0-Phenanthroline stability constants

Table XVI shows a selection of stability constants and redox potentials for iron(II) and iron(III) complexes. This Table covers a wide range of the latter, showing how the relative stabilities of the iron(II) and iron(III) complexes are refiected in. B (Fe /Fe ) values. A more detailed illustration is provided by the complexes of a series of linear hexadentate hydroxypyridinonate and catecholate ligands, where again high stabilities for the respective iron(III) complexes are refiected in markedly negative redox potentials (213). The combination of the high stabilities of iron(III) complexes of hydrox5rpyridinones, as of hydroxamates, catecholates, and siderophores, and the low stabilities of their iron(II) analogues is also apparent in Fig. 8. Here redox potentials for hydroxypyranonate and hydroxypyridinonate complexes of iron are placed in the overall context of redox potentials for iron(III)/iron(II) couples. The -(Fe /Fe ) range for e.g., water, cyanide, edta, 2,2 -bipyridyl, and (substituted) 1,10-phenanthrolines is...

Substituent effects in 1,10-phenanthrolines have been comprehensively investigated201,202 from their pA s, stability constants in Fe(III) and Cu(II) complexes, and redox potentials the Ni(II) complexes have also been examined,203 as have other metal complexes.204 p- Values have been obtained for the three successive proton losses (1, 2, 3, respectively) from 4-substituted dications of 10-hydroxy- 1,7-phenanthrolines (23).205... 

Charton (J52) has also applied the extended Hammett equation to the oxidation-reduction potentials of 5-substituted phenanthroline complexes of iron in various acidic media (95, 97, 651) and of bis-5- and 4,7-substituted phenanthroline complexes of copper in 50% dioxane (404). Thus, one should expect an overall similarity between the variations in pAa, stability constant, and oxidation-reduction potential data for the various ligands. The variations in a and )3 values found for various substitution positions and the tautomerism in the LH+ ions show that the correlation need not be good. A similar point may also be made about the comparison of data for the transoid bipyridylium ions and their cis complexes. Plots of A versus pA for various systems (95, 404) show a linear dependence to differing extents. As would be expected, the data for analogous complexes of iron (28), ruthenium (214, 217, 531), and osmium (111, 218, 220) show very good correlation. The assumption (152) that the effects of substituents are additive is borne out by these potential data, where the changes in potential on methyl substitution are additive (97). 

Sutton (666) and Kul ba (458-460) have prepared a number of bipyridyl and phenanthroline complexes of various thallium(III) salts those with nitrate and perchlorate are generally bis-chelate compounds, whereas the halides give compounds of stoichiometry TIX3L. Stability constants have been reported for some of the complexes (457). Other mixed ligand species containing ethylenediamine (461) and oxalate (456) and salts containing both bipyridyl and phenanthroline coordinated to the same thallium(III) ion (456) are also claimed. 

These ligands are now well represented in complexes with the lanthanides, but were not investigated until 1962, when a study in aqueous solution using a Job s plot method, e.g. at 451 nm for Ho ", showed that a complex Ho(phen)2" was formed. Pr, Nd and Er tripositive ions were also studied, but no solid products were characterized. Indeed, stability constants in water are possibly too low for a solid complex to be isolated. A little later, there were several reports of the isolation of bipyridyl and 1,10-phenanthroline complexes from ethanolic solution. It is perhaps of interest that at this time, the lanthanides were believed not to form stable amine complexes and the role of higher coordination numbers in lanthanide complexes was not fully appreciated. Complexes isolated included the stoichiometries M(N03)s(bipy)2, where M = M(NCS)3(bipy)3, where M = La, Ce, Dy M(MeC02)3(bipy),... 

First, let us discuss the shapes of the chelated complexes. In most cases they are tetrahedral, but iron (both ferrous and ferric) forms octahedral complexes. However, the most remarkable contribution to selectivity is made by cupric ions which form planar complexes. This means that copper is uniquely sensitive to a type of steric effect imposed by bulky groups. Thus, in Table 11.2, the stability constants for the cupric complexes ofbipyridyl [11,19), phenanthroline [11.18), and folic acid lie below those of the corresponding nickel complexes, which is contrary to the natural order discussed in Section 11.2. 

Kirschner and Magnell have reported that the magnitude of the Pfeiffer effect increases for the series of tris(ort/io-phenanthroline) complexes of zinc(II), cadmium(II), and mercury(II). The stability constants of these complexes, however, have been shown to be in the reverse order, indicating that the mercury complex is the most labile of the three, and, therefore, would be expected to be able to undergo the Pfeiffer effect with the greatest facility and to the greatest extent. 

Otherwise, Fe-(ni) may be characterized with o-phenanthroline after reduction to Fe(II) with sodium hydrogen sulfite or with ascorbic acid. Fe + may also be characterized by the complexes it gives with 2,2 -dipyrydile or with terpyridine, which yield a very stable red color with ferrous salts. The [Fe(o-phen)3] + complex exhibits the stability constant P3 = 10 (see Chap. 23). Fe " can also be characterized with hexacyanoferrate(ni), formerly called the ferricyanide ion. In this case, a blue color called Turnbull blue arises. For a long time, it was believed that this color was due to the formation of ferrous ferricyanide Fe3[Fe(CN)6]2, but this was wrong. Actually, hexacyanoferrate(in) (which is an oxidant) oxidizes Fe " " to Fe + by reducing itself to hexacyanoferrate(II) ... 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

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