Confacial bioctahedron

Nine-metal arrays in conventional coordination clusters are the confacial bioctahedron and two M4 cubanes linked through a non-cluster metal atom among naked metal clusters the M9 polyhedron is the tricapped trigonal prism or the monocapped square antiprism, and in gold clusters the metal array is the centred cube, the centred crown, or a fragment of the centred icosahedron. 

An alternative way of thinking about this structural type (as opposed to a confacial bioctahedron) is as an [EX6]3 octahedron in which one face of the octahedron is capped by a neutral EX3 unit. This is a somewhat contrived point of view but, in later sections, this capping principle will be quite useful in rationalizing a number of structural types. 

For T1I4 the stability of iodide in contact with Tl111 is a result of the stability of the ion, since thallium(III) iodide is itself unstable relative to T1 (I3). Thallium alone forms the ion Tl2Clg-, which has the confacial bioctahedron structure shown in Fig. 19-5a. 

Fig. 19-5. Two types of structure in which a full range of M—M interaction from strongly bonding to net repulsive can be observed, (a) The confacial bioctahedron, (b) The carbox-ylato-bridged dinuclear molecules.

Fig. 4. Increasing octahedral condensation with decreasing alkali metal halide content, (a) A section of the infinite cis-connected zig-zag chain of octa-hedra in CS2RECI5 (ft) a confacial bioctahedron as the characteristic structural feature of A RE Xq type compounds in both the CS3TI2CI9 and CsjC Clg structure types.

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