Chromium kinetic inertness

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. 

This is the most common and stable state of chromium in aqueous solution. The Cr3+ ion, with 3d3 electrons, forms mainly octahedral complexes [CrX6], which are usually coloured, and are kinetically inert, i.e. the rate of substitution of X by another ligand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

Unlike desferrioxamine analogs designed for specific therapeutic purposes described above, chiral DFO analogs that form conformationally unique complexes with iron(lll) were designed to serve as chemical probes of microbial iron(lll) uptake processes. As mentioned above, ferrioxamine B can form a total of five isomers when binding trivalent metal ions, each as a racemic mixture. Muller and Raymond studied three separate, kinetically inert chromium complexes of desferrioxamine B (N-cis,cis, C-cis,cis and trans isomers), which showed the same inhibition of Fe-ferrioxamine B uptake by Streptomyces pilosus. This result may indicate either that (i) ferrioxamine B receptor in this microorganism does not discriminate between geometrical isomers, or that (ii) ferrioxamine B complexes are conformationally poorly defined and are not optimal to serve as probes. 

The electronic301 and magnetic properties of mononuclear chromium(III) complexes are quite well understood however there is a distinct tendency for octahedral symmetry to be invoked in cases where the true symmetry is much lower. Chromium(III) is a hard Lewis acid and many stable complexes are formed with oxygen donors. In particular hydroxide complexes are readily formed in aqueous solution, and this may be a problem in synthesis. Substitution at chromium(III) centres is slow302,303 and may well have some associative character in many cases. The kinetic inertness of chromium(III) has led to the resolution of many optically active complexes this work has been extensively reviewed.304... 

Chromium(III) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been reported.797,798 The kinetic inertness of the chromium(III) complexes allows the facile separation of isomers for the model complex tris(iV-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D-cis, L-cis and the l/d-trans isomers have been separated.798 The chromium complexes of desferrioxamine B (191) have been investigated the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate800 and dihydroxamate (rhodotorulic acid) complexes have also been studied.801... 

Chromium(III) forms stable complexes with adenosine-S -triphosphate.840,841,842 These are kinetically inert analogues of magnesium ATP complexes and may be used to study enzyme systems. The complexes prepared are chiral and may be distinguished in terms of chirality at the metal centre (198,199).843 The related complex of chromium(lll) with adenosine-5 -(l-thiodiphosphate) has been prepared the diastereoisomers were separated.844 The stereospecific synthesis of chromium(III) complexes of thiophosphates has been reported845 by the method outlined in equation (47), enabling the configuration of the thiophosphoryl centre to be determined. The availability of optically pure substrates will enable the stereospecificity of various enzyme systems to be investigated.845... 

Chromium and cobalt are the metals most commonly used in dyestuffs for polyamide fibres and leather because of their kinetic inertness and the stability of their complexes towards acid. Since the advent of fibre-reactive dyestuffs, chromium and cobalt complexes have also found application as dyestuffs for cellulosic fibres, particularly as black shades of high light-fastness. Copper complexes are of more importance as dyes for cellulosic fibres and are unsuitable for polyamide fibres because of their rather low stability towards acid treatments. 

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

Kochi and Buchanan53 state that reaction (41) is rate-determining, and hence equilibrium (40) must be set up rapidly. Although this might be feasible for an octahedral complex of chro-mium(II), it does not seem feasible if (XIV) is a derivative of chromium(lll) since octahedral complexes of chromium(III) are invariably kinetically inert towards nucleophilic substitution (see ref. 54). 

E.s.r. data have been reported for the complexes [CrOX2 S2P(OEt)2 ] (X = Cl or NCS) and discussed in terms of the electronic structure of these complexes.233 Two new and kinetically inert complexes of chromium(v) have been reported. [CrO(iVN)-Cl3] (NN = bipy or phen) have been obtained by the dehydrochlorination of the corresponding (H2iViV)[CrOCl5] salt in a dry C02 atmosphere at 80 °C.234 Cr02Cl2 reacts with hexamethylmelamine (L) in anhydrous EtOAc to produce [Cr02ClL].235... 

There are literally thousands of chromium(III) complexes that, with a few exceptions, are hexacoordinate and octahedral. An important characteristic of these complexes in aqueous solutions is their relative kinetic inertness. Ligand displacement reactions of Cr111 complexes are only 10 times faster than those of Co111, with half-times in the range of several hours. It is largely because of this kinetic inertness that so many complex species can be isolated as solids and that they persist for relatively long periods of time in solution, even under conditions of marked thermodynamic instability. 

The kinetic inertness of chromium (III) has allowed establishment of the primary hydration of this ion by six water molecules (3), as represented in these equations. The exchange of oxygen-18 between hexaaquachro-mium(III) ion and water in aqueous solution (3,4) and the substitution of coordinated water in hexaaquachromium(III) ion by another ligand (e.g., methyl alcohol (5)) are very slow ... 

The isomers of kinetically inert chromium sid- rophore complexes have been used as biological probes to elucidate microbial siderophore uptake systems, answering the following questions ... 

Coordinatively saturated metal complexes that are kinetically inert with respect to ligand substitution may undergo outer-sphere electron transfer reactions with dioxygen. Typical examples include oxidations of six-coordinate chromium(II) complexes29 (Equation 4.6) and oxidations of polyoxometallate anions.30... 

Chromium(iH) Complexes.15 There are literally thousands of chro-mium(m) complexes which, with a few exceptions, are all hexacoordinate. The principal characteristic of these complexes in aqueous solutions is their relative kinetic inertness. 

The chromium(IIl) compound contains a central tridentate oxide ligand (Fig. 2), an arrangement which is typical of other oxo-centred derivatives of chromium(III), such as the carboxylato and sulfato complexes. In the diethylcarbamato derivative, the central oxide ligand deviates by only 0.07 A from the plane of the three chromium atoms. It should be noted that the chromium derivative still retains a chloride ligand in its coordination as the starting material is anhydrous CrCl3, this corresponds to incomplete removal of chloride from the kinetically inert 3d chromium(III) cation. 

Cobalt complexes have been the most generally studied, although other metal ions have been incorporated, such as platinum(IV), rhodium(III) and chromium(III). The cobalt(II) complex of (19) is optically stable and kinetically inert (no exchange observed using labelled Co + over a period of 24 hours). Electron transfer studies show that the [Co(sep)f couple undergoes electron exchange at a rate 10 -fold faster than [Co(en)3) . [Co(sep)] also quantitatively reduces O2 to HjOj via a superoxide radical ion O2 intermediate. ... 

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