Coordination complex reactivity substitution reactions

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

Our alternative approach has been to synthesize the solvento intermediate and then study its reactions in isolation. We thereby hope to show that its reactivity and steric course is inconsistent with the postulate stating that it is an intermediate in the substitution reactions. Complexes of the type cis- and trans-[Co en2 CH3OH q]+2 (7)f and cis-[Co en2 (CH3)2SO Cl]+2 32) have been prepared. We have shown in the first case that the lability of the coordinated methanol does not sufficiently explain the nonappearance of the solvento complex in the reactions of cis- and trans-[Co en2 Cb]"1" in methanol unless it is not an intermediate in the reaction. In dimethyl sulfoxide solution, cis- and trans-[Co en2 Cb] have been shown to isom-erize to an equilibrium mixture that also contains the solvento intermediate 32). 

Reagent Reactivity. One of the most interesting aspects of substitution reactions of square planar complexes is that the reaction rates depend on the nature of the reagent. This permits a thorough investigation of the factors responsible for reagent reactivity towards these substrates. Note that this has not been possible for the reactions of most six-coordinated metal complexes, since their rates do not depend on the reagent. 

In general terms, both steric effects and electronic factors are expected to play a role in determinating the reactivity of square-planar platinum complexes. The presence of planar amine ligands in cis- or /ran.y-Pt(anion )2 complexes and their orientation with respect to the coordination plane, as well as their substituents, can reduce the rates of DNA binding or thio binding compared to aliphatic ammine and amine complexes. Especially, substituents close to the coordination site should be expected to slow down axial substitution reactions at Pt. As there is now little doubt that DNA platina-tion is a key event (or THE key event) in the mechanism of action of platinum anticancer drugs, attention to the process of formation of the major adduct (GG) as an intrastrand cross-link between N(7) atoms of two adjacent guanine (G) residues, will remain important. 

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