Other Low-Spin Iron II Complexes

In the equilibrium and kinetic study of the reaction of iron(II) phthalocyanine with carbon monoxide in dimethyl sulfoxide, rate laws, rate constants, and activation enthalpies and entropies were determined.  

Substitution Reactions—Nos. 6 and Above Other Inert Centers 

A similar study has been made of reaction of the 14-ane and 15-ane (12 and 13, respectively) complexes of iron(II) with carbon monoxide and with 

For solvolysis of the water-soluble 4-AT,N,N-trimethylanilinium porphyrin complex of iron(II), rates are proportional to the second power of proton concentration. Rates measured for this complex, cobalt(II) and nickel(II) complexes of the same ligand, and for other metal(II)-porphyrin complexes, have been correlated with Buchler s stability index. This index incorporates charges, radii, and Pauling electronegativities.  

Addition of trimethyl phosphite to acetonitrile solutions of iron(II) salts results, eventually, in the replacement of most of the solvent molecules 

1 and S.2.2.3), in that ligand oxidation occurs without oxidation of the iron(II). The net change is shown in the partial formulas (31)(32).  

Several references deal with slow or relatively slow substitution at iron(III), while a few are directly relevant to analogous iron(II) systems discussed in Sections 8.2.1 and 8.2.2. Indeed iron(III)-diimine complexes have already been mentioned in connection with covalent hydration (Section 8.2.2.4). Substitution at [Fe(phen)3] is a very much less popular area of study than that of its iron(II) analogue (Section 8.2.2). Dissociation of the iron(III) complex in aqueous acetone is claimed to be first order with respect to [Fe(phen)3] , second order with respect to acetone, and reciprocal first order with respect to This information is derived from observations at 620 nm a more complicated picture emerged from kinetic studies carried out at 470 nm The interpretation offered rather conceals the key role of acetone in solvating the leaving 1,10-phenanthroline, though this is mentioned in the final sentence. It is regrettable that the authors do not report their primary experimental results, namely their rate constants. 

Attack of cyanide at polyaminocarboxylatonickel(II) complexes has been much studied kinetically. Now the first report has appeared on the kinetics of cyanide reaction with an analogous iron(III) complex, [Fe(edta)(OH)]. The reaction sequence involves substitution steps giving [Fe(CN)5(OH)] and then [Fe(CN)6] , which finally is reduced by the edta to [Fe(CN)6] . The initial reaction between [Fe(edta)(OH)] and cyanide has a complicated dependence on cyanide concentration, but the subsequent reaction of [Fe(CN)5(OH)] with cyanide follows simple second-order kinetics. The kinetics of the reverse step, [Fe(CN)5(OH)] with edta, were also examined. Brief irradiation of [Fe(CN)5(OH2)f in aqueous solution in the presence of 1,10-phenanthroline yields [Fe(phen)3] and [Fe(CN)6]. Longer irradiation results in some reduction to iron(II), producing [Fe(phen)3] and [Fe(phen)(CN)4] .  

Finally, two studies of the kinetics of j8-diketone replacement should be mentioned, involving the reactions of [Fe(acac)3] with tfacH and vice versa, and of [Fe(bzac)3] with acacH. Solvent effects in the latter system, especially the accelerating effect of t-butyl alcohol on the reaction in hexane, suggest a free-radical mechanism. 

Another important group of low-spin iron(II) complexes comprises those with tetradentate cyclic ligands, which generally have four nitrogen donor atoms, for example porphyrins (porph) and phthalocyanines (pc). Here kinetic studies of substitution generally are concerned with the axial ligands in the fifth and sixth coordination positions. The mechanism of substitution in reactions of iron(II) phthalocyanine complexes, 

The stepwise replacement of acetonitrile by trimethyl phosphite in [Fe(MeCN)6] has been monitored by nmr and by ultraviolet-visible spectroscopy. It is possible to derive some approximate kinetic data from these observations. The coordinated acetonitrile is replaced fairly slowly—indeed it takes several days to replace the fifth acetonitrile to obtain [Fe(MeCN) P(OMe)3 s]  

The kinetics of formation of chelating dicarbene products from the reaction of the hexakisisonitrile complex [Fe(CNMe)6] with hydrazine and with methyl- 

Substitution Reactions of Inert Metal Complexes—6 and Above 

Several kinetic studies have dealt with iron(III) complexes and their reactions which complement iron(II) studies mentioned above. Such studies will be reported in this section such reactions as complex formation at Feaq are dealt with at appropriate places elsewhere in this volume. 

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Two of the papers presented at the Fifth International Conference on Non-aqueous Solvents are of direct relevance to this Report. They deal with solvent effects on kinetics, in the areas of ligand substitution reactions at labile centres, and of preferential solvation in such systems." Another review on preferential solvation and its consequences deals primarily with chromium(iii) complexes, such as the [Cr(NCS)6] anion, in binary aqueous mixtures, but also mentions other groups of inorganic substrates such as low-spin iron(ii) complexes. A short article on the effectiveness of a solvent in catalysis considers such topics as affinities for nf-electrons and polarization potentials. ... 

As in previous volumes of this series, this chapter deals at some length with low-spin iron(II) complexes and with complexes of ruthenium(II) and (III) and rhodium(III) more briefly with complexes of iridium(III), osmium, and platinum(IV) and touches on a few other centers of marginal interest in the present context. 

The sterically hindered dianionic bidentate phenoxide ligand (205) gives several tetrahedral iron(II) complexes, e.g., [Fe (205)(THF)2], [Fe (205)(py)2], [Fe (205)(bipy)], and [Fe (205)(2,6-xylylNC)2]. The first of these is prepared from FeCl2 and (205)H2 in tetrahydrofuran the others are prepared from the dimer [Fe2(205)2]. The 2,6-xylylNC complex is low-spin, the others high-spin. There is also a five-coordinate iron(III) complex, red-black [Fe(205)(bipy)Cl], whose structure is intermediate between trigonal bipyramidal and square pyramidal." ... 

In the context of model studies of iron enzymes involved in oxygen transfer, the pentapyridine ligand 7 (R = OMe), the tetrapyridine ligand 11, and a number of other, less symmetrical N5 ligands have been used to prepare iron(II) complexes that react with hydrogen peroxide to generate transient low-spin Fem-OOH intermediates (17,18,21,117), with... 

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

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