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

JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Coordination Compounds

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 ______.

Answer: 6

The electron counts are

S c^{3 +}

18,

C r^{2 +}

22,

M n^{3 +}

22,

C o^{3 +}

24 and

F e^{3 +}

23, so the isoelectronic pair is

C r^{2 +}

and

M n^{3 +}

and

n = 2

. Two moles of AgCl per mole of complex means two chloride ions lie outside the coordination sphere, fixing the complex as

[Co (en)_{2} (N H_{3}) Cl] C l_{2}

. The central

C o^{3 +}

d^{6}

and, with the strong-field ethylenediamine and ammonia ligands, is low-spin octahedral with configuration

t_{2 g}^{6} e_{g}^{0}

. The number of electrons in the

t_{2 g}

set is six.

JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Integer answer