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

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  

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I 2 Stereochemical Activity of Lone Pairs in Heavier Main-group Element Compounds 2.3.1... 

A second problem occurs as a result of the possible stereochemical non-activity of the lone pair of electrons associated with the Group 15 element, and this is a problem common to the heavier elements of the neighbouring main groups in the Periodic Table. The VSEPR approach to molecular structure of the compounds of these heavier elements is tantamount to saying that d orbitals become part of any hybridization scheme and all the valence electrons are stereochemically active. 

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