The complex ion [Co(en)2Cl2]^+ (en = ethane-1,2-diamine, a strong-field bidentate ligand) is prepared. The total number of stereoisomers possible for…

Cobalt is in the $+3$ state ($\mathrm{Co^{3+}}$, $3d^6$): the ion carries $+1$, en is neutral and two chlorides give $-2$. Ethylenediamine is a strong-field ligand, so the six $d$ electrons pair up in the $t_{2g}$ set ($t_{2g}^6e_g^0$, $d^2sp^3$), with no unpaired electron, so $\mu=\sqrt{n(n+2)}=0$ BM. For the octahedral $[M(AA)_2X_2]$ type there are two geometrical isomers. The trans isomer has a mirror plane and is optically inactive (one form). The cis isomer lacks any mirror plane or inversion centre and exists as a pair of non-superimposable mirror images ($\Delta$ and $\Lambda$). Total stereoisomers $=1+2=3$. Counting only the geometrical isomers gives 2, while a high-spin $d^6$ ion would give $4.90$ BM.