Iron II halides

Ferrous ions usually have a non-cubic environment and give a large quadrupole splitting in paramagnetic complexes. Many ferrous compounds are antiferromagnetic at low temperatures and the spin and electric field gradient axes are not necessarily collinear. This causes considerable difficulty in interpretation, and computer methods must be used as already outlined in Chapter 3.6. Accordingly the results will be quoted without detailed proof in the discussion of actual spectra. 

By contrast, high-spin ferric complexes show only small quadrupole splittings due to the lattice terms, and in the magnetically ordered state the field is basically the Fermi term. Consequently less information is obtainable, with the notable exception of electronic relaxation times which can be deduced from the broadening which often occurs in ferric complexes. 

Appropriate illustrations are given of the principal features of high-spin ferrous and ferric spectra, and examples of the Mossbauer parameters observed are tabulated for many of the complexes. The tables are not intended to be completely comprehensive, but virtually all the significant and reliable results have been incorporated. Where possible, data are quoted for room temperature (RT), liquid nitrogen temperature (LN), and liquid helium temperature (HE). 

For FeF2 it was found that D = 1-36 0-03, (f = 0-325 0-005, and the hyperfine field at the absolute zero Hq — 329 2 kG. This behaviour is remarkably similar to that of MnF2 despite the appreciably lower anisotropy in this latter compound. 

The appearance of more than six Zeeman lines indicates that the Zeeman 

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Iron halides react with halide salts to afford anionic halide complexes. Because iron(III) is a hard acid, the complexes that it forms are most stable with F and decrease in both coordination number and stability with heavier halides. No stable I complexes are known. [FeF5(H20)]2 is the predominant iron fluoride species in aqueous solution. The [FeF6]3 ion can be prepared in fused salts. Whereas six-coordinate [FeClJ3 is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeClJ can be isolated if large cations such as tetraphenylarsonium or tetraalkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to iron(II) and bromine. Complex anions of iron(II) halides are less common. [FeClJ2 has been obtained from FeCl2 by reaction with alkali metal chlorides in the melt or with tetraethylammonium chloride in deoxygenated ethanol. 

Almost simultaneously with our first investigations About Reactions and Derivatives of Iron Carbonyl (Section II,A) we concerned ourselves at Heidelberg with the effect of halogens on iron pentacarbonyl, initially in the expectation of obtaining a pure surface-active iron(II) halide. However, to our surprise at the time, reaction occurred according to the equation... 

By substitution reactions with amines, especially pyridine and o-phen-anthroline (57), and later with isonitriles (11), and phosphines and similar compounds (60), we obtained the complete series of iron(II) halide (especially iodide) compounds with one to four CO groups per molecule. 

Halides. The best known species are the tetrahedral [FeX]2 but discrete homoleptic dinuclear iron(II) halide anions, [Fe2X6]2, are also known.9 More common are complexes with Fe2X2 cores.10... 

Nitrogen Ligands. Iron(II) halides and other salts absorb NH3 in excess giving the [Fe(NH3)6]2+ anion but ammonia complexes are stable only in saturated excess ammonia. Stable complexes with chelating amines are formed, for example, for ethylenediamine ... 

Iron(II) halides Pep2 is white PeCL is pale yellow-grey PeBr2 is yellow-green and Pel2 is grey. 

The coordination chemistry of Fe(II) is well developed and only a brief introduction to simple species is given here. Iron(II) halides combine with gaseous NH3 to give salts of... 

Table 6.1 Mossbauer parameters for high-spin iron(II) halides... 

The tris-(2-aminomethylpyridine)iron(II) halides, [Fe(2-NH2pic)3]X2 (X=C1, Br, I), also show a spin crossover [5]. In this instance the halide anions have a very strong effect on the equilibrium, as shown by the magnetic moment versus temperature plot in Fig. 8.2. No interpretation can be formulated using a simple thermal excitation model. The chloride gives Mossbauer spectra indicating T2 at room temperature, Ai at 4-2 K, and both states in coexistence at 77 K, with no detectable interconversion within the observation time-scale. 

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