Question

Which one of the following complexes has least number of stereoisomers?

• A. \([Co(NH_{3})_{4}Cl_{2}]Cl\)
• B. \([Co(en)(NH_{3})_{2}Cl_{2}]Cl\)
• C. \([Co(en)_{2}Cl_{2}]Cl\)
• D. \([Co(en)_{3}]Cl_{3}\) \bigskip

Hint:

For octahedral complexes: \[ [MA_4B_2] \] shows only cis-trans isomerism, whereas complexes containing en usually exhibit optical isomerism as well.

Answer 1

The Correct Option is A

Concept: The number of stereoisomers depends upon:
• Geometrical isomerism (cis-trans)
• Optical isomerism
• Presence of bidentate ligands such as ethylenediamine (en) Complexes containing only monodentate ligands generally exhibit fewer stereoisomers than complexes containing bidentate ligands.

Step 1: Examine option (A).
\[ [Co(NH_3)_4Cl_2]^+ \] This octahedral complex shows only cis and trans forms. Hence number of stereoisomers \[ =2 \] Neither form is optically active.

Step 2: Examine option (B).
\[ [Co(en)(NH_3)_2Cl_2]^+ \] Because of one bidentate ligand, cis forms become optically active. Total stereoisomers are more than 2.

Step 3: Examine option (C).
\[ [Co(en)_2Cl_2]^+ \] Cis form exists as optical isomers. Thus stereoisomers exceed those of option (A).

Step 4: Examine option (D).
\[ [Co(en)_3]^{3+} \] This complex exists as two optical isomers: \[ \Delta \text{ and } \Lambda \] Hence it is optically active.

Step 5: Compare all possibilities.
The complex having the minimum stereoisomerism is \[ [Co(NH_3)_4Cl_2]Cl \] \[ \boxed{\text{Answer = (A)}} \]