Cations radius

The ratio between the anionic and cationic radii leads to coordination numbers, the lowest of which is 6, which correspond to a octahedral polyhedron of anions around a central cation [135]. In this case, the compound structure type depends on the ratio of total number of anions and cations. The total number of anions (X) is calculated by summing up the number of oxygen (O) ions and of fluorine (F) ions X=0+F, while the total number of cations (Me) is the number of tantalum ions, niobium ions and other similar cations. 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

Since the repulsive forces are determined by the true sizes of ions, and not their crystal radii, the radius ratios to be used in this connection are the ratios of the univalent cation radii to univalent anion radii.12 Values of this ratio for small ions are given in Table II, together with predicted and observed coordination numbers, the agreement between which is excellent. 

At variance with the evaporated samples, Am and did not change much for the sol-gel ones, in spite of the difference between AE cation radii size (Fig. lb, c). It can be suggested that the sol-gel method succeeded in better introduction of Nd into a solid solution (supported by the TPD results) which also depended to a lower extent on the cation radii size match. The increase of the lattice anisotropy AO (Fig. Id) and the trend of the local strain values to decrease or remain about constant (Fig. lc) indicated that there was competition between disorder sources of different nature dispersed lattice defects and Nd3+ agglomerates. 

This showed that, in general, for at r atom (A) the inter-atomic distance d(AA) can be divided into two Golden sections, which give the anioiuc and cationic radii,... 

On the basis of cationic radii reported in the scientific literature (53 pm for Cooct and 40 pm for... 

But their system has recently been criticised from two directions. Firstly, Slater 3) has proposed a system of general-purpose radii, admittedly less accurate than Pauling s (roughly, constant to 0.1 A instead of 0.02 A). Secondly, a considerable literature has arisen (4) from a proposal (Gourary-Adrian) (5) for a new system of ionic radii making monovalent cation-radii about 0.2 A greater (hence anion-radii 0.2 A less) than Pauling s values. 

The discrepancies have to be attributed to differences in AGd, i.e. deformation energy to adjust the size of the preformed cavities to the cationic radii. It seems reasonable to assume a smaller cavity for monensin, because of its shorter skeleton and somewhat higher flexibility (fewer rings) as compared to nigericin. Thus, the differences in selectivity behavior of nigericin and monensin can qualitatively be accounted for. 

Cation radii used in this paper are from Pauling (29) unless otherwise specified. 

Fig. 5 Polyanions of group 14 and their dependence on averaged cation radii and degree of reduction...

Effective Cation Radii of Some Metal Ions and Their Average Observed Coordination Numbers for Different Ligands in Structures of Small Molecule ... 

Fig. 4. Radius ratios for various coordination numbers. Also shown are the types of coordination polyhedra, and cation radii that fit these criteria. Note that most metal ions of biological interest have coordination numbers of 6 or more.

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