Mixed valences

Walker G 0, Barbara P F, Doom S K, Dong Y and Hupp J T 1991 Ultrafast measurements on direct photoinduced electron transfer in a mixed-valence complex J. Rhys. Chem. 95 5712-15... 

Tominaga K, Kliner D A V, Johnson A E, Levinger N E and Barbara P F 1993 Femtosecond experiments and absolute rate calculations on intervalence electron transfer of mixed-valence compounds J. Chem. Rhys. 98 1228—43... 

Beratan D N and Hopfield J J 1984 Calculation of electron tunneling matrix elements in rigid systems mixed valence dithiaspirocyclobutane molecules J. Am. Chem. Soc. 106 1584-94... 

Elliott C M, Derr D L, Matyushov D V and Newton M D 1998 Direct experimental comparison of the theories of thermal and optical electron-transfer studies of a mixed-valence dinuclear iron polypyridyl complex J. Am. Chem. 

A mixed valency pale yeHow crystalline iron pentafluoride heptahydrate, FeF 7H20, is prepared by dissolving iron powder in 40% HF in the presence of air (16). No appHcations have been reported for this material. 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Hydroxide. Freshly precipitated cerous hydroxide [15785-09-8] Ce(OH)2, is readily oxidized by air or oxygenated water, through poorly defined violet-tinged mixed valence intermediates, to the tetravalent buff colored ceric hydroxide [12014-56-17, Ce(OH)4. The precipitate, which can prove difficult to filter, is amorphous and on drying converts to hydrated ceric oxide, Ce02 2H20. This commercial material, cerium hydrate [23322-64-7] behaves essentially as a reactive cerium oxide. 

Cobalt metal is significantly less reactive than iron and exhibits limited reactivity with molecular oxygen in air at room temperature. Upon heating, the black, mixed valence cobalt oxide [1308-06-17, Co O, forms at temperatures above 900°C the oHve green simple cobalt(II) oxide [1307-96-6] CoO, is obtained. Cobalt metal reacts with carbon dioxide at temperatures greater than 700°C to give cobalt(II) oxide and carbon monoxide. 

Cobalt(II) chloride hexahydrate [7791-13-1], C0CI2 6H20 is a deep red monoclinic crystalline material that deflquesces. It is prepared by reaction of hydrochloric acid with the metal, simple oxide, mixed valence oxides, carbonate, or hydroxide. A high purity cobalt chloride has also been prepared electrolyticaHy (4). The chloride is very soluble in water and alcohols. The dehydration of the hexahydrate occurs stepwise ... 

Cobalt(II) nitrate hexahydrate [10026-22-9], Co(N02)2 6H20, is a dark reddish to reddish brown, monoclinic crystalline material containing about 20% cobalt. It has a high solubiUty in water and solutions containing 14 or 15% cobalt are commonly used in commerce. Cobalt nitrate can be prepared by dissolution of the simple oxide or carbonate in nitric acid, but more often it is produced by direct oxidation of the metal with nitric acid. Dissolution of cobalt(III) and mixed valence oxides in nitric acid occurs in the presence of formic acid (5). The ttihydrate forms at 55°C from a melt of the hexahydrate. The nitrate is used in electronics as an additive in nickel—ca dmium batteries (qv), in ceramics (qv), and in the production of vitamin B 2 [68-19-9] (see Vitamins, VITAMIN B22)-... 

Cobalt(Il) dicobalt(Ill) tetroxide [1308-06-17, Co O, is a black cubic crystalline material containing about 72% cobalt. It is prepared by oxidation of cobalt metal at temperatures below 900°C or by pyrolysis in air of cobalt salts, usually the nitrate or chloride. The mixed valence oxide is insoluble in water and organic solvents and only partially soluble in mineral acids. Complete solubiUty can be effected by dissolution in acids under reducing conditions. It is used in enamels, semiconductors, and grinding wheels. Both oxides adsorb molecular oxygen at room temperatures. 

Figure 16.15 Structures of some mixed-valence bromopolyselenate(II,IV) anions.

Tetrazole with [(T) -C3Hj)2Rh(acac)] gives the trinuclear complex 191 (86JCS(D)2193). This product with [Rh( j.-Cl)(CO)2]2 produces the mixed-valence trinuclear species 192. 

The mixed-valence ion has an intervalence charge transfer band at 1562nm not present in the spectra of the +4 and +6 ions. Similar ions have been isolated with other bridging ligands, the choice of which has a big effect on the position and intensity of the charge-transfer band (e.g. L = bipy, 830 nm). 

In general, the dimers have three chlorine bridges, and Ru3C18(PBu3)4 resembles the mixed-valence chloro complex Ru3Cl 2. A similar, but less extensively studied, pattern of behaviour has been found with other alkyl phosphines. 

---