What are the facial and meridional isomers ? Explain with one example

Isomers are compounds that share the same molecular formula but differ in how their atoms are arranged or bonded.

In chemistry, octahedral molecular geometry refers to compounds where six atoms or groups of atoms (ligands) are symmetrically arranged around a central atom, forming the shape of an octahedron. An octahedron has eight faces (hence the name octa). Although it is one of the Platonic solids, in octahedral molecules the central atom is surrounded by ligands, and there are no bonds between the ligands themselves.

Examples of octahedral compounds include sulfur hexafluoride (SF₆) and molybdenum hexacarbonyl (Mo(CO)₆). The term “octahedral” is often used to describe the shape around the central atom, even if the ligands are different. For instance, the complex [Co(NH₃)₆]³⁺ is considered octahedral, even though the N–H bonds are not perfectly symmetrical.

When two or more types of ligands are attached to the central atom in an octahedral complex, isomerism can occur. The naming of these isomers depends on how the ligands are arranged:

• Facial (fac) isomer: When three identical ligands occupy one face of the octahedron (all adjacent to each other).
• Meridional (mer) isomer: When three identical ligands lie along a plane (some are cis and some are trans to each other).

For example, in the complex [Co(NH₃)₃Cl₃], there are two possible isomers:

• In the fac isomer, all three chlorine ligands are located on one face, while the three ammonia ligands are on the opposite face.
• In the mer isomer, the ammonia and chlorine ligands are arranged across different faces of the octahedron.