Self exchange reactions cobalt

We conclude with a consideration of a few other cobalt self-exchange reactions. The reaction in Eq. (9.33) is faster than that involving the ammine complexes (Eq. 9.30) because the water is a weaker-field ligand than ammonia. Thus, the activation energy for the formation of the electronically excited states is lower, as is the change in Co-ligand distances in the two oxidation states. 

Electron self-exchange reactions in macrobicyclic cobalt complexes have intensively been investigated. The rate constant of such reactions obtained for a variety of complexes, listed in Table 52, differ by several orders of magnitude (from 0.011 and 0.02 for the [CoCdiMesAMHsar)] and [Co(diAMHsar)]° cations to 2.8x10 for the hexathioether macrobicyclic [Co(diMEsar-S6)] + cation). The available data allow one to determine certain rules for the variation in the rate of electron self-exchange in macrobicyclic cobalt complexes. 

It should be noted that not all self-exchange reactions between Co(III) and Co(ll) are slow. The nature of the bound ligand has a significant influence on the reaction rate. In particular, ligands with it systems provide easy passage of electrons. For [Co(phen)J]5+/[Co(phen)J]2, exchange, for example, k is 40 M s-1, many orders of magnitude faster than for the cobalt ammine system.42... 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

The [Fe(CN)6] Self-exchange Reaction as a Case Study General Considerations The Significance of Kei in Outer-sphere Electron Transfer Reactions of Transition Metal Complexes Observations on Cobalt(III)-(II) Systems FranckA7ondon Factors... 

Observations for the ion-pair spectra in Table 2 are consistent with the assumption that 1 for these anion/cation cross-reactions. As noted in the discussion of the [Fe(CN)e]", [Fe(CN)6] ion pair, there are indications that Kgi may be less than 1 for some reactions. This is especially a concern for the Co Co self-exchange reactions since many of the cobalt(II) complexes listed in Table 2 are high spin (h e (i-orbital configuration) while the cobalt(III) complexes are all low spin The self-exchange reaction can be represented in terms of octahedral (7-orbital configurations as,... 

An electron transfer (ET) reaction is defined here as an oxidation-reduction reaction that occurs between two coordination compounds. The compounds may be the same species, but where the metals have different oxidation states (a selfexchan reaction), or are completely different species (a cross-reaction). An example of a self-exchange reaction is given in Equation (17.41), where Co is an isotopically labeled cobalt a cross-reaction is given by Equation (17.42). 

Figure 9-7. The self-exchange electron transfer reaction between vibrationally excited cobalt(ii) and cobalt(iii) complexes.

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

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). ... 

Reactions of electron self-exchange in macrobicyclic cobalt complexes... 

For Co(NH3)b / self-exchange (equation 46), Kg is small and the reaction non-adiabatic. The major contributor to the decreased magnitude for Kg is the fact that changes in oxidation state at cobalt not only cause significant changes in molecular structure but also a change in spin state from high-spin Co (t2g e/) to low-spin Co Uig )-... 

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