Definition of linkage isomerism

Linkage isomerism occurs when an ambidentate ligand can coordinate to the central metal through two different donor atoms.

Common examples:

• $NO_2^-$ : nitro / nitrito
• $SCN^-$ : thiocyanato-$S$ / isothiocyanato-$N$

Check each option

Option A:

$[Pd(PPh_3)_2(NCS)_2] \quad \text{and} \quad [Pd(PPh_3)_2(SCN)_2]$

Here, the ligand $SCN^-$ is ambidentate:

• $NCS$ means coordination through $N$
• $SCN$ means coordination through $S$

Since the same ligand is attached through different atoms, these are linkage isomers.

So, A is correct.

Option B:

$[Co(NH_3)_5NO_3]SO_4 \quad \text{and} \quad [Co(NH_3)_5SO_4]NO_3$

In the first compound, $NO_3^-$ is inside the coordination sphere and $SO_4^{2-}$ is outside. In the second, $SO_4^{2-}$ is inside and $NO_3^-$ is outside.

This is ionisation isomerism, not linkage isomerism.

So, B is incorrect.

Option C:

$[PtCl_2(NH_3)_4]Br_2 \quad \text{and} \quad [PtBr_2(NH_3)_4]Cl_2$

Here, $Cl^-$ and $Br^-$ exchange positions between inside and outside the coordination sphere. This is again ionisation isomerism, not linkage isomerism.

So, C is incorrect.

Option D:

$[Cu(NH_3)_4][PtCl_4] \quad \text{and} \quad [Pt(NH_3)_4][CuCl_4]$

Here, ligands are redistributed between two complex ions containing different metals. This is coordination isomerism, not linkage isomerism.

So, D is incorrect.

Final conclusion

The pair representing linkage isomers is:

$\boxed{A}$