Iridium , inert metal complexes

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Comparison of data in Tables II and III for the cobalt triad indicates that aquation proceeds, on average, about the same for Rh(III) as for Co(III) and substantially slower for iridium(III). As has been observed earlier, however, lability of CF3SO3" complexes is actually enhanced relative to other complexes (such as Cl" complexes) for iridium(III), and for second- and third-row metal ions generally. The property of CF3SO3" as a good leaving group is essentially constant, but the rate of displacement is clearly mediated by the metal ion aquation of the iridium(III) complex is over 100 times slower than for cobalt(III) whereas loss from the exceptionally inert platinum(IV) is slower still (84). 

Rhodium and iridium are unreactive metals they react with O2 or the halogens only at high temperatures (see below) and neither is attacked by aqua regia. The metals dissolve in fused alkalis. For Rh and Ir, the range of oxidation states Table 19.3) and the stabilities of the highest ones are less than for Ru and Os. The most important states are Rh(III) and Ir(III), i.e. which is invariably low-spin, giving diamagnetic and kineticaUy inert complexes (see Section 25.2). 

These auto-reduction processes of platinum tetrammine complexes, whether performed in vacuum or in inert atmosphere, lead to particle sizes within the range 1-4 run encapsulated in the zeolite grains. Different states of iridium dispersions obtained by auto-reduction of [IrfNHjljCl] in Y zeolites were reported by the same authors [116]. Auto-reduction processes are difficult to control and are not well suited to obtain well-defined and homogeneous states of metal dispersion in zeolites. 

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