JEE Advanced 2018 · Paper 2 · Chemistry Q1

ChemistryMultiple correct+4 / -2Question 19 of 54

The correct option(s) regarding the complex $[\mathrm{Co(en)(NH_3)_3(H_2O)}]^{3+}$ (en = $\mathrm{H_2NCH_2CH_2NH_2}$ ) is (are)

It has two geometrical isomers

It will have three geometrical isomers if bidentate ‘en’ is replaced by two cyanide ligands

It is paramagnetic

It absorbs light at longer wavelength as compared to

[\mathrm{Co(en)(NH_3)_4}]^{3+}

Answer and worked solution

Official answer: A, B, D

(A) The bidentate en must span two cis positions. Of the remaining four positions, one is trans to each en nitrogen and two are trans to each other. $\ce{H2O}$ can sit trans to an en N, or in one of the mutually trans pair. That gives 2 geometrical isomers. True.
(B) Replacing en by two $\ce{CN-}$ gives $\ce{[Co(CN)2(NH3)3(H2O)]}$ , of type $\ce{Ma2b3c}$ . With the two CN trans there is 1 isomer. With them cis, water can be trans to a CN or trans to an $\ce{NH3}$ , giving 2. Total 3. True.
(C) Co(III) is $d^6$ . With N-donor ligands it is low spin $t_{2g}^6$ , so diamagnetic. False.
(D) $\ce{H2O}$ is a weaker-field ligand than $\ce{NH3}$ . Replacing one $\ce{NH3}$ lowers $\Delta_o$ , so the complex absorbs at a longer wavelength than $\ce{[Co(en)(NH3)4]^{3+}}$ . True.

(A), (B), (D)

JEE Advanced 2018 · Paper 2, Chemistry question 1