Stereochemical activity

Bismuth Trifluoride. Bismuth(III) duoride is a white to grey-white powder, density 8.3 g/mL, that is essentially isomorphous with orthorhombic YF, requiring nine-coordination about the bismuth (11). It has been suggested that BiF is best considered an eight-coordinate stmcture with the deviation from the YF stmcture resulting from stereochemical activity of the bismuth lone-pair electrons. In accord with its stmcture, the compound is the most ionic of the bismuth haUdes. It is almost insoluble in water (5.03 0.05 x 10 M at pH 1.15) and dissolves only to the extent of 0.010 g per 100 g of anhydrous HF at 12.4°C. 

The reaction of Te(N3)4 with ionic azides generates the [Te(N3)6] anion (Eq. 5.8). The stmeture of this anion is strongly distorted from octahedral by the stereochemically active lone pair on the tellurium atom, which gives rise to substantial differences in the Te-N bond lengths. Eour of these bonds are in the range 2.09-2.24 A, cf. [Te(N3)5] ,... 

No completely general and quantitative theory of the stereochemical activity of the lone-pair of electrons in complex halides of tervalent As, Sb and Bi has been developed but certain trends are discernible. The lone-pair becomes less decisive in modifying the stereochemistry (a) with increase in the coordination number of the central atom from 4 through 5 to 6, (b) with increase in the atomic weight of the central atom (As > Sb > Bi), and (c) with increa.se in the atomic weight of the halogen (F > Cl > Br > 1). The relative energies of the various valence-Ievel orbitals may also be an important factor the F(a) orbital of F lies well below both the s and the p valence... 

Stereochemical Activity of Lone Pairs in Heavier Main-group Element Compounds... 

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... 

When Does a Lone Pair of Electrons Become Stereochemically Active. -Observations... 

So far, many rules of thumb have appeared in the Hterature and have also found their way into general and inorganic chemistry textbooks [19]. Unfortunately, aU these mles of thumb lack deeper explanation or quantification, and their predicting power is generally low. The most common mles (together with some examples) of when a lone pair is expected to become stereochemically active are ... 

The stereochemical activity of the lone pair of a central atom ... 

Heavier main-group elements like T1(I), in complex compounds, generally exhibit a stereochemically active lone pair when the donor atoms are nitrogen, oxygen or fluorine [23]. 

C is enhanced by ligands that form more covalent bonds to the central atom The tendency for the lone pair, for example, on Pb(II) to become stereochemically active grows as the tendency of the donor atoms on the ligand increases to form more covalent metal-ligand bonds [24]. 

Because of the comparatively large space requirements of a lone pair, low coordination numbers favor the expression of a stereochemically active lone pair [25]. Specific ligand design can trigger the stereochemical activity of a lone pair in complex compounds. 

Lighter elements show a stronger tendency to develop a stereochemically active lone pair than their heavier homologues. For instance, for antimony(III) more distorted structures are known than for bismuth(III) ]29]. 

The repulsion of the thallium and oxygen lone pairs lead to a distortion of the Tl-6s lone pair, which is no longer totally spherical as one would intuitively expect for an s orbital. The thallium contributions involved in the anti-bonding combination are indeed of 97.7% s-, 1.8% p- and 0.5% d-character. Thus, a much less than expected p-character is found for the thallium orbital, which shows that extensive s-p mixing or hybridization is not essential for the lone pair to become stereochemically active. 

I 2 Stereochemical Activity of Lone Pairs in Heavier Main-group Element Compounds -ICOHP (eV/cell) -ICOHP (eV/cell)... 

In summary, one can state that s-p-hybridization on the heavier main group metals is not responsible for the stereochemical activity of a lone pair. Instead, the general conclusion can be drawn that anti-bonding metal ns-ligand np interactions lead to structural distortions in order to minimize these unfavorable interactions. 

In cases where elements in compounds retain unused exposed electrons, especially those in pairs, they occupy space and are stereochemically active. They are often sites of reaction. 

The free selenite ion has a pyramidal shape (C3v symmetry) owing to the lone electron pair at the selenium atom. Thus, the Se032- ion can be treated as a pseudo-tetrahedral anion and the lone electron pair often acts as an invisible ligand within the crystal structures of selenites. This observation is called the stereochemical activity of the lone electron pair and it will turn out as one of the... 

---