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Molecular Physics
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computational shortcoming (which could easily disappear when changing the
computational level) and the redox transformation a very asynchronous concerted
process occurring in two phases. Furthermore, it is important to note that the redox
transformation appears to be the rate-determining step of the whole process. This
would be in agreement with the results of experimental kinetic studies [11] for
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mixtures Pd(OAc) 2 /PPh 3 that show that the rate-limiting step of the reaction is an
intra-molecular endothermic process affording a Pd(0) species. On the other hand, it is
important to mention that the presence of a third ligand does not imply that the
reaction is first order in phosphine, since it remains always at a non bonding distance
from the palladium atom, and it is not involved in the bond breaking and forming
process (TS3). Indeed, the interaction between the metallic centre and such a PPh 3 is
very week and could be suppressed for instance if the solvent was considered
explicitly, so the proposed mechanism is still zero order in PPh 3 , as it was determined
experimentally.
The binding of two additional phosphines to Pd(PPh 3 ) 2 again involves the
preliminary formation of weakly bound intermediates corresponding to shallow
minima on the potential surface. One phosphine ligand weakly interacts with
Pd(PPh 3 ) 2 and affords M8 without overcoming any activation barrier. M8 is 45.8 kcal
mol -1 below the asymptotic limit (35.8 kcal mol -1 at the PCM level). The energy
barrier ( E(TS5) – E(M8)) for the definitive phosphine binding leading to M9 is very
low, being only 0.23 kcal mol -1 (0.92 kcal mol -1 when solvent effects are included).
M9 is a planar tri-coordinated palladium complex Pd(PPh 3 ) 3 which lies 55.6 kcal mol -
1 below reactants (45.0 kcal mol -1 in the presence of solvent). The addition of a further
phosphine follows the same pattern. M10 is the preliminary weakly bound complex,
(60.1 kcal mol -1 more stable than reactants; 43.2 kcal mol -1 at the PCM level) and
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