Cobalt complexes ligand substitution reactions

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

The rates of complex formation and ligand substitution reactions of the polymer-bound Co(III) complexes depend on the dynamic property of the polymer domains. Reports on the kinetics of complex formation and ligand substitution of macromolecule-metal complexes are, however, relatively scarce. They include investigations on the complexation of poly-4-vinylpyridine with Ni2+ by the stopped conductance technique 30) and on a ligand substitution reaction of the polymer-bound cobalt(III) complexes 31>. 

The most obvious examples are in the preparation of octahedral Co(III) complexes. The most readily-available cobalt compounds are Co(II) salts in the presence of suitable ligands - usually N-donors - these are oxidised to give Co(III) complexes by air or hydrogen peroxide. A few such easily-prepared complexes open up pathways to the vast number of known octahedral Co(III) complexes via substitution reactions. For example, [Co(NH3)5(H20)]CI3 is readily converted into [Co(NH3)5X]Cl2 via anation reactions of the type discussed in Section 9.5, and salts containing the [Co(NH3)4(C03)]+ ion (where the carbonate is bidentate, taking up two cis positions) are useful for the formation of cw-[Co(NH3)4X2]+. 

Combined Kinetic and Thermodynamic Data fob Ligand Substitution Reactions in Some Cobalt(III) Complexes... 

Finally, one can include qualitative observations. Ligand substitution reactions of cobalt(III) porphyrin complexes, for example, appear to be fast or instantaneous, although many have been studied in nonaqueous solvents and are therefore not really comparable. Cobalt(III) hematoporphyrin is, however, a clear-cut case the substitution of H2O by CN or py in aqueous solution is instantaneous (123). 

We discuss a few ligand substitution reactions to give you a feel for the properties and reactions of coordination complexes. These simple reactions are aptly named one or more ligands are simply substituted for one another. We have already discussed one example of a series of ligand substitution reactions the exchange of NH3 and CP in Werner s cobalt complexes. 

The preparation of many metal complexes often involves an accompanying oxidation-reduction reaction. For the thousands of cobalt(III) complexes that have been prepared, the starting material was almost always some cobalt(II) salt. This is because the usual oxidation state of cobalt in its simple salts is 2. The oxidation state of 3 becomes the stable form only when cobalt is coordinated to certain types of ligands (Section 5.2). Furthermore, it is convenient to start with salts of cobalt(n) because cobalt(ii) complexes undergo substitution reactions very rapidly, whereas reactions of cobalt(m) complexes are very slow (Section 6.4). The preparation of cobalt(ni) complexes, therefore, proceeds by a fast reaction between cobalt(ll) and the ligand to form a cobalt(ll) complex which is then oxidized to the corresponding cobalt(ni) complex. For example, reaction (19) is presumed to involve first the formation of [Co(NH3)6] ... 

The rapidity of substitution reactions at a metal surrounded by a porphyrin or corrin group seems to be connected with r-delocalization and a strong in-plane ligand field which is not present in normal complexes. The substitution reactions (4) of five-co-ordinate neutral dithiolen complexes (where M is iron or cobalt and L and [M(S2CaPh2)2X] + L—> [M(S2C2Ph2)2L] + X (4)... 

One of the commonest reactions in the chemistry of transition-metal complexes is the replacement of one ligand by another ligand (Fig. 9-3) - a so-called substitution reaction. These reactions proceed at a variety of rates, the half-lives of which may vary from several days for complexes of rhodium(iii) or cobalt(m) to about a microsecond with complexes of titanium(iii). 

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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