JEE Advanced 2026 Paper 2 Chemistry Question 13 Solution

In an octahedral complex $MA_4B_2$ ( $cis$ isomer), the two $B$ ligands (Cl) are adjacent and share one edge of the octahedron. This edge is shared by exactly two triangular faces. Since all other vertices are $A$ (N), these two faces will have the composition $\{Cl, Cl, N\}$ . Thus, count for $cis$ = 2.
In an octahedral complex $MA_3B_3$ ( $mer$ isomer), the three $B$ ligands (Cl) are arranged such that they occupy two adjacent edges in a 'T' shape (meridional). Specifically, if Cl are at positions $(1,0,0), (0,1,0), (-1,0,0)$ , the edges are between $(1,0,0)-(0,1,0)$ and $(0,1,0)-(-1,0,0)$ . Each of these two edges is shared by 2 faces that also contain an $N$ atom (axial positions). Total faces with $\{2 Cl, 1 N\}$ = $2 + 2 = 4$ .
Sum = $2 + 4 = 6$ .

Question 13