Cobalt self exchange

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. 

Figure 9-6. The consequences of a self-exchange electron transfer between a ground state cobalt(ii) and a ground state cobalt(iii) complex.

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

The difference in the self-exchange rates of the two cobalt couples favors the oxidative pathway by a factor of 300. (For a further discussion of the above and other self-exchange rates, see B. S. Brunschwig, C. Creutz, D. H. Macartney, T.-K. Sham, and N. Sutin, Faraday Discuss. Chem. Soc., No 74, in press). Evidently the difference in the intrinsic barriers is large enough to compensate for the less favorable driving force for the oxidative pathway. As a result the latter pathway can compete favorably with the reductive pathway. 

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. 

Table 10.2. Observed and calculated electron self-exchange rates of hexaamine cobalt(III/II) complexes11321.

Macrocyclic N-Donors. Glick et al." have proposed that the greater difference in Co—X axial bond length between the cobalt(n) and cobalt(m) complexes of (16) compared with the corresponding complexes of (17) accounts for the unusually slow self-exchange rate of the former. The electronic spectra of the five-co-ordinate cobalt(n) complexes of the macrocycles (18) and (19) have been reported.100... 

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). ... 

For homoleptic cobalt(III/II) amines, a relatively large number of self-exchange rates have been directly determined the rate constants... 

Fig. 12. Plot of number of amine protons versus log of the self-exchange rate constant (M s ) for cobalt hexaamine complexes at 25°C. No correction has been made for ionic strength differences. The data include some nonhomoleptic complexes. (1) [CoCNHslg], (2) [Co(en)3]3+ 2+, (3) [Co(chxn) ] + 2+ (4) [Co(tmen) ]3+ 2+, (5) [Co(dien)"]"+ 2+, (6) [Co(pet) P+ 2+, (7) [Co(linpen)P" 2 + (g) lCo(medien)(9) [Co(tacn)(dien)]3+ 2+, (10) [Co(tacn) (pet) (11) [Co(tacn) (etdien) (12) [Co(tacn) (budien) p+ 2+ 3 [Co(tacn)(medien)P, (14) [Co(diAmsar)] , (15) [Co(taptacn)P, (16) [Co(metacn) ] 2+, (17) [Co(diAmsar)P 2+, (18) [Co(sar)(19) [Co(sep)P 2, (20) [Co(dtne)] , (21) [Co(Amsartacn)], (22) [Co(Amsartacn)] , (23) [Co-(diAmchxnsar)] , (24) [Co(diAmchxnsar)] . The data for homoleptic complexes are taken from Table IV the other data are from reference U02). The line was calculated without the data for the sep and sar derivative cages and the [Coftmenls] couple.

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

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. 

Electron self- exchange rate constants for macrobicychc 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... 

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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