Why do isomeric bis-ethylenediamine cobalt complexes react differently?

A compound $\ce{Co(en)2(NO2)2Cl}$ has been prepared in a number of isomeric forms.One form undergoes no reaction with $\ce{AgNO3}$ or $\ce{(en)}$ and is optically inactive.


A second form reacts with $\ce{AgNO3}$ but not with $\ce{(en)}$ and is optically inactive.


A third from is optically active and reacts with both $\ce{AgNO3}$ and $\ce{en}$. Identify each of these isomeric forms. Name and sketch each of the structures.


*My attempt:*


I understood how to use the information of reaction with $\ce{AgNO3}$ and optical activity but couldn't understand what does reaction with $\ce{en}$ represent (I know $\ce{en }$ means ethylene diamine). 


In solution, it is given that, if the compound reacts with en it means monodentate ligands are cis to each other,if not it is trans to each other.
I couldn't understand why it is so. please help.


