Octahedral unit

A fascinating variety of discrete (or occasionally polymeric) polynuclear halogeno complexes of As, Sb and Bi have recently been characterized. A detailed discussion would be inappropriate here, but structural motifs include face-shared and edge-shared distorted (MXe) octahedral units fused into cubane-like and other related clusters or cluster fragments. Examples (see also preceding paragraph) are ... 

Se3Bri3- < > SeCls , TeClj-, TeCle ", etc.< > The anion structures are much as expected with the Se species featuring square planar (pseudo-octahedral) units, and the trinuclear Se " anions as in the tellurium analogue above. See also p. 776. There are, in addition, a fascinating series of bromoselenate(II) dianions based on fused planar SeBr4 units, e.g. Se3Brg ", Se4Bri4 ,... 

Periodates form numerous complexes with transition metals in which the octahedral unit acts as a bidentate chelate. Examples are ... 

Many attempts have been made to rationalize the structures and inechanir-mb of formation of pcilynsevillatcs. Lipscomb observed th t no individual MOn octahedral unit ever has more than twii unshared, i.c. tcrmiacil oxygens (exceptions jppi to be scible only in the solid state) and this has been explained on the basis of T-bondi e between the metal and terminal oxygen Moms awire thnn two of thc,sc... 

The most usual type of coordination in compounds of Hg with other donor atoms is a distorted octahedron with 2 bonds much shorter than the other 4. In the extreme, this results in linear 2-coordination in which case the bonds are largely covalent. Hg(CN)2 is actually composed of discrete linear molecules (C-bonded CN ), whereas crystalline Hg(SCN)2 is built up of distorted octahedral units, all SCN groups being bridging ... 

A further decrease in the X Me ratio, to 4, leads to linkage of the octahedral units by sharing more than one ligand so as to achieve coordination saturation. Sharing of two vertexes (two comers of the each octahedron) leads to the formation of compounds with layered-type structures. 

It is an intensely reactive and hygoscopic yellow-brown substance (m.p. 75-78°C) its volatility suggests a low molecular mass Mossbauer spectra indicate 6-coordinate gold while the Raman spectrum is interpreted in terms of cw-bridged octahedral units. In the gas phase at 170°C, it comprises dimers and trimers [29] (electron diffraction). 

The appearance of a flake of clay reflects its internal structure, which is something like an untidy stack of papers (Fig. 14.45). Sheets of tetrahedral silicate units or octahedral units of aluminum or magnesium oxides are separated by layers of water molecules that serve to bind the layers of the flake together. Each flake of clay is surrounded by a double layer of ions that separates the... 

With diammonium ions [H3N(CH2) NH3]2+ (2 < n < 6) as counterions, several linear and layered polymeric Cd sulfates are generated. In the chain anion oo1[CdCl4/2(S04)2/2]2 ( = 2), octahedral units are linked by both /r-chloro and /r-sulfato bridges a similar structural situation is met in the compounds with oo1[CdX4/2(S04)2/2]2 anions (X = C1, Br n = 6). A layered structure of the anion oo2[Cd2(H20)2(S04)3T (n = 3) is achieved by linking two different octahedral CdOfi units by S04 bridges exclusively. 0... 

All of the above-mentioned active materials show a crystal structure characterized by a continuous network of corner shared octahedral units of metal cations (TaC>6, NbOg, Ti06), which presumably allows for high charge carriers mobilities and efficient charge separation. A relevant role of local electric dipole moments cannot be excluded [111] (see also below). 

Figure 17.1. A cubo-octahedral unit composed of Si04 and AIO4 tetrahedral...

The preparation of single crystals is difficult, but is successful in some case, so that we are well informed about the structures (1,6,17,21,22,23). The structures of the plastic phases are related to the well-known intermetallic phase Li3Bi, where the centres of the polycyclic P7 or Pn anions surround the positions of the Bi atoms in LisBi. The orientation of the polyanions is disordered (dynamically ). For these structures this orientation leads to a typical electron density distribution of a seemingly octahedral unit. In contrast the orientation of the anions is fixed for the crystalline phases. The symmetry of the unit cells as well as the distribution of cations and anions in these M3P7 and M3P11 type structures reflect the direct relationship to the structures of the plastic phases. 

Figure 1.10 (a) Replacement of a BOg octahedron by a PO4 tetrahedron in an octahedral unit and (b) accommodation of a diphosphate group. 

Nb(OMe)s appears to be the only one of these pentaalkoxides whose structure has been solved.172 It revealed the presence of two crystallographically different dimers with distinct conformations in the unit cell. Both are centrosymmetric and consist of two approximately octahedral units with a shared edge, but with either actsora tram arrangement of the terminal methoxo groups on each metal atom with respect to the equatorial plane (Figure 3). 

Figure 1 Prototypal structures for small oligomeric metal alkoxides (a) the cubane-M404 unit in [TKOMe) , and [Na(OBu )]4 (b) die planar [CufOBu1) , structure (c) and (d) fused trigonal and fused trigonal-tetrahedral units as seen in [Be(OR)2) compounds, where n = 3 and 2 (e) fused tetrahedral-octahedral units seen in [Al(OPri)3]4 (f) edge-shared tetrahedra as in [Al(OBu )3]2 (g) fused octahedra as in [MfOEt), , where M = Ti, V, W and (h) edge-shared octahedra as...

In figure 8.6 every octahedron shares the four OH -ions in the sheet with four neighbouring octahedrons, i.e. 1/4 of each of those four ions belongs to the octahedron. The OH-ions above and below the Al3+-ion fully belong to the octahedron. The formula of an octahedral unit in the sheet is thus Al3+ + 4x1/4 OH + 2 x OH = Al(OH)3. Now that we have been introduced to the O and T sheets, the next step is to link these sheets and in this way form clay minerals. This process in represented in figure 8.7. 

Figure 44. Supramolecular association in the orthorhombic form of Ni(S2COEt)2 (a) showing the links between alternating chains of molecules in the ab plane, (b) viewed down the axes of the chains, and (c) showing the connectivity between the tetragonally distorted octahedral units.

Both rutile and anatase crystallize in a tetragonal lattice, and their bulk structure can be described basically in terms of a three-dimensional arrangement of TiOe octahedra. The two polymorphs differ by the degree of distortion of each octahedral unit and by the manner in which the TiC>6 building blocks are spatially assembled. The structural differences described previously result in different physicochemical properties, such as density (4.250 g cm 3 for rutile and 3.894 g cm-3 for anatase) and stabihty (rutile is more stable than anatase by about 4.9 kJ mol-1). 

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