Heterolytic Ligand Dissociation

The second type of system in which heterolytic cleavage is favored arises with a metal-ligand complex having an atomic metal ion (with a s°dI1+1 configuration) and a two electron donor, L 2. A prototype is (Ag C6H6)+ which was observed to photodissociate to form X + Y = Ag(2S, s M10) + C6H6+(2B j) rather than the lower energy (heterolytically cleaved) dissociation limit Y + X2 =... 

Ligand dissociation can involve either heterolytic or homolytic processes. Most biological processes involve the former, although examples of the latter are known. For example, adenosylcobalamin (vitamin B-12 coenzyme), in association with an appropriate apoenzyme, catalyzes a variety of rearrangements via a 1,2-hydride shift (see Chapter 13). The hrst step in these reactions is thought to be homolytic dissociation of a Com—alkyl bond as shown ... 

First-Order Kinetics, K[A] Unimolecular processes, such as ligand dissociation from a metal center or a simple homolytic or heterolytic cleavage of a single bond, provide a straightforward example of a first-order reaction. The kinetics of this simple scheme, Equation 8.5, is described by a first-order rate law, Equation 8.6, where A stands for reactants, P for products, [A]0 for initial concentration of A, and t for time. The integrated form is shown in Equation 8.7 and a linearized version in Equation 8.8. 

Heterolytic and homolytic ligand dissociation processes occur, and are of importance in the context of the catalytic roles of metal complexes, e.g. ... 

The Mo -trihydropterin radical view accommodates all the experimental data observed for the Mo complexes of reduced pterins. For example, tetrahydropterin ligand dissociation suggests a heterolytic cleavage of the Mo-N5 bond with the pterin regaining its original lone pair electrons. The laek of... 

A heterolytic rupture will, in the case of an ordinary covalent bond, lead to formation of a cation and an anion in the case of a coordinate bond, the ligand simply departs along with the electron pair it contributed to form the bond. A dissociative mechanism exhibits first-order kinetics its rate is independent of the concentration of the incoming group Z. The intermediate EX in the overall reaction ... 

In Sections III-V, we speculated about the nature of active sites on chromia and the relations of such speculations to chemisorption and heterogeneous catalytic reactions. In particular, we suggested that many types of active sites would involve coordinatively unsaturated surface (cus) ions of Cr3+ and 0 - and that the following types of chemisorption might occur at such sites simple coordinative adsorption at Cr3+(cus), adsorption of generalized acids at 02-(cus), heterolytic dissociative adsorption at pair sites of Cr +(cus) and 02-(cus), and reductive adsorption. In addition, we considered the possibility of ligand displacement adsorption which does not depend upon (cus) ions. 

The mechanistic conclusions may be summarized as follows, reference being made to the classification of processes at the beginning of Section X,A,1. The basic reactions are five heterolytic dissociative adsorptions and one reaction analogous to a ligand insertion reaction. The first five reactions below represent reaction of Cr3+(cus) + 02 (cus) with deuterium or a hydrocarbon. 

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