The kinetic
s of reaction of the dihydrogen complex
trans-[FeH(
s/gifchar
s/eta.gif" BORDER=0 >
2-H
2)(dppe)
2]
+ with an exce
ss ofNEt
3 to form
cis-[FeH
2(dppe)
2]
show
s a fir
st-order dependence with re
spect to both the metal complex andthe ba
se. The corre
sponding
second-order rate con
stant only
show
s minor change
s when the
solvent i
schanged from THF to acetone. However, the pre
sence of
salt
s containing the BF
4-, PF
6-, and BPh
4- anion
scau
se
s larger kinetic change
s, the reaction being accelerated by BF
4- and PF
6- and decelerated in thepre
sence of BPh
4-. The
se re
sult
s can be interpreted con
sidering that the ion pair
s formed by the complexand the anion provide a reaction pathway more efficient than that going through the unpaired metal complex.From the kinetic re
sult
s in acetone
solution, the
stability of the ion pair
s and the rate con
stant for theirconver
sion to the reaction product
s have been derived. Theoretical calculation
s provide additional informationabout the reaction mechani
sm both in the ab
sence and in the pre
sence of anion
s. In all ca
se
s, the reactionoccur
s with proton tran
sfer from the
trans-dihydride to the ba
se through intermediate
structure
s showingFe-H
2···N and Fe-H···H···N dihydrogen bond
s, i
somerization to the ci
s product occurring once the protontran
sfer
step ha
s been completed. Optimized geometrie
s for the ion pair
s show that the anion
s are placedclo
se to the H
2 ligand. In the ca
se of BPh
4-, the bulky phenyl
s hinder the approach of the ba
se and makethe ion pair
s unproductive for proton tran
sfer. However, ion pair
s with BF
4- and PF
6- can interact with theba
se and evolve to the final product
s, the anion accompanying the proton through the whole proton tran
sferproce
ss, which occur
s with an activation barrier lower than for the unpaired metal complex.