Fluorides complex anion

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

All the tetrahalides, but especially the chlorides and bromides, behave as Lewis acids dissolving in polar solvents to give rise to series of addition compounds they also form complex anions with halides. They are all hygroscopic and hydrolysis follows the same pattern as complex formation, with the chlorides and bromides being more vulnerable than the fluorides and iodides. TiCU fumes in and is completely hydrolysed by... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

There is evidence that in KSb2F7 the complex anion [Sb2F7] contains a central fluoride ion coordinated to two antimony(III) fluoride molecules (36) however, this structure has recently been criticized (37). 

Discrete [F(SbF3)4]" units are present in the crystals of KSb4Fi3 (39) and these consist of fluoride ions tetracoordinated by antimony(III) fluoride molecules through the antimony atoms. The coordinate bond lengths are 2.87 A and hence longer than the antimony-fluorine distances in the antimony(III) fluoride molecules, while the Sb-F bonds in the complex anion under consideration are 2.01 A and thus slightly longer than in the uncoordinated SbFs molecules (Fig. 4). 

Double Fluorides of Niobium Pentafluoride.—Niobium pentafluoride shows a strong tendency to form stable double fluorides with the fluorides of other metals. These are conveniently prepared by the action of carbonates of the metals on solutions of niobium pentoxide in a large excess of hydrofluoric add, or by the addition of a large excess of hydrofluoric acid to solutions of the oxyfluorides of the metals. In the absence of excess of hydrofluoric add hydrolysis takes place as usual with the formation of niobium oxytrifluoride, NbOF3. The precipitation of these double fluorides indicates the probable existence in solution of niobium pentafluoride stability is imparted by the formation of complex anions containing several fluorine atoms. 

The term demasking is used rather broadly to describe any process which reverses the process of masking. Control of the pH can be used to mask calcium ions in admixture with copper(II) whereby, in sufficiently acid solution, the latter is precipitated by oxine whereas the concentration of Ox- is reduced sufficiently by protonation to prevent the precipitation of calcium oxinate. Raising the pH will increase [Ox-] and both metals can now be precipitated. A more obvious example is the addition of fluoride ions to form the very stable complex anion SnF62- and so to mask tin(IV) against its precipitation as SnS2- Addition of boric acid will give BF4- and so demask the tin. 

A CE determination of fluoride in rain water was compared with IC and ISE potentiometry the IC response was related to the total concentration, whereas CE and ISE responded to free fluoride [50]. The fluoride concentrations obtained by CE and ISE were systematically lower than those obtained by IC due to the fluoride complexation with aluminium. The detection limits for IC and ISE were similar (0.2 and 0.3 pmol/l) and somewhat lower than those for CE (0.6 xmol/l). CE was evaluated as an alternative method to the EPA ion chromatographic method for the determination of anions in water and a better resolution and a shorter analysis time were found for CE [51]. 

In acid medium, fluoride reacts instantaneously with zirconyl-dye lake which is composed of zirconyl chloride octahydrate, ZrOCl2 8 H20, and sodium 2-(parasulfophenylazo)-l,8-dihydroxy-3,6-naphthalene disulfonate (SPADNS), displacing the Zr2+ from the dye lake to form a colorless complex anion, ZrF62 and the dye. As a result, the color of the solution lightens as the concentration of F increases. 

Figure 4.21 X-ray structures of (a) the chloride complex of 4.49 and (b) the fluoride complex of the analogous cyclohexyl derivative, showing the better fit of the F anion.

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