Mixed-valence compounds classification

Mixed-valence compounds are divided into three classes following proposals by Robin and Day [98] and Hush [99]. The basis for this classification is the... 

Classification of Mobile Charge Carriers in Mixed-Valence Compounds... 

Analogy with chemical classification of molecular compounds. Chemists are perhaps more familiar with the Robin-Day classification of mixed-valence compounds, which was later refined by Mayoh and Day. ... 

Robin Day (1967) have proposed a classification of mixed-valence compounds based on the valence delocalization coefficient, a, the magnitude of which depends on the energy difference between the two states and Mb, where A... 

Figure 3 exhibits some typical electronic spectra of mixed-valence conplexes of the [Mo(CN) Jr ion with various other metal ions. Mixed-valence compounds formed By interaction of Mo(IV), W(IV), Fe(ll), and Rj(ll) cyanometallates with metal ions such as Fe(lll), Cu(II), UO2, and A) belong to the class II of the Robin-Day classification ... 

The IR spectroelectrochemistry of the mixed-valent heteropolyanions [PMo O ]"- (where n= 4, 5, 6, 7) suggests that these anions belong to the class II system in Robin and Day s classification of mixed-valence compounds. The spectra also showed evidence for decreased Mo=0 bond strength after reduction.171... 

A general classification scheme for the discussion of the properties of mixed valence compounds has been introduced by Robin and Day 17). Although by no means all the deeply colored transition metal cyanides M [BM(CN) ]2H2O are mixed valence compounds in the strict sense, we will use this classification scheme for the whole group. The classification of the polynuclear cyanides is made on a purely phenomenological basis using the electronic spectra as a criterion. 

The concept of binodal orbital aromaticity in reduced early transition-metal polyoxometalates may be related to their classification as mixed valence compounds. Robin and Day divide mixed valence compounds into the following three classes ... 

The above classification of mixed valence compounds has been illustrated (Fig. 2) using as an example a symmetric redox system (i.e., one which. 

In the mixed valence BFD salts (in addition to the TCNQ compound, the picrate and tetrafluoroborate were also studied), the metal-metal interactions are so strong [Class III in the classification of Robin and Day (23)] that the metals are equivalent in every measurable respect. The doubly oxidized (24) Fe(III)-Fe(III) species is diamagnetic which is further evidence for strong interactions. Since the irons in BFD are 3.98 A apart (25), direct metal-metal interactions are not likely. We must then assume that in this case there exist metal-ligand-metal interactions of the magniture in which we are interested. We propose that the BFD system be considered a delocalized aromatic species. 

This review is divided into four major sections (i) a general survey of n molecular conductors, including the chemistry and the structural classification of radical-ion salts (ii) a summary of the physics of these electronic and magnetic low-dimensional systems (iii) a discussion of the present "state-of-the-art" in these materials, with an emphasis on their mix -valence character, and the impetus to create new compounds with specific low-temperature physical properties (iv) a description of the current trends, which show an explosion in different directions, l cause one can get new materials with specific properties, by playing with the molecular organization in stable or metastable phases. 

Table 4. A comparative classification of mixed-valence (MV) molecular compounds.

In conclusion, the valence classification of Yb compounds, based on the SIC-LSD total energies, maps very well onto the physical properties observed experimentally. In particular, this allows to identify the third group of compounds as the heavy-fermion and mixed-valent systems on the trivalent side, and, on the divalent side, those systems that are likely to imdergo pressure-induced valence transitions. 

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