Explanation

(a)
Octahedral $Co\left( {NH}_{3} \right)_{4}{Br}_{2}Cl$ shows ionisation and geometrical isomerism.
In ionisation isomerism ligands show different coordination sphere and the anions present outside the coordination sphere.
These are exchanged with each other as follows
$Co\left( {NH}_{3} \right)_{4}{Br}_{2}Cl$ $\rightleftharpoons \ \ \ \ \left\lbrack Co\left( {NH}_{3} \right)_{4}{Br}_{2} \right\rbrack^{+} + {Cl}^{-}$
I
$\lbrack Co\left( {NH}_{3} \right)_{4}BrCl\rbrack Br\ \rightleftharpoons \left\lbrack Co\left( {NH}_{3} \right)_{4}Br\ Cl \right\rbrack^{+} + {Br}^{-}$
II
In geometrical isomerism, coordination number of central atom (cobalt) is six and shape is octahedral, so it shows following geometrical isomers.
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