Stereochemically active lone pair electrons

For systems which possess stereochemically active lone-pair electrons in the crystalline state, a definitive experimental answer to the title question is at least in principle available from difference density analysis of X-ray results where the potential for deriving electron distributions from elastic diffraction data is now being realized (13, 31, 104). In one application to subvalent molecules (164), (CH3)2TeCl2 was shown to possess a peak of 0.27 e/A3 centered at 0.9 A from the Te(IV) atom in the position expected for a lone pair of electrons (175). 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

Two factors combine to lend a greater diversity in the stereochemistries exhibited by bivalent germanium, tin and lead compounds, the increased radius of Mn compared with that of Mw and the presence of a non-bonding pair of electrons. When the non-bonding pair of electrons occupies the isotropic valence level s orbital, as in, for example, the complex cations Pb[SC(NH2)2]6+ and Pb[antipyrine]6+, or when they are donated to conductance band levels, as in the binary tin and lead selenides or tellurides or the perovskite ternary phases CsMX3 (M = Sn, Pb X = Cl, Br, I), then the metal coordination is regular. However, in the majority of compounds an apparent vacancy in the coordination sphere of the metal is observed, which is usually ascribed to the presence of the non-bonding pair of electrons in a hybrid orbital and cited as evidence for a stereochemically active lone pair . 

Complexes of arseniqlll). anrimony(III), (electron configuration = ( - I lead(II). and bismulh(lll) I(n - 21/14 (n - l)r/ 0/ 2] with polydentate ligands occupying six coordination sites have been found 10 have a stereochemically active lone pair. However, the dichotomy of behavior of the heavier elements that have a lone pair is reflected m Ihe crystal chemistry of Br - When forced into sites of high symmetry, the Bi,+ ion responds by assuming a spherical shape in crystals of lower symmetry the lone pair asserts itself and becomes stereochemically active. 

The octalluoroxenaies are the most stable xenon compounds known they can be heated to 400 °C without decomposition. The anions have square antiprismatic geometry. They, too, present a problem to VSEPR theory analogous to that of XeF6 since they should also have a stereochemically active lone pair of electrons that should lower the symmetry of the anion. If the steric crowding theory is correct, however, the presence of eight ligand atom/, could force the lone pair into a stereochemically inert s Orhital. 

Fig. 12. Valence bond interpretation of the bending in Group 14 metallocenes. The bent structure stems from a stereochemically active lone pair of electrons, which is approximately sp1 hybridized.

For lead with its relatively large radius, higher coordination numbers have been established, but in some of these there is no evidence for stereochemically active lone pairs as in the 10-coordinate Pb( r -N 03)2(2,9-diformyl-1,10-phenanthrolinedi-semicarbazone). However, in the 1,4,7-triazacyclononane complex, tacnPb(N03)2, the electron pair is stereochemically active. 

BiXe configurations. The polymeric anion Bi2l7 is also known. HCl reacts with BiCh in ether solution to produce the acid H[BiCLi]-(solvent)x, in which a complex anion (5) exists. As seen in the structures, the bismuth atoms which are hypervalent may or may not show stereochemically active lone pairs of electrons. 

Stable monomeric complexes can also be obtained without the presence of bulky ligands. Intramolecular donation of electrons into the empty valency orbital of Ge from nitrogen renders them stability, as seen in (11), which is stable to dry oxygen, but readily hydrolyzes. The same phenomenon occurs in the acetylacetonato complex of Ge(II) and Ge(acac)I, in which germanium is three-coordinate with a stereochemically active lone pair. 

The complex PhBi(S2COMe)2 contains two bidentate xanthogenato chelate ligands which lie equatorially in the coordination octahedron of bismuth, while the phenyl group and the stereochemically active lone-pair of electrons occupy the axial positions. ... 

Actinide compounds with heavy oxoanions containing a stereochemically active lone-pair of electrons... 

The purpose of this chapter is to address the role of heavy oxoanions containing a chemically inert, but stereochemically active, lone-pair of electrons on the structures of purely inorganic actinide compounds. This work is not intended to be a comprehensive survey of all compounds that fall into this class, but rather its goal is to highlight key compounds with atypical structures where the influence of the lone-pair of electrons can be ascertained, if only in part. 

Figure 16. A depiction of part of the polar, three-dimensional network structure of Np02(103) showing the alignment of the stereochemically active lone pair of electrons on the iodate anions along the c-axis.

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