The oxidative addition of the F−CH3 bond to
coordinatively unsaturated
trans-M(X)(PH3)2 (M = Rh,
Ir;
X = CH3, H, Cl) was theoretically investigated by density
functional theory. All of the stationary points were
determined at the B3LYP/LANL2DZ level. A configuration mixing
model based on the theory of Pross and Shaik
has been used to develop an explanation for the barrier height as well
as the reaction enthalpy. Our theoretical
findings suggest that the singlet−triplet splitting
(ΔE
st = E
triplet −
E
singlet) of the ML3 species can be
used as a basis
to predict its reaction activity for oxidative additions; i.e., the
smaller the ΔE
st of ML3, the lower
the barrier height
and the larger the exothermicity, in turn, the faster the oxidative
addition reaction. Considering the substituent
effect, and the nature of the central metal, the following conclusions
therefore emerge: for the 14-electron trans-M(X)(PH3)2 complex, a stronger π-donor
ligand (such as Cl) as well as a heavier transition metal center (the
third-row) will result in a smaller ΔE
st, and thus
will provide a potential model for the oxidative addition of
saturated
C−F bonds. In this work, an energetically feasible reaction
mechanism which should not have radical intermediates
involved is suggested.