Octahedral sites

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

Titanium Dhodide. Titanium diiodide is a black soHd p = 499(0) kg/m ) that has the cadmium iodide stmcture. Titaniums occupy octahedral sites in hexagonaHy close-packed iodine layers, where a = 411 pm and c = 682 pm (144). Magnetic studies indicate extensive Ti—Ti bonding. Til2 reacts rapidly with water to form a solution of titanous iodide, Til. ... 

T > 120 C) are centered in their respective octahedral sites, and the polarizabilities of the Ti—O bonds yield a net value of zero for the (3 terms of the TiOg groups. 

The least compHcated clay minerals are the 1 1 clay minerals composed of one tetrahedral (T) layer and one octahedral (O) layer (see Fig. 1). These 1 1 clay minerals are also referred to as TO minerals. The TO package has a basal spacing (nominal thickness) of 0.7 nm (7 E) and they are commonly referred to as 7 E minerals. Kaolinite, the dioctahedral 1 1 mineral, has filling two of three octahedral sites, and serpentine [12168-92-2J, (Mg)3Si205(0H)4, the trioctahedral 1 1 mineral has filling all three octahedral sites. The kaolin minerals have limited substitution in the octahedral... 

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

Fig. 2. Structures for the solid (a) fee Cco, (b) fee MCco, (c) fee M2C60 (d) fee MsCeo, (e) hypothetical bee Ceo, (0 bet M4C60, and two structures for MeCeo (g) bee MeCeo for (M= K, Rb, Cs), and (h) fee MeCeo which is appropriate for M = Na, using the notation of Ref [42]. The notation fee, bee, and bet refer, respectively, to face centered cubic, body centered cubic, and body centered tetragonal structures. The large spheres denote Ceo molecules and the small spheres denote alkali metal ions. For fee M3C60, which has four Ceo molecules per cubic unit cell, the M atoms can either be on octahedral or tetrahedral symmetry sites. Undoped solid Ceo also exhibits the fee crystal structure, but in this case all tetrahedral and octahedral sites are unoccupied. For (g) bcc MeCeo all the M atoms are on distorted tetrahedral sites. For (f) bet M4Ceo, the dopant is also found on distorted tetrahedral sites. For (c) pertaining to small alkali metal ions such as Na, only the tetrahedral sites are occupied. For (h) we see that four Na ions can occupy an octahedral site of this fee lattice.

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

Figure 8.10 The fee strueture of K3C60, showing potassium ions oceupying the tetrahedral (O) and octahedral ( ) sites. The shortest K-K distanee is 617 pm (mueh larger than in metallie K) and the diameter of C6o is 708 pm.

A range of shiny metallic compounds featuring trigonal planar anions SnX3 (X = As, Sb, Bi) have been characterized with composition M6(SnX3)Oo.5 (M = Rb, Cs) the Sn and X atoms in SnX3 (isostructural with C03 ) are coordinated by trigonal prisms of 6M+, and the ions occupy octahedral sites in the M+ lattice. 

Finally, oxygen is soluble in metallic titanium up to a composition of TiOo.s with the oxygen atoms occupying octahedral sites in the hep metal lattice distinct phases that have been crystallographically characterized are TieO, TisO and Ti20. It seems likely that in all these reduced oxide phases there is extensive metal-metal bonding. 

The sulfides have been less thoroughly examined than the oxides but it is clear that a number of stable phases can be produced and nonstoichiometry is again prevalent (p. 679). The most important are the disulfides, which are semiconductors with metallic lustre. TiS2 and ZrS2 have the Cdl2 structure (p. 1211) in which the cations occupy the octahedral sites between alternate layers of hep anions. 

When is approximately the same size as Ti (i.e. M = Mg, Mn, Fe, Co, Ni) the stmcture is that of ilmenite, FeTi03, which consists of hep oxygens with one-third of the octahedral interstices occupied by and another third by Ti This is essentially the same structure as corundum (AI2O3, p. 243) except that in that case there is only one type of cation which occupies two-thirds of the octahedral sites. 

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

By heating the metal with appropriate oxides or carbonates of alkali or alkaline earth metals, a number of mixed oxides of Ru and Os have been made. They include NasOs Og, LifiOs Og and the ruthenites , M Ru 03, in all of which the metal is situated in octahedral sites of an oxide lattice. Ru (octahedral) has now also been established by Ru Mdssbauer spectroscopy as a common stable oxidation state in mixed oxides such as Na3Ru 04, Na4Ru2 07, and the ordered perovskite-type phases M Ln Ru Og. 

Of the anhydrous dihalides of iron the iodide is easily prepared from the elements but the others are best obtained by passing HX over heated iron. The white (or pale-green) difluoride has the rutile structure the pale-yellow dichloride the CdCl2 structure (based on cep anions, p. 1212) and the yellow-green dibromide and grey diiodide the Cdl2 structure (based on hep anions, p. 1212), in all of which the metal occupies octahedral sites. All these iron dihalides dissolve in water and form crystalline hydrates which may alternatively be obtained by dissolving metallic iron in the aqueous acid. 

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