Proton and P-31 NMR data reveal that the molecules Os(H)(2)X(2)L(2) (X = halide, L = (PPr3)-Pr-i) exist in solution as two rapidly interconverting isomers, one having C-2 symmetry (as seen in the solid-state structure) and one having no symmetry. Three distinct intramolecular rearrangements were detected and quantified (Delta G(double dagger)), and P/H coupling constants were employed to estimate P-Os-H angles in the nonsymmetric isomer. The geometries of these two isomers were determined at the MP2 level by gradient method with effective core potential ab initio calculations. The geometry calculated for the symmetric isomer is a distorted octahedron with C-2v symmetry; The solid-state structure, which resembles the calculated symmetric structure, is shown to be the result of opposing forces : steric interactions between phosphine and chlorine ligands and bonding preference of the chlorine ligands. The structure of the nonsymmetric isomer is related to the structure calculated for the symmetric isomer by exchange of one hydride for one chloride together with further slight geometric distortions. Using the calculated structures, physical mechanisms for all observed fluxionality are proposed.