Inversed mixed-valency

Secondly, the EPR method is also appropriate to study inverse mixed-valency , involving metal-bridged ligands of different redox states.In those cases, the hopping of unpaired electrons may be noted through linewidth effects as for system [Ru(abpy)2(bpy)] (10). ... 

A highly unusual trinuclear Sm(n)/Sm(m) mixed valence inverse sandwich was obtained when Cp Sru[//-OSi(Ollu )3]3Sm was treated with either phenylacetylene or 4-Mc-2,6-Bu 2( V,I I2OII (=ArOFI) in toluene (Scheme 103). Most of the products were structurally characterized by X-ray analyses, and the reactions illustrated in Schemes 100 to 103 represent only part of the interesting chemistry discovered with this system.506... 

Magnetite, Fe30>4, is a unique material. It is a mixed-valency compound (Fe " and Fe " on crystallographic sites of the same symmetry) with a low electrical conductivity below 120K, and a nearly metallic conductivity above this temperature. Between 120 and 770 K, magnetite is an inverse spinel, (Fe )[Fe Fe ]04, and all the transition phenomena occur in the octahedral sites. For T > 770 K, the redistribution of Fe on tetrahedral sites becomes non-negligible (Wu Mason, 1981). The reversible, sharp discontinuity in conductivity was first observed by Okamura (1931). Ferroelectric features have also been observed at low temperatures (Rado Ferrari, 1975 Kato et al, 1983). 

Table 11.8 reveals that most spinels involving Fe (AFe .04) have the inverse structure. The d Fe ion will have a CFSE of zero for both tetrahedral and octahedral coordination, so if there is to be a site preference it will be due to the A(ll) ion. This is cleatly the case for NiFe204. for example, the Nr ion having an octahedral site preference energy of 86 kJ mol. In magnetite. Fe,04, both A and B ions are iron, with some in the +2 oxidation state and others in +3 Fe"Fe" 04. the f" Ft ion, octahedral coordination is more favorable than tetrahedral by about 13 kJ tiu>l, whieh, although only a modest amount, is apparently sufficient to invert the structure. In contrast, the similar oxide Mn]04 has the normal structure. In this instance, the Mn has no CFSE in either octahedral or tetrahedral fields, but d Mn shows a preference of 106 kJ mol for octahedral sites. For Q>,04. another mixed-valence oxide, there is an additional fiictor to take into account—Co is low... 

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