Experimental and Theoretical Studies of Nonclassical d0 Cyclopentadienyl Polyhydride Complexes of Molybdenum and Tungsten

Abstract

Low-temperature protonation of compounds Cp{sup *}MH{sub 5}(PMe{sub 3}) (M = Mo, 1; W, 2) by HBF{sub 4}{center_dot}Et{sub 2}O in CD{sub 2}Cl{sub 2} or CDFCl{sub 2} affords the thermally unstable hexahydride derivatives [Cp{sup *}MH{sub 6}(PMe{sub 3})]{sup +} (M = Mo, 3; W, 4). The corresponding protonation of 1- and 2-d{sup 5} affords 3- and 4-d{sup 5}, respectively. The {Delta}{delta} on going from H{sub 6} to HD{sub 5} is small for both compounds, but positive for 3 and negative for 4, and no isotopic perturbation of resonance (IPR) is observed. The T{sub 1min} at 400 MHz for [Cp{sup *}MH{sub 6}(PMe{sub 3})]{sup +} apparently doubles on going from Mo to W (52 ms for 3 and approximately 100 ms for 4). Optimized geometries at the restricted Hartree-Fock (RHF) and second-order Moeller-Plesset (MP2) levels and energy calculations at higher levels of theory show that these complexes are dihydrogen complexes [Cp{sup *}M(H{sub 2})(H){sub 4}(PR{sub 3})]{sup +}. The theoretical determination of a dihydrogen complex is consistent with the fact that the experimental T{sub 1min} values lie within the expected range for dihydrogen complexes. Examination of the potential energy surface at the MP2 level gives two mechanisms for hydride dihydrogen ligand about its axis. The barrier formore » the hydride exchange ({approximately}4 kcal/mol) is consistent with the inability to decoalesce the proton NMR signal.« less