Co-EDTA

A few cases of optical isomerism are known for planar and tetrahedral complexes involving unsymmetrical bidentate ligands, but by far the most numerous examples are afforded by octahedral compounds of chelating ligands, e.g. [Cr(oxalate)3] and [Co(edta)] (Fig. 19.13). 

The rate of isotopic exchange in the solid state, between cobalt in the cation and in the anion of [60Co(H2O)6] [Co(edta)]2 4 H20, was increased [1144] by irradiation (100 Mrad) of the reactant. It was concluded that exchange occurred via vacancies, rather than through motion of a ring of cobalt atoms, one from a cationic site and the other from a neighbouring anionic site. 

With a large excess of the cobalt reactant, the equilibration process followed first-order kinetics. Table 3-1 presents the data. The plot of kL, versus [Co(edta)2-] is linear, as displaced in Figure 3-2. The line gives k = (3.36 0.04)x 104 L mol ls"1 and k-1 =... 

A plot of the first-order rate constant for equilibration in reaction (3-23) is shown as a function of [Co(edta)2 ]. the reagent present in large excess. The plot is linear as expected from Eq. (3-23). Data, from Ref. 1. are given in Table 3-1. 

Under conditions of a large excess of Co(EDTA) and the rate is given by... 

Co(EDTA) Fe(CN)6 , and Ar bs the observed (first-order) rate coefficient. In accord with the rate law, plots of log (A, — A ) versus time are linear, where A... 

This means that either /r 2 or k is 0.21 M order redox process which (1.117) represents, but is very unlikely for k 2 since formation of the bridged adduct must involve Co "-0 bond cleavage in Co(edta) and such a process would be expected to be much slower (Ch. 4). For this, and other reasons mechanism (b) is strongly preferred. 

The second-order rate constants for thiocyanate anation vs pH are shown in Fig. 1.13. The full line represents (1.216) with the values shown in scheme (1.217). This profile had been earlier recognized in the ring closure of the three analogous pH-related forms of Co(III)-edta to give Co(edta) in which the edta is completely coordinated.In the Co(lll) case the reactivities of the three forms are much closer. A plot of A [H+] -1-[H+] is a quadratic curve from which / ah2> ah be obtained. 

The plot of ogk vs p /l + p for the second-order reaction between Co(edta)" and Fe(CN)g- (Sec. 1.6.3) showed an initial linear slope at low p of 3.8, reached a maximum at p 0.1 M and then decreased. The ionic strength was supplied by NaC104. Give a reasonable explanation for this behavior. 

A number of Co(III) complexes, such as Co(edta) and Co(phen)3, can be resolved into optical isomers and are extremely stable towards racemization. The Co(II) analogs are configurationally labile and resolution has proved impossible. Suggest how with a double mixing apparatus it might be possible to measure half-lives in the 10 -1 s range for the first-order racemization of the Co(Il) complexes. 

Table 5.9 Ratio (R) of Reactivities of Co(edta)CE and CofedtalHjO- towards Reductants ...

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