Mixed-valent compound

Also mercury(II) selenite is polymorphic and two new modifications, p-HgSe03 and y-HgSe03, have been recently described.134 They show the mercury atoms in sevenfold coordination of oxygen atoms, and the [I Ig07] polyhedra are linked by the pyramidal selenite ions. A remarkable and unusual mercury selenite is the mixed valent compound (HgSe03)3E[gSe.134 As a characteristic feature, the crystal structure contains [Hg3Se] pyramids with distances Hg Se of 2.489 A. 

On this basis it would seem that the biferrocenium ion can be classified as a mixed-valent (Fen-Fem) compound displaying localized charge. At this point it is useful to digress briefly and consider more deeply the concept of mixed-valent compounds. 

In order to understand the nature of mixed-valent compounds one must consider both electrostatic and magnetic interactions between centres which have different oxidation states. Nevertheless, as schematically shown in Figure 14, simple electrostatic effects can help to clarify the so-called Robin-Day classification.24 This is based on the degree of electronic delocalization in (usually binuclear) mixed-valent compounds. 

For the response illustrated in Figure 12, in which the difference between the two redox potentials is AE° = 0.34 V, one can calculate that Acom 105 - 106. This value is typical of a mixed-valent compound which, on the electrochemical time scale, is at the border between... 

In the crystal structure of the mixed valent compound Hg402(N03)2 there is a two-dimensional framework i[(HgII)2/20(Hg2)1/2]+.564 A three dimensional structure is obtained by additional weak Hg—O contacts via NOJ anions. A novel mercury(I, II) compound has been reported.565... 

F.U. Hillebrecht and M. Campagna, Bremsstrahlung isochromat spectroscopy of alloys and mixed valent compounds 425... 

It has also been pointed out that XAS is a powerful tool for the determination of the 4/ occupancy number, in particular in rare-earth mixed-valent compounds (151a). 

The oxo-bridged mixed-valent compound [(phen)2MnII,( -0)2Mn v(phen)2]3+, a model for part of the oxygen-evolving complex in photosystem II, oxidizes dihydroanthracene with high selectivity to anthracene.162... 

Several dinuclear units have been assembled to form linear Au5 + and Ane complexes. However, they were shown to be Au An mixed-valent compounds. The normal Au -Au distances are in the range 2.5-2.7A. The only binuclear gold(II) complex (15) with unsupported Au Au bond has been shown to possess interesting photophysical and thermal redox properties." ... 

Dimers of M centers occur in the mixed valent compounds GayXs (X = Br, I) = (Ga" )2 [GayXe] " which contain ethane-type [GayXe] " units with Ga-Ga single bonds. As in the dihalides, mixtures of complex and simple ions combine also in InsXy = (In+)3[In2Br6]Br, which contains the ethane-like units with M-M single bonds as weU. ... 

At lower temperatures, the a phase transforms through a disorder-order process into a series of ordered, fluorite-related phases which can be described with the generic formula CenO n jm- These mixed-valent compounds have compositions between CeO[ 7,4 (Ce70,2. n=7, m=I) and Ce02. The seemingly continuous variation of X at low temperatures is due to the co-existence of two phases with different stoichiometries (n, n-t D. Pure Ce 02 .2mi in contrast, exists only in a narrow composition range. 

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