Cations coordination polyhedra

The structure can be analyzed in terms of cation coordination polyhedra surrounded by a variable number of oxygens, as shown in Fig. 8 [9]. 

One might assume that additional edge sharing of the cation coordination polyhedra is a means to bind excess d-electrons of the cations. However, while the V—V distance in the straight V chains along the 6-axis is 6 = 2.99 A, the additional contacts in the zigzag chains are even looser, 3.15 A. The asymmetry of the cation coordination looks even as if it were caused by the V—V repulsion. These distances are definitely too large to represent cation-cation bonds, as... 

Fig. 118. Stacking of the cation-coordination polyhedra in the orthorhombic structure of WTe2. The zig-zag chains formed by the cations are indicated.

Figure 14.30 Cationic coordination polyhedra a) tysonite modification I, b) tysonite modification II and c)...

Coulomb interactions mean that like signs should be as far apart as possible and opposite signs as close together as possible. Crystal structures are thus at their most stable when the cations have the maximum CN allowed by rx. In many well-known ceramics, the cation coordination polyhedron is the basic building block. 

The dihydrates M03-2H20 can be precipitated at room temperature from acidic aqueous molybdate and tungstate solutions. Mo03-2H20 grows in the form of lemon yellow transparent prisms from a HCl solution of M0O3 [438]. In these modifications the water is present as H2O molecules as follows from H-nuclear-resonance and infra-red studies. Half the water forms part of the cation coordination polyhedron which is a distorted octahedron [Mo004/2(H20)]. The structure... 

Fig. 233. The cation coordination polyhedron in Ce2SeF4. The stacking sequence of the hexagonal layers is indicated on the right.

A coordination polyhedron of anions is formed around every cation. The cation-anion distances are determined by the sum of the ionic radii, and the coordination number of the cation by the radius ratio. 

The distances d(MX) within the coordination polyhedron of a cation M vary in the same... 

The recently [25] published Mg(NH3)6Hg22 is an outstanding new member of this group of amalgams. It is characterized by a coordination polyhedron of 32 Hg atoms around each cationic Mg(NH3)6 group. 

An alternative approach is to use the bond graph to propose a coordination polyhedron for each of the cations and to generate the full structure by examining the different ways in which these polyhedra can link together to form an infinite framework. 

More than 60 years ago, the important proposal3 was made that the strength, s, of an X—O bond in an XO -coordinated polyhedron in a crystal like a silicate should depend on the valence, z, and the coordination number, n, of the X+z metal cation such that s = z/n. With this simple definition, it was found for instance that the sum of the strengths of each of the t bonds in a crystal reaching a given oxide ion,... 

The lack of a strong correlation between isomer shifts and Fe-oxygen distances has been attributed to covalency effects beyond the immediate coordination polyhedron surrounding an iron cation (12). ... 

The situation becomes more complicated, however, when each oxygen belonging to an Fe coordination polyhedron is bound to different populations of next-nearest cations in their own coordination sites as a result of extensive atomic substitution of Fe2, Fe3, Ti4, etc., for Mg and Al. Such chemical and bonding variations for different mineral structures and the variability of inductive effects from adjacent cations are probably sufficiently strong to account for large ranges of isomer shifts exhibited within a specific coordination symmetry in silicate minerals. 

Sandwich and triple-decker complexes are routinely observed for Por and Pc ligands and have attracted most attention (Fig. 21 Buchler, WeiB). The sandwich systems are thermally stable and sublimable and show only limited moisture sensitivity. The lanthanide cations are located between two saucershaped macrocycles usually forming a square antiprismatic coordination polyhedron. The separation of the two macrocyclic rings is very close, starting at... 

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