For a complex, MX3Y3 possessing trigonal prismatic geometry, the number of possible isomer is 3. These are as follows:
fac- and mer- isomerism occurs in octahedral coordination entities of the type [Ma3b3]. If 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 meridional (mer) isomer.