Molecular shape stereoisomerism

An isomer is one of several species that have the same atomic composition (molecular formula), but have different constitutional formulae (atom connectivities) or different stereochemical formulae. Isomers exhibit different physical and/or chemical properties. 

In this section we discuss stereoisomerism. Examples are taken from both p- and /-block chemistry. Other types of isomerism are described in Section 20.8. 

Sqttare planar species of the general form EX2Y2 or EX2YZ may possess cis- and traus-isomers. 

There are two types of stereoisomerism associated with octahedral species. In EX2Y4, the X groups may be mutually cis or trans as shown for [SnF4Me2) (2.34 and 2.35). In the solid state structure of [NH4]2[SnF4Me2], the anion is present as the /rans -isomer. 

If two species have the same molecular formula and the same atom connectivity, but differ in the spatial arrangement of different atoms or groups about a central atom or a double bond, then the compounds are stereoisomers. 

It is important to note that whereas the VSEPR model may be applicable to p-block species, it is not appropriate to apply it to J-electron configurations of transition metal compounds. 

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 nonbonding role in a molecule or ion is termed the stereochemical inert pair effect. 

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MO theory heteronuclear diatomics Molecular shapes The VSEPR model Stereoisomerism... 

This concept for the cycle- and nnsatnration-free control of molecular shapes is likely to find multiple applications. For example, fonr stereoisomeric vicinal diflnorinated analogues of the natural cyclic heptapeptide unguisin A were found to adopt dramatically different secondary strnctnres as a function of stereoelectronic and steric interactions. The preferred conformations of PEG (polyethylene glycol) groups (popular solubilizing substitnents for many biomedical applications) are likely to be affected by the gauche effect as well. 

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