Octahedral complexes anation reaction

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

The most obvious examples are in the preparation of octahedral Co(III) complexes. The most readily-available cobalt compounds are Co(II) salts in the presence of suitable ligands - usually N-donors - these are oxidised to give Co(III) complexes by air or hydrogen peroxide. A few such easily-prepared complexes open up pathways to the vast number of known octahedral Co(III) complexes via substitution reactions. For example, [Co(NH3)5(H20)]CI3 is readily converted into [Co(NH3)5X]Cl2 via anation reactions of the type discussed in Section 9.5, and salts containing the [Co(NH3)4(C03)]+ ion (where the carbonate is bidentate, taking up two cis positions) are useful for the formation of cw-[Co(NH3)4X2]+. 

In addition to the thermal transformations described in the preceding sections, a variety of other reaction t)rpes may be induced by heat treatment. These include linkage isomerizalion,84 thermal conversions of coordinated ligands,85 reductive elimination reactions, thermal conversions of counterions, and the transformations of square-planar and octahedral complexes through counter-ion anation reactions or via dehydration. ... 

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 ... 

Palladium(n).—Solvolysis and Anation. Kinetics of solvolysis of sterically hindered (pseudo-octahedral) palladium(ii) complexes catalysed by mercury(ii) have been monitored in methanol. A relationship holds between the catalytic efficiency and the stability constant of the complex between the mercury(ii) and the leaving group. Reaction of Pdaq with ethene is thought, by analogy with earlier results on the reaction between Pd q and quinone, to be the key step in the kinetics of oxidation of ethene to acetaldehyde catalysed by palladium(ii) in the presence of phospho-molybdovanado-heteropolyanions. ... 

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