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What I fail to understand is why does trans isomer contain $2$ $\ce{Cl}$ at axial positions but $2$ $\ce{NH3}$ are not taken to be so. Isn't the two $\ce{NH3}$ in the trans isomer in the picture cis to each other?

  • 2
    $\begingroup$ Trans and cis here are relative terms: the structures shown are with respect to Cl, while it can also be made with respect to NH3 as well $\endgroup$ Jan 26 '19 at 8:29
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    $\begingroup$ According to IUPAC nomenclature, in case of octahedral systems, the terms cis and trans are only used for octahedral centres of the form $\ce{[Ma2b4]}$, i.e. with four ligands of one kind and two of another. $\endgroup$
    – user7951
    Jan 26 '19 at 8:42

The way I justify it relates to the main plane of the complex: referring to your compounds, the plane I am talking about it the one which includes Co, the two NH3 and ethylenediamine. If you want to "join together" with a line the two chlorides, you have to "cross that plane", therefore your compound is trans.

In the other case (the compound on the right), the plane includes Co, ethylenediamine, chloride and NH3. If you want to do the same thing, i.e. linking the two chlorides or the two ammonia together, you don't cross that plane. In the end, your compound is cis.

Also, take a look at this website as well. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Coordination_Chemistry/Properties_of_Coordination_Compounds/Isomers/Stereoisomers%3A_Geometric_Isomers_in_Transition_Metal_Complexes_II

Feel free to add more questions about this!


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