Number of complexes which show optical isomerism among the following is ________.
cis- [Cr(ox)₂ Cl₂]³⁻,[Co(en)₃]³⁺, cis- [Pt(en)₂ Cl₂]²⁺, cis- [Co(en)₂ Cl₂]⁺, trans- [Pt(en)₂ Cl₂]²⁺, trans- [Cr(ox)₂ Cl₂]³⁻

Solution

Chemistry ma’am · 3 steps

Look for a mirror plane: cis bis-chelates and tris-chelates have none, trans ones always do.

1. What chirality needs. A complex shows optical isomerism only when its mirror image cannot be superimposed on it, that is, when it has no plane of symmetry. In octahedral chelate complexes, [M(AA)₃] and cis-[M(AA)₂B₂] are chiral. Trans-[M(AA)₂B₂] has a mirror plane through the two chelate rings, so it is not chiral.

   Complex	Type	Mirror plane?	Optically active?
   cis-[Cr(ox)₂Cl₂]³⁻	cis-[M(AA)₂B₂]	no	yes
   [Co(en)₃]³⁺	[M(AA)₃]	no	yes
   cis-[Pt(en)₂Cl₂]²⁺	cis-[M(AA)₂B₂], octahedral Pt(IV)	no	yes
   cis-[Co(en)₂Cl₂]⁺	cis-[M(AA)₂B₂]	no	yes
   trans-[Pt(en)₂Cl₂]²⁺	trans-[M(AA)₂B₂]	yes	no
   trans-[Cr(ox)₂Cl₂]³⁻	trans-[M(AA)₂B₂]	yes	no

2. Check the platinum. In [Pt(en)₂Cl₂]²⁺, x + 0 + 2(-1) = +2 gives x = +4. Pt(IV) is octahedral with coordination number 6, so its cis form is chiral like the cobalt one. It is not square planar Pt(II).
3. So. The two trans complexes are inactive and the other four show optical isomerism. The answer is 4.

The common slip: 3. Taking platinum as square planar, which leaves out cis-[Pt(en)₂Cl₂]²⁺, gives 3. Here Pt is +4 and octahedral, so its cis form is chiral.

The idea to keep. For octahedral chelates, [M(AA)₃] and cis-[M(AA)₂B₂] are optically active, and trans forms never are. Always find the metal's oxidation state before assuming a geometry.

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