Octahedral shape described

Platinum (IV) Structures. The oxidation state of the platinum atom in platinum coordination compounds determines the steric configuration of the molecule platinum(II) structures are planar molecules, while platinum(IV) derivatives assume an octahedral shape. Though it was hoped that these differences could be used to circumvent platinum resistance, the two compounds developed in the clinic, iproplatin and ormaplatin, have not proven useful. In the case of the former, testing in Phase-II trials failed to reveal activity. In the case of ormaplatin, the platinum(IV) configuration is not maintained under biological conditions conversion to a platinum(II) metabolite occurs within minutes [14], A series of novel platinum(IV) and mixed ammine/amine derivatives being developed at the Institute for Cancer Research are described in this volume by Kelland. 

Problem The explanation for the symmetric octahedral shape of the crystal lies in its chemical structure which can be described as a cubic packing of hydrated potassium ions whose octahedral and tetrahedral gaps are filled with hydrated aluminum ions and sulfate ions, respectively. If one assumes the simple particle model of alum particles and chooses the spherical model for one alum particle, then one can explain the simple structure as a cubic close packing of many similar spheres. It is possible to reconstruct such a sphere packing in an octahedral form and it can be recognized as a structural model when compared with the original crystal. 

Three common molecular shapes are associated with octahedral electron group geomehy. Most often, an inner atom with a steric number of 6 has octahedral molecular shape with no lone pairs. Example uses a compound of xenon, whose chemical behavior is described in the Chemical Milestones Box, to show a second common molecular shape, square planar. 

Most of these comments also apply to octahedral complexes, which tend to show longer bonds also. Again, there are examples ranging from near-regular to shapes better described as distorted tetrahedral with two long interactions. 

Solv=MeOH, EtOH and PrOH), and l,4-bis(4-pyridyl-butadiyne) (bpb, n= 0.5, Solv=MeOH). Like the btr derivative, compressed [FeN6] pseudo-octahedral sites define the knots of the square- or rhombus-shaped windows, which constitute the layered grid structure of the three compounds. Stacking of these layers in the crystal defines their most important structural differences, which are determined by the ligand size and crystal packing efficiency. In principle, the 2D grids are organised in a fashion similar to that described for the [Fe(btr)2(NCX)2]-H20 system the parallel layers are alternated so that the iron atoms of one layer lie vertically above and below the centres of the squares formed by the iron atoms of the adjacent layers. 

Octahedral tunnel structures. Metal-oxygen octahedra, BO, form host lattices characterized by large tunnels where cations are located. Thus, perovskites form a large family which are described by four-sided tunnels. There are a large number of tunnel structures which can be classified according to the size and the shape of the tunnels. Tunnel structures with angles of 90° or 60 -120° occur commonly. We shall briefiy examine the general features of tunnel structures. (Note that the bronzes we discussed earlier are tunnel structures). 

Crystal structures may be described in terms of the coordination polyhedra MX of the atoms or in terms of their duals, that is, the polyhedra enclosed by planes drawn perpendicular to the lines M-X joining each atom to each of its neighbours at the mid-points of these lines. Each atom in the structure is then represented as a polyhedron (polyhedral domain), and the whole structure as a space-filling assembly of polyhedra of one or more kinds. We can visualize these domains as the shapes the atoms (ions) would assume if the structure were uniformly compressed. For example, h.c.p. and c.c.p. spheres would become the polyhedra shown in Fig. 4.29. These polyhedra are the duals of the coordination polyhedra illustrated in Fig. 4.5. These domains provide an alternative way of representing relatively simple c.p. structures (particularly of binary compounds) because the vertices of the domain are the positions of the interstices. The (8) vertices at which three edges meet are the tetrahedral interstices, and those (6) at which four edges meet are the octahedral interstices. Table 4.9 shows the octahedral positions occupied in some simple structures c.p. structures in which tetrahedral or tetrahedral and octahedral sites are occupied may be represented in a similar way. (For examples see JSSC 1970 1 279.)... 

Although the angles between faces remain constant, the relative sizes of faces may vary from crystal to crystal. This behaviour is described as crystal habit, and the growth of the same substance from different solutions can result in different shapes. For example, sodium chloride crystals grown in water are cubic, but if urea is added to the water they become octahedral (a regular octahedron and a cube have the same overall symmetry). 

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