Cobalt complexes kinetic effects

The kinetic effects of C02 in the base catalyzed hydrolysis of some carboxylato amine cobalt(III) complexes have been reported (80-82). In the base catalyzed hydrolysis of oxalatopentaammine-cobalt(III) (80), C02 retarded the reaction due to the formation of a virtually unreactive ion-pair, f (N H .) r, 2 2 COi ]. The equilibrium constant for formation of carbonate ion-pairs with (glycinato-O) (tetraethylene-pentamine)cobalt(III), (81) and (o-methoxybenzoato) (tetraethylenepentamine)cobalt(III) (82) were, however, much smaller than for the oxalatopentamminecobat(III) and a very weak rate retardation and virtually no effect was observed in the base catalyzed hydrolysis of the latter two complexes. 

No reaction at all took place at 25°C in the absence of carbon so that the measured rates could be completely ascribed to the action of the catalyst, Decolorizing Charcoal Cl77. The concentrations of both cobalt complexes were spectrophotometrically monitored with time and it was noted that the sums of the concentrations of the two species were always 2-3% short of the initial concentrations. Since the intercepts of the first-order rate plots at zero time also gave concentrations 2-3% lower than the initial values, these apparent discrepancies clearly pointed to a small amount of fast adsorption. The rates were independent of the shaking speed which marked the catalysis as surface-controlled. The kinetics of this surface reaction were, however, extremely complicated. Mureinik systematically varied the concentrations of the relevant species he found that the plot of the effective first-order rate... 

Until fairly recently it was assumed that large kinetic effects of the type found in platinum(II) complexes did not occur in cobalt(III) complexes. The situation has now been reversed. The rate constants for the reaction... 

Silver(i) is another catalyst for aquation. Unfortunately, a kinetic study of its catalytic effect on aquation of [CoI(NH8)5l + is complicated by heterogeneous catalysis by the silver iodide produced. Indeed this topic of heterogeneous catalysis of aquation of cobalt(iii) complexes has recently been reviewed and extended by studies of the kinetic effects of a range of diverse solids, e.g. mercury(ii) sulphide, silver bromide, and platinum metal. ... 

As in organic chemistry where m values for bromides are rather lower than for chlorides, for example m — 0.92 for t-butyl bromide compared with m = 1.00 for t-butyl chloride, m values for cobalt(ra)-amine-bromide complexes are, at around 0.2, rather lower than for the analogous chlorides. Whereas in this work the effect of solvent structure on reaction rates has been used to gain further insight into reaction mechanisms, the opposite approach has also been used, in a study of aquation of tra/u-[Co(en)2Cl2]+ in alcohol-water mixtures, in which variation of rate with solvent composition has been used as a probe of solvent structure variation. Rates of aquation of both cis- and trans-[Co ea)2C have been determined in aqueous acetonitrile (0 < mole fraction MeCN < 0,104). For both complexes aquation rates decrease only slightly as the proportion of acetonitrile increases, with the cir-complex slightly more sensitive to solvent variation. The kinetic effects observed here are smaller than those observed in t-butyl alcohol-water solvent mixtures. ... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

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