Atom coordination number

It can be readily confirmed thaf by decreases as the number of bonds N increases and/or llieir length (r ) decreases. This relationship between the bond strength and the number of neighbours provides a useful way to rationalise the structure of solids. Thus the high coordination of metals suggests that it is more effective for them to form more bonds, even though each individual bond is weakened as a consequence. Materials such as silicon achieve the balance for an infermediate number of neighbours and molecular solids have the smallest atomic coordination numbers. 

The structure of LiTa02F2, as reported by Vlasse et al. [218], is similar to a ReC>3 type structure and consists of triple layers of octahedrons linked together through their vertexes. The layers are perpendicular to the c axis, and each layer is shifted, relative to the layer below, by half a cell in the direction (110). Lithium atoms are situated in the centers of the tetragonal pyramids (coordination number = 5). The other lithium atoms are statistically distributed along with tantalum atoms (coordination number = 6) at a ratio of 1 3. The sequence of the metal atoms in alternating layers is (Ta-Li) - Ta - (Ta-Li). Positions of oxygen and fluorine atoms were not determined. The main interatomic distances are (in A) Ta-(0, F) - 1.845-2.114 Li-(0, F) - 2.087-2.048 (O, F)-(0,F) - 2.717-2.844. 

The X-ray absorption fine structure (XAFS) methods (EXAFS and X-ray absorption near-edge structure (XANES)) are suitable techniques for determination of the local structure of metal complexes. Of these methods, the former provides structural information relating to the radial distribution of atom pairs in systems studied the number of neighboring atoms (coordination number) around a central atom in the first, second, and sometimes third coordination spheres the... 

Here Zp is the Fermi energy, that is, the energy of the highest occupied level Zc is a limiting atomic coordination number below which the local magnetic moment of that atom adopts the value pdim of the dimer, and one can choose... 

The geometry of the coordination compounds can be similarly predicted based on the coordination number of the central atom. Coordination numbers 2 and 3 are both relatively rare and give linear and planar or pyramidal geometries, respectively. The most important coordination numbers are 4, 5 and 6 with the latter being the most important one as nearly all cations form 6-coordinate complexes. Table 2.4 shows the geometries corresponding to the commonest coordination numbers in biological systems. 

M in Mt Valence Dissociation Energy per MOx (kcal/g-atom) Coordination Number Single-Bond Strength (kcal/g-atom) ... 

The assignment of descriptors and configuration is sometimes arbitrary, at best, when based on model structures and pseudo-atom coordination numbers. A more explicit stereochemical notation is achieved by using this notation, which states within the stereodescriptor the model structure on which the notation is based. In this notation the pseudo-square pyramidal structure is [5PF-5-14-C(i )]. This structure can be expected to result in geometric isomers when one of the... 

The size of an arsenic atom depends on its valence state and the number of surrounding atoms (its coordination number). When valence electrons are removed from an atom, the radius of the atom not only decreases because of the removal of the electrons, but also from the protons attracting the remaining electrons closer to the nucleus (Nebergall, Schmidt and Holtzclaw, 1976), 141. An increase in the number of surrounding atoms (coordination number) will deform the electron cloud of an ion and change its ionic radius (Faure, 1998), 91. Table 2.2 lists the radii in angstroms (A) for arsenic and its ions with their most common coordination numbers. 

If the immediate neighbor atoms (coordination number) m is known such as in a crystalline solid, the question can be answered readily by invoking the binomial theorem. Since in the liquid state the coordination number m varies dynamically throughout the system, the answer to this question requires more thought. We know that for a given P (the atomic composition of B), there exists an average number of B s surrounding A and that the number n (P) can be written as ... 

When silicon is deposited from the vapor phase at ambient temperature, it solidifies as amorphous silicon. Vapor deposited bilayers and multilayers of silicon with metals thus consist of polycrystallinc metal and amorphous silicon. The earliest observations of amorphous silicide formation by SSAR were made on such diffusion couples [2.51, 54], Similar results were also obtained earlier by Hauser when Au was diffused into amorphous Tc [2.56], Figure 2.15 shows an example of an amorphous silicide formed by reaction of amorphous silicon with polycrystallinc Ni-metal at a temperature of 350"C for reaction times of 2 and 10 s [2.55,57], The reaction experiments were carried out by a flash-healing method (see [2.55] for details). In this example, the amorphous phase grows concurrently with a crystalline silicide. The amorphous phase is in contact with amorphous Si and the crystalline silicide in contact with the Ni layer. As in the case of typical mctal/metal systems, the amorphous interlayer is planar and uniform. It is also interesting that the interface between amorphous silicon and the amorphous silicide appears to be atomically sharp despite the fact that both phases are amorphous. This suggests that amorphous silicon (a covalently bonded non metallic amorphous phase with fourfold coordinated silicon atoms) is distinctly different from an amorphous silicide (a metallically bonded system with higher atomic coordination number). These two phases are apparently connected by a discontinuous phase transformation. 

XAS,EXAFS, NEXAFS Extracts the evidence about the chemical state environment like the oxidation state, symmetry or the local charge distribution. It also gives the local environment of atoms (coordination number, distance between the neighboring atom etc.) absorbing the incident radiation. 

Define the following terms coordination compound, ligand, donor atom, coordination number, chelating agent. 

The stereochemical features of the polymerization were satisfactorily accounted for by considering the nonbonded interactions between the monomer molecule undergoing insertion and the ligands on the vanadium atom. Coordination number five for vanadium was chosen to justify the more relevant data concerning the propylene polymerization and ethylene, 1-butene, and cis-2-butene copolymerization. 

Coordination number zero corresponds to an isolated atom coordination number one occurs for very simple molecule combinations such as Ni-NN that are stable only in very cold matrices such as argon. Coordination... 

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