Dissociative ligand exchange

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

A comparison with cross-linker 4a proves the underlying dynamics are controlled by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster than for 4b but the association thermodynamics are very similar, and the effect of those kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of lOOmgmL 4a-PVP is only 6.7 Pa s, a factor of 80 less than that of the isostmctural network 4b PVP. Although the association constants are not identical, the effect of the thermodynamics would be to increase the viscosity of 4a PVP relative to 4b PVP, the opposite direction of that observed. The kinetics dominate even the extent of cross-linking 5% 4a PVP is less viscous by a factor of 5 than is 2% 4b PVP. 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

A main objective of this work is to develop the relationship between the many reaction pathways leading to ligand substitution at square-planar molecules. Concentrating on more recent results to illustrate the processes under discussion, we examine in detail the evidence for operation of the less common and sometimes controversial routes such as dissociative ligand exchange (6). It cannot be stressed too much, however, that the field is still dominated by associative reactions, so to maintain a balance, as well as to provide the now necessary comparative evidence, we also cover the essential features of nucleophilic ligand replacements. 

To confirm fhat a dissociative ligand exchange mechanism is in operation, the kinetics of fhe rapid reaction of fhese complexes wifh efhyl vinyl ether was studied by NMR and UV/VIS spectroscopies. The results were in complete agreement with a dissociative hgand exchange mechanism for aU complexes in this series, with the possible exception of fhe diiodo derivatives [101]. 

Dissociative Ligand Exchange Prior to Reductive Elimination of AuR3(PPh3)... 

In dissociative ligand exchange, an equilibrium between four and five coordinate compounds is responsible for the rapid intermolecular fluorine exchange (without racemization at silicon) in compound 196 (p. 163) (321). A similar equilibrium between open and closed forms is also implicated in amino-arylsilanes 213 (p. 174) (133). 

Table 3.4. Degree of Inertness and Lability of Octahedral Transition Metal Complexes for Dissociative Ligand Exchange ...

Fig. 1.14. Reaction diagram for the dissociative ligand exchange mechanism for d-l cations.

While these studies established the electronic preference for a dissociative ligand exchange, the steric effects of the experimentally used ligands also play a role. Chen and coworkers [5] completed the mechanistic picture with a DFT (density functional theory) analysis of the full experimental ligands. It was found that steric interactions increased the barrier and reaction energy of the associative mechanism by more than 5 kcal mol , while the dissociative pathway energetics were... 

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