NMR studies of Ru-3(CO)(10)(PMe2Ph)(2) and Ru-3(CO)(10)(PPh3)(2) and theirH-2 addition products: Detection of new isomers with complex dynamic behavior

The clusters Ru-3(CO)(10)L-2, where L = PMe2Ph or PPh3, are shown by NMR sp ectroscopy to exist in solution in at least three isomeric forms, one with both phosphines in the equatorial plane on the same ruthenium center and th e others with phosphines in the equatorial plane on different ruthenium cen ters. Isomer interconversion for Ru-3(CO)(10)(PMe2Ph)(2) is highly solvent dependent, with DeltaH(double dagger) decreasing and DeltaS(double dagger) becoming more negative as the polarity of the solvent increases. The stabil ities of the isomers and their rates of interconversion depend on the phosp hine ligand. A mechanism that accounts for isomer interchange involving Ru- Ru bond heterolysis is suggested. The products of the reaction of Ru-3(CO)( 10)L-2 with hydrogen have been monitored by NMR spectroscopy via normal and para hydrogen-enhanced methods. Two hydrogen addition products are observe d with each containing one bridging and one terminal hydride ligand. EXSY s pectroscopy reveals that both intra- and interisomer hydride exchange occur s on the NMR time scale. On the basis or the evidence available, mechanisms for hydride interchange involving Ru-Ru bond heterolysis and CO loss are p roposed.