Cobalt complexes outer-sphere reactions, 994

Reactions of cobalt complexes of macrocyclic tetramine ligands with various couples (including the highly oxidizing MnCl /, hitherto little studied) have been reported. Inner-sphere and outer-sphere reactions involving, for example, [Ru(NH3)6py] +/ + have been characterized. A free-energy correlation for reactions between [Co(N4)(OH2)2] with inner-sphere oxidants shows the expected slope 9AG /3AG 0.5 down to a limit of AG a 7 kcal mol, which is believed to represent diffusion control. Application of the Marcus equations (1) and (3) to reactions of both types leads to an assessment of the factors con-... 

Reactions of a number of substrates with 12-tungstocobaltate(iii), [Co04Wi2036], have been examined. Both oxidized and reduced forms of the complex ion have cobalt in a tetrahedral environment with four oxygen donors. The complex is inert to substitution and is therefore suitable for the study of outer-sphere reactions. 

The rates of reduction of a number of pyridine derivatives by and Eu + have been determined from a study of catalysis in the reduction of cobalt(iii) complexes by metal-ion reductants. - If an outer-sphere mechanism were operating in both cases, the ratio of rate constants for and reductions would be around 0.26 whereas the observed ratios are 10 —10 . An inner-sphere mechanism is proposed for the europium(ii) reactions to give Eu + and a pyridine-related radical. Values for the rate constants (Table 5) show considerable variation with substrate structure, but for all are less than 401 mol s and consequently no distinction between an inner-sphere and an outer-sphere process may be made in this case. The relative rates for reduction of bipyridyl derivatives do, however, lie within the range for outer-sphere reactions. 

The very slow outer-sphere reactions of Eu and Eu " have long been considered good candidates for nonadiabaticity, in view of the poor overlap of / orbitals. New data from europium cryptate complexes do not support this view. From cross reactions, self-exchange rates of two cryptate couples [Eu(2.2.1)] and [Eu(2.2.2)] t are calculated to be much higher than for EUaq", whereas for nonadiabatic reactions the rate is expected to fall with increasing metal-metal distance. Studies of cobalt ammines, including the encapsulated [Co(sep)], [Co(sar)], and [Co(azacapten)] , have led to a similar conclusion. " ... 

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... 

One example of outer-sphere electron transfer is the reaction between the dipotassium cycloocta-tetraene (KjCgHg) and the cobalt complex of bis(salicylidenediamine) (Co Salen) (Levitin et al. 1971). 

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... 

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. 

Several studies of bimetallic complexes in which the donor and acceptor are linked across aliphatic chains have demonstrated that these are generally weakly coupled systems. " Studies of complexes linked by l,2-bis(2,2 bipyridyl-4-yl)ethane (bb see Figure 5), indicate that these are good models of the precursor complexes for outer-sphere electron-transfer reactions of tris-bipyridyl complexes. A careful comparison of kinetic and spectroscopic data with computational studies has led to an estimate of //rp = 20cm for the [Fe(bb)3pe] + self-exchange electron transfer. In a related cross-reaction, the Ru/bpy MLCT excited state of [(bpy)2Ru(bb)Co(bpy)2] + is efficiently quenched by electron transfer to the cobalt center in several resolved steps, equations (57) and (58). ... 

The reaction of /r-superoxo complexes with reducing metal ions generally follows an outer sphere mechanism, and kinetic data have been reported for reduction by Fe and cobalt(II) chelates =>, Mo(V) ), [Ru(NH3)6] and... 

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. 

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]. ... 

The reactions of cobalt(III) in mixed-metal outer sphere electron transfer reactions are usually slow also, as a result of the influence of effects discussed above. The reaction (5.56) of the hexaamminecobalt(III) ion with the hexaaquachromium(II) complex... 

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