Coordination of cations

The stmctures of STP-I and -II differ primarily in the ionic coordination of cations. In STP-II all sodium ions are octahedraHy coordinated by oxygen, whereas in STP-I some sodium ions are surrounded by only four oxygen atoms. In STP-II a distinct sheet-like arrangement occurs. The faster hydration rates are attributed to these properties. 

Fig. 5.21 Coordination of cations by (a) crown ethers and (b) bicyclic cryptands indicating that the cation is better shielded from the anion in (h).

Replacing THF with toluene allows increased coordination of cation and ester group, and thus enhances stereoselectivity. 

Many simple minerals, especially simple salts like halite, NaCl, sulfides, sulfosalts and oxides, have structures based upon cubic or hexagonal closest-packed arrays of either cations or anions. Coordination geometries of metal ions in many of these kinds of minerals are thus confined to more or less regular octahedra and tetrahedra. The occupancy of the two types of sites is dictated by the stoichiometry of the mineral, the radius of the ions involved and their preferred coordination geometries. Coordination of cations in mineral species in terms of bonding and crystal field effects has been extensively reviewed.16-21 Comprehensive lists of ionic radii relevant to cation coordination geometries in minerals have also been compiled.16,21... 

G-quadruplex formation absolutely requires the participation of cations. Thus, no in-depth treatise on quadruplex nucleic acids would be complete without ample discussion of G-quadruplex-cation interactions. The basic building block of a G-quadruplex is the G G G G quartet, which is composed of four hydrogen-bonded guanine nucleotides in a horizontal planar arrangement (Figure 1). The coordination of cations by the closely spaced... 

Evidence for intramolecular phenyl coordination of cationic monobenzyl-substituted zirconocene complexes (101) has also been established by low-... 

In contrast to coordination of cations or the binding of anions, the recognition of neutral substrates requires binding interactions that are non-electrostatic, or at least non-ionic, in nature. This has made neutral substrate recognition a problem of formidable challenge in... 

The example which works best in agreement to the Rule 1 Pauling referees to the coordination of cations in minerals of silicates type. 

Apparently, the (2, l)-type structure is closely related to the (1, l)-type. It can be expressed by the use of U and T layers as -U-T-T-U-T-U-T-T-U-T-. Both anions and cations occupy the triangular lattice points and the coordinations of cations are the same with the R ion at an octahedral site and the M or M ion at a trigonal bipyramidal site, as shown in fig. 40. It should be noted, however, that single T layers appear in the stacking sequence of the (2, l)-type structure, besides double T layers found in the (l,l)-type structure. According to the... 

A typical procedure adopted to monitor the coordination of cations to POE chains, based on the inspection of the skeleton vCOC modes in the FT-IR spectra, follows. The example given deals with a study previously carried out by our group on d-U(2000) LiCF3S03 di-ureasil ormolytes. In the... 

Analysis of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) of mixtures of salts and zeolites before and after heat-treatment enables us to prove or disprove that a solid-state reaction has occurred. Moreover, EXAFS is a tool to determine the coordination of cations introduced via SSIE into the zeolite matrix. If these cations are reducible to the zero-valent state as, e.g., Pt + or Pd + cations are, EXAFS also provides information about the size of the metal particles generated by reduction. To date, unfortunately, only a few pertinent studies have been reported (cf., e.g., [40] and Sect. 5.4.2). 

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