Pentacyanoferrates II

There has been a marked resurgence of interest in this area. Recently published kinetic parameters for ligand dissociation from pentacyano-ferrates(II), [Fe(CN)5L] , are shown in Table 8.1, which also contains earlier kinetic data for a few key complexes for comparison. It may be presumed that these dissociations take place by the limiting D mechanism. 

Kinetic data for formation of pentacyanoferrate(II) complexes [Fe(CN)5L] from [Fe(CN)5(OH2)f are shown in Table 8.2. The Eigen-Wilkins mechanism applies here overall formation rate constants kf are determined by outer-sphere association constants that is, mainly by electrostatics. However, there is an unexpected inconsistency if one compares N2H4/N2H5 with en/enH, though this may well arise in part from solvation effects (en much less hydrophilic and thus less heavily solvated than N2H4) (cf. previous paragraph),There seems to be some difficulty 

Pyrazine- iV-oxide Pyrimidine (9) Quinoxaline (10) Dimethyl sulfoxide Thiourea Thioacetamide Dithiooxamide 

The mechanism involves successive loss of the five cyanides from one aquopentacyanoferrate(II) anion, with each released cyanide being scavenged by another [Fe(CN)5(OH2)] to give [Fe(CN)6r /  

Such ligands as the cyanopyridines can coordinate at two different positions to the iron of the [Fe(CN)5] moiety. The kinetics of isomerization (14) (15) (and the analogues 3-cyanopyridine reaction) were studied 

Dissolution of the sodium salts of the [Fe(CN)s(NH3)] or [Fe(CN)s(S03)] anions gives the aquo ion [Fe(CN)s(OH2)] and its dimer. For the ammine complex in acidic solution the rate-determining step is dimerization, for which the rate constant is 1.1 dm mol s at 298 K, but in alkaline or neutral solution the rate-determining step is 

The kinetics of dissociation and of formation of the pentacyanofer-rate(II) complexes of cysteine, penicillamine, glutathione, and 2-mercapto-ethylamine have been established pyrazine was used as incoming ligand in the dissociation studies. The pH dependence of both dissociation and formation can be ascribed to protonation equilibria of the ligands. Kinetics both of dissociation and of formation have also been studied for the 2-methyl pyrazine complex [Fe(CN)s(2Mepz)] , 2. Dissociation was 

Given a suitable potential bridging ligand (Y), such as 4,4 -bipyridyl, 4-cyanopyridine, or pyrazine, metal complexes can be used as ligands for pentacyanoferrate(II) as in equation (1). The value of kf is about a thousand 

Substitution Reactions—Nos. 6 and Above Other Inert Centers 

A study of photochemical aspects of the nitroprusside anion in relation to its use in pharmacy gives some kinetic information, including the pH dependence of rates. 

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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). 

Solvatochromism and piezochromism of a range of pentacyanoferrates(II) have been examined in binary aq ueous solvent mixtures, " and their solvatochromism in micelles and reversed micelles. The solvatochromism of [Fe(CN)5(nicotinamide)] has been established in several ranges of water-rich binary solvent mixtures, " of [Fe (CN)5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.The solvatochromism of [Fe(CN)5(4Phpy)] and [Fe(CN)5(4Bu py)] has been proposed as an indicator of selective solvation in binary aqueous solvent mixtures. ... 

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)] " +... 

Kinetic parameters k, often also and AS, occasionally AV ) for formation and dissociation of several pentacyanoferrate(II) complexes [Fe(CN)5L]" have been established. Ligands L include several S- and A-donor heterocycles,4-methyl- and 4-amino-pyridines, a series of alkylamines, 3- and 4-hydroxy- and 3- and 4-methoxy-pyridines, several amino acids, nicotinamide, " 4-pyridine aldoxime, 3-Me and 3-Ph sydnones, several bis-pyridine ligands,neutral, protonated, and methylated 4,4 -bipyridyl, 1,2-bis(4-pyridyl)ethane and traTO-l,2-bis0-pyridyl)ethene, pyrazine- 4,4 -bipyridyl- and bis(4-pyridyl)ethyne-pentaammine-cobalt(III), edta-ruthenium(III), and pentaammineruthenium-(II)and-(III) complexes of... 

Equilibrium constants for formation of complexes [Fe(CN)5L] can be derived from kinetics and independently from spectroscopic determinations. Values are given in many of the papers cited above stability constants for several pentacyanoferrate(II) complexes have been compared with those for their pentacyanoruthenate(II) analogues. " ... 

Table 6 Kinetic parameters for dissociation of pentacyanoferrates(II), [Fe(CN)5L] , in aqueous solution...

Reactivities of pentacyanoferrates(II) in micelles and reversed micelles have been studied. The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)] + CN. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation. 

Enantiomerically pure pipecolic acid (6) is accessible essentially by two well-established synthetic routes (i) cyclization of l- or D-lysine by reaction with disodium nitrosyl-pentacyanoferrate(II) with preservation of configuration at C2 215 216 (ii) ring closure of A ,Ae-bis(A-nitroso-A-tosyl) derivatives of l- or D-lysine, again with retention of chirality at C2. 217 Stereoselective synthesis of pipecolic acid derivatives, substituted in position 4, is achieved using the aza-Diels-Alder reaction of imines with dienes 218-220 or via an ene-iminium cyclization. 221 222 ... 

The pentacyanoferrate(II) group is known to have a strong affinity for aromatic A -heterocycles and rapidly binds to both ends of the self-assembled a-cyclodextrin/ligand complex to yield the rotaxane 56. Alternatively, the product... 

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... 

Limiting rate constants for loss of pyrazine or of piperidine from their respective pentacyanoferrate(II) derivatives are affected to only a very small extent by the acetone content of binary aqueous mixtures.Probably here, as in the earlier case of the 4-cyanopyridine complex in aqueous alcohols, " this minor effect on rate constants conceals large but almost equal effects on the initial and transition states. This proved impossible to assess, due to the authors failure to find salts of appropriate solubility for the requisite measurements and transfer chemical potential derivations. It may be that the recently characterized transition metal(II) salts could lead to an answer. 

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