STABILITY OF THE FLUORIDE COMPLEXES

Hydrofluoric acid is best known for its ability to dissolve silica, but even small quantities exert a marked catalytic effect on the nitric acid dissolution of many refractory oxides, including ignited plutonium oxide, due to the high stability of the fluoride complexes. Consequently, use of hydrofluoric acid in leaching procedures generally gives more accurate results. However, erratic... 

From Table V we see that the high stability of the fluoride complexes of the lanthanide ions is attributed to a large positive entropy in contrast to Ihe negative values found for the formation of the mono-... 

The high stabilities of the fluoride complexes, and the very low stabilities of the complexes formed by the heavier halides, are evident. So also are the very high stabilities of fluoride complexes of ions relative to those of MO2 and... 

This result is explained by the fact that addition of a hard base (fluoride ions) to the chloride melt containing a weaker hard base (chloride ions) favours stabilization of the halide complexes formed by a harder acid (in our case it is Am3+) as compared with a weaker acid (Am2+). This causes the shift of the equilibrium (1.1.39) to the right, and the disproportionation reaction runs. 

The halides vary in hydrolytic stability. Thus the fluoride is not hydrolyzed by refluxing aqueous ammonium hydroxide, while the chloride hydrolyzes completely in 45 hours and the iodide in 45 minutes. The complexes increase in solubility in chloronaphthalene in the sequence F < Cl < I, which is interpreted to imply increasing covalent character to the bond, in that sequence (195). The variation in hydrolytic stability has also been interpreted in terms of increasing covalent character. The hydrolytic stability of the fluoride may also be due to its low solubility which renders attack by hydroxyl groups kinetically very slow. 

A wide variety of anionic actinide halide complexes are well known and typically are isolated with alkali or alkahne-earth metal ions. The tendency and stability of the anionic complexes follow the trend F Cl > Br I. The trivalent fluorides and chlorides typically form complexes of the form AnX4 and AnXe ". Plutonium has also been shown to give the following complexes PuCb ", Pu2Cl7, and PuClg . The anionic tetravalent actinide fluorides represent a broad class of complexes, for example, M AnFj, (x = 1, y = 5 x = 2, y = 6 V = 3, y = 7 x = 4, y = 8). Tetravalent actinide chloro, bromo and iodo complexes can be isolated from aqueous solutions in the form of octahedral AnCle " ions. 

Anhydrous KPbla is curious, in that when reattacked by water it immediately forms bright yellow lead(II) iodide rather than KPbl3.2H20. This reaction is used for the detection of traces of water in gases or organic solvents. The stabilities of the halide complexes of Ieadi93-i96 are in the order I > Br > Cl- > F no fluoride complexes are detectable in solution. It is believed that chlorine, bromine and iodine utilize partial double bonding to lead in these complex ions >7. 

Chaudhuri, N.K., and Sawant, R.M. (1997) Stability constants of the fluoride complexes of actinides in aqueous solution and their correlation with fundamental properties. Bhabha Atomic Research Centre report, BARC/1997/E/022, 45 p. 

Thermodynamic data show that the stabilities of the caesium chloride-metal chloride complexes are greater than the conesponding sodium and potassium compounds, and tire fluorides form complexes more readily tlrair the chlorides, in the solid state. It would seem that tire stabilities of these compounds would transfer into tire liquid state. In fact, it has been possible to account for the heats of formation of molten salt mixtures by the assumption that molten complex salts contain complex as well as simple anions, so tlrat tire heat of formation of the liquid mixtures is tire mole fraction weighted product of the pure components and the complex. For example, in the CsCl-ZrCU system the heat of formation is given on each side of tire complex compound composition, the mole fraction of the compound... 

In solvent extraction with TBP, the fluoride complexes form solvated complexes with TBP. The stabilities of such complexes depend on the acidity/concentration of hydrofluoric... 

Oxidative addition of a silyl-protected 4-(bromomethyl)phenol precursor to (tme-da)Pd(II)Me2 (tmeda = tetramethylethylenediamine), followed by ethane reductive elimination, resulted in formation of the benzylic complex 16 (Scheme 3.10). Exchange of tmeda for a diphosphine ligand (which is better suited for stabilizing the ultimate Pd(0) QM complex), followed by removal of the protecting silyl group with fluoride anion, resulted in the expected p-QM Pd(0) complex, 17, via intermediacy of the zwitterionic Pd(II) benzyl complex. In this way a stable complex of p-BHT-QM, 17b, the very important metabolite of the widely used food antioxidant BHT20 (BHT = butylated hydroxytoluene) was prepared. Similarly, a Pd(0) complex of the elusive, simplest /)-QM, 17a, was obtained (Scheme 3.10). 

Exposure of ferri-heme-hemopexin to imidazole or KCN can displace one or both of the heme coordinating His residues, but millimolar concentrations are required (138). Other potential ligands such as azide or fluoride are inactive. This coordination stability of the ferri-heme-hemopexin bis-histidyl complex, despite the exposed heme site, is home out by thermal imfolding studies (Section IV,F). Reduction of the heme-hemopexin complex, however, has dramatic effects on its stability. 

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