Coordination isomers occur when two complex ions have:

Coordination isomers arise when two complex ions have the same composition but differ in the connectivity of their ligands to the central metal ion. This can occur in cases where a ligand can bond to the metal in more than one way, creating isomers that are structurally distinct despite having the same overall formula and types of ligands. Therefore, when we refer to different bonding orders, it relates to how the ligands are attached to the metal center—some might be bonded in a bidentate manner while others might be monodentate, for example.

While the choices concerning donor atoms, ligands, and identical structures may seem appealing, they do not accurately capture the nature of coordination isomers. Different donor atoms reflect an entirely different ligand, same ligands would not lead to isomerism, and identical structures contradict the premise of isomerism, which fundamentally relies on structural differences in bonding. Thus, the presence of different bonding orders accurately describes the essential characteristic of coordination isomers.