Class II mixed-valence compound

The cyclobutadiene derivatives [Co(Tj -C4Fc Ph4 )Cp] (n = 1-3) (Cp = 17 -cyclopentadienyl) undergo one-electron oxidation to stable cations (27) which show charge-transfer bands in the near infrared region. An analysis of the spectra led to the conclusion that the cations are Class II mixed-valence compounds and that the charge-transfer transition arises from a weak interaction between Co(I) and Fe(III) rather than between Fe(II) and Fe(III). Each compound showed further oxidations at more positive potentials, including an irreversible process ascribed to electron loss from the cobalt center. 

TABLE 5. Electron transfer parameters for class II mixed-valence compounds. 

The spectroelectrochemical study of the IVCT transition in the asymmetric complex II has confirmed that this species is a class II mixed-valence compound and several physical parameters pertaining to thermal electron transfer rates have been evaluated. Further studies on other asymmetric FSBO complexes are now in progress to enhance the work presented here. 

Electric fleld gradient, 22 214-218 Electroabsorption spectroscopy, 41 279 class II mixed-valence complexes, 41 289, 291, 294-297 [j(jl-pyz)]=+, 41 294, 296 Electrocatalytic reduction, nickel(n) macro-cyclic complexes, 44 119-121 Electrochemical interconversions, heteronuclear gold cluster compounds, 39 338-339 Electrochemical oxidation, of iron triazenide complexes, 30 21 Electrochemical properties fullerene adducts, 44 19-21, 33-34 nickeljll) macrocyclic complexes, 44 112-113... 

By further evaluation of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents (Section III,A), the first examples of bromoselenates-(II,IV) containing selenium in both oxidation states +2 and +4 were prepared. Three dinuclear, trinuclear, and tetranuclear types of anions are reported in this novel class of mixed-valence compounds Se2Brg, SeaBrio , and Se4Bri2 . They are shown in Fig. 31. Similar to the bromoselenates(II), they were obtained by nucleophilic addition reactions of bromide ions within the complex reaction mixtures of selenium bromides. Evidently, the formation of Bra" is important for the stabilization of Se(II) in addition to Se(IV), shifting the equilibrium... 

Figure 1.46). ° The monocation 97 can be considered as a push-pull chromophore and exhibits a / i.o6 (HRS) value twice that of its neutral precursor 97 and the fully oxidised compound 97. Complex 97 is a class II mixed-valence complex, retaining charge localisation, but with enough electronic coupling between the redox centres to permit intervalence charge transfer transitions. As the redox couples 97/97" and... 

The compound 6 is a model of the Fe2Se unit found in ferredoxin proteins. Electrochemical and homogeneous chemical redox reactions establish the existence of the series Fe(III)Fe(III), Fe(III)Fe(II), and Fe(II)Fe(II), and epr and Mossbauer spectra show the middle members to be of the class II mixed-valence type. " As yet, however, the IT band has not been detected. 

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 ... 

An important qualitative description of the spectral behavior of class II compounds was presented by Robin and Day. This simple model has found apphcabihty to the discussion of the spectra of numerous mixed valence compounds in which some delocalization occurs. In this model, it is assumed that the ground-state wave function contains the function, a, which describes mixing of the wave function for site A with the wave function of site B. 

In class I mixed valence species, electron transitions between sites are energetically difficnlt to accomplish. In the sohd state, these materials are generally insnlators. However, with class II and class III compounds, electron transfer becomes much easier so that conducting and superconducting materials are obtained (see Sulfur Organic Polysulfanes). 

Another special spacer consists of porphyrin (either metallated or not) substituted in the 5,15 positions (see Figure 12b). Intervalence transitions have been observed when the ferrocene groups are electrochemically oxidized [36]. The metal-metal distance is >10 A. No estimation of the l b coupling was reported, but the mixed-valence compounds would correspond to class III, or II/IIl. 

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... 

Figure 5 Potential energy curves for mixed-valence compounds corresponding to Class I, Class II,...

Polynuclear transition metal hexacyanometallates, e.g., PB (iron(III) hexacyanoferrate(II)), are a class of insoluble mixed-valence compounds that have the general formula M j.[M"(CN)g] where M and M" are... 

Robin Day (1967) have classified mixed valence compounds on the basis of the eneigy required to transfer an electron between the low valence site and the high valence site. In Class I compounds, the ground state M(II)/ M (III) electronic configuration (where M and M refer to different iron(II) and iron(lll) sites in a compound) is much lower in eneigy than the excited state M(III)/M (II) configuration which may result from an intervalence electron transfer. In this type of compound the two iron sites may be... 

One of the most well-known mixed valence compounds is the pigment Prussian Blue which is a Class II compound. There has been considerable controversy as to whether Prussian Blue, which may be produced by the reaction,... 

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