Interpenetrating distorted

Figure 1.7 Interconversion of the highly porous, homochiral 3D MOF [Ni3(btc)2(py)6(l,2-pd)3]- (guest) upon guest desorption and exposure to ethanol vapour (a), involving transformation from a doubly interpenetrated, distorted (10,3)-a network phase to one in which four regular (10,3)-a nets interpenetrate (b). Reprinted with permission from C.J. Kepert, T.J. Prior and M.J. Rosseinsky, /. Am. Chem. Soc., 122, 5158. Copyright (2000) American Chemical Society...

The dodecahedral coordination is produced by two interpenetrating tetrahe-dra (slightly distorted) of chlorine and arsenic atoms. 

Figure 25.9 Some non-haem iron proteins (a) rubredoxin in which the single Fe is coordinated, almost tetra-hedrally, to 4 cysteine-sulfurs, (b) plant ferredoxin, [Fe2S2(S-Cys)4], (c) [Fe4S4(S-Cys)4] cube of bacterial ferredoxins. (This is in fact distorted, the Fe4 and S4 making up the two interpenetrating tetrahedra, of which the latter is larger than the former).

Fig. 3.—The structure of trisilver cobalticyanide. There are three mutually interpenetrating rhombohedral frameworks, distorted from the cubic frameworks of Fig. 2 in such a way as to achieve increased density and closer approximation to the optimal packing of the bulky cobalticyanide groups.

To start with a relatively simple case, Rb[Cd Ag(CN)2 3] (from RbCl, AgN03, Cd(N03)2, and NaCN in H20) is discussed. In its structure (P312, Z = 1), Cd and the rod-like N-coordinating NC—Ag—CN units create three independent, interpenetrating, 3-D frameworks Cd (symmetry 32 —Z)3) has trigonally distorted octahedral coordination (r(Cd—N) 232.4 pm). Rb occupies one of two types of trigonal channels.218... 

An important contribution to the structure analysis of intermetallic phases in terms of the coordination polyhedra has been carried out by Frank and Kasper (1958). They described several structure types (Frank-Kasper structures) as the result of the interpenetration of a group of polyhedra, which give rise to a distorted tetrahedral close-packing of the atoms. Samson (1967, 1969) developed the analysis of the structural principles of intermetallic phases having giant unit cells (Samson phases). These structures have been described as arrangements of fused polyhedra rather than the full interpenetrating polyhedra. 

Figure 22. Two distorted interpenetrating (10,3)-a nets in the structure of [Ag2(2,3-dimethylpyra-zine)3](SbF6)2. Circles represent Ag.

A number of interesting modes of coordination are found with simple organic molecules as ligands, e.g. a crystal structure determination of the tetrameric methylzinc(II) derivative MeZn(OMe) 4684 shows the structure to be as in (101). Closely related to this is the structure of the polymerization catalyst Zn(OMe)2(EtZnOMe)6 (102).686 The centrosymmetric complex consists of two enantiomorphic distorted cubes which share a comer the zinc atoms occupy the comers of a tetrahedron and oxygen atoms the comers of an interpenetrating, but smaller, tetrahedron (Zn—O = 2.06 A). 

A special case of pyramidal M30 units involves the cubane framework (cf. 26) consisting of a tetrahedron of M atoms linked in threes by the four /x3-O ligands comprising the interpenetrating tetrahedron of oxygens. This framework is generally distorted, often having only Z)2 symmetry. It... 

Finally in this section, we note that multicentre bonding must sometimes be admitted and cannot be simplified into a VB (or pseudo-VB) description. An example is methyllithium, which consists in the solid state of Li4(CH3)4 molecules. The structure is a distorted cube, which may be seen as interpenetrating Li4 and (CH3)4 tetrahedra ... 

From the above discussion, it is evident that mixed cluster ions of the type Ar M exhibit strong magic numbers at values of (n + m) = 13, 19, 55, 71, and 147 in a variety of different studies. These values correspond to the completion of the first, second, and third icosahedral shells occurring at 13, 55, and 147 whereas 19 and 71 correspond to especially stable subshells formed by interpenetrating double icosahedron structures. The size and symmetry of the dopant moiety appear to be the most important factors in observing magic numbers that can be rationalized on the basis of icosahedral-like structures. The inability to observe magic numbers has been attributed to the distortion of the icosahedral structure due to size and steric factors associated with the dopant ion which destroys the delicate balance between the monomer interactions. One of the issues that has been interpreted differently involves the location of the dopant atomic/molecular... 

Rao et al. recently reported a related structure that is based upon modular self-assembly 59 an organic network formed by trithiocyanuric acid (TCA) and bipy. Adjacent layers are aligned parallel to each other and there is no interpenetration. The resulting open framework structure exhibits channels with an effective diameter of 10 A. An interesting feature of this compound is that the cavities in the layers, and therefore the channels, can vary in size depending on the solvent of crystallization that is used to template the modular self-assembly process. (Scheme 8). It should be noted that the two architectures are not simply distorted or stretched variants of one another, they have distinct hydrogen-bonding patterns. Another salient feature is that there are sulfur atoms accessible in the cavities, which could... 

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