Cobalt complexes 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. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

The sequence of Reactions 19, 20, and 21 indicates that substitution on the metal ion by at least one of the substrates is faster than the over-all reaction rate. This can be understood easily for manganese (III) complexes (substitution labile). It is, however, more difficult to rationalize for cobalt (III) unless the rate of cobalt (II)-cobalt (III) exchange is very fast in this system. 

More recently, the parent mono- and di-methylenecyclopropane cyclopentadienyl-cobalt -complexes were prepared by ligand exchange reactions583. Reaction of methyl-enecyclopropane (and its tetramethyl derivative) with CpCo(ethylene)2 in pentane affords... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

Under the influence of UV radiation the cobalt complex CofPhNNNNPhXCjHj) undergoes a remarkable transformation to the diimine derivative Co(HNC6H4NPh)(C5H5) (93, 96). More details of this and some related reactions are given in Section III,E,3. Exchange reactions between tetrazadiene complexes and free aryl azides lead to partial or complete substitution (169). 

Figure 3 Comparison of the spectra for the cobalt complexes in the solution and the anion-exchanger phase in equilibrium A 4 M HCl solution of Co(II) B AG 1-X8 equilibrated with solution A. (From Ref. 13.)...

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. 

Rates of Electron Exchange for Some Cobalt Complexes... 

We discuss a few ligand substitution reactions to give you a feel for the properties and reactions of coordination complexes. These simple reactions are aptly named one or more ligands are simply substituted for one another. We have already discussed one example of a series of ligand substitution reactions the exchange of NH3 and CP in Werner s cobalt complexes. 

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