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

JEE Main2026Jan 28, Shift 2Chemistry

The number of isoelectronic species among Sc $^{3 +}$ , Cr $^{2 +}$ , Mn $^{3 +}$ , Co $^{3 +}$ and Fe $^{3 +}$ is ' $n$ '. If ' $n$ ' moles of AgCl is formed during the reaction of complex with formula $CoCl_{3} (en)_{2} NH_{3}$ with excess of AgNO $_{3}$ solution, then the number of electrons present in the $t_{2 g}$ orbital of the complex is ____ .

Your answer

Solution

Electron count of each ion (Z - charge):

Sc $^{3 +}$ : 21 - 3 = 18; Cr $^{2 +}$ : 24 - 2 = 22; Mn $^{3 +}$ : 25 - 3 = 22; Co $^{3 +}$ : 27 - 3 = 24; Fe $^{3 +}$ : 26 - 3 = 23.

Isoelectronic pair: Cr $^{2 +}$ and Mn $^{3 +}$ (both 22 electrons). So $n = 2$ .

Identify the complex giving 2 AgCl: $CoCl_{3} (en)_{2} NH_{3}$ has 3 chlorides; only the ionisable ones precipitate with AgNO $_{3}$ . For $n = 2$ AgCl, two Cl $^{-}$ are ionic and one is coordinated. Formula: $[Co(en)_{2} (NH_{3}) Cl] Cl_{2}$ .

$t_{2 g}$ count in the cation: Co is $+ 3$ , i.e. $d^{6}$ . The ligand set (en, NH $_{3}$ ) is strong-field $\Rightarrow$ low-spin $\Rightarrow t_{2 g}^{6} e_{g}^{0}$ .

Electrons in $t_{2 g}$ $= 6$ .

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