Introduction geometric isomerizations

The introduction of more than one C- or jV-substituted en ring into the coordination sphere considerably increases the isomeric complexity. Many of these ligands are now unsymmetrical and their complexes may exhibit geometric isomerism dependent upon the end-for-end orientation.257 Thus there are 24 distinct configurational and conformational forms expected for Co(R,5-pn)3+, 258 Table 6 lists a variety of C- and JV-substituted ethylenediamine type ligands that have been investigated. 

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

The purpose of the present paper is to review this field of research - a summary of this review appeared in (4). After a brief introduction (part 2) to the XPS technique itself, specially oriented to the study of the polymer valence bands, given in order to stress upon the inherent difficulties (and limitations) of the experiment, we shall present specific examples of the various types of informations that can be acquired on the polymer molecular and electronic structure (part 3) substitution effects in the valence bands (part 4) structural isomerism, stereoisomerism, and geometrical crystalline structure (part 5) other types of information. 

Introduction of a different ligand alters the relative stabilities of cis- and trans- isomers, so there may be a driving force for isomerization, leading to this occurring concomitant with substitution. In circumstances where two geometric isomers are energetically similar, both may exist in solution once thermodynamic equilibrium is reached, with the amount of each form present defined by the relative stability of the isomers. 

---