being the trinuclear species [Pt3(CO)3(PPh3)4]. The complex [Pt(PPh3)2(C2H4)] is also able to catalyse the selective substitution of one carbonyl by PPh3 in [ReBr(CO)5]. At low temperature the reaction between [Pt(PPh3)2(C2H4)] and [ReH(CO)5] gives, reversibly, the adduct [(CO)4Re(mu-H)(mu-CO)Pt(PPh3)2], isolated at 193 K, and characterised spectroscopically, C-13-, P-31- and H-1 NMR variable-temperature spectra of this species have revealed the occurrence of two mutual exchange processes, one equalizing the four carbonyls cis to the H ligands (fast even at 173 K), and the other one exchanging the phosphines. The kinetic parameters of the latter process have been estimated by band shape analyses of the H-1- and P-31 NMR spectra (E(a) = 42.6 +/-1.4 kJ mol-1; DELTAH# = 40.8 +/- 1.4 kJ mol-1; DELTAS# = 2 +/- 6 J mol-1 K-1). The addition of a tertiary phosphine, at 193 K, causes the instantaneous cleavage of the adduct, restoring free [ReH(CO)5]. At temperatures higher than 240 K, the adduct decomposes, leading to the formation of the same products obtained in the reaction at room temperature. The possibility that the adduct is the intermediate in the CO/PPh3 exchange between Re and Pt is discussed, in the light also of some kinetic experiments.