In crystal structures of the molybdenum complexes [(1,2,4-(C5H2Bu3)-Bu-t)Mo(PMe3)(2)H-3] ((CpBu3)-Bu-t) and [((C3HPr4)-Pr-i)Mo(PMe3)(2)H-3] ((CpPr4)-Pr-i), the Mo-bound hydrogen positions were resolved for (CpBu3)-Bu-t, but not for (CpPr4)-Pr-i. NMR experiments revealed the existence of an unknown mechanism for hydrogen atom exchange in these molecules, which can be "frozen out" for (CpBu3)-Bu-t but not for (CpPr4)-Pr-i. 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 (CpPr4)-Pr-i complex are unresloved because of their disorder, which does not occur for (CpBu3)-Bu-t. 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 (CpPr4)-Pr-i than for (CpBu3)-Bu-t.