If Ni²⁺ is replaced by Pt²⁺ in the complex [NiCl₂ Br₂]²⁻, which of the following properties are expected to get changed ?
A. Geometry
B. Geometrical isomerism
C. Optical isomerism
D. Magnetic properties

Solution

Chemistry ma’am · 3 steps

Swap Ni for Pt and the shape flips from tetrahedral to square planar; then check each property against that.

1. Nickel complex. Ni²⁺ is 3d⁸, and Cl⁻ and Br⁻ are weak-field ligands, so the d electrons do not pair up. The complex is sp³, tetrahedral, with 2 unpaired electrons, so it is paramagnetic.
2. Platinum complex. Pt²⁺ is 5d⁸. For a 5d metal the crystal field splitting is large even with weak-field ligands, so all 8 electrons pair. The complex is dsp², square planar, and diamagnetic.

   Property	[NiCl₂Br₂]²⁻	[PtCl₂Br₂]²⁻	Changed?
   A. Geometry	tetrahedral	square planar	yes
   B. Geometrical isomerism	none: every position in a tetrahedron is adjacent to every other	cis and trans forms exist	yes
   C. Optical isomerism	none: MA₂B₂ has a mirror plane	none: the molecular plane is a mirror plane	no
   D. Magnetic properties	paramagnetic, 2 unpaired electrons	diamagnetic, no unpaired electrons	yes

3. So. Geometry, geometrical isomerism and magnetic behaviour all change. Optical isomerism is absent in both complexes, so it stays the same. The answer is (b): A, B and D.

The idea to keep. For d⁸ ions, 3d metals with weak-field ligands go tetrahedral, but 4d and 5d metals like Pt go square planar. Fix the geometry first, and the isomers and magnetism follow from it.

Answer from the NTA final answer key. Solution by PrepWiser.