The plane hexagon net

If alternate atoms in the 6-gon layer are of different elements the composition is AB (Fig. 3.22), and we find hexagonal BN with plane layers and GeS (and SnS) with buckled layers similar to those in black P. 

Orthoboric acid is a trihydroxy-compound, B(OH)3, and in the crystalline state each molecule is hydrogen-bonded to neighbouring molecules by six 0—H-0 bonds. These are in pairs to three adjacent molecules, an arrangement similar to that in the dimers of carboxylic acids  

In the crystalline hydrates of some acids a proton is transferred to the water molecule forming the ion, which can form three hydrogen bonds. 

Trithiane, S3(CH2)3 a chair-shaped molecule like cyclohexane, forms many metal complexes. Two silver compounds have layer structures based on the 6-gon net. In both structures Ag is tetrahedrally coordinated, the fourth ligand (X in Fig. 3.23(a)) being H2O in Ag(trithiane)C104. H2O and 0 of NOJ in Ag(tri-thiane)N03. H2O (JCS A 1968 93). Silver tricyanmethanide, Ag[C(CN)3], has a 

If A atoms at the points of a 6-gon net are joined through X atoms (Fig. 3.22) the result is a layer of composition A2X3. Crystalline AS2S3 (the mineral orpiment) is built of layers of this kind (Fig. 3.24(a)), and one of the forms of the trioxide (claudetite) has a very similar layer structure. (The other form, arsenolite, is built of AS4O6 molecules with the same type of structure as the P4O6 molecule.) 

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The structures of crystalline As (Sb and Bi) and of black P illustrate two different ways of distorting the simple cubic packing so that each atom has three nearest and three more distant neighbours. Both are layer structures which may alternatively be described as forms of the plane hexagon net. They are described and illustrated in the section on the plane hexagon net (p. 88). 

FIG. 7.4(c) and (d). Two of the (unknown) AXj structures in which octahedral AX groups share three edges as in the CrCU and Bilj layers, the A atoms being situated at the points of 3D 3-connected nets instead of the plane hexagon net as in the layer structures. In the structure (c), based on the cubic (10,3) net of Fig. 3.28, the X atoms occupy three-quarters of the positions of cubic closest packing—compare the ReOj structure. The spheres indicate the positions of the missingc.p. atoms. In the structure (d), based on the (10,3) net of Fig. 3.29, the X atoms occupy all the positions of cubic closest packing. 

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