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Which one of the following is expected to exhibit optical isomerism ?
(en = ethylenediamine)
(en = ethylenediamine)
A
cis-$[Pt(NH_3)_2Cl_2]$
B
cis-$[Co(en)_2Cl_2]$
C
trans-$[Co(en)_2Cl_2]$
D
trans-$[Pt(NH_3)_2Cl_2]$
Detailed Solution
A complex shows optical isomerism if it has no plane of symmetry, so that its mirror image cannot be superimposed on it.
cis-$[Co(en)_2Cl_2]^+$ is octahedral with the two chloride ions adjacent to each other; the two en rings are arranged like the blades of a propeller.
It has no plane of symmetry, so it exists as a pair of enantiomers (d and l forms).
trans-$[Co(en)_2Cl_2]^+$ has the two chloride ions opposite each other and has a plane of symmetry, so it is optically inactive.
$[Pt(NH_3)_2Cl_2]$ is square planar; both its cis and trans forms lie in a plane, which is a plane of symmetry, so neither is optically active.
So the complex expected to exhibit optical isomerism is cis-$[Co(en)_2Cl_2]$.
cis-$[Co(en)_2Cl_2]^+$ is octahedral with the two chloride ions adjacent to each other; the two en rings are arranged like the blades of a propeller.
It has no plane of symmetry, so it exists as a pair of enantiomers (d and l forms).
trans-$[Co(en)_2Cl_2]^+$ has the two chloride ions opposite each other and has a plane of symmetry, so it is optically inactive.
$[Pt(NH_3)_2Cl_2]$ is square planar; both its cis and trans forms lie in a plane, which is a plane of symmetry, so neither is optically active.
So the complex expected to exhibit optical isomerism is cis-$[Co(en)_2Cl_2]$.