Dodecahedral cation

Attempts to prepare related hpp complexes of tantalum were only partially successful. Reaction of 2 equivalents of either (hpplSiMes or in situ generated Li[hpp] with Tads afforded a solid that analyzed correctly for the compound (hpp)2TaCl3. X-ray structural analysis of the crystallized product identified the coordination isomer [Ta(hpp)4][TaCl6], with a distorted dodecahedral cation and a regular octahedral anion (Figure 28). °... 

The ribbon width is variable and involves both glide and dimh, so that it may he nonequilibrium. The 1/4(111) fault vector causes initially empty sites in the perfect structure to he filled hy tetrahedral and dodecahedral cations, so that the SFE should be high. Equation (22) could not he detected in dislocations introduced into silicate... 

Also of interest are the octacyano complexes, (M(CN)g] (M = Mo, W), whieh are commonly prepared by oxidation of the M" analogues (using MnO,) or Ce" ) and whose structures apparently vary, aceording to the environment and counter cation, between the energetically similar square-antiprismatic and dodecahedral forms. 

Agreement is also poor concerning entropy and volume excess terms. Because divalent cations (Mg, Ca, Fe, Mn) occupy only dodecahedral sites whereas octahedral sites are reserved for trivalent cations (Cr, Fe, Al), each cation has only one site at its disposal and permutability is fixed by stoichiometry (cf. section 3.8.1). As regards the occupancy on tetrahedral positions, we have already seen that analyses of natural specimens show silicon deficiencies, compensated by AF ... 

Tetrakis O, O -dialkylthiophosphates have been obtained for niobium starting from oxidation states +V or +IV (Table 40). These chelates display a distorted dodecahedral geometry in the solid (X-ray) as well as in solution (ESR).561 An unusual cation, [ Nb(Cp)2 2(PS4)]+, in which a PS4 moiety is doubly bridging the terminal NbCp2 units, has been obtained from [NbCl2(Cp)2] and P4Si0. The metal has a distorted tetrahedral environment of approximate symmetry, while symmetry at phosphorus is reduced to C. ... 

Figure 3.11 Orgel diagram for transition metal ions possessing rD spectroscopic terms in octahedral crystal fields of increasing intensity. The right-hand side applies to 3d1 (e.g., Ti3+) and 3d6 (e.g., Fe2+) cations and the left-hand side to 3d4 (e.g., Mn3+) and 3d9 (e.g., Cu2+) cations in octahedral coordination. The diagram in reverse also applies to the cations in tetrahedral, cubic and dodecahedral coordinations.

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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