geometric (cis-trans and fac-mer) isomerism

Imagine a square table with four seats and you must seat two guests of one kind and two of another. You can put the matching guests side by side (adjacent) or directly across from each other (opposite). Same guests, same table — but a genuinely different seating. Geometric isomerism in complexes is exactly this: same ligands, same metal, same connectivity, but a different placement around the metal.

The most common form is cis-trans isomerism. In a square planar MA2B2 complex (like the platinum drugs), the two A ligands can sit next to each other (cis, 90 degrees apart) or opposite each other (trans, 180 degrees apart). The same happens in an octahedral MA4B2: cis if the two B ligands are adjacent, trans if they are on opposite poles. A different pattern appears in octahedral MA3B3: the three A ligands can occupy one triangular face of the octahedron (facial, abbreviated fac) or lie in a meridian plane spanning from pole to pole (meridional, abbreviated mer). These are not interconvertible without breaking bonds, so they are isolable, distinct compounds. Note that perfectly tetrahedral complexes show no cis-trans isomerism, because all four positions are equivalent and equally adjacent.

Geometric isomers are a type of stereoisomer (specifically diastereomers — not mirror images), and they can differ dramatically in properties. The most famous case is cisplatin, cis-[PtCl2(NH3)2], a major anticancer drug, whose trans isomer is far less effective against tumors — same atoms, same bonds, but the cis arrangement lets it bind DNA in the way that matters. Identifying cis versus trans, or fac versus mer, is therefore not a paper exercise; it can be the difference between a medicine and an inactive look-alike.