Cobalt catalyzed aquation

Rate constants have been determined for mercury(II)-catalyzed aquation of the hexachlororhenate(IV) anion in a range of binary aqueous solvent mixtures. With the aid of ancillary information on Gibbs free energies of transfer for the reactant ions from water into the solvent mixtures it proved possible to analyze the observed reactivity trends into initial state and transition state components. The results are discussed below, in conjunction with a parallel study of mer-cury(II) catalysis of aquation of chloro-cobalt(III) complexes. 

Acid-catalyzed aquation has already been covered at appropriate points in Section 5.7.1. Most of the present section deals with catalysis of removal of halide from cobalt(III) with the aid of mercury(II). The final paragraphs of the section deal with the question of catalytic removal of a variety of groups and the relation of the results to the question of the possible intermediacy of transient five-coordinated intermediates. 

The most important t5q)es of homogeneous catalysis in water are performed by acids, bases and trace metals. A wide variety of mechanisms have been outlined for acid/base catalysis and are presented in kinetics texts (e.g. Moore and Pearson, 1981 Laidler, 1965). A number of bases have been observed to catalyze the hydration of carbon dioxide (Moore and Pearson, 1981 Dennard and Williams, 1966). Examples are listed in Table 9.7 for OH and the base Co(NH3)gOH2. The most dramatic effect is the catalysis of HS-oxidation by cobalt-4,4, 4",4"-tetrasulfophthalocyanine (Co-TSP ). At concentrations of 0.1 nM Co-TSP the reaction rate was catalyzed from a mean life of roughly 50 h to about 5 min. The investigators attributed the reason for historically inconsistent experimentally determined reaction rates for the H2S-O2 system by different researchers partly to contamination by metals. Clearly, catalysis by metal concentrations that are present in less than nanomolar concentrations is likely to be effective in aquatic systems. We shall see that similar arguments apply to catalysis by surfaces and enzymes. 

The observation that charcoal catalyzes the basic aquation of hexaammine-cobalt(III) had been noted in literature, and JB observed that this catalysis was accelerated in the presence of cobalt(II). These observations were the basis for the equilibrium studies that now will be described. 

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