Cobalt complexes inner-sphere reactions

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... 

Pyridine derivatives of the type known to catalyze the outer-sphere reduction of Co(III) catalyze the reduction of [Coensby The catalysis is inhibited by U +, and the catalyst is slowly consumed (102). The catalyst, for example, isonicotinamide, is reversibly reduced by an inner-sphere reaction with the In another group of papers, it is shown that can bring about reduction by an inner-sphere mechanism involving attachment that is remote from the cobalt atom. The oxidants were dinuclear complexes of the type of 6. 

On the other hand, when one thinks in terms of electrochemical reductions or oxidations, special attention is devoted to the coreactant, that is, to the electrode that provides or accepts electrons. Thus, in order to discuss or compare electrochemical reactions with their organic analogs, it is of the utmost importance to use more precise terms than the so inaccurate reduction of oxidation notions. A similar problem has been addressed in the inorganic and organometallic fields. Indeed, it was early recognized that oxidation-reduction reactions at metal centers must be classified according to two types outer sphere or inner sphere reactions. A typical example of this dichotomy is given in Eqs. (14) and (15), which relate to chromium (II) oxidations by cobalt (III) complexes. 

The five-coordinate cobalt(II) species presumably immediately picks up a water molecule to fill its sixth coordination position and then hydrolyzes rapidly to [Co(H20)J . Formally, such an inner sphere reaction consists of the transfer of a chlorine atom from cobalt to chromium, decreasing the oxidation stale of the former but increasing that of the latter. In addition to the self-consistency of the above model (inert and labile species) and the observed formation of a chlorochromium complex, further evidence for this mechanism has been obtained by running the reaction in the presence of free radioisotopes of chloride ion in the solution. Very little of this labeled chloride is ever found in the product, indicating that the chloride transfer has indeed been through the bridge rather than indirectly through free chloride. 

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 ... 

Syntheses and characterization of a number of cobalt(III) and cobalt(II) cage complexes have been reported. These species are unusual in that both oxidation states are inert to substitution and the closed structural framework precludes inner-sphere reactions. Facile resolution of the complexes into optical isomers provides a convenient method for investigating self-exchange rates which are much faster (see Table 2.3) than for other simple cobalt(III)/(II)-amine complexes. 

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... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Kinetics of oxidation of iron(II) by the surfactant complex ions d.v-chloro/bromo (dodecylamine)bis(ethylenediamine)cobalt(III) have been reported. The second-order rate constant remains constant below the critical micelle concentration (cmc), but increases with cobalt(III) concentration above the cmc. The rate of reaction was not affected by the added hydrogen ions. It is suggested that the reaction proceeds by an inner-sphere mechanism.74... 

A number of other metal complexes can decompose hydrogen peroxide via reactions analogous to Eqs. (45) and/or (46), including those of cerium813 b copper,823 b cobalt,833 b manganese,84 and silver.85 Many of these electron transfer reactions are thought to proceed via inner-sphere complexes of metal-hydrogen peroxides (M—OOH).84 86... 

The inner sphere reduction of 7 cobalt dioxygen complexes by a second cobalt(II) ion is extremely well established in the oxygenation reactions of cobalt(II) solutions and is discussed in many reviews . Endicott has recently investigated the kinetics of the two step reaction... 

Cobalt(III) complexes are reduced readily to Co(II) by radiation-generated reducing radicals" including aliphatic radicals derived from alcohols, acids, amines, and aldehydes". However, the rate constant for the reaction of a-hydroxyalkyl radicals with Co(NH3)jP is pH-dependent (high in neutral solution, low in acidic solution) indicating an inner-sphere mechanism" the yield of Co follows the same pH dependence. ... 

In both cases, the cobalt containing product is the aqua complex because H2O is present in abundance, and high-spin d complexes of Co(II) are substitution labile. However, something that distinguishes the two pathways is the composition of the vanadium-containing product. If [V(N3)(OH2)s] is the product, then the reaction has proceeded via an inner-sphere pathway. If [V(OH2)6] " is the product, then the electron-transfer reaction is outer-sphere. The complex [V(N3)(OH2)5] is inert enough to be experimentally observed before the water molecule displaces the azide anion to give [V(OH2)6]. ... 

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