Octahedral complexes stereoisomerism

Electrochemically induced cis-to-trans postitional stereoisomerization of ligands has been found on oxidation-reduction of several types of octahedral complexes of groups VIIB metals [510-519]. The key step is isomerization of cationic cw-intermediates formed by one-electron oxidation. 

Bis(tridentate) complexes (i.e. octahedral complexes containing two identical linear tridentate ligands) may exist in three stereoisomeric forms, and there will be more if the tridentate ligands do not themselves contain some symmetry elements. The three isomers of the simplest case are represented below (Examples 1, 2 and 3), along with their polyhedral symbols (Section IR-9.3.2.1) and configuration indexes (Section IR-9.3.3.4). Complexes of A-(2-aminoethyl)ethane-1,2-diamine and iminodiacetate can be described by these diagrams. 

Octahedral complexes of the type MX3Y3 show a different type of stereoisomerism. If the three X ligands are in a plane that includes the M ion (with the three Y ligands in a plane at right angles), then the structure is called the mer-isomer ( mer stands for meridional). If the three X (and Y) ligands are all on a face of the octahedron, the structure is the fac-isomer ( fac stands for facial). See also E-Z convention... 

The term cis describes geometric isomers in which two groups are attached on the same side of a double bond in an organic molecule, or along the same edge of a square in a square-planar complex, or at two adjacent vertices of an octahedral complex. (See also geometric isomerism.) cis-trans isomerism is a type of stereoisomerism. 

Stereoisomerism is commonly shown by octahedral (six co-ordinate) complexes associated with bidentate ligands. An example is the complex containing nickel as the transition metal and 1,2-diaminoethane (NH2CH2CH2NH2) as the bidentate ligand (Figure 24.9). The two isomers are stereoisomers because the two different molecules are mirror images of each other and cannot be superimposed. 

---