Spiral chains

Gep2 is formed as a volatile white solid (mp 110°) by the action of Gep4 on powdered Ge at 150-300° it has a unique structure in which trigonal pyramidal [GePsI units share 2 P atoms to form infinite spiral chains (Pig. 10.2). Pale-yellow GeCl2 can be prepared similarly at 300° or by thermal decomposition of GeHCls at 70°. Typical reactions are summarized in the scheme ... 

Some of the alkali metal-group 15 element systems give compounds of stoichiometry ME. Of these, LiBi and NaBi have typical alloy stmc-tures and are superconductors below 2.47 K and 2.22 K respectively. Others, like LiAs, NaSb and KSb, have parallel infinite spirals of As or Sb atoms, and it is tempting to formulate them as M+ (E )" in which the (E )" spirals are iso-electronic with those of covalently catenated Se and Te (p. 752) however, their metallic lustre and electrical conductivity indicate at least some metallic bonding. Within the spiral chains As-As is 246 pm (cf. 252 pm in the element) and Sb-Sb is 285 pm (cf. 291 pm in the element). 

Tellurium has only one crystalline form and this is composed of a network of spiral chains similar to those in hexagonal Se (Fig. 16.1c and d). Although the intra-chain Te-Te distance of 284 pm and the c dimension of the crystal (593 pm) are both substantially greater than for Scjt (as expected), nevertheless the closest interatomic distance between chains is almost identical for the 2 elements (Te Te 350 pm). Accordingly the elements form a continuous range of solid solutions in which there is a random... 

In addition, for the sodium-antimony alloy, where the Zintl anions are Sb spiral chains, ab initio MD simulations are in progress. 

The arrangement of the atoms in a Te crystal is shown in Figure 5a. The atoms are arranged in spiral chains. The bonds between adjacent atoms on the same chain are covalent whereas between chains they are... 

In the solid state, GeF2 has a unique structure in which trigonal pyramidal GeF3 units share two F atoms to form an infinite spiral chain. Reaction between GeFi and F gives GeF3, which is a trigonal pyramidal ion. 

The compound (f-Bu)2Sn(DMTT) is unassociated196. The tin center in solid PhMeSn (DMIT) (24a) is five-coordinate with a trigonal-bipyramidal geometry the molecules are linked into spiral chains via intermolecular S(thione) —> Sn interactions (Table 6). This was confirmed by the single 5119Sn value (94.9 ppm) in the solid state 119Sn NMR spectrum197. 

Grey selenium (mp 494 K, metallic) is the stable form. It may be obtained crystalline from hot solutions of Se in aniline or from the melt. The structure, which has no sulfur analogue, contains infinite, spiral chains of selenium atoms. Although there are fairly strong single bonds between adjacent atoms in each chain, there is evidently weak metallic interaction between the neighboring atoms of different chains. Selenium is not comparable with most true metals in its electrical conductivity in... 

The structure of Te4 has been determined (102) from the crystal structures of Te4(AlCl4)a and Te4(AlaCl7)2. In both cases the Te4 ion lies on a center of symmetry and is almost exactly square planar. The tellurium-tellurium distance of 2.66 A is significantly shorter than the tellurium-tellurium distance of 2.864 A within the spiral chain in elemental tellurium (105). This is consistent with a structure exactly analogous to that for 804 + in which each bond has 25% double bond character. The Raman spectra of Te4 in solution and the solid state are analogous to those of 804 and 84 but shifted to lower frequency (Table VIII). The magnetic circular dichroism (75) and visible and... 

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