Tetrahedral, Octahedral

Equilibria between tetrahedral and octahedral cobalt(II) complexes in nonaqueous solution are well characterized. Kinetic data are sparse but those available from T-jump experiments in pyridine solution are interpreted in terms of (X = Cl and Br)  

Stereochemical nonrigidity is ubiquitous with seven and higher coordinated complexes because the geometries associated with them are easily interconverted by relatively small atomic displacements. Intramolecular rearrangements are complex but their understanding is helped considerably, but not solved, by nmr techniques.  

Magnetic susceptibilities of solutions — These are useful parameters for determining equilibrium constants for reactions involving spin changes. The Evans nmr method utilizes the observed shift in the resonance line (say of a proton of t-BuOH or hexamethyldisilox-ane) in solution when a paramagnetic substance is added. The paramagnetic shift A/ is related to the magnetic moment (jj, of the solution at TK by the approximate expression 

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Buckminsterfullerene is an allotrope of carbon in which the carbon atoms form spheres of 60 atoms each (see Section 14.16). In the pure compound the spheres pack in a cubic close-packed array, (a) The length of a side of the face-centered cubic cell formed by buckminsterfullerene is 142 pm. Use this information to calculate the radius of the buckminsterfullerene molecule treated as a hard sphere, (b) The compound K3C60 is a superconductor at low temperatures. In this compound the K+ ions lie in holes in the C60 face-centered cubic lattice. Considering the radius of the K+ ion and assuming that the radius of Q,0 is the same as for the Cft0 molecule, predict in what type of holes the K ions lie (tetrahedral, octahedral, or both) and indicate what percentage of those holes are filled. 

Cation type Radius [A] [14] tetrahedral octahedral cation / O " ratio f e foct ... 

Figure 11.9 Arrangementt of ions in silicate garnet (grossularite). Showing tetrahedral, octahedral, and quasi-cubic groups. After Geller (1967).

Geometry pyramidal triangular bipyramidal tetrahedral octahedral tetrahedral... 

Mineral Chemical structure Structure Charge per half unit ceU Tetrahedral Octahedral Structural charge, cmol /kg... 

In the structures cited in Table 12.3, except for pure silicon dioxide, metal ions are required for overall electrical neutrality. These metal ions are positioned in tetrahedral, octahedral, etc. positions in the silicate-like lattice. Sometimes they replace the silicon atom. Kaolinite asbestos has aluminum substituted for silicon in the Gibbosite sheet. Further, sites for additional anions, such as the hydroxyl anion, are available. In ring, chain, and sheet structures neighboring rings. 

Magnetite differs from most other iron oxides in that it contains both divalent and trivalent iron. Its formula is written as Y[XY]04 where X = Fe , Y = Fe " and the brackets denote octahedral sites (M sites). Eight tetrahedral sites (T sites) are distributed between Fe" and Fe", i.e. the trivalent ions occupy both tetrahedral and octahedral sites. The structure consists of octahedral and mixed tetrahedral/octahedral layers stacked along [111] (Fig. 2.13a). Figure 2.13b shows the sequence of Fe- and O-layers and a section of this structure with three octahedra and two tetrahedra is depicted in Figure 2.13 c. 

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...

Fig. 15. Schematic representation of a planar, tetrahedral, octahedral and double-helical arrangement of external interaction sites (arrows) on ligands organized around metal ions of given coordination geometry in metallo-exo receptors a linear strand is also shown.

Although some tetrahedral-octahedral combinations are more stable than others, the relative abundance of certain octahedral compositions (Fig.25) may be in part due to the restrictions imposed by the major natural environments. For example, the environmental conditions (and source material) under which most clays form is such that either A1 or Fe may be relatively abundant, but seldom both. The plot also shows that the octahedral sheet of attapulgite tends to be composed of approximately 50% Mg and 50% A1 and their compositional field does not overlap that of the 2 1 dioctahedral and trioctahedral clays. 

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