Alkaline earth metal complexes ionic radii

By studying a series of complexes, it is possible to observe the differences in structural type that occur with change of cation radius. Table 6 shows the ionic radii for the alkali and alkaline earth metal cations, together with the average ligand cavity radii for simple polyethers.33 From this information it can be seen that the predicted optimal fit situation for 1 1 complexes would arise for Li+ and 12-crown-4 (74) for Na+ and 15-crown-5 (75) and for K+ and Ba2+ and 18-crown-6 (76). For 24-crown-8 (77) all of the cations have smaller radii than that of the ligating cavity. 

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor... 

When Z is a simple aquacation, two types of complex are formed depending upon the ionic radius of Z. For alkali, alkaline earth and most transition metal cations the product contains Z"+ in quasi-octahedral coordination. Equilibrium constants for reaction (6) have been determined for Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni, Cu2+ and Zn2+.93 For the transition metals, log K lies between 3 and 9, and is sensitive both to Z and to the lacunary polyanion involved. Larger cations, Sr24, Ba2+, and tri- and tetra-valent lanthanides and actinides are also able to bind two lacunary ligands in a manner similar to that illustrated in Figure 18. Although the stepwise formation of 1 1 and 2 1 complexes of the... 

Fig. 13. Plot of 2 versus radius, r (in pm)/ionic charge, for the complexation of (29) with a series of alkaline earth and lanthanide metal cations.

Spectra have also been reported for alkaline earth complexes of bipyridyl, terpyridyl, and substituted bipyridyls and phenanthrolines (106,107) of the type M +(L )2. For Be, Mg, Ca, and Sr the spectra show the presence of a ground, orvery lowexcited, triplet state consistent with a divalent metal cation in a tetrahedral environment with one electron on each chelate ligand. The similarity in splittings for Be and Mg is thought to show that interligand interactions prevent the achievement of the small Be + ionic radius. Treatment of bipyridyl with zinc amalgam does... 

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