The oxidation number of the central atom in a complex is defined by considering the charge it would have if all ligands and shared electron pairs are removed. It is represented by a Roman numeral in parenthesis following the coordination ...

Homoleptic complexes, like [Co(NH₃)₆]³⁺, involve a metal bound to only one kind of donor group. Heteroleptic complexes, such as [Co(NH₃)₄Cl₂]⁺, feature a metal bound to more than one kind of donor group.

Optical isomers are mirror images that cannot be superimposed, known as enantiomers. Enantiomers are chiral and rotate the plane of polarized light either to the right (dextro – d) or to the left (laevo – l) in a polarimeter.

In octahedral coordination entities like [Co(NH₃)₃(NO₂)₃], if ligands occupy adjacent positions at the corners of an octahedral face, it is the facial (fac) isomer. When they are around the meridian, it is the meridional (mer) isomer.

Geometrical isomerism occurs in complexes of the formula [MX₂(L–L)₂] when didentate ligands like ethane-1,2-diamine (en) are present.