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Molecular Physics
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bond. In particular, the replacement of one oxygen with a phosphorous atom allows a
more regular and stable square planar coordination of the metal.
(Table 1 here)
(Figure 1 here)
The energy profile for the addition of a second phosphine ligand is almost
identical to that previously discussed. A slight stabilization accompanies the
formation of a new preliminary complex M3 (4.3 kcal mol -1 lower than M2 and 33.8
kcal mol -1 lower than M0). The weak interactions that stabilize this intermediate
(again weak H-bonds) are totally lost in condensed phase, where M3 is 26.7 kcal mol -
1 below M0 and hence 1.6 kcal mol -1 higher than M2. Even if this marked
destabilization of M3 is partly due to the frozen geometry (only PCM single point
calculations have been carried out), these results suggest that the intermediate M3
does not probably exist in condensed phase or that it is in equilibrium with M2.
(Scheme 1 here)
The definitive binding of the second phosphine to palladium is a crucial point
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of the process. As a matter of fact, the resulting complex Pd(OAc) 2 (PPh 3 ) 2 can exist in
two different configurations (cis and trans) and each of them originates a unique
reaction pathway (denoted here as cis and trans channel) leading to palladium
reduction.
We first discuss the trans-channel. The corresponding energy profile is
depicted in Figure 2. The various structures located along the reaction path are
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