Variable cation stoichiometry

In an attempt to better understand the mechanism for encapsulation of cobaltocenium inside 46 we performed careful titrations of this redox-active cation with variable concentrations of host 4. To our surprise, we quickly realized that only 2-3 equivalents were necessary to fully shut down the electrochemical response of cobaltocenium.51 This result is in strong contrast to the NMR data, which clearly indicate a 6 1 [host/guest] stoichiometry for full encapsulation. There are two important differences between these two types of experiments. In the NMR experiments, solutions were prepared in pure, deuterated CD2CI2 and the only solutes present are cobaltocenium hexafluorophosphate (ca. 1 mM) and host 4 (0-8 mM). In the electrochemical experiments, solutions were prepared in isotopically unenriched CH2CI2 also containing 0.1 M tetradodecylammonium bromide as supporting electrolyte. The concentrations of cobaltocenium hexafluorophosphate and host 4 were similar to those used in the NMR experiments. It clearly became evident that the nature of the supporting electrolyte, especially the nature of its anion, was crucial to... 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

By non-stoichiometry it is implied that the metal to non-metal atom ratio is not exactly that given by the chemical formula, even though the compound is electrically neutral. These can only be reconciled by assuming that either the anion or the cation exhibits variable valency on its sub-lattice. It is much more likely that the metal or... 

It was revealed that the introduction of cations with variable valence (In, Cu) leads to a specific effect caused by the presence of NS vacancies and consisting in the recharging of cations under changing concentration of impurity atoms or deviation from stoichiometry. 

The electroneutrality condition decreases the number of independent variables in the system by one these variables correspond to components whose concentration can be varied independently. In general, however, a number of further conditions must be maintained (e.g. stoichiometry and the dissociation equilibrium condition). In addition, because of the electroneutrality condition, the contributions of the anion and cation to a number of solution properties of the electrolyte cannot be separated (e.g. electrical conductivity, diffusion coefficient and decrease in vapour pressure) without assumptions about individual particles. Consequently, mean values have been defined for a number of cases. 

Widespread medicinal use of colloidal bismuth subcitrate (CBS) has prompted extensive studies of bismuth compounds involving the citrate anion. Bismuth citrate is essentially insoluble in water, but a dramatic increase in solubility with increasing pH has been exploited as a bio-ready source of soluble bismuth, a material referred to as CBS. Formulation of these solutions is complicated by the variability of the bismuth anion stoichiometry, the presence of potassium and/ or ammonium cations, the susceptibility of bismuth to oxygenation to Bi=0, and the incorporation of water in isolated solids. Consequently, a variety of formulas are classified in the literature as CBS. Solids isolated from various, often ill-defined combinations of bismuth citrate, citric acid, potassium hydroxide, or ammonium hydroxide have been assigned formulas on the basis of elemental analysis data or by determination of water and ammonia content, but are of low significance in the absence of complementary data other than thermal analysis (163), infrared spectroscopy (163), or NMR spectroscopy (164). In this context, the Merck index lists the chemical formula of CBS as KgfNHJaBieOafOHMCeHsCbh in the 11th edition (165), but in the most recent edition provides a less precise name, tripotassium dicitrato bismuthate (166). 

To overcome this problem an extension of the sublattice model was proposed by Hillert et al. (1985) which is now known as the ionic two-sublattice model for liquids. As in the previous case it uses constituent fractions as composition variables, but it also considers that vacancies, with a charge corresponding to the charge of the cations, can be introduced on the anion sublattice so that the composition can move away from the ideal stoichiometry and approach an element with an electropositive character. The necessary neutral species of an electronegative element are added to the anion sublattice in order to allow the composition to approach a pure element. The sublattice formula for the model can then be written as... 

While high surface area and metallic conductivity are beneficial to electrocatalysis, they do not alone explain the high catalytic activity. We speculate that the variable oxygen stoichiometry of the pyrochlore lattice, and the multiple valence states of the cations, particularly the ruthenium, are essential to the catalytic activities of these pyrochlores. 

A first observation is that the volume is different in nature from the other variables. Some questions raised are Should this volume be considered at all Should it be used to compensate for dilution effects These questions are not answered here but the answer depends on the goal of the study. A second observation is that the pH is not a concentration. Should it be made into a H+ concentration or not It is easily done but whether it is appropriate is not always obvious. A third problem is typical for chemistry or rather stoichiometry. The concentrations in ppb are not in stoichiometric form. To get real chemical meaning they may have to be expressed in molarity, molality or normality values. In this way the balance between cations and anions may be studied. Again, the choice is not always easy. For dilute samples, molarity and molality are close to each other, but normality may be determined by the type of reaction under consideration. 

Complete structural characterization of a material involves not only the elemental composition for major components and a study of the crystal structure, but also the impurity content (impurities in solid solution and/or additional phases) and stoichiometry. Noncrystalline materials can display unique behavior, and noncrystalline second phases can alter properties. Both the long-range order and crystal imperfection or defects must be defined. For example, the structural details which influence properties of oxides include the impurity and dopant content, nonstoichiometry, and the oxidation states of cations and anions. These variables also influence the point-defect structure, which in turn influences chemical reactivity, and electrical, magnetic, catalytic, and optical properties. 

The pyrochlore structure is three-dimensional framework of corner-shared (Ti,Ta,Nb)06 octahedra. The A site lies within this framework and is 8-coordinated. The octahedra can articulate to allow for a fair range in sizes of the A cation, which accounts for the variable compositions of most of these compounds. Part of the structure is depicted in Fig. 5. The A coordination is a distorted cube, the size and degree of distortion depending on the amount of tipping of the B octahedra. Many pyrochlores show deviations from the/42 206(0,OH,F) stoichiometry, some of which are probably defect structures that result from charge balance effects and coupled substi-... 

Therefore, PEC act as a model material with the same local molecular structure of the complex, but have the advantage of a variable stoichiometry and known ion content. In PEC, the content of small cations and anions is known because it depends on the mixing ratio of the poly ions. Furthermore, systems with mainly one type of counterion can be prepared if excess salt is removed by dialysis. In this way, conductivity data in dependence of the composition can be related to the conductivity contribution of a single type of charge carrier [40, 41]. For this purpose, solid PEC complexes have to be prepared from complexes formed in aqueous solution. The broad composition range includes both water-soluble as well as insoluble complexes, i.e. complex coacervates. Both can be treated by drying and subsequently pressing the polymer material to form a dense solid [40]. 

---