Determination of coordination number

If the identity of the backscatterer is known, then the interest is in determining the number of near neighbors. In this case, one needs to compare the amplitude of the EXAFS of the material of interest (unknown) to that for a compound of known coordination number and structure. However, unlike transferability of phase, which is generally regarded as an excellent approximation, the transferability of amplitude is not. This is because there are many factors that affect the amplitude and, except for the case of model compounds of very similar structures, these will not necessarily (and often will not) be the same. As a result, determination of coordination numbers (near neighbors) is usually no better than 20%. 

The practical determination of coordination numbers in agglomerates is technically impossible, particularly because of the extreme smallness of the particles forming an agglomerate. Theoretically it is possible to determine the coordination number / (jc) of a spherical particle with diameter x which is surrounded by smaller spherical particles of size as... 

The non-location of hydrogen atoms presents major difficulties both in the determination of coordination numbers for metal atoms, and for correct identification of ligands (e.g. for distinguishing methoxide from methanol). Ambiguous cases were excluded (e.g. M-OCH3 and M-0(H)CH3 both present in a structure in which hydrogen-atom positions were not reported). 

Some critical comments concerning the interpretation of the data should now be made. First what can be said about the degrees of dimerization It is known that the determination of coordination numbers hy means of EXAFS spectroscopy is generally very diflicult and can have an error of about 3C) /i. It follows that the tendency determined for the decree of dimerization cerlainlv is... 

Ebert M, Mair V, Tessadri R, Hoffmann P, Ortner HM (2000) Total-reflection X-ray fluorescence analysis of geological microsamples. Spectrochim Acta 55 205-212 Eisenberger P, Lengeler (1980) Extended X-ray absorption fine-structure determination of coordination numbers limitations. Phys Rev 22 3551-3562... 

Determination of coordination numbers or number of backscatterers is fraught with difficulties. The Debye-Waller factor is strongly correlated with the coordination number, and one must have recourse to other information to narrow the range that is possible from curve fitting analysis alone. It is useful to compare the spectra from the unknown complex to some known model complexes (assuming that there is evidence that the structure resembles that of the model complex) and then use Debye-Wal-... 

The equal valence rule has some simple but far-reaching implications in crystal chemistry. We may infer from its general applicability that low energy configurations of solids are those in which valences are as nearly as equal as possible. Thus we might expect an important factor in the determination of coordination numbers to be the ability of a structure with given coordination numbers to accommodate equal valences. Some simple examples, taken froiti oxide emd nitride chemistry, illustrate this point... 

Over the last decades, the concept of bond-valence calculations has been widely applied in solid state chemistry for prediction of bond lengths, as well as for determination of coordination numbers in corresponding compoimds. Besides, this concept serves as a criterion to verify the reliability of structure determinations (Brown, 1978 Brown and Altermatt, 1985 Altermatt and Brown, 1987). According to their approach, the atomic valence of an atom, Vi, is defined as the sum of bond valences z>ij... 

Molecular geometry, determination of cis- and trans isomers, distortion from cubic symmetry (Jahn-Teller effect), effect of polymerization, finding the right structure of polynuclear complexes, identification of possible structural position occupied by the Mossbauer atom, determination of distribution of Mossbauer atoms among the sites, determination of site preference, determination of coordination number, effect of neighbors... 

Laves P (1967) Space limitation on the geometry of the crystal stmctures of metals and intermetallic compounds. Phase transition in metals alloys. In Rudman PS, Stringer J, Jaffee RI (eds) Phase stability in metals and alloys. McGraw-HiU, New York Serezhkin VH, Mikhailov YuN, Buslaev YuA (1997) The method of intersecting spheres for determination of coordination numbers of atoms in crystal structures. Russ J Inotg Chem 42 1871-1910... 

Poroshina lA, Berger AS, Batsanov SS (1973) Determination of coordination number of metals of groups I and II in silicates from refractometric data. J Struct Chem 14 789-793 Bokii GB, Batsanov SS (1954) About quantitative characteristics of trans-influence. Doklady Academii Nauk SSSR 95 1205-1206 (in Russian)... 

Example 1 Determination of coordination numbers. If one considers the spectra of vitreous silica and its different crystalline allotropic forms (quartz, cristobalite, etc.), one notices the presence of an infrared absorption band centered at 9.1 /rm in all spectra. As the structure of all those varieties is based on a tetrahedrally coordinated silicon atom, it is natural to assign this band to one of the fundamental vibrations of the Si04 tetrahedron. 

I ratio phys chem The ratio of the radius of a cation to the radius of an ion relative ionic radii are pertinent to crystal lattice structure, particularly the determination of coordination number. I rad-e-as, ra-sho )... 

Consideration of the effective atomic number rule and the determination of coordination number by the electron-donating properties of ligands has led to the arranging of compounds in isostructural and/or isoelectronic... 

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