For a complex, $MX_3Y_3$ possessing trigonal prismatic geometry, the number of possible isomer is $3$. These are as follows :
fac- and mer- isomerismoccurs in octahedral coordination entities of the type $[Ma_3b_3]$. 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.
A coordination compound holds a central metal atom or ion surrounded by various oppositely charged ions or neutral molecules. These molecules or ions are re-bonded to the metal atom or ion by a coordinate bond.
Coordination entity:
A coordination entity composes of a central metal atom or ion bonded to a fixed number of ions or molecules.
Ligands:
A molecule, ion, or group which is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is commonly called a ligand. It may be either neutral, positively, or negatively charged.
