Substituted pentacyanoferrate complexes

Reviews " of pentacyanoferrate substitution kinetics have included a detailed consideration of high-pressure studies of thermal and photochemical substitution and electron transfer reactions of pentacyanoferrates-(II) and -(III). Photochemical activation can result in the loss of L or of CN . The best way to study the latter is through photochemical chelate ring closure in a pentacyanoferrate complex of a potentially bidentate ligand LL [Fe(CN)5(TL)]" rFe(CI 4(LL)] " +... 

The preparation of substituted pentacyanoferrate(II) ion complexes involves a series of ligand exchange reactions at the iron(II) metal center. Equations (4.1)-(4.3) outline the synthesis of amino acid (AA) metal complexes in aqueous solution. Starting from sodium nitroprusside ion, [Fe(CN)5(NO)]2, equation (4.1), the nitrosyl ligand, NO+, is replaced by an ammine moiety, NH3. The aquapentacyanoferrate(II) ion, [Fe(CN)5(H20)]3, is then generated in situ, equation (4.2), followed by reaction with an AA to yield the desired [Fe(CN)5(AA)](3+n) complex, equation (4.3). 

This experiment involves advanced theory and substantial reagent preparation that requires outside-lab prep time. The goal of this experiment is to determine the standard reduction potentials (E°, V) for a series of substituted pentacyanoferrate(II) complexes. By comparing the electrochemical behavior of each AA ligand system, information about electronic structure and solution properties will be obtained. An introduction to cyclic voltammetry is given in Appendix 2. 

Results Summary for the CV Analysis of Substituted Pentacyanoferrate(II) Complexes,... 

EXPERIMENT 4.5 SEMI-EMPIRICAL CALCULATIONS IN THE STUDY OF SUBSTITUTED PENTACYANOFERRATE(II) COMPLEXES... 

A = +14 cm3 mol-1 for both the forward and the reverse reaction. That this AV value is markedly less than the partial molar volumes of water and of ammonia (25 and 18 cm3 mol-1, respectively) indicates limiting dissociative (D) activation (133), as do the A values of close to +70JK-1mol-1 in both directions. Overall, the current situation with regard to thermal substitution at pentacyanoferrates(II) appears to be that an I,i mechanism can also operate for reactions of [Fe(CN)5(H20)]3-, whereas the D mechanism operates for all other [Fe(CN)5L]" complexes (134). 

The kinetics of formation of nitroprusside from [Fe VCN)5(H20)] indicate a mechanism of complex formation in which outer-sphere reduction to [Fe (CN)5(Fl20)] precedes substitution."" Reduction of the dimeric pentacyanoferrate(III) anion [Fe2(CI io]" by thiourea is a multi-stage process the first step is one-electron transfer to give [Fe2(CN)io], which dissociates to give [Fe(CN)5(tu)]2- and [Fe(CN)5(H20)] -.""... 

Espenson JH, Wolenuk SGJ. (1972) Kinetics and mechanisms of some substitution reactions of pentacyanoferrate(III) complexes. Inorpr Chem 11 2034-2041. 

Competition experiments again feature prominently in another discussion of the possible role of transient five-coordinate [Co(NH3)5] in induced and in spontaneous aquation of pentaaminecobalt(III) derivatives. " The operation or nonoperation of the D mechanism at various cobalt(III) centers and at penta-cyanoferrate(II) still requires a few experiments providing unambiguous results. Its operation at molybdenum(O)- and tungsten(0)-penta or tetracarbonyl complexes seems more firmly based. The question of its operation at pentacyanoferrates(III) does not seem to have caused much concern. The only recent paper which mentions kinetics of such a reaction, replacement of 2-methyl imidazolate in [Fe "(CN)5(2-Meimid)] ", reports that the limiting first-order rate constant is 2.3 x 10 s at 298 K, but is more preoccupied with redox catalysis by traces of iron(II) than with simple substitution. 

Most substitutions at iron(III) are fast, and are therefore discussed elsewhere in this report (see Chapter 9), but several are slow enough to monitor by conventional techniques and are therefore mentioned here (though pentacyanoferrate(III) complexes are in Section 8.3.1). The first system bridges this and the preceding sections, for it involves relatively slow fac mer isomerization for tris-hydroxamato complexes of iron(II) and of iron(III). These complexes containing ligand (36) have been known for some time, but isomer details have only been sorted out in the course of the present kinetic study. Kinetics of formation of several iron(III)-hydroxamate complexes have also been reported. ... 

A study of photosubstitution in six (substituted) pyridine and pyrazine pen-tacyanocobaltate(III) complexes [Co(CN)5L]" complements an earlier study of similar pentacyanoferrate(II) complexes. Irradiation in ligand field bands results in 100% replacement of L by water quantum yields range from 0.12 to 0.40. The results are similar to those for the iron(II) complexes, with such differences as are observed assignable to the large difference in importance of tt back bonding to cobalt(III) and to iron(II). ... 

Table 8.1. Kinetic Parameters for Substitution at Pentacyanoferrate(II) Complexes [Fe(CN) L], in Aqueous Solution at 298.2 K...

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