Cobalt inert complexes, 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. 

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... 

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. 

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. 

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)... 

Two distinct tendencies can be recognized in the literature dealing with diastereo-selective reactions on metal complexes. On one side, stability differences are determined on the base of equilibrium data or product ratios. Inert cobalt(III)-complexes play a large part in this investigation. The procedures and the methods used in these investigations correspond to those already known in coordination chemistry. The aim of these studies is a better understanding of stereochemical interactions in the coordination sphere of metal complexes. The nature of ligand molecules is not modified in the systems studied. These reactions correspond to the first step of the scheme shown in Fig. 1. 

The stereoselective reduction of spinach plastocyanin with several cobalt cage complexes (Scheme 26) has been reported, too [60]. These cage complexes are very useful for investigation of outer-sphere electron transfer reactions because of their inertness to hydrolysis and to loss of ligands in the redox reaction. 

Chromium(III) and cobalt(III) complexes are substitu-tionally inert (no exchange of ligands) under conditions of the experiment. However, chromium(II) and cobalt(II) complexes can exchange ligands very rapidly. One of the products of the reaction is Cr(H20)5CI2+. Is this consistent with the reaction proceeding through formation of... 

Kinetically inert low-spin cobalt (III) clathrochelates are reversibly reduced by accepting one electron to yield kinetically labile cobalt(II) complexes. In the case of the usual amines (for instance, ammonia), the reduction is, as a rule, accompanied by irreversible decay of the amine cobalt complex. This reaction is slower for chelating amines macrocyclic and especially macrobicyclic amines produce complexes with cobalt(II) ion that are stable over a long time. This fact facilitates the study of the reduction of cobalt(III) complexes to cobalt(II) ones. In most cases, the reactions of macrobicyclic ligands do not interfere with this process. 

So far, we have generally assumed that the complexation reaction is fast and that equilibrium is attained. This is often, but not always, the case. The differentiation of the thermodynamic terms, stable and unstable, from the kinetic terms, labile and inert (or robust), should be made. The classical example of a kinetically inert complex is the hexamine cobalt(III) cation in acid solution ... 

Valence d electrons of transition metals impart special properties (e.g., color and substitution reactivity) to coordination complexes. These valence electrons can also be removed completely from (oxidation) or added to (reduction) metal d orbitals with relative facility. Such oxidation-reduction (redox) reactions, like substitution reactions, are integral to metal complex reactivity. Consider the role of redox chemistry in the synthesis of [Co(NH3)5C1]+, equation (1.8). In general, the preparation of cobalt(III) complexes (Chapters 2 and 5) starts with substitutionally labile cobalt(II) salts that are combined with appropriate ligands with subsequent oxidation of the metal by H202 or 02 to the substitutionally inert (robust) +3 state. 

The bridging group, X, does not necessarily transfer from A to B however, if this happens, it is strong evidence that an inner-sphere reaction has taken place. Indeed, this sort of evidence was how Taube first solved the puzzle. In the reduction of [Co(NH3)5C1]2+ by [Cr(H20)6]2+, the Cl- on the inert cobalt(III) complex readily displaces a H20 molecule at the labile chromium(II) center to form the bridged species ... 

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