Complexes of the Alkaline-Earth Metals

Some typical complexes of beryllium have already been considered, and it was seen that complex formation is favored if the complexing atoms become incorporated into a cyclic structure. Indeed, in the case of the heavier alkaline-earth metals (whose radii are far greater and 

Chlorophyll (Fig. 6-3) and the calcium complex of yersene (Fig. 6-2) are, like the complexes of beryllium shown on page 110, chelate structures. 

Sidgwick, N. V. The Chemical Elements and Their Compounds, 59-102, 193 -261, Oxford University Press, London (1950). 

Moeller, T. Inorganic Chemistry— An Advanced Textbook 18-824, 848-852, John Wiley and Sons, Inc., New York 0952). 

Kennedy, J. J. The Alkali Metal Cesium and Some of Its Saits. Chem. Reviews. 23, 159 (1938), 

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The complexation of the alkaline earth metals is reminiscent of the behaviour of several of the naturally occurring antibiotics and, like the latter, the crown often exhibits remarkable selectivity for particular ions. The thermodynamic factors underlying the selectivity of many of the crowns have been studied in some depth and the results related to such parameters as cavity size, number of donor atoms present, possible ring conformations on complex formation and the solvation energies of the various species involved. 

Ate complexes of the alkaline earth metals are most prominent for magnesium, and to a significantly lesser extent for the heavier metals. The compounds are often observed in the presence of encapsulating donors such as crown ethers or cryptates a representative example is shown in Figure 28. ... 

Matthias and Warhurst 34) report that the preparation of organic complexes of the alkaline earth metals Mg and Ca succeeded only with the aid of the amalgams of the metals. This may be caused by a lower ionization potential of the metal alloy as compared with that of its components. 

Michel O, Dietrich HM, Lidabn R, Tomroos KW, Maichle-Mossmer C, Anwander R. Tris(pyrazolyl)borate complexes of the alkaline-earth metals alkylaluminate precursors and Schlenk-type rearrangements. Organometallics. 2012 31 3119-3127. 

Sawada, K. Miyagawa, T. Sakaguchi, T. Doi, K. "Structure and thermodynamic properties of aminopolyphosphonate complexes of the alkaline-earth metal ions", J. Chem. Soc. Dalton Trans. 1993, 3777-3784. 

Atienza et al. [657] reviewed the applications of flow injection analysis coupled to spectrophotometry in the analysis of seawater. The method is based on the differing reaction rates of the metal complexes with 1,2-diaminocycl-ohexane-N, N, N, A/Metra-acetate at 25 °C. A slight excess of EDTA is added to the sample solution, the pH is adjusted to ensure complete formation of the complexes, and a large excess of 0.3 mM to 6 mM-Pb2+ in 0.5 M sodium acetate is then added. The rate of appearance of the Pbn-EDTA complex is followed spectrophotometrically, 3 to 6 stopped-flow reactions being run in succession. Because each of the alkaline-earth-metal complexes reacts at a different rate, variations of the time-scan indicates which ions are present. 

Even though the hydrogen halides HC1, HBr, and HI are strong acids, HF is not a strong acid, as explained in Box 6-3. For most purposes, the hydroxides of the alkaline earth metals (Mg2+, Ca2+, Sr2+, and Ba2+) can be considered to be strong bases, although they are far less soluble than alkali metal hydroxides and have some tendency to form MOH1 complexes (Table 6-3). 

Adducts prepared in aqueous media generally possess one or more molecules of water of hydration per molecule, the number being a function of cation, anion, and the combining ratio of carbohydrate to salt. Available data on complexes of simple carbohydrates indicate that three molecules of water per molecule may be the maximum for adducts of alkali metal salts as many as seven have been reported for those of the alkaline-earth metal salts. Most complexes, however, possess only one or two molecules per molecule. Generally, the higher the combining ratio, the smaller is the number of water molecules that can be accommodated by a molecule of the adduct. 

The dynamics of the alkaline earth metal reactions with alkali halides appear to closely resemble the exchange reactions of alkali atoms with alkali halides [208, 216, 296] for which no direct energy disposal measurements have been reported. They proceed through a long-lived collision complex which is identified with a well in the reaction potential-energy surface. 

Alkaline Earths. 1.0 M stock solutions of the alkaline earth metal chlorides were prepared by dissolving the chloride salt in D2O (99.9% D). Standardization was either by direct EDTA titration (for CaCl2) or back titration of the metal-EDTA complex with standard M (C104)2 using Eriochrome Black T indicator. The solutions were buffered to pH 10 with NH3/NH4CI and the determinations were done in triplicate. 

Studies have continued on the intercalation of the alkaline-earth metals into lattices of the transition-metal sulphides. Calcium and strontium are intercalated into M0S2 from liquid ammonia solution. X-Ray data reveal a lowering of the crystal symmetry of the sulphide and an increase in the complexity of the structure on intercalation. The intercalation compounds begin to superconduct at ca 4 K (for Ca) and 5.6 K (for Sr), and they show considerable anisotropy with respect to the critical magnetic field. Calcium in liquid ammonia also intercalates with TiS2- Two Ca,TiS2 phases have been identified. The limits of the first are 0.03 0.50, for which a relationship between x and cell parameters a and c... 

Many salts of the alkaline earth metals are insoluble and this feature is enhanced in the case of barium. Barium is less electronegative than calcium and it is less likely to form cationic complexes. One radioisotope exists, i.e., °Ba with a half-life of 12.8 days. 

The order can be explained by the decrease in metal-ion radius fix m Mn to Zn, and by the increase of crystal-field stabilization energy fi om Fe to Cu. The d orbitals of Zn (II) are full, so no stabilization energy is gained through the complex. That is the main reason that Zn (II) AcAc behaved similarly to that of the alkaline earth metal ions and the lanthanide and actinide ions, initiating curing at low temperature but giving poor mechanical properties after cure. 

J. Na, J. Zhou, and X. Wen. Polarography of the alkaline-earth metals—II the adsorption wave for the magnesium-eriochrome black T complex. Talanta 32 479-487, 1985. 

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