Geometrical and optical isomerism

Isomers share a formula but differ in how the atoms are arranged. Stereoisomers have the same bonds in different spatial arrangements; structural isomers have different bonds.

Geometrical isomerism appears in heteroleptic complexes of CN 4 and 6. In square planar [MX₂L₂], the two X ligands sit side by side (cis) or across from each other (trans), as in cis- and trans-[Pt(NH₃)₂Cl₂].

Square planar [MABXL] with four different unidentate ligands has three geometrical isomers: two cis and one trans.

Tetrahedral complexes never show geometrical isomerism: every position is adjacent to every other, so the ligands' relative positions are all alike.

Octahedral [MX₂L₄] ([Co(NH₃)₄Cl₂]⁺) and [MX₂(L–L)₂] ([CoCl₂(en)₂]⁺) also form cis and trans isomers.

Octahedral [Ma₃b₃], such as [Co(NH₃)₃(NO₂)₃], gives fac and mer isomers: three identical donors on the corners of one face (facial), or along a meridian of the octahedron (meridional).

Optical isomers (enantiomers) are non-superimposable mirror images, and such molecules or ions are chiral. The dextro (d) form rotates plane-polarised light to the right and the laevo (l) form to the left in a polarimeter.

Optical isomerism is common in octahedral complexes with didentate ligands, e.g. [Co(en)₃]³⁺. For [PtCl₂(en)₂]²⁺ only the cis isomer is optically active; the trans form has a mirror plane.

Of cis- and trans-[CrCl₂(ox)₂]³⁻, only the cis form is chiral.