Coordination number barium

Fig. 11.11, p. 112). Incommensurate structures related to bismuth-III are also observed for strontium and barium. Magnesium, calcium and strontium are remarkable in that they transform from the normal closest-packing of spheres to a body-centered packing upon exertion of pressure. Even more remarkable is the following decrease of the coordination number to 6 for calcium and strontium (Ca-III, a-Po type Sr-III, /3-tin type). 

These have coordination numbers at barium that range from three to eleven. Monomeric barium amido complexes can be obtained if sterically demanding amido ligands and neutral donor co-ligands are employed as in [Ba N(SiMe3)2 2(thf)2] (Fig. 3.10). 

The number of donor atoms can be influential in complex formation. Ideally, as the incoming ligand is comparable to an inner solvation sphere, the number of donor atoms should match the preferred coordination number of the cation. An example of this factor comes from a comparison of two similarly sized ligands, [2.2.2]cryptand and [2.2.C8]cryptand, the latter having one dioxoether chain replaced by a C8 unit.478 A reversal of the Ba2+/K+ selectivity of the order of 106 is found, the ratio being 104 for [2.2.2]cryptand and <10 2 for [2.2.C8]cryptand the preferred coordination numbers are six for potassium and eight for barium. 

The structural basis for the instability may be found in the inability of Ba " " to adjust to low coordination numbers. Barium ions in naturally occurring minerals and oxides have coordination numbers ranging from 6 to 10 (21). Some Baz+ ions in hydrated Bag-A have a coordination number of 6, which, upon dehydration, would be reduced to 3, the partially Baz+-exchanged structures indicate, if dehydrated Bag-A were to form. 

Moreover, pore and channel size and shape can be tuned by using different alkaline-earth cations (Mg2+, Ca2+, Sr2+, Ba2+) having different sizes and coordination numbers. The heat of water adsorption ranges from —46 to —52 kj mol-1, values similar to those of silica-containing zeolites. As recently reported, the properties of these materials can be further differentiated through the incorporation of transition (i.e. Co2+) or alkaline cations (i.e. K+) into the channels of barium-linked materials composed of metal-assembled cages. 

The calcium, strontium, barium, and lead 80) complexes of 160 and 161 have also been reported. In these two ligands the six donor atoms are essentially confined in a plane these complexes thus permit study of unusual coordination geometries in species of high coordination number. Attempts to form alkali metal complexes with 160 and 161 under the same conditions as employed for the alkaline earth metal complexes have failed. The successful syntheses of complexes of the latter type indicate that the higher charge to radius ratio is of consequence when spherically charged cations are employed. Such metal ions have no apparent coordinative discrimination as the template ion 87). 

Pr)2Ba(/i-0-i-Pr)2]2. This observation is intriguing in heterometallic alkoxide chemistry. The analytical as well as other physicochemical data could not have distinguished between the expected and the rather unusual actual formulation, which was established by X-ray crystallography (Fig. 37). This structure is consistent with the preferential characteristics of the two metals involved (barium and cadmium) for attaining coordination numbers 6 and 4, respectively. 

A Owing to the systematic removal of oxygen from the lattice, yttrium and barium have different coordination numbers. For yttrium the coordination number is eight, but for barium it is ten. 

No significant modification of the crystal cell structure can be attributed to the substitution of barium (ionic radius 175 pm, coordination number XII) by K that has the same coordination number and a very close ionic radius (178 pm). 

Hydroxo salt is the term used for a group of complexes where the central atom of the complex anion is a metal to which hydroxyl ions are bound as ligands. The niunber of these ions depends on the normal coordination number of the metal. The cation of a hydroxo salt is usually an alkali metal, particularly sodium, or the alkaline earth metals barium, strontium and, in some cases, calciiun. Heavy-metal salts can be prepared from a few hydroxo anions via a double decomposition reaction. 

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