Coordinates, atomic crystal

The kind of bond between neighboring atoms also has to be considered. For instance, the coordination number for a chlorine atom in the CC14 molecule is 1 when only the covalently bonded C atom is counted, but it is 4 (1 C + 3 Cl) when all atoms in contact are counted. In the case of molecules one will tend to count only covalently bonded atoms as coordinated atoms. In the case of crystals consisting of monoatomic ions usually only the anions immediately adjacent to a cation and the cations immediately adjacent to an anion are considered, even when there are contacts between anions and anions or between cations and cations. In this way, an I- ion in Lil (NaCl type) is assigned the coordination number 6, whereas it is 18 when the 12 I- ions with which it is also in contact are included. In case of doubt, one should always specify exactly what is to be included in the coordination sphere. 

There has been much discussion of the probable limits of error of atomic coordinates in crystal structures, the upshot of which is the conclusion that earlier estimates of accuracy were too optimistic. Cruickshank (1949) showed that the relation between the standard deviation a x) of a coordinate of an atom and the differences between... 

In the a-modifications of silver(I) diethyldithiocarbamate irregular hexamers are linked in chains by weak bonds only (Ag—-S = 299pm). Two of the six silver atoms have four-fold coordination, the others three-fold. Two ligands coordinate to only two metal atoms, the others to four. Five short metal-metal distances occur in each of the hexamers.323 In the /3-modification, which is a true high polymer, all the silver atoms have four-fold coordination and all the ligands are linked to three metal atoms, (39). There are only two short metal-metal distances per six silver atoms. Crystals of the /3-form were obtained from CS2 solutions cooled to -40 °C. Ag—S bond lengths were in the range 251-274 pm.324... 

J. S. Leigh and K. H. Whitmire, Acta Crystallogr., Sect. C. C46,732 (1990)], which contain tetra-hedrally coordinated tin and lead atoms. Crystal structure data has been presented for [Sn Fe2-(CO)8) Fe3(CO)n ], which contains a tetrahedral, spiro, i/4-tin atom bridging the Fe—Fe bond in the Fe2(CO)8 fragment and one Fe—Fe vector in the triangular Fe3(CO)n unit [S. G. Anema,... 

Z = 4. The crystal structure is shown in the Figure 4.43. The La coordination polyhedron can be depicted as a folded butterfly configuration. Viewed from the two flexible -CH2-CH2- lateral units, the whole molecule can be divided into two hemispheres, one above and one below the donor atom plane. This structure effectively relaxes the repulsion among the coordination atoms and allows the central metal ion to attain its highest possible coordination number, 12. The donor atoms consist of six nitrogen atoms from one macrocyclic 24 and four nitrogen atoms from two bidentate nitrates on one side of the macrocycle and the other bidentate nitrate on the opposite site. 

A crystal structure is described by a collection of parameters that give the arrangement of the atoms, their motions and the probability that each atom occupies a given location. These parameters are the atomic fractional coordinates, atomic displacement or thermal parameters, and occupancy factors. A scale factor then relates the calculated structure factors to the observed values. This is the suite of parameters usually encountered in a single crystal structure refinement. In the case of a Rietveld refinement an additional set of parameters describes the powder diffraction profile via lattice parameters, profile parameters and background coefficients. Occasionally other parameters are used these describe preferred orientation or texture, absorption and other effects. These parameters may be directly related to other parameters via space group symmetry or by relations that are presumed to hold by the experimenter. These relations can be described in the refinement as constraints and as they relate the shifts, Ap,-, in the parameters, they can be represented by... 

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