Silicate, cation complexes

Silicates produce hard, dense, gray to grayish brown scales of variable composition. Silicates are complex materials and usually are associated with several cations, including sodium, magnesium, iron, and calcium. 

Hydroformylation. Although cationic complexes such as Rh(diene)(PPh3) are active for olefin hydroformylation, they are not suitable for intercalation in layered silicates, because the active species formed under hydroformylation conditions are electrically neutral (19, 20). Since neutral complexes have little or no affinity for the negatively charged silicate sheets, extensive desorption of rhodium occurs during the reaction and most of the observed catalytic activity occurs in the solution phase. 

Structure of Water Soluble Silicates with Complex Cations... 

The results of X-ray determinations of the crystal structures of silicates with complex cations are disciissed. Vfaterso-luble single crystals of silicates with Ni(en), Cu(en)p Co(en), UCCH,). and cations have been studied. It is shown that in all structures determined the silicate anions have the foim of double trigonal and tetragonal rings... 

Among a small group of silicates that can be obtained from aqueous solutions at room temperatuiTe and normal pressure are silicates with complex cations. Silicates with chelate complexes of transition metals were first obtained by V. Molchanov and N.Pri-khid ko at the Institute of Silicate Chemistry (jl,). Silicates with alkylammonium cations were prepared in aqueous solutions by S. Glixelli and T. Krokowski ( ) and D. Hoebbel and W, Wieker (3),... 

These considerations prompted a study of the structure of these compounds by methods of X-ray structural analysis. In recent years silicates with complex cations found also important practical application. 

It can be expected that the use of silicates with complex cations and their solutions will permit one to obtain in the nearest future some compounds of great importance for chemical technology. 

Rapid formation of soluble colored complexes when lichen compounds or ground lichen thalli were shaken with water suspensions of minerals and rocks was reported by Schatz (1963) and Syers (1969) no information was obtained, however, for the amounts of cations complexed. Of the six lichen compounds investigated by Iskandar and Syers (1972), four formed soluble, colored complexes with biotite. The extract obtained from the interactions of lecanoric acid, a depside of the orcinol type, and biotite was reddish-yellow in color. The formation of a reddish-yellow complex when lecanoric acid is allowed to react with calcium salts is used as a specific color test for this acid (Culberson, 1969). Because a complex may be colorless or adsorbed by the silicate phase, the fact that a lichen compound does not form a colored complex does not necessarily indicate that complex formation has not occurred. Iskandar and Syers (1972) showed that significant amounts of Ca, Mg, Fe, and Al were complexed by lichen compounds. In general, greater amounts of divalent than trivalent cations were complexed. For a particular cation, a similar amount was released from the silicates by solutions of the lichen compounds and by solid lichen compounds. These findings suggest that lichen compounds are sufficiently soluble in water to form soluble metal complexes. 

Carbonate, bicarbonate and silicate ions form moderately strong complexes with most metal ions. For instance, the association constants of alkaline earth cations for carbonate and bicarbonate are in the range of 10-1000 [107,108]. Similarly, silicate ions complex readily with many metal ions, e.g. with calcium (fci = 1230) and magnesium ki = 1.5 x 10 ) [108]. There seems to be no data on metal ion association with aluminate ions but it is likely that ion association is moderately strong here also. 

More complex (and more common) structures result when some of the sili-con(IV) in silicates is replaced by aluminum(III) to form the aluminosilicates. The missing positive charge is made up by extra cations. These cations account for the difference in properties between the silicate talc and the aluminosilicate mica. One form of mica is KMg (Si1AlO10)(OH)2. In this mineral, the sheets of tetrahedra are held together by extra K+ ions. Although it cleaves neatly into transparent layers when the sheets are torn apart, mica is not slippery like talc (Fig. 14.40). Sheets of mica are used for windows in furnaces. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

The viscosity and non-Newtonian flooding characteristics of the polymer solutions decrease significantly in the presence of inorganic salts, alkali silicates, and multivalent cations. The effect can be traced back to the repression of the dissociation of polyelectrolytes, to the formation of a badly dissociating polyelectrolyte metal complex, and to the separation of such a complex fi"om the polymer solution [1054]. 

---