JEE Main 2026 — Jan 28, Shift 2 — Chemistry solution

JEE Main2026Jan 28, Shift 2Chemistry

The correct increasing order of spin-only magnetic moment values of the complex ions $[MnBr_{4}]^{2 -}$ (A), $[Cu (H_{2} O)_{6}]^{2 +}$ (B), $[Ni (CN)_{4}]^{2 -}$ (C) and $[Ni (H_{2} O)_{6}]^{2 +}$ (D) is:

A
A = B $<$ C $<$ D
B
A = B $<$ C $<$ D
C
C = D $<$ B $<$ A
D
C $<$ B $<$ D $<$ A

Solution

Count unpaired electrons ( $n$ ) for each metal centre:

A. $[MnBr_{4}]^{2 -}$ : Mn $^{2 +}$ is $d^{5}$ ; Br $^{-}$ is weak field; tetrahedral always high spin. $n = 5$ .

B. $[Cu (H_{2} O)_{6}]^{2 +}$ : Cu $^{2 +}$ is $d^{9}$ . $n = 1$ .

C. $[Ni (CN)_{4}]^{2 -}$ : Ni $^{2 +}$ is $d^{8}$ , CN $^{-}$ is strong field $\Rightarrow$ square planar, all electrons paired. $n = 0$ .

D. $[Ni (H_{2} O)_{6}]^{2 +}$ : Ni $^{2 +}$ is $d^{8}$ , octahedral high-spin gives $t_{2 g}^{6} e_{g}^{2}$ . $n = 2$ .

Order ( $μ \propto n (n + 2)$ ): C $<$ B $<$ D $<$ A.

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