Molecular solids trigonal pyramidal

The crystal and molecular structures of two spiroarsoranes of type XXI (R = Ph, R = R" = CH3 and R = OH, R = R" = H) have been determined by single-crystal X-ray diffraction analyses (68, 69). The crystal data for these compounds are summarized in Table V. Both compounds have a geometry at the arsenic atom that lies on the Berry coordinate between rectangular-pyramidal and trigonal-pyramidal. These structures show close parallels between the structures of related arsenic and phosphorus systems. It has been concluded that, since the solid-state structures of these compounds lie close to the Berry coordinate, the dynamic process in solution is distortion along that coordinate (68, 69). 

The structures of the element trihalides EX3 are covered in a number of textbooks on structural inorganic chemistry (4, 5), and these will not be discussed in great detail here. It is, however, worth mentioning some of the salient structural features. In most cases, a molecular trigonal pyramidal EX3 unit consistent with VSEPR theory predictions is readily apparent in the solid-state structure, although there are usually a number of fairly short intermolecular contacts or secondary bonds present. A general description of the structures as molecularly covalent but as having a tendency toward macromolecular or polymeric networks is therefore reasonable. Only in the case of the fluorides is an ionic model appropriate. 

In the solid, liquid and gas states, ASF3 and ASCI3 have molecular, trigonal pyramidal structures. With an appropriate reagent, ASF3 may act as either an F donor or acceptor (equations 14.79 and 14.80) compare this with the behaviours ofBrF3 (SectionS.lO) and ASCI3 (equation 14.81) which finds some use as a non-aqueous solvent. 

With the molecular geometry for a carbanion described as a trigonal pyramid the question is whether or not carbanions can display chirahty. After all, when the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in racemization. However, solid evidence exists that carbanions can indeed be chiral, for example, in research carried out with certain organoUthium compounds. 

Halides R MX and RMX are formed by all the elements P to Bi. In general they have pyramidal molecular structures. The rather high melting points of MeBiCl (242°C) and Me BiCl (116 C) suggest that these compounds may be associated in the solid state. Phosphorus forms a complete set of halides Me PX, (Table 4.4). The fluorides are monomeric, volatile species, which have trigonal bipyramidal structures. Electron diffraction studies show that the fluorine atoms preferentially occupy the axial positions. The axial P—bonds are longer than the equatorial P—n.m.r. studies confirm these findings for Me PF ... 

---