Structures based on 3D 3-connected nets

There are very few examples as yet of simple inorganic compounds with structures based on 3-dimensional 3-connected nets. We might have expected to find examples among the crystalline compounds of boron, an element which forms three coplanar bonds in many simple molecules and ions. The structure of the normal form of B2O3 is in fact based on a simple 3-connected net however, boron is 4-coordinated in many borates, and both triangular and tetrahedral coordination occur in many compounds. The complex crystalline forms of elementary boron are not simple covalent structures but electron-deficient systems of a quite special kind, the structures of which are briefly described in Chapter 24. 

There are two 3-connected nets (10, 3) with four points in the simplest unit cell, but if the nets are constructed with equal bonds and interbond angles of 120° they have eight points in their unit cells and cubic and tetragonal symmetry respectively. The cubic net (Fig. 3.28) is clearly the 3-connected analogue of the diamond net. It 

0- H 0 distances of 2-70 A and HO-OH (intramolecular) equal to 1-47 A. An interesting property of this net is that it is enantiomorphic accordingly, crystalline H2O2 is optically active. 

Examples of more complex 3-connected 3D nets formed by dihydroxy-compounds include the structures of a- and 3-resorcinol (m-dihydroxybenzene) and of OH. Si(CH3)2- CfiH4. Si(CH3)2. OH. 

The projection of the cubic (10, 3) net on a face of the cubic unit cell, the full circles and lines of Fig. 3.28(b), shows that the net is built of 4-fold helices which are all anticlockwise upwards. The figures indicate the heights of the points in terms of c/8, where c is the length of the cell edge. A second net can be accommodated in the same volume, and if the second net is a mirror-image of the first in no case is the distance between points of different nets as short as the distance between (connected) points within a given net. In the second net of Fig. 3.28(b) (dotted circles and lines) the helices are clockwise. This type of structure, which would be a 3D racemate, is not yet known, but in view of its similarity to the /3-quinol structure described later, there is no reason why it should not be adopted by some suitable compound. 

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Table 3.12 summarizes the examples we have described of structures based on 3D 3-connected nets. 

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