Facial (fac) and Meridional (mer) Isomerism
Understanding geometrical isomerism in octahedral $MA_3B_3$ coordination complexes.
1. Introduction to $MA_3B_3$ Complexes
In coordination chemistry, geometrical isomerism arises when ligands can be arranged in different spatial orientations around a central metal ion. While square planar and general octahedral complexes exhibit cis and trans isomerism, a special type of geometrical isomerism occurs in octahedral complexes with the formula $MA_3B_3$.
Here, M is the central metal ion, and A and B are two different types of monodentate ligands, each present in a quantity of three. These complexes exist in two distinct stereoisomeric forms: Facial (fac) and Meridional (mer) isomers.
Facial (fac) Isomer
Ligands 'A' occupy the corners of one triangular face.
Meridional (mer) Isomer
Ligands 'A' occupy a plane bisecting the octahedron (meridian).
2. The Facial (fac) Isomer
In the facial isomer, three identical ligands are situated on the corners of one of the triangular faces of the octahedron.
- Bond Angles: Any two identical ligands in a fac isomer are mutually cis to one another. The bond angle between any pair of identical ligands is exactly $90^\circ$.
- Symmetry: The molecule possesses a $C_3$ axis of symmetry passing through the center of the face.
3. The Meridional (mer) Isomer
In the meridional isomer, three identical ligands are positioned such that they map out a meridian (an arc or equatorial plane) of the octahedron.
- Bond Angles: The three identical ligands are arranged in a "T" shape. Two of the identical ligands are trans to each other (bond angle $180^\circ$), while the third is cis to the other two (bond angle $90^\circ$).
- Structural Visual: Imagine the equator of a globe; the three ligands wrap around this equator.
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