Alkali metal complexes ionic radii

Figure 7 Stability constants (log Ks) of various podand alkali metal complexes as a function of ionic radius (a) stepwise loss of terminal groups (b) ligands with stiffened chains (c) variation of terminal group (reproduced with...

Crowns with Stilbene Fluorophores. Stilbenes undergo cis-trans isomerism on irradiation and this feature is exploited in (3.82). In the case of the frany-isomer, large ionic radius ions such as K+, and Cs, give fluorescent complexes whilst the smaller Lb and Na " cause quenching. UV irradiation to the dy-isomer causes a considerable change in the conformation of the crown ether substructure and increases its ability to extract alkali metals, including Lb and Na+, from water into benzene. 

As concerns the spatial fit of host and guest, 44 forms the most stable complex with K+ (Figure 3.1) [10], since its radius of ca. 138 pm is approximately equal to the ionic radius of the guest. The dependence of stability constants of the complexes of 47, 48 and 44 with alkali metal cations on the ion diameters is shown in Figure 3.2. The complicated character of the depicted relations indicates that more factors (e.g., solvent effect) are at play in the ions recognition. 

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... 

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. 

For the trivalent lanthanides99-100 and actinides,99 as well as for yttrium and scandium,75 the equilibrium constant for the extraction reaction has been shown to vary inversely with the ionic radius of the metal ion. It has therefore been concluded that the extracted complexes are all of the M(HA2)3 type, involving predominantly ionic metal—ligand bonds.75 The similarity of the IR spectra of the scandium(III) and thorium(IV) complexes of D2EHPA to those of the alkali metals is also indicative of the importance of ionic bonding.102... 

Alkali-earth metals (calcium, barium, and magnesium) complex with polysaccharides extensively (Reisenhofer et al., 1984). Calcium has a smaller atomic and ionic radius than does sodium and, because it has two valence electrons, it is endowed with greater polarizing and bonding ability than Na+. Ca and Ca2+ easily form insoluble complexes with oxygenated compounds. Polysaccharide salts of alkali-earth metals are generally insoluble. 

Reactions 1 and 2). These substances were initially of interest because they provided a neat inorganic study of the relative importance of the lattice energies of MX and M [AIR3X] or M [AbReX]. Thus, Lehmkuhl (13) concluded that complexation occurs most readily when the ionic radius of the alkali metal is large, the alkyl chain is short, and the complex ion is small. 

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