Equivalent ligands or, more completely, chemically equivalent ligands in an organic molecule are like ligands that have identical chemical properties under all or a specified set of conditions. A test known as the replacement test is used to find if two like ligands in a molecule are equivalent.
Apply the replacement test to find if the two hydrogen atoms in 1 are equivalent:
Label the two hydrogen atoms in 1 for identification purposes.
Replace H (a) in 1 with a hypothetical atom (X).
Replace H (b) in 1 with the same hypothetical atom, X.
Compare molecules 2 and 3. They are superimposable on each other, meaning that they are identical. Identical molecules have identical chemical properties under all conditions. Since the replacement of H (a) and of H (b) in 1 with the same atom leads to molecules that have identical chemical properties under all conditions, H (a) and H (b) have identical chemical properties under all conditions, i.e., H (a) and H (b) are equivalent under all conditions.
see also homotopic, enantiotopic, diastereotopic, constitutionally heterotopic