The kinetics of reaction of the dihydrogen co
mplex
trans-[FeH(
mages/gifchars/eta.gif" BORDER=0 >
2-H
2)(dppe)
2]
+ with an excess ofNEt
3 to for
m cis-[FeH
2(dppe)
2] shows a first-order dependence with respect to both the
metal co
mplex andthe base. The corresponding second-order rate constant only shows
minor changes when the solvent ischanged fro
m THF to acetone. However, the presence of salts containing the BF
4-, PF
6-, and BPh
4- anionscauses larger kinetic changes, the reaction being accelerated by BF
4- and PF
6- and decelerated in thepresence of BPh
4-. These results can be interpreted considering that the ion pairs for
med by the co
mplexand the anion provide a reaction pathway
more efficient than that going through the unpaired
metal co
mplex.Fro
m the kinetic results in acetone solution, the stability of the ion pairs and the rate constant for theirconversion to the reaction products have been derived. Theoretical calculations provide additional infor
mationabout the reaction
mechanis
m both in the absence and in the presence of anions. In all cases, the reactionoccurs with proton transfer fro
m the
trans-dihydride to the base through inter
mediate structures showingFe-H
2···N and Fe-H···H···N dihydrogen bonds, iso
merization to the cis product occurring once the protontransfer step has been co
mpleted. Opti
mized geo
metries for the ion pairs show that the anions are placedclose to the H
2 ligand. In the case of BPh
4-, the bulky phenyls hinder the approach of the base and
makethe ion pairs unproductive for proton transfer. However, ion pairs with BF
4- and PF
6- can interact with thebase and evolve to the final products, the anion acco
mpanying the proton through the whole proton transferprocess, which occurs with an activation barrier lower than for the unpaired
metal co
mplex.