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 need to consider the geometry and possible ligand arrangements around the central metal ion.

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

This complex is a coordination compound where chromium (Cr) is the central metal, coordinated by three chloride ions (Cl) and three pyridine molecules (py). This complex shows octahedral geometry because Cr is typically coordinated by six ligands in an octahedral shape.

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

• Facial (fac) isomer: Three identical ligands occupy adjacent positions at the corners of an octahedral face.
• Meridional (mer) isomer: Three identical ligands occupy positions in a straight line across the complex.

Thus, 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 a central metal ion, two chloride ions (Cl), and two oxalate ions (ox). Oxalate is a bidentate ligand, meaning each oxalate ligand coordinates to the metal at two coordination sites, forming chelate rings.

This type of complex can also display octahedral geometry. For this complex, possible stereoisomers are created due to different arrangements of the ligands, which are:

• 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 due to chiral arrangements.

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

• Two cis-trans isomers:
  • One cis form
  • One trans form
• In cases where oxalato ligands are trans, chiral centers are not being developed. However, when oxalato ligands form a cis structure, optical isomers arise due to non-superimposable mirror images leading to two additional isomers (enantiomers), counting as:
  • One optically active pair (left-handed and right-handed)
  • Total 3 stereoisomers (1 trans, 1 pair of optical for cis)

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

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

Answer 2

1. $\left[\mathrm{CrCl}_{3}(\mathrm{py})_{3}\right]$: - Facial and meridional isomers are possible. - Total stereoisomers = 2. 
2. $\left[\mathrm{CrCl}_{2}(\mathrm{ox})_{2}\right]^{3-}$: 
- Geometrical isomers: cis and trans. 
- Optical isomers for cis: 2. 
- Optical isomers for trans: 1. 
- Total stereoisomers = 3. 
Therefore, the correct answer is (3) 2 & 3.