Carbonato complexes chelated

The monodentate carbonato complex may undergo ring closure to the chelated carbonato complex [see Eq. (18)]. 

The acid catalyzed aquation of the chelated carbonato complexes involves three steps (i) the initial rapid protonation pre-equilibrium of the exocyclic oxygen, (ii) carbonate ring opening, and (iii) decarboxylation of the resulting bicarbonate complex [see Eq. (30)]. 

Few well-defined Con-carbonate complexes are known. The two structures given in Table 68 are probably representative of chelated and monodentate carbonate. K2[Co(C03)2(H20)4] crystallizes as rose-coloured plates from warm solutions of Co 1 salts in the presence of K2C03s7° and contains trans monodentate carbonate. Bidentate carbonate in [Co(C03)(H20)2(imH)2] appears to exert a measurable structural tram influence on the trans water molecules (CoO, 216.6 pm vs. 209.4 in Co(H20)5+)330 and a similar effect is found in Coin-carbonato complexes. Violet crystals of [Co(C03)(H20)2(NHpy2)] have been isolated on reacting Na3[Co(C03)3] with 2,2 -bipyridylamine and presumably contain chelated carbonate (IR n = 5.23 BM).871... 

The hydrolysis of chelated carbonato complexes of cobalt(III) is much faster in acid than in neutral solution. Explain. 

The unidentate and bidentate (chelating) coordinations shown below are found in the majority of carbonato complexes. 

Individual steps in chelate ring opening can be followed kinetically during base hydrolysis of [Co en2C03]+, in contrast to acid aquation where only one stage can be monitored kinetically. Thus, while the intermediate formation of a unidentate carbonate complex in acid aquation is a postulate, its formation in alkaline solution is kinetically proven. Further discussion on mechanisms of reactions of carbonato complexes will be found in the kinetic section of a general review on the chemistry of these complexes. ... 

There are some cases of carbonato complexes, as pointed out by Jolivet et al. (1982), in which water molecules can even modify the coordination state of the carbonate ion. This is also the case of the Na2Cu(C03)2-3H20 complex, which contains both bidentate chelate and bridged carbonate ions, whereas its dehydrated form is exclusively a bridged structured (see also Fig. 8). Concerning our carbonato complex, it would be interesting to get more information about the actual influence of H2O on the coordination. Since apparently the H2O has... 

Fujita, J. Martell, A. E. Nakamoto, K. (1962). Infrared Spectra of Metal Chelate Compounds. VIII. Infrared Spectra of Co(III) Carbonato Complexes. The Journal of Chemical Physics, Vol.36, No. 2, pp. 339-345. 

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