Substitution reactions of the type trans-[FeH(H-2)(DPPE)(2)](+) + L --> trans-[FeHL(DPPE)(2)](+) + H-2 (L = MeCN, PhCN, DMSO) occur in a single measurable kinetic step. Although the observed rate constants, k(obs), in THF solution show a saturation behavior in [L] with a curvature that is sensitive to the steric requirements of L, the limiting rate constant is almost independent of L (ca. 7 x 10(-3) s(-1) at 30 degrees C) and agrees with the values obtained under solvolytic conditions. The rate law in acetone solutions is simpler, with k(obs) being independent of [L] and very close to the limiting value in THF. The thermal and pressure activation parameters for the limiting rate constants of the reaction with MeCN have been determined in THF, acetone and neat acetonitrile. The values of Delta H double dagger are close to 80 kJ/mol for the three solvents while Delta S double dagger is slightly negative in all cases. The activation volumes are very negative and solvent dependent : -23 +/- 1 cm(3)/mol (THF), -18 +/- 1 cm(3)/mol (acetone), and -35 +/- 2 cm(3)/mol (acetonitrile). As a whole, the kinetic and activation parameters do not agree with a mechanism in which a direct substitution of H-2 for the incoming ligand takes place; instead a mechanism is proposed in which the initial opening of a DPPE chelate ring leads to an intermediate containing a monodentate DPPE and a weakly bound solvent molecule. Thus, the rate-determining step is an associative attack of L to this intermediate to form a species containing both coordinated L and H-2. The final substitution product is formed in a rapid intramolecular attack of the dangling PPh2 arm followed by a cis/trans isomerization.