Dissociation mechanism for substitution

As we have seen, an area of major importance and of considerable interest is that of substitution reactions of metal complexes in aqueous, nonaqueous and organized assemblies (particularly micellar systems). The accumulation of a great deal of data on substitution in nickel(II) and cobalt(II) in solution (9) has failed to shake the dissociative mechanism for substitution and for these the statement "The mechanisms of formation reactions of solvated metal cations have also been settled, the majority taking place by the Eigen-Wilkins interchange mechanism or by understandable variants of it" (10) seems appropriate. Required, however, are more data for substitution in the other... 

Returning to the dissociative mechanism for substitution reactions permits a crude picture of the oxalate reaction between the one-bonded and two-bonded forms. 

In addition, dissociative mechanisms for substitution of d complexes can occur when the geometry is distorted from square planar or if the coordination number is higher than four. For example, replacement of the monophosphine in the highly distorted [cis-PtfSiMePhjljfPMejPhlj] with a bisphosphine has been shown recently to occur by dissociative substitution. The dissociation is favored in this case because of the strong trans... 

To conclude this section, we have seen thatvarious pieces of data from (1) rates of water exchange, (2) anations, and (3) various aquation reactions all seem to generally favor a dissociative mechanism for substitution reactions of octahedral compounds. 

A non-dissociative mechanism for interconversion of the tautomers of 6-substituted-2-pyridones (116) has been found (Bensaude et al., 1977). In... 

Ford et al. (95) further prepared 9-substituted 9-mesityl-2-isopropylfluorene (55) to disprove the dissociation mechanism for the exchange. In these com-... 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

Whilst the majority of the available evidence may be interpreted in terms of a dissociative type of mechanism for substitution, there is a substantial number of reactions for which the kinetic data indicate a purely associative SN2 mechanism. In particular, it was found that reactions of cobalt(m) ammines in basic conditions obeyed pure second-order kinetics (Fig. 5-39). 

On the basis of the reported activation parameters, a dissociative mechanism for the first substitution step and a dissociative or dissociative interchange (Id) mechanism for the second substitution step are suggested for the reaction in MeOH and EtOH. 

The Dissociative Mechanism In the D mechanism for substitution of X in ML X. the M-X bond is completely broken and the ML intermediate has lost all memory of X before bonding to the incoming ligand A begins. Thus, applying the steady-state approximation (d[ML ]/dt 0) to the mechanism 8.9... 

Let us now look further at experimental trends that are consistent with dissociative (D or If) mechanisms for substitution in octahedral complexes is supported in very many instances. 

The hapticity of the Cp ring in CpCo (CO) 2 has been the topic of much debate. If an associative mechanism for substitution is followed, a hapticity seeHapticity) of rj from ring slippage would result in a 19-electron intermediate and 17-electron product, while a hapticity of rf would result in a more unstable 21-electron intermediate. However, it has been shown that CO substitution follows a dissociative mechanism... 

The majority of the above systems involve a bimolecular substitution mechanism. Evidence for the transient existence of RaSi as an intermediate, and hence for a dissociative mechanism of substitution at silicon, has been sought but not obtained. Another variant on simple 8 2 substitution at... 

Solvent effects on and for reaction of Co(NO)(CO)3 have been studied. Effects on k are fairly small and are consistent with the effect of varying dielectric constant on the bimolecular rate-determining step. Values of ki for these reactions conducted in the poor solvents cyclohexane, toluene, and nitromethane are similar. This similarity, and the determined activation parameters, are consistent with a dissociative mechanism for this reaction pathway. But values for reaction in tetrahydrofuran, acetonitrile, and dimethyl sulphoxide are different from the former k values and from each other, indicating an associative mechanism involving the solvent for the k term. The balance between the k and k pathways for substitution in Co(NO)(CO)2L depends both on the 7r-bonding and steric characteristics of L. This dependence on the bulk of L is perhaps surprising in view of the tetrahedral stereochemistry and consequent lack of crowding in the substrate. The reverse reaction of Co(NO)(CO)2L with carbon monoxide has also been studied, so that rates in both directions and equilibrium constants are known for ... 

Positive values for the entropy of activation and volume of activation suggest a dissociative mechanism. For example, AS =28J/Kmol and AV = 1.2 cm /mol for the octahedral substitution reaction of [Co(NH3)5H20] +. 

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

The mechanism for substitution of X at five-co-ordinate [Ir(cod)(phen)X] (X=I or SCN) by ethylenediamine is dissociative. Substitutions and exchange at five-co-ordinate nickel and palladium complexes containing tertiary phosphines and uni- or bi-dentate carbaboranyldithiocarboxylate ligands have been studied, but quantitative kinetic data are not yet available. ... 

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