Objectives

Fig. 9.3: Geometrical isomers (cis and trans) of [Co(NH ) Cl ]⁺
octahedral complexes of formula [MX₂L₄] in which the two ligands X may be oriented cis or trans to each other (Fig. 9.3).

3 4 2


251 Coordination Compounds

Fig. 9.4: Geometrical isomers (cis and trans) of [CoCl (en) ]
This type of isomerism also arises when didentate ligands L– L [e.g., NH₂ CH₂ CH₂ NH₂ (en)]
are present in complexes of formula [MX₂(L– L)₂] (Fig. 9.4).
Another type of geometrical isomerism occurs in octahedral coordination entities of the type

^{2 2} [Ma b ] like [Co(NH ) (NO ) ]. If
3 3 3 3 2 3

Fig. 9.5
The facial (fac) and meridional (mer) isomers of
[Co(NH₃ )₃(NO₂ )₃]
three donor atoms of the same
ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer (Fig. 9.5).

Why is geometrical isomerism not possible in tetrahedral complexes having two different types of unidentate ligands coordinated with the central metal ion ?

Tetrahedral complexes do not show geometrical isomerism because the relative positions of the unidentate ligands attached to the central metal atom are the same with respect to each other.