Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

In crystal structures of the molybdenum complexes [(1,2,4-C 5 H 2 t Bu 3 )Mo(PMe 3 ) 2 H 3 ] (Cp t Bu 3 ) and [(C 5 H i Pr 4 )Mo(PMe 3 ) 2 H 3 ] (Cp i Pr 4 ), the Mo-bound hydrogen positions were resolved for Cp t Bu 3 , but not for Cp i Pr 4 . NMR experiments revealed the existence of an unknown mechanism for hydrogen atom exchange in these molecules, which can be “frozen out” for Cp t Bu 3  but not for Cp i Pr 4 . Density functional theory calculations of the most stable conformations for both complexes in the gas phase and in a continuum solvent model indicate that the H’s of the Cp i Pr 4  complex are unresloved because of their disorder, which does not occur for Cp t Bu 3 . A corresponding examination of alternative rearrangement pathways shows that the rearrangements of the H’s could occur by two mechanisms: parallel to the cyclopentadienyl (Cp) ring in a single step and perpendicular to the Cp ring in two steps. The parallel pathway is preferred for both molecules, but it has a lower energy barrier for Cp i Pr 4  than for Cp t Bu 3 .