Mechanisms of substitution associative

These data are in accord with an associative mechanism of substitution with the ligands that bond preferentially to Pt(II), giving faster substitution reactions. From these and similar studies, it is possible to arrange a series of ligands on a scale of relative rates of substitution, with the order being... 

In summary, transfer of tetraphenylcyclobutadiene might be assumed to involve an associative mechanism of substitution with the metal increasing its coordination number in the transition state. 

Fig. 9. Eight-coordinate transition state in an interchange associative mechanism of substitution reactions on a pentagonal-bipyramidal complex (S=solvent molecule, A =anionic monodentate ligand).

Figure 11.3 Energy profiles for the associative mechanism of substitution reactions, A in the Langford-Gray classification, showing the relation between the intermediate and the two TSs (a) The bond-breaking TS has higher energy, (b) The bond-making TS has higher energy.

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

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. 

It has always been of some interest to examine the extent to which associative activation dominates the mechanism of substitution of four-coordinate planar cP metal complexes. The coordination unsaturation of these formally 16-electron valence shell complexes predicts that a substitution pathway with increased coordination number (18-electron valence shell) will be favoured over one with a reduced coordination number (14 electrons). This was well understood by workers in the field438 long before Tolman94 published his rules. The first attempt to force a dissociative mechanism was made by Basolo and Baddley513>514 who reasoned that since the steric requirements of associative substitution (rates reduced by steric hindrance from the cis position) were opposite to those of a dissociative mechanism (rates increased or unchanged by increased steric hindrance), sufficient congestion in the substrate should reduce the rate of the associative process to the point where dissociative activation took over. If this did not produce a change in mechanism it could at least indicate a lower limit to the difference of the two modes of activation. 

E2I.1 If the mechanism of substitution were to be associative, the nature of incoming ligand should affect the rate of the reaction. This is because the rate-limiting step would require the formation of M-X bonds (X = incoming ligand). In the present case, however, the rate of the reaction does not vary much with the nature or the size of the incoming ligand. Therefore, the more likely mechanism would be dissociative. 

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... 

Just as in conventional substitution reactions in Cr(III) complexes, there is considerable controversy as to the mechanism of substitution in Cr(III) meso-substituted porphyrins negative AS values suggest associative activation of the axial sites while positive AV values imply dissociative activation. A thorough study of the kinetic and equilibrium properties of the axial ligation reaction between imidazole and pyridine with [Cr(TPPS)(H20)2] " has now been made. Data were analyzed according to Scheme 1 and the rate and equilibrium constants are reported in Table 6.4. Data for the toluene soluble [CrCl(TPP)X] (X = py, quinoline, or PPhj) with 1-methyl imidazole (Meim) are not quite comparable, as the reaction is not first order in Meim. Kinetic parameters for the dissociation of X from [CrCl(TPP)X] in toluene are, in order X, k (s" ), AH, AS, AV py,... 

Solid State Reactions.—Kinetic and mechanistic studies of substitutions in the solid state, which are of some relevance to mechanisms of substitution in solution, include those of [Cr(NH3)5(OH2)]X3, where there seems to be some associative character to the mechanism for X = Br, SOg - or S042- 13 of [Cr(NH3)6]Cl3, [CrCl(NH3)5]Cl2, and c -[CrCl2(NH3)4]Cl, for which rates and activation parameters were determined of [Cren3](NCS)3 and of the racemisation of K3[Cr(oxalate)3]. ... 

The original mechanism was modified to include associative substitutions of the 17-electron intermediates. Associative mechanisms for substitution of 17-electron intermediates were shown to occur after this initial study that illustrated a radical chain mechanism. 

Analogous behavior has been observed for Rej(CO),The trapping of Mn(CO)j by PPhj has been observed to depend on [PPhj] and was taken to indicate an associative mechanism for substitution on the 17-electron radical. 

Mechanisms of reactions of this type are broadly varied from the classical associative mechanism of bimolecular substitution Sn2 (I-+II-+III) to the classical dissociative mechanism SnI which requires the formation of the free carbocation VI, including the intermediate mechanisms of Winstein and Sneen where the contact IV or the solvent-separated V ion pairs serve as the active forms, see Ref. [12] for a detailed review. 

Mechanisms of substitution at [WFa] by various anionic nucleophiles, in particular azide and cyanide, have been investigated. Kinetic parameters have been obtained for [WF Ns] and [WFeCN], in liquid sulfur dioxide. Exchange takes place by an associative mechanism. A dissociative mechanism seems improbable in the light of thermochemical information (Aff ) on the previously characterized compound [WF5N3]. ... 

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