Chemistry - Coordination Chemistry Question with Solution | TestHub

The initial complex (P) is $[{\mathrm{Co}}_2(\mathrm{en}{)}_4(\mu -\mathrm{OH}{)}_2]{\mathrm{Cl}}_4$. Reaction with HCl gives $[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]\mathrm{Cl}$ (Q), which is resolvable (cis isomer). Isomer (R) is the trans isomer, $[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_2]\mathrm{Cl}$, which is not optically resolvable due to a plane of symmetry.

A. If one 'en' in (R) (trans-$[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_2{]}^{+}$) is replaced by two ${\mathrm{NH}}_3$ molecules, it becomes trans-$[\mathrm{Co}(\mathrm{en})({\mathrm{NH}}_3{)}_2{\mathrm{Cl}}_2{]}^{+}$. This complex is still optically inactive (has a plane of symmetry). So, statement A is incorrect.

B. If one 'en' in (R) is replaced by two ${\mathrm{NH}}_3$ and a second 'en' is replaced by two ${\mathrm{H}}_2\mathrm{O}$ molecules, it becomes trans-$[\mathrm{Co}({\mathrm{NH}}_3{)}_2({\mathrm{H}}_2\mathrm{O}{)}_2{\mathrm{Cl}}_2{]}^{+}$. This complex is still optically inactive (has a plane of symmetry). So, statement B is correct.

C. If one 'en' in (Q) (cis-$[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_2{]}^{+}$) is replaced by two ${\mathrm{NH}}_3$ molecules, it becomes cis-$[\mathrm{Co}(\mathrm{en})({\mathrm{NH}}_3{)}_2{\mathrm{Cl}}_2{]}^{+}$. This complex is chiral and optically resolvable. So, statement C is correct.

D. If one 'en' in (Q) is replaced by two ${\mathrm{NH}}_3$ and a second 'en' is replaced by two ${\mathrm{H}}_2\mathrm{O}$ molecules, it becomes cis-$[\mathrm{Co}({\mathrm{NH}}_3{)}_2({\mathrm{H}}_2\mathrm{O}{)}_2{\mathrm{Cl}}_2{]}^{+}$. This complex is chiral and optically resolvable. So, statement D is incorrect.

The final answer is $\boxed{B,C}$