This study is devoted to understanding the nature of the substitution and geometrical isomerization reactions for species of the series trans-[Os-II(en)(2)(eta(2)-H-2)L], where L is a variable ligand. In aqueous solution, when the counterion is CF3SO3-, the complex assumes the composition trans-[Os(en)(2)(eta(2)-H-2)H2O](2+), 1. Formation constants, K-f, vary from values in excess of 10(5) for certain S and N donors to <10(-3) for CF3SO3-. Evidence is adduced for the conclusion that substitution takes place by loss of a ligand, H2O in the case of 1. Considerable activation is required for the entry of a nucleophile into the coordination sphere of the resulting intermediate. This is evidenced by the fact that relative rates of entry of neutral ligands cover a range of 700. We infer that the intermediate is stabilized by rearrangement to [Os(en)(2)(H)(2)](2+), a dihydride of Os(IV). The foregoing reactions take place without isomerization to the cis form. The experimental results indicate that, in contrast to substitution, isomerization takes place by intramolecular rearrangement. In view of the known low-energy barrier for the conversion of an eta(2)-H-2 complex to a dihydride, it is reasonable to assume that this provides a path for isomerization; The H-1 NMR data (eta(2)-H-2) accumulated in the course of these studies are reported, enabling a comparison of delta, J(HD), and T-1 for cis and trans isomers. With the exception of those for the I- derivative, the values of these parameters differ little for the isomeric forms.