If $\mathrm{Ni}^{2+}$ is replaced by $\mathrm{Pt}^{2+}$ in the complex $\left[\mathrm{NiCl}_{2} \mathrm{Br}_{2}\right]^{2-}$, which of the following properties are expected to get changed ?

A. Geometry

B. Geometrical isomerism

C. Optical isomerism

D. Magnetic properties

The complex $\left[\mathrm{NiCl}_{2} \mathrm{Br}_{2}\right]^{2-}$ is a tetrahedral complex due to the fact that Nickel (Ni) in this complex is in its +2 oxidation state, and thus adopts a dsp² hybridization, resulting in a tetrahedral geometry. <br/><br/>If $\mathrm{Ni}^{2+}$ is replaced by $\mathrm{Pt}^{2+}$, the new complex would be $\left[\mathrm{PtCl}_{2} \mathrm{Br}_{2}\right]^{2-}$. Platinum (Pt) in its +2 oxidation state typically forms square planar complexes due to its preference for dsp² hybridization. <br/><br/><ol> <li><p><strong>Geometry</strong> :</p> <ul> <br/><li>$\left[\mathrm{NiBr}_2 \mathrm{Cl}_2\right]^{2-}$ is tetrahedral.</li> <li>$\left[\mathrm{PtBr}_2 \mathrm{Cl}_2\right]^{2-}$ is square planar.</li> <li>Thus, replacing $\mathrm{Ni}^{2+}$ with $\mathrm{Pt}^{2+}$ changes the geometry from tetrahedral to square planar.</li> </ul> </li> <br/><li><p><strong>Geometrical Isomerism</strong> :</p> <ul> <br/><li>Tetrahedral complexes, like $\left[\mathrm{NiBr}_2 \mathrm{Cl}_2\right]^{2-}$, do not exhibit geometrical isomerism.</li> <li>Square planar complexes, like $\left[\mathrm{PtBr}_2 \mathrm{Cl}_2\right]^{2-}$, can exhibit geometrical isomerism.</li> <li>Therefore, geometrical isomerism would be introduced by replacing $\mathrm{Ni}^{2+}$ with $\mathrm{Pt}^{2+}$.</li> </ul> </li> <br/><li><p><strong>Optical Isomerism</strong> :</p> <ul> <br/><li>Both the $\mathrm{Ni}$-based tetrahedral complex and the $\mathrm{Pt}$-based square planar complex are optically inactive. </li> <li>Optical isomerism is not observed in either case.</li> </ul> </li> <br/><li><p><strong>Magnetic Properties</strong> :</p> <ul> <br/><li>The magnetic properties depend on the electronic configuration of the central metal ion.</li> <li>There will be a change in magnetic properties due to the different electronic configurations of $\mathrm{Ni}^{2+}$ and $\mathrm{Pt}^{2+}$.</li> </ul> </li> </ol> <p>Based on this information, the properties expected to change when $\mathrm{Ni}^{2+}$ is replaced by $\mathrm{Pt}^{2+}$ are:</p> <ul> <li>Geometry (from tetrahedral to square planar)</li> <li>Geometrical Isomerism (introduced in the $\mathrm{Pt}$-based complex)</li> <li>Magnetic Properties (due to different electronic configurations)</li> </ul> <p>Optical isomerism remains unchanged (inactive in both cases).</p> <p>Therefore, the correct answer is :</p> <p>Option B: A, B, and D (Geometry, Geometrical Isomerism, and Magnetic Properties).</p>