Iron pentacyanoferrates

Further variations on the theme have been achieved54 by anchoring species such as [RuivO-(terpy)(py)]2+ or complexes of osmium.55 Iron complexes have also been studied for example, evaporation of a solution containing [Fe(CN)5(H20)]3 and PVP on to an electrode will immobilize the pentacyanoferrate as a pyridyl complex, one in three available pyridyl groups being used to avoid precipitation prior to evaporation of solvent. 

The markedly negative redox potentials of tris-catecholate and tris-hydroxamate iron complexes (Figure 4) may be ascribed to the high stabilities of the iron(III) complexes and the rather low stabilities of their iron(II) analogues. Table 9 details the relevant data (interconnected by a thermochemical cycle earlier applied to amino acid pentacyanoferrate complexes ), and documents the remarkably higher stabilities of tris-catecholate than of tris-hydroxamate complexes of iron(III). 

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). 

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). ... 

Rate studies on the formation of the iron(m) complex in the presence of nitrite ion confirm that the iron(n)-thiocyanate complex is a precursor to the oxidized species. In the corresponding reaction with thiourea in basic solution the order with respect to the thiol is complex, and plots of log Arobs (observed rate constant) against pH pass through a maximum at pH ll.S. The violet-coloured adduct formed in the reaction is considered to incorporate two pentacyanoferrate(n) groups and one mole of thiourea, but on addition of acid the reversible formation of a blue colour is observed. 

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