(REH2(CO)2(PME2PH)3)+, A DIHYDRIDE COMPLEX WITH CLASSICAL AND NONCLASSICAL TAUTOMERIC FORMS IN EQUILIBRIUM

Reactions of [ReH3(CO)(PMe2Ph)3] (1) with PMe2Ph or CO in refluxing THF give the monohydride complexes trans-ReH(CO)(PMe2Ph)4 (2) and cis,mer-[ReH(CO)2(PMe2Ph)3] (3), respectively. Protonation of 2 with HBF4.OMe2 in CH2Cl2 at 298 K gives the classical dihydride complex [ReH2(CO)2(PMe2Ph)3]+ (4), while protonation of 3 in CD2Cl2 at 193 K results in an equilibrium mixture of the classical dihydride complex [ReH2(CO)2(PMe2Ph)2]+ (5a) and its nonclassical dihydrogen tautomer [Re(eta(2)-H2)(CO)2(PMe2Ph)3]+ (5b). The eta(2)-HD isotopomer of 5b, formed by protonation of the corresponding monodeuteride complex, shows a 1J(HD) value of 31 Hz in the H-1 NMR spectrum. Variable-temperature H-1 NMR equilibrium studies for the conversion of 5a to 5b produce the thermodynamic parameters DELTA-H = 1.7 +/- 0.4 kcal mol-1 and DELTA-S = 8 +.- 3 cal mol-1 K-1. The lower limit of DELTA-G not-equal for the conversion of 5b to 5a at 223 K is estimated to be 11.5 +/- 0.3 kcal mol-1.