Question

Number of stereoisomers possible for the complexes, $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$ and $\left[\mathrm{CrCl}_{2}(\mathrm{ox})_{2}\right]^{3-}$ are respectively} (py = pyridine, ox = oxalate)

• A. $3 \& 3$
• B. $2 \& 2$
• C. $2 \& 3$
• D. $1 \& 2$

Hint:

Stereoisomers include geometrical and optical isomers.

Answer 1

The Correct Option is C

To determine the number of stereoisomers for the given complexes, we analyze their geometry and possible ligand arrangements around the central metal ion.

• Complex 1: \([\text{CrCl}_3(\text{py})_3]\)

This coordination compound features chromium (Cr) as the central metal, coordinated by three chloride ions (Cl) and three pyridine molecules (py). The complex exhibits octahedral geometry, with Cr typically bonded to six ligands in an octahedral configuration.

For octahedral complexes of the type \([\text{MA}_3\text{B}_3]\), two types of isomerism are possible:

• Facial (fac) isomer: Three identical ligands occupy adjacent positions on one octahedral face.
• Meridional (mer) isomer: Three identical ligands are arranged in a line across the center of the octahedron.

Consequently, the complex \([\text{CrCl}_3(\text{py})_3]\) can exist as two stereoisomers: fac and mer.

• Complex 2: \([\text{CrCl}_2(\text{ox})_2]^{3-}\)

The second complex involves chromium as the central metal ion, two chloride ions (Cl), and two oxalate ions (ox). Oxalate is a bidentate ligand, coordinating to the metal at two sites and forming chelate rings.

This complex also displays octahedral geometry. Stereoisomers arise from different ligand arrangements:

• Cis isomer: Ligands of the same type are adjacent.
• Trans isomer: Ligands of the same type are opposite each other.
• Optical isomerism: Non-superimposable mirror images resulting from chiral arrangements.

The complex \([\text{CrCl}_2(\text{ox})_2]^{3-}\) can exhibit:

• Two cis-trans isomers:
  • One cis form
  • One trans form
• Chiral centers are not formed when oxalate ligands are trans. However, when oxalate ligands are cis, optical isomers emerge due to non-superimposable mirror images, creating two additional isomers (enantiomers). This results in:
  • One optically active pair (left-handed and right-handed)
  • A total of 3 stereoisomers (1 trans, 1 cis pair which is optically active)

Therefore, the complex \([\text{CrCl}_2(\text{ox})_2]^{3-}\) has 3 stereoisomers.

In summary, the number of stereoisomers for \([\text{CrCl}_3(\text{py}_3)]\) is 2, and for \([\text{CrCl}_2(\text{ox}_2)]^{3-}\) is 3. The final answer is 2 & 3.