JEE Main 2021 · Chemistry · Coordination Compounds · medium

The compound CoCl3·4NH3 has a molar conductance corresponding to two ions per formula unit, and treatment with excess AgNO3 precipitates only one…

Single correct option

The compound

\mathrm{CoCl_3\cdot4NH_3}

has a molar conductance corresponding to two ions per formula unit, and treatment with excess

\mathrm{AgNO_3}

precipitates only one chloride per mole. Which statement about its complex ion is correct?

A. It is octahedral, uses $d^2sp^3$ hybridisation, is diamagnetic and exists as two geometrical isomers.
B. It is octahedral, uses $sp^3d^2$ hybridisation, is paramagnetic with four unpaired electrons and exists as two geometrical isomers.
C. It is octahedral, uses $d^2sp^3$ hybridisation, is diamagnetic and exists as three geometrical isomers.
D. It is tetrahedral, uses $sp^3$ hybridisation and is paramagnetic.

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Answer: (A) It is octahedral, uses $d^2sp^3$ hybridisation, is diamagnetic and exists as two geometrical isomers.

Two ions with one ionic

\mathrm{Cl^-}

means the formula is

[\mathrm{Co(NH_3)_4Cl_2}]\mathrm{Cl}

. Cobalt is

+3

(

3d^6

\mathrm{NH_3}

is a strong-field ligand, so the six electrons pair up in

t_{2g}

, leaving two

3d

orbitals for

d^2sp^3

hybridisation: octahedral, low spin, zero unpaired electrons, hence diamagnetic. The ion is of type

[\mathrm{MA_4B_2}]

, which has only cis and trans forms (two geometrical isomers). The

sp^3d^2

option would need an outer-orbital, high-spin complex, as in

[\mathrm{CoF_6}]^{3-}

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