A cobalt(III) compound of formula CoCl3.4NH3 behaves as a 1:1 electrolyte and gives 1 mol of AgCl per mole with excess AgNO3. Treating NH3 as strong-field on Co^3+, how many geometrical isomers does its octahedral complex ion have and what is its magnetic behaviour?

2 isomers; 4 unpaired electrons

JEE Advanced levelSingle correct optionDifficulty 3 of 3cc205

A cobalt(III) compound of formula $CoCl X_{3} \cdot 4 NH X_{3}$ behaves as a 1:1 electrolyte and gives 1 mol of AgCl per mole with excess $AgNO X_{3}$ . Treating $NH X_{3}$ as strong-field on $Co X^{3 +}$ , how many geometrical isomers does its octahedral complex ion have and what is its magnetic behaviour?

1:1 and one free chloride give

[Co (NH X_{3}) X_{4} Cl X_{2}] Cl

. The cation is of type

MA X_{4} B X_{2}

, with cis and trans forms, i.e. 2 geometrical isomers (a computer enumeration of the rotation group gives 2 stereoisomers). Co(III) low spin is

t_{2 g}^{6}

, so it is diamagnetic.