Iron spin-pairing transitions

Shock-wave experiments causing phase changes in wiistite, Fe094O, and Fe203 have also produced controversial evidence for spin-pairing transitions in iron cations (Jeanloz and Ahrens, 1980 Jackson and Ringwood, 1981 Goto et al., 1982 Syono etal., 1984). 

Figure 3.16 Energy level diagram for ferric iron matched to spin-forbidden crystal field transitions within Fe3+ ions, which are portrayed by the polarized absorption spectra of yellow sapphire (adapted from Ferguson Fielding, 1972 Sherman, 1985a). Note that the unassigned band at -17,600 cm-1 represents a paired transition within magnetically coupled Fe3+ ions located in adjacent face-shared octahedra in the corundum structure.

The UV spectral pattern of the spin-paired tris-bipy and -phen complexes of iron(II), ruthenium(II) and osmium(II) (see ref. (23)) is very similar to that of the complexes of other divalent metal ions. As compared with the latter complexes, the first internal ligand transition is shifted to larger wave numbers, the shift beinggreater for ruthenium(II) and osmium(II) than for iron(II). This shift was associated (23, 69) with the effect of back-donation which tends to push the filled and vacant i-levels of the ligands further apart. 

The electron transfer ) character of the visible absorption spectra of iron complexes of conjugating imine ligands was first recognized by WiUiams (77). From the opposite effect of methyl substituents on max of ferrous (or cuprous), and of ferric tris-phen complexes, it was concluded that the excitation involves a partial electron transfer from a filled metal electron transfer (ET) transitions in spin-paired iron(II) tris-diimine complexes, was further elaborated by Jorgensen (78). 

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