Octahedral substitution, kinetics

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. 

In addition to the presence of these elements in ores, they are also available from recycled feeds, such as catalyst wastes, and as an intermediate bulk palladium platinum product from some refineries. The processes that have been devised to separate these elements rely on two general routes selective extraction with different reagents or coextraction of the elements followed by selective stripping. To understand these alternatives, it is necessary to consider the basic solution chemistry of these elements. The two common oxidation states and stereochemistries are square planar palladium(II) and octahedral platinum(IV). Of these, palladium(II) has the faster substitution kinetics, with platinum(IV) virtually inert. However even for palladium, substitution is much slower than for the base metals so long as contact times are required to achieve extraction equilibrium. 

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 ... 

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

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 ... 

Octahedral substitution reactions also exhibit a kinetic chelate effect Coordination compounds containing a chelating ligand react more slowly than their counterparts containing two monodentate ligands with similar M-L bond strengths. For example, the rate constant for substitution of Ni[(bpy)] + is 3.3 X 10 s , almost 10 times slower than for [Ni(py)] +, which has a rate constant of 38.5. The pro-... 

R. van Eldik, ed.. Inorganic High Pressure Chemistry, Elsevier, Amsterdam, The Netherlands, 1986. High pressure coordination kinetics including solvent exchange, octahedral and four-coordinate substitution, electron transfer, photochemical, and bioinorganics are discussed. 

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

This is the most common and stable state of chromium in aqueous solution. The Cr3+ ion, with 3d3 electrons, forms mainly octahedral complexes [CrX6], which are usually coloured, and are kinetically inert, i.e. the rate of substitution of X by another ligand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

---