EXPERIMENTAL AND THEORETICAL-STUDIES OF NONCLASSICAL D(0) CYCLOPENTADIENYL POLYHYDRIDE COMPLEXES OF MOLYBDENUM AND TUNGSTEN

Low-temperature protonation of compounds CpMH5(PMe3) (M = Mo, 1; W, 2 ) by HBF4. Et2O in CD2Cl2 or CDFCl2 affords the thermally unstable ''h exahydride'' derivatives [CpMH6(PMe3)](+) (M = Mo, 3; W, 4). The corr esponding protonation of 1- and 2-d(5) affords 3- and 4-d(5), respecti vely. The Delta delta on going from H-6 to HD5 is small for both compo unds, but positive for 3 and negative for 4, and no isotopic perturbat ion of resonance (IPR) is observed. The T-1min at 400 MHz for [CpMH6( PMe3)](+) apparently doubles on going from Mo to W (52 ms for 3 and ap proximately 100 ms for 4). Optimized geometries at the restricted Hart ree-Fock (RHF) and second-order Moller-Plesset (MP2) levels and energy calculations at higher levels of theory show that these complexes are dihydrogen complexes [CpM(H-2)(H)(4)(PR3)](+). The theoretical deter mination of a dihydrogen complex is consistent with the fact that the experimental T-1min, values lie within the expected range for dihydrog en complexes. Examination of the potential energy surface at the MP2 l evel gives two mechanisms for hydride motion: exchange through a ''tri hydrogen anion'' transition state and rotation of the dihydrogen ligan d about its axis. The barrier for the hydride exchange (similar to 4 k cal/mol) is consistent with the inability to decoalesce the proton NMR signal.