Kinetics of octahedral substitution

Substitution Reactions in Square Planar Complexes 538 Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

Considerable investigation of the octahedral carbonyl complexes has been carried out. To a certain degree this is because definitive evidence for associative substitution in the case of type A complexes has been conspicuously lacking whereas for the type B compounds there seem to be several well-substantiated examples. A general summary of the main types of octahedral substitutions which have been kinetically examined is given in Table 15. 

It was clear in the 1950s that there was a need for detailed kinetic studies of ligand substitution reactions of platinum(ii) complexes, and our laboratory was prepared to do this because it was engaged in such studies of octahedral substitution. However, only a brief account of our studies is given in this article. At about this time Martin and his students initiated their investigations of aquation reactions of chloroammineplatinum(ii) complexes. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Kinetics and mechanisms of substitution reactions of octahedral macrocyclic amine complexes. C. K. Poon, Coord. Chem. Rev., 1973,10,1-35 (130). 

These data provide some qualitative guides to the kinetic behavior of octahedral species. If the change in the d-electron stabilization energy (i.e., the CFAE) is negative for a particular mechanism, the reaction is favored and the complex should be relatively labile—i.e., the substitution process should occur easily. Conversely, if the CFAE is positive, the reaction is disfavored and the complex should be relatively kinetically inert. 

Certain octahedral complexes, particularly the acido—amine complexes of cobalt(III), undergo substitution in protonic solvents at rates that are proportional to the concentration of the conjugate base of the solvent (e.g. OH- in water) or inversely proportional to the concentration of the conjugate acid of the solvent (e.g. retardation by H30+ in water or NH4+ in liquid ammonia). Such reactions have received considerable attention since systematic studies of ligand substitution commenced, and figured amongst the earliest kinetic studies in the field.298 The subject has been... 

Complexes of (( Ir(III) are kinetically inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of [IrBr(NH3)5]2+ [35884-02-7] at 298 K has been measured at -2 x 10-10 s-1 (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid greatly accelerates the rate of substitution via a transient, labile Ir(H) species (169). Optical isomers can frequently be resolved, as is the case of ot-[IrCl2(en)2]+ [15444-47-0] (170). Ir(III) amine complexes are photoactive and undeigo rapid photosubstitution reactions (171). Other iridium complexes... 

Most of the work on the kinetics and mechanism of aquation - the first step in octahedral substitution - has been done on cobalt(III) complexes, which are neither too inert nor too labile for exhaustive investigations. The aquation of Co(NH3)5X2+/3+ (the charge depends on whether X is neutral or anionic) has been studied in great depth. The rate law for such a process is found to take the form ... 

Far less information is available on olefin substitution in closed-shell octahedral complexes. Angelici and Loewen (86) have studied the kinetics of the reaction... 

Ingold, Nyholm and Brown (1954) have shown tliat the kinetic order in the substitution of the octahedral cobalt(III) complex l-cis-[Co(en)2Cl2] [en = H2N.CH2.CH2.NH2] depends upon the nucleophilic power of the substituting anion. Working in methanol solution and using polarimetric, spectrophotometric, titrimetric and, where necessary, radiochemical methods of following the kinetics, they found the rates of substitution ot Cl by the other ions shown in Fig. 286. For the first three reagents the kinetic order... 

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