MA3B3 fac and mer isomers


fac



mer
Stereoisomers are compounds that have the same atom-to-atom bonding sequence but a different arrangement of the atoms in space.

There are two possible stereoisomers for octahedral complexes of the general formula MA3B3.

These are referred to as fac and mer isomers.
The fac (facial) isomer has the three identical ligands at the corners of a face of the octahedron.
Recall that an octahedron is a polygon with eight triangular faces.

The mer (meridional) isomer has the three identical ligands in the same plane through the metal M.
Mer stands for meridional which is appropriate in this case because the three ligands of one type are on a meridian (like the equator).
 
In studying isomers, it becomes important to recognise the symmetry properties of a particular isomer of a coordination complex.  Both of the fac and mer isomers are symmetrical.

In the fac isomer the symmetry plane is in the plane of the page through the vertical A-M-B bonds.
The A pointing out of the plane of the page mirrors with the A pointing to behind the plane of the page, and likewise for the two B's. 

In the mer isomer the symmetry plane is perpendicular to the plane of the page through the M and the four groups pointing in and out of the plane of the page.
The B above the symmetry plane mirrors with the B below the symmetry plane. 
Dr Sheila Woodgate
© The University of Auckland 2002