Mixed valence species

Reaction of OsCl6 with acetic acid/acetic anhydride mixtures containing concentrated HC1 gives the diosmium compound Os2(OAc)4C12 (rather than mixed-valence species, see section 1.8.3) other carboxylates can be made by carboxylate exchange ... 

As with the trihalides , some formally platinum(III) compounds are in fact mixed-valence species [161]. Thus ... 

After a consideration of optical transitions in which MMCT plays a role, and after a characterization of the excited states involved, a short review of mixed-valence compounds and their spectroscopy is in order. For more extended reviews we refer to Refs. [60,97], At least 40 elements of the periodic table form mixed-valence species which are of importance in solid state physics and chemistry, inorganic chemistry, materials science, geology and bioinorganic chemistry. It is usually their colors which are their most striking property (see also above), but they have more intriguing properties, for example electrical and magnetic properties. 

The localized-valence halogen-bridged platinum(II)/platinum(IV) complexes (175) are of interest because of their highly anisotropic electrical and optical properties. The complexes are characterized by broad and intense intervalence bands in their electronic spectra.542 Partial oxidation of [PtCl(dien)]Cl with iodine yields the mixed valence species [PtI(dien)][PtI3(dien)]I2,543 but when 4Me-dien is used, the mixed valence compound is only obtained if it is trapped quickly, before isomerization of the meridional platinum(IV) complex to a facial form.544 Although these species nominally have localized valences, the extent of delocalization increases as the bridging halide... 

In addition to complexes of the type, trans-[Tc02(pyr)2]+ (pyr pyridine or imidazole), various species, such as Jrans-[TcO(RO)X2(pyr)2] (R CH3 or CH3CH2 X Cl or Br) were detected in alcohol. Further complicated mixed-valence species, [X2(pyr)3Tc-0-Tc(pyr)2X3] and [X(pyr)4TcO Tc(pyr)X4], appeared on long standing or heating in pyridine [44,45]. Rather peculiar features were found in the substitution reaction of trans-[Tc02(py)4] + with 4-aminopyridine (apy) in mixtures of methanol and toluene in the presence of excess pyridine ([py] — 0.14 M) [46]. Its rate was expressed as... 

The 2,2 -bipyrazine (bpz)-substituted RU3 cluster monomer 42, dimer 43, and trimer 44 could be accessed by reaction of triruthenium precursor 2 with different amounts of 2,2/-bipyrazine [30]. The trimeric species 44 containing two parent Ru30(0Ac)6(py)2 n m and one derivate Ru30(0Ac)5(py)2II m m units could be directly prepared by reaction of 3.8 equivalents of 2 with 2,2/-bipyrizine. It is also accessible by reaction of dimeric species 43 with 1.8 equivalents of 2. The bpz adopts ri1 (N),(x-r 1 (N),r(1 (N) and p,4-r 1(N),r 1(N),r 1(C),r 2(N,N) bonding modes in 42, 43, and 44, respectively. Reduction of 3+ trimer 44 by addition of aqueous hydrazine allowed isolation of neutral intracluster mixed-valence species 44b containing three Ru 11,111,11 units. Oxidation of 44b with two... 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

One of the most important redox reactions of this class of compounds is the oxidation of Pt(2.0+)2 by 02, for this is the main cause of the appearance of the blue, purple, or dark red colors of the mixed-valence species. Although no detailed examination has been performed, the kinetics of the 02 oxidation of I Pt,1 (NHo)4(a-pyrrolido-nato)2]2+ into I Pt1 VNH3)4(a-pyrrolidonato)2(H20)2 I41 (Eq. (7)) was spec-trophotometrically examined (117). The study showed that the reaction proceeds over several days at room temperature. The first-order rate constants in acidic media were in the range of kobs = 4.2 x KT6 s 1 (pH = 0.23) - 1.13 x lO6 s"1 (pH = 2.1) (at 25°C, in air, I = 1.5 M) (117). 

On the other hand, the electrochemistry of the mixed-valence species cannot be exactly explained. The difficulty lies in that a pure... 

The mineralized core of ferritin can be modeled by mixed valence species such as [Fe 4-Fe g02(0Me)i8(02CMe)6]-4.67MeCN, whose 3D close-packed layer structure mimics ferritin. This compound can be prepared by oxidizing a methanolic solution of iron(II) acetate and lithium methoxide with a slow stream of dioxygen it can be reduced to give [Fe 2Fe o02-(0Me)i8(02CMe)g]2-.i ... 

It Is postulated that mixed-valence species or complex salts (12) formed as a result of this field Induced redox reaction control the semiconducting behavior of these films and these complex salts exist In a solid-state equilibrium with the simple 1 1 salt. Since non-integral oxidation states are common In solids, It Is difficult to predict exact stoichiometry In the equilibrium equation, but a likely equation for switching In Cu-TCNQ, for example, may Involve... 

In most of the reactions studied, the appearance of biphasic kinetics showed that the full oxidation process is achieved through the stepwise oxidation of one of the metal centers, rendering a mixed-valence species which is further oxidized to the final product. The question arises in Eq. (35) as to the alternative first oxidation of iron or ruthenium. The stoichiometry of the first oxidation step with bridging 4- and 3-cyanopyridines (with the N-atom of py binding to... 

Silicon-based dendrimers 8 and 9 (Fc = ferrocenyl) also showed oxidative precipitation onto electrodes to give idealized electrochemistry as films.181 Specifically, the peak current was linear with scan rate and the potential difference between the anodic and cathodic waves was small (AE = 10 mV at a scan rate of 100 mV/s).182 This latter observation indicated that the rate of electron transfer was rapid. For molecule 9, the surface coverage was measured as = 2 x 10 10 mol/cm2. These molecules were also explored as mediators in amperometric biosensors.183 In contrast, molecule 10 showed two redox peaks, indicative of interaction between the two ferrocenyl units at each peripheral site. 181 When oxidation of one of the two interacting redox units results in some electron sharing between the two units (Robin-Day class II mixed valence species), the second oxidation is naturally... 

One of the key issues for the further exploitation and development of the area is the choice of the electrode material and the anchoring of the mixed-valence species. It is apparent from the above examples that the choice of the metal periphery and even the substituents on those ligands have material effects on the... 

Figure 13 Binuclear and trinuclear mixed-valence species on 2 films.

The structure of the few indium(II) compounds identified by early workers is one of the classical problems in group III chemistry, (cf. gallium, Section 25,3.3.3). It is now clear that both M—M-bonded compounds and ionic mixed valence species exist, but the factors which govern the relative stability of these species still remain to be elucidated. 

Mixed-valence species containing V11 and Viv bridged by an oxygen atom were intermediate in electron transfer between [V(Hedta)]- and [VO(Hedta)], and [V(edta)]2 and [VO(edta)]2-.815 Similarly bridged species were assumed in the reaction of [V(H20)6]2+ with [V0(H20)5]2+. Vanadium(III) dimeric complexes are the reaction products (see Section 33.4.9.3). 

In the case of two equivalent non-interacting metal ions, undergoing successive one-electron reductions, AE should be 35.6 mV, i.e. Kc = 4. Values of Kc > 4 are thus a reflection of the stabilization of mixed valence species. 

The study of mixed-valence species with two identical metal centres linked by a bridging ligand, e-g. 

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