EDTA and Its Analogs

Metal-sequestering agents are sometimes incorporated at low levels (10-100 mg per kg) to preclude any problems arising from trace metal contamination. Ethylenediaminetetraacetic acid (EDTA) and its various analogs tend to be the preferred type. 

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. 

At pH < 6, in the absende of excess bipy and with EDTA as electron donor, hydrogen production takes place by a different mechanism (reactions 32 to 37 and 39 to 42 for TEOA read EDTA). Presumably, the [Ru(bipy)3]3+ generated in reaction (40) is reduced by EDTA. However, the source of the electron in reaction (41) is unclear, although presumably it could arise from EDTA-formed by deprotonation of EDTA+ in an analogous reaction to that described above for TEOA. 

Figure 15.44. TOP reducing activities of betaxolol (Betoptic)and its oxime and methoxime analogs in New Zealand albino rabbits. Doses of 100 gL solutions formulated in saline with 0.01%EDTA and 0.01% benzalkoniumchloride (pH 6.0) were administered in one eye (1.84-1.86 mg). Data are from Alcon Laboratories.

It should be mentioned that EDTA decomposes at elevated temperatures and it was of interest to establish, if this process affects the interactions with and dissolution of iron oxides. The resulting fi-agments of EDTA at moderately elevated temperatures were identified as iminodiacetic (IDA) and N-hydroxyethyl-iminodiacetic (HIDA) acids. (At higher temperatures the decomposition proceeds to still smaller subunits). By using IDA and HIDA it was found that analogous dissolution results with hematite were obtained as with EDTA, although considerably less efficiently [37,38]. 

LCo(H20)6] ion, and bidentate /V-donor ligands such as cn, bipy and phen form octahedral cationic complexes [Co(L-L)3] , which are much more stable to oxidation than is the hexaammine [Co(NH3)6l . Acac yields the orange [Co(acac)2(H20)2] which has the tram octahedral structure and can be dehydrated to [Co(acac)2l which attains octahedral coordination by forming the tetrameric species shown in Fig. 26.3. This is comparable with the trimeric [Ni(acac>2]3 (p. 1157), like which it shows evidence of weak ferromagnetic interactions at very low temperatures. fCo(edta)(H20)] is ostensibly analogous to the 7-coordinate Mn and complexes with the same stoichiometry, but in fact the cobalt is only 6-coordinate, 1 of the oxygen atoms of the cdta being too far away from the cobalt (272 compared to 223 pm for the other edta donor atoms) to be considered as coordinated. 

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