Question

Which of the following is not optically active?
A.${\left[ {Co{{\left( {en} \right)}_3}} \right]^{3 + }}$
B.${\left[ {Cr{{\left( {ox} \right)}_3}} \right]^{3 - }}$
C.$cis - {\left[ {CoC{l_2}{{\left( {en} \right)}_2}} \right]^ + }$
D.$trans - {\left[ {CoC{l_2}{{\left( {en} \right)}_2}} \right]^ + }$

Answer 1

Hint: A substance or molecule, which is unable to rotate the plane of plane polarized light, is said to be optically inactive.

Complete step by step answer:
In order to answer this question, first we need to understand what optical activity is and on what factors it depends on. When we place solutions of complexes in the path of plane polarized light, then light rotates its plane through a certain angle either to the left side or right side, after interacting with the molecules of that solution. This property of complex, by virtue of which it is able to rotate the plane of polarized light, is termed as its optical activity. The complexes that possess this property are said to be optically active.
Studies has shown that optically active complexes are said to exist in the following forms,
Complexes that rotate plane of polarized light towards right side are said to be dextrorotatory(d-form)
Complexes that rotate planes of polarized light towards left direction are said to be laevorotatory (l-form).Complexes that are optically inactive are said to be forming a racemic mixture, i.e., it consists of both the l-form and d-form. Out of these two forms, if one form will rotate the plane of polarized light in one direction, then it will get balanced by another form that will rotate the plane of polarized light in the opposite direction.
Molecules will be non-super imposable on its mirror image.
 The most important property that a molecule should possess in order to be optically active is that the molecule should lack a plane of symmetry and must be asymmetrical.
Now, let us consider each option one by one, and check whether it is optically active or not.
${\left[ {Co{{\left( {en} \right)}_3}} \right]^{ + 3}}$ and ${\left[ {Cr{{\left( {ox} \right)}_3}} \right]^{3 - }}$ - These two complexes are of the form ${\left[ {M{{\left( {AA} \right)}_3}} \right]^{ \pm n}}$ type.

Clearly, these two mirror images are non-super imposable, because of their asymmetrical geometry. So, they are optically active.
${\left[ {CoC{l_2}{{\left( {en} \right)}_2}} \right]^ + }$ - This complex has two geometrical isomers, namely cis-form and trans-form. In cis isomer there is no plane of symmetry, hence it is optically active. But, the trans-form consists of a plane of symmetry and hence it is optically inactive.

 So, the correct option is D.

Note:
Resolution is the term used for the separation of racemic mixture into its components, l-form and d-form. Since d- and l-form are identical in their physical and chemical properties, it is not possible to separate them by ordinary methods such as fractional crystallization, fractional distillation etc.