Total number of relatively more stable isomer(s) possible for octahedral complex $$[Cu(en)_2(SCN)_2]$$ will be _____

We need to find the total number of relatively more stable isomers for the octahedral complex $$[Cu(en)_2(SCN)_2]$$.

This complex exhibits two types of isomerism. The first is geometrical isomerism, which arises as cis and trans forms due to the arrangement of en and SCN$$^-$$ ligands. The second is linkage isomerism, since SCN$$^-$$ is an ambidentate ligand that can bind through S (thiocyanato, S-bonded) or through N (isothiocyanato, N-bonded).

In an octahedral geometry for $$[Cu(en)_2(SCN)_2]$$, the cis isomer has the two SCN$$^-$$ ligands adjacent (90° apart). This arrangement is more stable because ethylenediamine (en), a bidentate chelating ligand, preferentially occupies cis positions. In contrast, the trans isomer has the SCN$$^-$$ ligands opposite (180° apart), which is relatively less stable for chelating ligands like en.

Since we seek the relatively more stable isomers, we focus on the cis geometrical isomer.

Within the cis form, the two SCN$$^-$$ ligands can coordinate in three ways. Both can bind through N to give $$[Cu(en)_2(NCS)_2]$$ — cis-bis(isothiocyanato). Alternatively, both can bind through S to give $$[Cu(en)_2(SCN)_2]$$ — cis-bis(thiocyanato). Finally, one can bind through N and the other through S to give $$[Cu(en)_2(NCS)(SCN)]$$ — cis-mixed.

These three linkage isomer variants represent the relatively more stable isomers in the cis form. Therefore, the total number of relatively more stable isomers is 3.