Mixed or complex oxides

Although the dioxides, MO2, are notable for their inertness, particularly if they have been heated, fusion or firing at high temperatures (sometimes up to 2500°C) with the stoichiometric 

When is approximately the same size as Ti (i.e. M = Mg, Mn, Fe, Co, Ni) the stmcture is that of ilmenite, FeTi03, which consists of hep oxygens with one-third of the octahedral interstices occupied by and another third by Ti This is essentially the same structure as corundum (AI2O3, p. 243) except that in that case there is only one type of cation which occupies two-thirds of the octahedral sites. 

however, is significantly larger than Ti (e.g. M = Ca, Sr, Ba), then the preferred structure is that of perovskite,Ca Ii03. This 

M2Ti04 (M = Mg, Zn, Mn, Fe, Co) have the spinel stmcture (MgAl204, p. 248) which is the third important stmcture type adopted by many mixed metal oxides in this the cations occupy both octahedral and tetrahedral sites in a cep array of oxide ions. Ba2Ti04, although having the same stoichiometry, is unique amongst titanates in that 

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

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Chemical reactions between solids are used to synthesize mixed powders or complex oxides. The reactants are usually simple oxides, carbonates, nitrates, sulfates, oxalates or acetates, which are mixed according to a target compound with a given stoichiometric composition. An example is the reaction between magnesia and alumina to form magnesium aluminate or spinel, with the following reaction equation ... 

Despite the pancity of well-characterized gold(0) compounds, there are numerous cluster complexes that contain gold in a mixed or fractional oxidation state that lies between Au(0) and Au(I). These clnsters are described in a later section on clusters and chains with fractional oxidation states. 

Most mixed and complex ammonium metal sulphates (and selenates) [948,949] lose NH3, H20 and S03 (or Se03) to form the simple metal sulphate (or selenate) some of the ammonia may be oxidized [949]. The basic aluminium ammonium sulphate [950], (NH4)20 3 A1203 4 S03 xH20 (x = 6—8), loses water at 473 K. Deammination and complete dehydration commences at >673 K, and S03 evolution starts at about 873 K to yield residual A1203 which contains traces of S03. a—Time data for most of the stages obeyed the contracting volume equation [eqn. (7), n = 3] [951]. 

Figure 1.37 Synthesis ofthe mixed-valence or completely oxidized complexes [p-C(OMe)=N(Me)Au]3l (n = 2-6).

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

For the purposes of molecular electronics, the formation of solid-state multiple linear stacks, causing electron delocalization, favours electrical conductivity. Further, (partial) oxidation of such linear materials can produce non-stoichiometric (or mixed-valent) complexes like K175Pt(CN)4-1.5H20,9 and K1.62Pt(C204)2-2H20,8b which possess higher conductivity than their precursors. 

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