Which pair of metal complexes are isomers?

Isomers are compounds that have the same molecular formula but differing structural or spatial arrangements of atoms. In the case of metal complexes, isomerism typically refers to different ways that ligands can coordinate to a central metal ion or differences in their structural arrangements.

The correct choice in this context is the pair consisting of [Cu(H2O)6]²⁺ and [Cu(H2O)5]+. Both complexes have the same copper ion and are made up of water as the ligand, but they differ in their coordination numbers. In the first complex, the copper ion is coordinated to six water molecules, while in the second complex, it is coordinated to only five. This difference in the number of ligands constitutes a form of isomerism known as coordination isomerism, where variations in ligand coordination can lead to different structural forms of the same metal complex.

Understanding this concept of isomerism is key for recognizing that coordination numbers and the arrangement of ligands around a metal center can result in distinct compounds, even if they share some similarities in their composition.