Cobalt inert complexes

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. 

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. 

If this complex now collapses, it will be the labile Co-Cl bond which is broken, as opposed to the inert Cr-Cl bond. The labile cobalt(ii) complex reacts further with bulk water to generate [Co(H20)6] (Eq. 9.37). The key feature is that a necessary consequence of this inner-sphere reaction is the transfer of the bridging ligand from one center to the other. This is not a necessary consequence of all such reactions, but is a result of our choosing a pair of reactants which each change between inert and labile configurations. In the reaction described above, the chloride... 

Inert centers facilitate the detection and characterization of intermediates. Recently investigated systems involving cobalt(III) complexes include [Co(L)(OH)(H20)]2+-promoted hydrolysis of 2,4-dinitrophenyl... 

Although Co(III) is often considered the classical representative of inert behavior, there are a number of cobalt(III) complexes that react rapidly enough to require that the rates be determined by flow methods. Table 8.11 shows a representative selection of such labile complexes. 

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

Consequently, reduction of cobalt(III) ammines in basic solution is not favorable. A variety of reducing agents has been used to effect reaction (11). The fortunate coincidences that cobalt(III) complexes are substitution inert while cobalt(II) systems are labile and that cobalt(II) is resistant to oxidation or further reduction in acid solution offer many advantages in the study of redox processes. Not surprisingly, work with cobalt(III) complexes forms the basis for much of the present understanding of oxidation-reduction reactions. 

A mechanism represented by Equations 5, 6, 7, and 8 could be applied to cobalt (III), but the rate-limiting step would have to be the first substitution reaction to account for the experimental rate equation (Equation 2). It is known that cobalt (III) complexes are substitution inert (6, 23) unless significant amounts of cobalt(II) are present (I, 8, 23), and hence one could visualize the first and slow step as follows ... 

The Ojima group has extended their studies of silylformylation to include more complex substrates, such as alkenyne, dialkyne, alkynyl nitrile, and ethynyl pyrrolidinone. Use of rhodium or rhodium-cobalt metal complexes catalyzes the silylformylation of these substrates with high chemoselectivity, as the other functionalities present are inert to the reaction.122b,c d... 

It was not until 1965 that 1 1 cobalt(III) complexes of tridentate azo compounds were prepared49 by the interaction of the azo compound and a cobalt(II) salt in aqueous medium in the presence of excess ammonia under an inert atmosphere. In every case, e.g. (41), the coordination sphere of the cobalt ion was completed by three molecules of coordinated ammonia and oxidation to the cobalt(III) state occurred at the expense of the azo compound, some of which was reduced. The scope of the reaction is wide and 1 1 cobalt(III) complexes of this type have been prepared from a wide range of tridentate metallizable azo compounds. 

Its oxidation potential is such that it is oxidized to the kinetically inert 2 1 cobalt(III) complex (equation 6). The relevant redox potentials have not been measured, but support is provided by the isolation by Wittwer46 of 2 1 cobalt(II) complexes of tridentate azo compounds. These are stable under only a very limited range of conditions and readily undergo oxidation to the corresponding 2 1 cobalt(III) complexes. 

In contrast, the oxidation potential of the 1 1 cobalt(II) complex dyestuff in which the coordination sphere of the metal is completed by three molecules of ammonia is such that oxidation to the kinetically inert 1 1 cobalt(III) complex occurs rapidly (equation 6a). 

The intramolecular hydrolysis involving coordinated hydroxide (equation 10) was first detected in kinetically inert cobalt(III) complexes and these reactions are considered in detail in Section 61.4.2.2.3. 

The above studies indicate that metal ions catalyze the hydrolysis of amides and peptides at pH values where the carbonyl-bonded species (25) is present. At higher pH values where deprotonated complexes (26) can be formed the hydrolysis is inhibited. These conclusions have been amply confirmed in subsequent studies involving inert cobalt(III) complexes (Section 61.4.2.2.2). Zinc(II)-promoted amide ionization is uncommon, and the first example of such a reaction was only reported in 1981.103 Zinc(II) does not inhibit the hydrolysis of glycylglycine at high pH, and amide deprotonation does not appear to occur at quite high pH values. Presumably this is one important reason for the widespread occurrence of zinc(Il) in metallopeptidases. Other metal ions such as copper(II) would induce amide deprotonation at relatively low pH values leading to catalytically inactive complexes. 

The use of kinetically inert cobalt(III) complexes has led to important developments in our understanding of the metal ion-promoted hydrolysis of esters, amides and peptides. These complexes have been particularly useful in helping to define the mechanistic pathways available in reactions of this type. Work in this area has been the subject of a number of reviews.21-24 Although most of the initial work was connected with cobalt(III), investigations are now being extended to other kinetically inert metal centres such as Rhin, lrni and Ru111. 

Most of the work on the kinetics and mechanism of aquation - the first step in octahedral substitution - has been done on cobalt(III) complexes, which are neither too inert nor too labile for exhaustive investigations. The aquation of Co(NH3)5X2+/3+ (the charge depends on whether X is neutral or anionic) has been studied in great depth. The rate law for such a process is found to take the form ... 

Figure 2-17. The first step in the SN1 ch mechanism for a kinetically inert cobalt(m) complex containing ammonia ligands involves a deprotonation of the co-ordinated NH3 to generate an amido complex.

Although very dramatic rate enhancements have been observed with labile metal ions such as copper(n) and nickel(n), most studies have involved kinetically inert d6 cobalt(m) complexes. In general, copper(n) complexes have been found to be the most effective catalysts for these reactions. 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

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