Divalent fluorides

PZG is characterized by a high lead content, which results in a high refractive index for that glass. ZnSB, with 80-90% Zn—Sr—Ba, is an example of divalent fluoride glass. CNBZn and CNBK are chlorofluoride glasses, which contain 22 and 33 mol% CdCl2 in addition to divalent and monovalent fluorides. 

Except for BeF2, glasses based on divalent fluorides show a broader IR transparency than glasses based on indium or zirconium, as reported for ZnSB and ZnSBC in Fig. 1 [10]. They exhibit a full transparency up to 9 /an for the former and 10 nm for the latter which is a pure MF2 fluoride glass containing Zn, Sr, Ba, and Cd. 

Divalent noble-gas compounds are undoubtedly the most intensively studied species in noble-gas chemistry and the divalent fluorides, NgF2 (Ng = Kr, Xe or Rn), is the... 

Te2Fio, and oxide fluorides, e.g. TeFjOTeFs, are also formed during the fluor-ination of tellurium oxides, tellurium, organic derivatives Tellurium forms organic derivatives in the +2 and +4 slates. The +2 compounds are similar to divalent sulphur derivatives although less stable. Tellurium(IV) derivatives are comparatively unstable. 

Putile Ceramic Pigments. StmcturaHy, aH mtile pigments are derived from the most stable titanium dioxide stmcture, ie, mtile. The crystal stmcture of mtile is very common for AX2-type compounds such as the oxides of four valent metals, eg, Ti, V, Nb, Mo, W, Mn, Ru, Ge, Sn, Pb, and Te as weH as haHdes of divalent elements, eg, fluorides of Mg, Mn, Fe, Co, Ni, and Zn. 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

These rod-shaped ligands share a sterically efficient terminal N-donor and their divalent Co chemistry is well established. They will be discussed here only with selected examples. [Co (NCMe)6](TFPB)2 (TFPB- = tetrakis(3,5-bis(trifhioromethyl)phenyl)borate)) has been synthesized and characterized in the solid state along with a number of other divalent transition metal analogs.357 As a result of the extremely poor coordinating ability of the anion and facile loss of MeCN ligands from the cation, the salt is an excellent source of naked Co2+ ions. Thermolysis up to 100 °C leads to the loss of one MeCN and formation of a r -bound nitrile, whereas above 130 °C decomposition occurs with loss of MeCN and abstraction of fluoride from the anion to form CoF2. 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

The characteristic feature of the divalent tin compounds is that they have both a vacant orbital and a lone pair of electrons (Figure 2.5). In this glycosylation reaction, it is assumed that SnCl2 behaves as a Lewis acid, where the vacant orbital accepts one of the three lone pairs in the fluorine atom of the glycosyl fluoride. As a result of this interaction, the C—F bond cleaves to give the oxocarbenium ion intermediate that is then attacked by an alcohol to give the glycoside. 

Figure 2.5 Activation of the C—F bond of glycosyl fluoride by the divalent tin species, giving rise to oxocarbenium ion intermediate.

The three-dimensional structure of insulin remained recalcitrant in spite of the knowledge of its primary sequence. The early crystals had been found by Scott (1936) to contain zinc which could be replaced by other divalent metals. The zinc atom is not heavy enough to be unambiguously distinguishable. Eventually it proved possible to introduce uranyl acetate and uranyl fluoride into the insulin molecule and to obtain the three-dimensional structure, first at 2.8 A resolution and then at 1.9 A (see Blundell, Dodson, Hodgkin, and Mercola, 1972). 

With the exception of the alkaline hydroxides NaOH, KOH and possibly the fluorides KF, NaF, the direction of the e.m.p. indicates that all monovalent anions are repelled from the free surface of waters less than the cations Li , Na , K ,Cs . Divalent ions show on the other hand only slight differences. As has already been noted the fact that the surface tension of water is raised by the addition of salts indicates that repulsion of the salt as a whole from the surface must take place. 

We must, however, ensure that there is no contaminant such as free Fe3+ in the solution before the potassium iodide is added otherwise, more I- will be oxidized than there is Cu2+ in the sample (reaction 15.12). The effective concentration of Fe3+ can be reduced to negligible levels by adding sodium fluoride to complex it. (The divalent copper ion is little affected.)... 

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