Solids divalent halides

The dihalides of Si and Ge are polymeric solids that are relatively unimportant compared to those of Sn and Pb. The latter elements are metallic in character and have well-defined +2 oxidation states. Physical data for the divalent halides are shown in Table 11.1. The compounds of Si(II) are relatively unstable because the reaction... 

Starting from the corresponding hydroxymethyl-benzocrown, it has been possible to generate the immobilized system (186) by reacting the above precursor with chloromethylated polystyrene (which is available commercially as Merrifield s resin). Typically, systems of this type contain a polystyrene matrix which has been cross-linked with approximately 1-4% p-divinylbenzene. In one study involving (186), a clean resolution of the alkali metal halides was achieved by HPLC using (186) as the solid phase and methanol as eluent (Blasius etal., 1980). In other studies, the divalent alkaline earths were also separated. 

Brown, I. D. and Duhlev, R. (1991). Divalent metal halide double salts in equilibrium with their aqueous solutions II. Factors determining their crystal structures. J. Solid State Chem. 95, 51-63. 

The chemistry of phases with intermediate compositions REX (2.0 < n < 3.0), which are complex mixed valent RE(II)/RE(III) halides, reminds on the AX2/REX3 systems (A = Ca, Sr, Ba). In fluorides REF (n 2.0-2.2) divalent ions and trivalent ions occnpy the sites within a flnorite-type variant, in which, for charge compensation, interstitieUhahdes are incorporated into the primitive anion snblattice and solid solutions REX2/REX3 are observed. For higher n, the anions form clusters and line phases are formed which crystallize as so-called anion ordered excess flnorite-type variants or in other complicated structure types. 

Preparation and Identification of Divalent Lanthanide Ions as Dilute Solutes in Alkaline Earth Halide Solid Solutions... 

Membranes prepared from cast pellets of silver halides have been used successfully in electrodes for the selective determination of chloride, bromide, and iodide ions. In addition, an electrode based on a polycrystalline Ag2S membrane is offered by one manufacturer for the determination of sulfide ion. In both types of membranes, silver ions are sufficiently mobile to conduct electricity through the solid medium. Mixtures of PbS, CdS, and CuS with Ag S provide membranes that are selective for Pb-, Cd-+, and Cu-", respectively. Silver ion must be present in these membranes to conduct electricity because divalent ions are immobile in crystals. The potential that develops across crystalline solid-state electrodes is de.scribed by a relationship similar to Equation 21-10. 

Copper occurs in soil solids and solutions almost exclusively as the divalent cation Cu ". However, reduction of Cu " (cupric) to Cu (cuprous) and Cu (metallic copper) is possible under reducing conditions, especially if halide or sulfide ions ( soft bases) are present to stabilize Cu" (a soft acid). Copper is classified as a chalcophile, owing to its tendency to associate with sulfide in the very insoluble minerals, CU2S and CuS. In reduced soils, then, copper has very low mobility. Most of the colloidal material of soils (oxides of Mn, Al, and Fe, silicate clays, and humus) adsorb strongly, and increasingly so as the pH is raised. For soils with high Cu accumula-... 

A variety of halide and haloamine complexes have been prepared which show evidence for one-dimensional structures in the solid state. The materials can be broken down into (1) complexes containing metals in the same oxidation state and (2) complexes comprised of metals in different oxidation states. Type (1) complexes may be dianions and dications, for example Magnus Green Salts or alternatively chains of neutral molecules, for example, Pt(en)Cl2. Type (2) complexes are comprised of alternating square planar rf metal complexes (metal = Pdii,Pt ,Aui i) and octahedral / metal complexes (metal = Pd, Pt ), or linear platinum complexes are formally trivalent while the gold complexes are formally divalent. 

At this point let us briefly consider the formation of associates. The formation of associates between cation vacancies and divalent impurities in alkali halides has already been given as an example. Such reactions are homogeneous solid state reactions, and so the relaxation time for the formation of associates can be calculated in a completely analogous manner to the calculation of the relaxation time for the equilibration of Frenkel defects. The result of such calculations is precisely the same as the result given in eq. (6-5). It is only necessary, in the case of association, to replace the concentration c (eq) = in the denominator by the nearly constant concentration of the corresponding majority defect. In general, in the case of the formation of defect associates, we can conclude that the equilibrium concentration is attained rapidly compared to the time required by defect reactions which occur at sites of repeatable growth. 

The discussion so far has concentrated on the structural chemistry of the electron rich clusters of the [MgXg] type. To prepare for the next paragraph, interest is now focussed on metals with smaller numbers of valence electrons which then prefer to form [MgX j type clusters. A tremendous amount of work has been performed on [MgX,2] type halide clusters of the metals Nb and Th both in the solid state and in solution. Ousters with 14, 15, and 16 electrons in M-M bonding states are well known. Examples of compounds with this cluster type but formed with divalent anions had been very scarce, being represented by only two which contained the [NbgOi2] cluster. [57, 58] Recently, a rather broad chemistry of reduced oxoniobates based on this cluster has emerged. [16]... 

All divalent, trivalent, and tetravalent chlorides, bromides, and iodides are hygroscopic and an appreciable solution chemistry has been characterized, but only aspects of solution chemistry that relate to hydrates are considered here. The solid-state chemistry of the lower oxidation states (to IV) of cations in combination with the F, Cl, Br, and I atoms, and combinations of these with main-group cations is considered. The numerous reactions some of these halides undergo with organic reagents and solvents are considered only to the extent they relate to solid-state chemistry. 

Homogeneous Catalysis Lanthanide Halides Organometallic Chemistry Fundamental Properties Tetravalent Chemisiry Inoiganic Tetravalent Chemistry Organometallic The Divalent State in Solid Rare Earth Metal Halides The Electronic Structure of the Lanthanides. 

The enthalpy of vaporization of the metals refers to the process Ln s) — LU(g). This has an influence in the stability of oxidation states of the lanthanides (see Variable Valency, The Divalent State in Solid Rare Earth Metal Halides, and Tetravalent Chemistry Inorganic) and the variation of AHvap across the series is shown in Figure 3. 

The Divalent State in Solid Rare Earth Metal Halides... 

In order to fully understand the crystal chemistry of the anhydrous LnXs and their solvates ([LnXj(solv) ]), the Ln atomic properties of these species must be considered. The predominant oxidation state for LnX species is the +3 state however, for a number of these cations, tiie +2 (see The Divalent State in Solid Rare Earth Metal Halides) and +4 (see Tetravalent Chemistry Inorganic) states are available. Since the bonding in these compounds is mainly ionic, the cation size and sterics of the binding solvent play a significant role in determining the final crystal structures isolated. The ionic nature of the LnX complexes makes... 

The pattern as seen in Figure 5 may be transferred to a periodic table of the rare earth elements, see Figure 6. Only elements underlaid in red form clusters. The lower I3 is, the easier it is to produce cluster complexes. Elements underlaid in blue form stable divalent compounds, for example EUCI2 the divalent state with the electronic configuration 4f 5d° (with n =7, 14, 6, 13 for R = Eu, Yb, Sm, Tm) has the highest stability and, thus, is the easiest to achieve when the third ionization potential is the highest. The divalent chemistry of these elements is alkaline-earth and saltlike this is described in The Divalent State in Solid Rare Earth Metal Halides. 

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