Langford-Gray Classification

The mechanisms of substitution reactions at carbon atoms lead to a direct relation between the order of the reaction and its molecularity. This may cause the impression that 8j4l and 

Sn2 mechanisms encompass all possible substitution reaction mechanisms. However, the most general case is 

Langford and Gray presented another classification for the mechanism of substitution reactions that is particularly appropriate for metal complexes [5], but also englobes the classification of Ingold and co-workers for organic substitutions. This classification makes a distinction between the stoichiometric mechanism and the activation mechanism. The stoichiometric mechanism concerns the nature of the intermediate. It distinguishes between  

The activation mechanism is related to the nature of the activation step. It distinguishes 

An example of an interchange mechanism, I, which is essentially an 8 2 mechanism, is the reaction 

---

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.

Figure 11.2 Relationship between the mechanisms of substitution reactions and their energy profiles, and the classifications of Hughes-Ingold and Langford-Gray.

Figure 11.2 presents the relation between the classification of Ingold-Hughes and that of Langford-Gray. An example of a dissociative mechanism, D, which corresponds to the S,., mechanism in the classification of Ingold-Hughes, is reaction (11. VII) and... 

The mechanistic classification generally accepted for ligand substitution reactions was proposed by Langford and Gray in 1965 (19). This classification was often discussed in the literature and its principles are only summarized here for convenience. 

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... 

H. B. Gray and C. Langford (1968) have classified metal ions into four categories based on the rates of H20 exchange between their aqua complexes and bulk solvent. The classifications that they have proposed are shown in Table 20.1. 

---