Lone pair, stereochemical influence

Lone pair cations exhibit external pairs of electrons which do not participate in the bonds but can influence dramatically the geometry of the structures (52). This is the case of cations like Bi(III), Pb(II) or T1(I) whose 6s2 lone pairs have been shown to present an important stereochemical activity. Such cations which can be found in the rock salt type layers are capable of influencing the oxygen framework and may consequently affect the superconducting properties of the layered cuprates. 

M(II) organometallic compounds are found with cyclopentadienyl. Sn(C5H5)2 has a bent sandwich structure 2, where the stereochemical influence of the lone-pair is apparent. M(IV) organometallic compounds with M C s bonding are extremely varied and include simple tetraalkyls MR4 and compounds with Sn Sn bonds similar to those of Si and... 

The structures of these compounds are octahedral with trans sulfur atoms showing no evidence (cf. TeXi-) of stereochemical influence of the lone pair. 

For both of these complexes the lone-pair electrons are not stereochemically active with respect to the polyhedral coordination, but stabilize the 6s and 5s level, respectively (inert pair effect). The interactions between metals and arenes are rather weak but significantly influence the coordination sphere in comparison to BiCl3 or SbCl3. 

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. 

It is generally accepted, that stereochemical anomalies due to the influence of a lone pair can be analyzed by applying a pseudo-Jahn Teller (PJT)-type treatment.6,7 If G and E are the irreducible representations of the ground state and of a low-lying excited state, respectively,... 

If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 

In contrast with amines, the inversion at the phosphorus atom in phosphanes is much more hindered and, therefore, the influence of the lone pair of electrons at the phosphorus atom becomes more readily apparent if the substituents prefer a particular orientation. This can be enforced by using bulky substituents (Scheme 27). The effect has first been observed for /( P, C) in aminophosphanes, where the J values are large and positive for a sy/j-orientation and small and negative for anti-orientation of the respective N-C bonds and the assumed orientation of the lone electron pair at the phosphorus atom. The analogous stereochemical dependence is observed for V( P,N, Si) as shown for the A-silylaminophosphanes 135 and 136 (Scheme 27). ... 

The loss of selectivity observed made clear the influence of the anomeric alkoxy group, but also showed that a strong stereochemical bias was inherent in the molecule, responsible for the high facial selectivity. The reason for that stereochemical preference could be ascribed to the equatorial substituent at C-5 (as advanced by Prof. Franck) rixing a preferred conformation, or possibly to the presence of the ring oxygen with two astereotopic lone pairs. 

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