Anion contact

Figure 2.4. Limiting conditions jor cation-anion contact octahedral structure)...

This theoretical result is completely substantiated by experiment. Goldschmidt,31 from a study of crystal structure data, observed that the radius ratio is large for fluorite type crystals, and small for those of the rutile type, and concluded as an empirical rule that this ratio is the determining factor in the choice between these structures. Using Wasastjerna s radii he decided on 0.67 as the transition ratio. He also stated that this can be explained as due to anion contact for a radius ratio smaller than about 0.74. With our radii we are able to show an even more satisfactory verification of the theoretical limit. In Table XVII are given values of the radius ratio for a large number of compounds. It is seen that the max-... 

Ionic crystals can be cleaved in certain directions. Fig. 19.1 shows why the exertion of a force results in cleavage if two parts of a crystal experience a mutual displacement by a shearing force, ions of like charges come to lie side by side and repel each other. The displacement is easiest along planes which have the fewest cation-anion contacts. In... 

With the long alkyl chain substitutions on the A-heterocyclic carbenes, lamella-structured silver(i) carbene complexes 27a and 27b (Figure 14) were isolated.74 It is interesting to note that the synthetic procedures for the two complexes are the same except for the use of different solvents of crystallization. The dinuclear 27a was obtained from recrystallization in dichloromethane- -hexane while the tetranuclear 27b was obtained from acetone. The structure of 27a could be interpreted as the dimeric form of [Ag(carbene)Br] bridged by intermolecular Ag-Br interactions. The Ag-G bond has a distance of 2.094(5) A. The tetranuclear 27b, on the other hand, could be regarded as two monocationic bis(carbene)silver(i) bridged by an [Ag2Br4]2 anion, with the presence of short Ag(cationic)-Ag(anionic) contact (3.0038(18) A) and comparable Ag-G bond distances (2.0945(5), 2.138(13) A). A related... 

Figure 12 The polyiodide network determined by I- I cation/anion contacts in 4-ferrocenium-ethylene-l,3-dithiole-2-thione I2 adducts balanced by V-shaped pentaiodides...

Figure 16 (a) Cation-anion chains of (tBu3PSe)2I+ I3, (b) cation-anion contacts in... 

Morph) 3PScT I-, and (c) cation-anion contacts in tBu2iPrPSel+ 17... 

Fig. 3 (a) Crystal structure of (DMET)2FeBr4. The dotted and dashed lines denote the intermo-lecular anion—anion and donor-anion contacts, respectively, (b) Fermi surfaces obtained for a donor layer around z = 1/2 using the tight-binding approximation. The solid arrow represents the nesting vector Q (a b )/2... 

Alternative equations of state may be derived from Eq. (15). For example, one can obtain an equation of state in terms of variables expressed relative to their values at the melting point. At the melting point, the reduced variables 7rm, rm, and 6m are universal constants. This is illustrated in Table I where rm is given for several salts. Except for the lithium halides, which are not expected to follow this development since the small radius of the lithium ion leads to anion-anion contacts, rm is reasonably... 

Fig. 6-29. Change in potential profile across a compact layer due to anionic contact adsorption at constant potential on a metal electrode solid line = without contact anion adsorption broken line = with contact anion adsorption 4m - inner potential of metal electrode, 4s = inner potential of solution 4ihp = inner potential at IHP.

The testing of a rigid-sphere approximation must however be logically consistent, and if the anions are big enough they must be in anion-anion contact. The geometry of this is well-known (7) from the halide-radii just obtained from potassium salts, the anions should be in contact in lithium bromide and iodide conversely from their experimental anion-anion distances one would infer for r ... 

These same arguments are now applied to the alkali-metal hydrides. The following Table shows the nearest-neighbour distances 24) and the anion-radius calculated on the assumption of anion-cation and anion-anion contact if one consistently assumes anion-anion contact in the... 

Salt D assuming anion- cation contact assuming anion- anion contact... 

The essential difference is that aU the other methods give isoelectric anions considerably larger than cations, while the Electron Density Minimum method does not. Yet, for example, the mineralogy of oxides and silicates suggests large oxide anions often in contact (47) and there is evidence for anion-anion contact in sulphides and selenides (2). This evidence for anion-anion contact is the strongest evidence for the older systems of ionic radii. 

This distinction between electron-cloud radii and structural radii is then used to refine the system of ionic radii due to Pauling and Goldschmidt. Some further examples of anion-anion contact are discussed, and a value deduced for the crystal radius of the hydride anion. These cases of anion-anion contact argue for the Pauling tradition and against the new electron-density-minimum (EDM) radii. 

One possible criterion for designating a compound complex , that the ligand is exclusively held by one metal, is too restrictive because it excludes the / -complexes. However, it is reasonable to state that if all anions contact more than three cations the solid does not contain complexed cations. Compounds made by addition of ligands to M(I)... 

FIGURE 1.46 (a) Anions packed around a cation on a horizontal plane, (b) anion-anion contact on a horizontal... 

Assuming that anion-anion contact occurs as in Figure 1.46(b), the iodide ion 300... 

Nevertheless, in some cases, cation- -anion contacts give rise to extended structures, as in both the colorless and the yellow polymorph of [Au(C = NC6H11)2][PF6] [13], where the gold cations aggregate via aurophilic contacts into infinite linear chains, which are further connected through Au- F contacts to afford two-dimensional... 

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