Ligand substitution reactions dissociative mechanism

The determination of activation parameters, normally strongly associated with the nature of the transition state complex, may also be very useful in distinguishing plausible mechanisms in ligand substitution reactions. Dissociative mechanisms will be associated with a relatively high enthalpy change as well as to positive AS values. Associative processes will indeed show a not very high endothermic effect but a considerable decrease in entropy. 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

H. M. Marques, J. C. Bradley, and L. A. Campbell, J. Chem. Soc., Dalton Trans., 2019 (1992). Ligand Substitution Reactions of Aquacobalamin Evidence for a Dissociative Interchange Mechanism. 

The mechanism of ligand substitution reactions in the carbyne complexes /rar7.v-M(CR)X(CO)4 (M = Cr, W R = Me, Ph, NEtj X = Cl, Br, 1, SePh) was investigated by H, Fischer and co-workers (JOO). The influence of the metal center, the trans ligand, and the carbyne substituent on the M—CO dissociation step was determined. The reactions with PPhj in 1,1,2-trichloroethane [Eq. (62)] all follow first-order kinetics, with activa-... 

The transformation of M --C to (MC) in Eq. 2 may be further divided into individual elementary steps, MC, 1 —s-MC/, representing the successive replacements of the solvent molecules associated with the cation by the donor atoms of the ligand. As with ligand substitution reactions on transition metal ions, there are two limiting hypothetical mechanisms dissociative and associative. The first mechanism is a dissociative or pseudomonomolecular process with consecutive dissociation (Eq. 3) and recombination (Eq. 4) steps ... 

The ligand substitution reactions of carbene complexes such as (CO)s-CrC(OCH3)CHj allow the synthesis of many phosphine- and phosphite-substituted carbene complexes. It is expected that these complexes will have modified reactivity and will provide a means of fine tuning reactions of carbene complexes. More importantly, the substitution reactions of carbene complexes proceed by a dissociative mechanism involving coordinatively unsaturated intermediates. Study of the ligand substitution reactions can give valuable information about these coordinatively unsaturated intermediates which are also involved in the important cyclopropanation, alkene scission, and thermolysis reactions of metal-carbene complexes. 

Demonstration of the nature of the intermediates is in general a very difficult experimental task, therefore most ligand substitution reactions are assumed to be interchange processes. Nonetheless, kinetic studies often permit us to establish the intimate reaction mechanism which assigns to the interchange pathways either an associative or a dissociative character. 

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