Dissociation of ligands

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

This behavior is illustrated in Fig. 4 for the case where the first ligand binds with a dissociation constant that is 30 times lower than the equilibrium constant for dissociation of ligand from the second site. The dotted line shows the behavior of a Scatchard plot in the absence of second-site binding in this case, the dotted line extrapolates to a value of 1.0 on the horizontal axis. 

Some special considerations apply when palladium complexes are to be used in solid-phase chemistry. Two obvious concerns are penetration of relatively large palladium complexes into resins and the effects of the resin microenvironment on dissociation of ligands from fully coordinated palladium complexes to give active species. The success of the chemistry reported to date has been an empirical test of these questions, but such factors may explain instances in which, for example, the ligandless... 

The observation that dissociation of ligand is required to induce reactivity, in this and many of the cases described below, led to Tolman s proposal that most catalytic processes occur through discreet 16- and 18-electron intermediates (24). This proposal has remained a cornerstone for the study of homogeneous catalysis. 

Oxidative addition occurs with coordinatively unsaturated complexes. As a typical example, the saturated Pd(0) complex Pd(Ph3P)4 (four-coordinate, 18 electrons) undergoes reversible dissociation in situ in solution to give the unsaturated 14-electron species PdlPhjP) (23), which is capable of undergoing the oxidative addition. Various (T-bondcd palladium complexes 24 are formed by the oxidative addition. In many cases, dissociation of ligands to supply a vacant coordination site is the first step of catalytic reactions. 

The photo substitution pathway probably involves intramolecular energy transfer from an f-f excited state to an intraligand state leading to the dissociation of ligand thd from the complex. Excitation of ligand absorption bands of Tb(thd)3 also leads to the same substitution reaction [111],... 

The association and dissociation of ligands at metal centers is a key reaction type in organometallic chemistry, including organometallic radical chemistry. Typically,... 

Many other examples of the effect of ligand bulk on the dissociation of ligands have been reported in the chemical literature. For many dissociation reactions, the effect of ligand crowding may be more important than electronic effects in determining reaction rates. 

The palladium complexes (DPPF)2Pd and (BINAP)2Pd also undergo dissociation of ligand, this time to form a (chelate)Pd(O) intermediate that undergoes oxidative addition. Addition of phenyl bromide to (BINAP)2Pd(0), conducted with high ratios of aryl bromide to free phosphine, were close to zero order in phenyl bromide.178 Under these... 

Brown and coworkers reported studies of the addition of aryl iodides to a series of palladium complexes of trialkylphosphines with the general formula Pd[(P(Cy) (Bu-f)3 )]2 (n = 0-3)181. They concluded that the complexes of the smaller phosphines (n = 2, 3) react with Phi through an associative mechanism and undergo oxidative addition directly to the PdL2 complex. They also concluded that oxidative addition of Phi to complexes of the bulkier phosphines (n = 0-1) proceed after dissociation of ligand to generate PdL. In unpublished work, Barrios-Landeros and Hartwig have found that the kinetic behavior of the reactions of these complexes is complex and that these reactions occur with profiles that are characteristic of autocatalysis. 

Dissociation of ligands from square-planar complexes is also important in ligand redistribution reactions of square-planar complexes ... 

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