Changes of coordination number

1 Changes of coordination number A guiding principle of crystal chemistry is that the coordination number of a cation depends on the radius ratio, RJR, where Rc and / a are the ionic radii of the cation and anion, respectively. Octahedrally coordinated cations are predicted when 0.414 7 c// a 0.732, while four-fold (tetrahedral) and eight- to twelvefold (cubic to dodecahedral) coordinations are favoured for radius ratios below 0.414 and above 0.732, respectively. The ionic radii summarized in Appendix 3 

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The metal ion in conformer II of TPPEr(dpm) is shifted out of the porphyrin plane, as suggested by an increase in the Q(0,0) intensity and a comparable red shift of the Q and B bands, while that of conformer I is in the plane as TPPErOH in methanol/ethanol. However, the conformation change might be due to a change of coordination number by solvent coordination rather than the shift of the central metal ion. Excitation spectra of the emission of... 

Figure 15.12 (a) Fourier transforms of Pt P-weighted EXAFS functions x(k) for a HjPtCle/HMM-l sample with MeOH/HjO under the UV-irradiation forO, 4, 8, 12, 24 and 48h at 300 K, and Pt foil (b) changes of coordination numbers for Pt-Pt (red line) and... 

Quite a number of interesting solid-state reactions have been observed that do not quite fit into the categories of earlier sections. These include reactions of coordinated ligands, changes of coordination number or geometry and polymerization reactions. Limitations of space do not permit detailed discussions of these reactions. 

Coordinative-addition may be accompanied by ligand displacement from the coordination sphere of the metal, in which case there is no change of coordination number ... 

The dichloro ion T1C12 appears to be particularly stable, presumably because it is linear and analogous to the linear, very stable and isoelectronic HgCl2 (cf. also Me2Tl+ below). The thermodynamic quantities are also consistent with a change of coordination number on formation of the tetrahedral T1C14 ion, e.g.,... 

Change of coordination number is involved in a kinetic and mechanistic study of the ruthenium(II)-salen-triphenyl phosphine system. The rate constant for dissociation of [Ru(salen)(PPh3)2], by what must be a dissociative mechanism, is 1.2 X 10 " s in benzene at 25... 

The transformation P has higher enthalpy (several authors give the values from 1424 to 2722 J/mole) [130, 131], caused by breaking the bonds Ca-O, linked with the change of coordination number of calcium ions. 

With the help of alkoxy groups, intramolecular bonding inaeases which leads to the change of coordination numbers. 

In various stages of a catalytic reaction, the change of oxidation state (OS), the change of coordination number (A ), and the change of the number of valence electrons (NVE) may take place in a complex. Changes of these quantities in various elementary reactions are given in Table 1.4. ... 

Note that, as was shown above, the variations in the relative changes of the interatomic distances upon a change of coordination numbers are several times smaller than the variations of the absolute values of the bond lengths. The relative change in interatomic distances on the Nc(3) Nc(8) transition in structures of binary... 

The structure of the KAu4Sn2 type is related to the CuAh type (Sinnen and Schuster 1978). An analysis of the structure (Skolozdra 1993) shows, that it can be obtained from the CuAh structure by filling the m symmetry tetrahedra in the CuAh structure with Au atoms. In the case of a compression of the structure, a change of coordination numbers and polyhedra in comparison with the structure type CUAI2 is observed. 

The interpretation of these variations is not yet entirely clear. There is no satisfactory correlation with the bulk properties of the solvent, such as dielectric constant or polarisability nor would such a correlation be expected, since the solvent here takes part in the reaction as a nucleophilic entering and leaving group, as well as a solvating medium. Factors that might be expected to be important are the ion-solvent interaction energies, the nucleophilicity and steric requirements of the solvent molecule, the difference of solvation between the reactants and the transition state, and (for some cations) the possibility of a change of coordination number. No one of these factors appears to dominate, and a fuller interpretation must await more... 

The expression for the forward rate (Nc) depends on the nature of the local rearrangements occurring at the interface. In the case of diffusion-limited crystallization, which involves significant changes of coordination numbers and local order at the interface, is given by N 6D/X, where N is the number of surface atoms of... 

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