UNEXPECTED MECHANISM FOR SUBSTITUTION OF COORDINATED DIHYDROGEN IN TRANS-[FEH(H-2)(DPPE)(2)](+)

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 sin gle measurable kinetic step. Although the observed rate constants, k(o bs), in THF solution show a saturation behavior in [L] with a curvatur e 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 degre es C) and agrees with the values obtained under solvolytic conditions. The rate law in acetone solutions is simpler, with k(obs) being indep endent 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/m ol for the three solvents while Delta S double dagger is slightly nega tive in all cases. The activation volumes are very negative and solven t 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 a ctivation parameters do not agree with a mechanism in which a direct s ubstitution of H-2 for the incoming ligand takes place; instead a mech anism is proposed in which the initial opening of a DPPE chelate ring leads to an intermediate containing a monodentate DPPE and a weakly bo und solvent molecule. Thus, the rate-determining step is an associativ e attack of L to this intermediate to form a species containing both c oordinated L and H-2. The final substitution product is formed in a ra pid intramolecular attack of the dangling PPh2 arm followed by a cis/t rans isomerization.