Coordination Compounds: NEET previous year questions

$[\mathrm{Pt}(\mathrm{NH}_{3})_{2}\mathrm{Cl}_{2}]$ shows cis/trans (geometrical) isomerism (III); $[\mathrm{Co}(\mathrm{en})_{3}]^{3+}$ is a tris-chelate that is chiral, showing optical isomerism (I); $[\mathrm{Co}(\mathrm{NH}_{3})_{5}\mathrm{NO}_{2}]\mathrm{Cl}_{2}$ shows linkage isomerism through the ambidentate $\mathrm{NO}_{2}^{-}$/$\mathrm{ONO}^{-}$ ligand (IV); and $[\mathrm{Cr}(\mathrm{H}_{2}\mathrm{O})_{6}]\mathrm{Cl}_{3}$ shows solvate (hydrate) isomerism (II). Hence A-III, B-I, C-IV, D-II.

An ambidentate ligand has two different donor atoms and can bind through either one. Thiocyanate (SCN-) can coordinate through sulphur (-SCN) or nitrogen (-NCS), so it is ambidentate. Ethane-1,2-diamine and oxalate are bidentate chelating ligands and ethylenediaminetetraacetate ion (EDTA) is hexadentate, but each of their donor sets is fixed, so they are not ambidentate.

[PtCl$_{2}$(NH$_{3}$)$_{2}$] uses dsp$^{2}$ hybridisation at Pt$^{2+}$ → square planar (III); [Co(NH$_{3}$)$_{6}$]$^{3+}$ is d$^{2}$sp$^{3}$ → octahedral (I); [NiCl$_{4}$]$^{2-}$ has sp$^{3}$ Ni$^{2+}$ with weak-field Cl$^-$ → tetrahedral (IV); [Fe(CO)$_{5}$] is sp$^{3}$d/dsp$^{3}$ → trigonal bipyramidal (II). This gives P-III, Q-I, R-IV, S-II.

A tris-chelate of type $[\mathrm{M}(\mathrm{AA})_{3}]$ has no plane or centre of symmetry and exists as the non-superimposable delta and lambda propeller forms, so $[\mathrm{Fe}(\mathrm{ox})_{3}]^{3-}$ is indeed chiral. For the bis-chelate $[\mathrm{Cr}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{ox})_{2}]^-$, only the CIS isomer is chiral; the TRANS isomer, with the two aqua ligands on opposite sides, possesses a plane of symmetry and is therefore achiral. Statement-II names the wrong isomer. NCERT Class 12, Coordination Compounds (isomerism).

The spectrochemical series orders ligands by the size of the crystal field splitting they produce. Carbon monoxide is the strongest field ligand in this set because it accepts electron density into its $\pi^{*}$ orbitals; ammonia follows as a stronger sigma donor than water; and chloride, being a weak-field pi donor, is the weakest. The accepted sequence is $\mathrm{CO} > \mathrm{CN}^- > \mathrm{NH}_{3} > \mathrm{H}_{2}\mathrm{O} > \mathrm{F}^- > \mathrm{Cl}^-$. NCERT Class 12, Coordination Compounds (crystal field theory).

All four are octahedral with strong-field or moderate-field ligands, so the electrons pair up in $t_{2g}$ before entering $e_g$. Counting d electrons: $\mathrm{Mn}^{3+}$ is $d^4$, giving low-spin $t_{2g}^4$ with TWO unpaired electrons; $\mathrm{Fe}^{3+}$ is $d^5$, giving $t_{2g}^5$ with one; $\mathrm{Co}^{3+}$ is $d^6$, giving $t_{2g}^6$ with none; $\mathrm{Ti}^{3+}$ is $d^1$ with one. Since $\mu = \sqrt{n(n+2)}$ rises with n, the manganese complex has the highest moment at about 2.83 Bohr magneton (BM). NCERT Class 12, Coordination Compounds.

The name lists the coordination sphere first: four ammine, one aqua and one chlorido ligand around cobalt(III). Charge balance inside the brackets gives $+3 + 0 + 0 - 1 = +2$, so two chloride counter-ions are needed outside to neutralise the complex cation. The word 'chloride' at the end names those counter-ions, not a ligand. NCERT Class 12, Coordination Compounds (IUPAC nomenclature).

Facial and meridional isomerism is specific to octahedral complexes of the type $[\mathrm{Ma}_{3}\mathrm{b}_{3}]$. In the fac isomer the three identical ligands occupy one triangular face; in the mer isomer they lie in a plane through the metal. $[\mathrm{Cr}(\mathrm{py})_{3}\mathrm{Cl}_{3}]$ has exactly this 3:3 arrangement. NCERT Class 12, Coordination Compounds (geometrical isomerism).

Only the ions OUTSIDE the coordination sphere are released in solution; ligands bound to the metal stay put. $[\mathrm{Co}(\mathrm{NH}_{3})_{6}]\mathrm{Cl}_{3}$ has all six ammonia molecules coordinated and three chlorides as counter-ions, so it dissociates into one complex cation and three chloride ions - a 1 : 3 electrolyte. NCERT Class 12, Coordination Compounds (Werner's theory, primary and secondary valence).

In the spectrochemical series relevant here, ligand field strength rises as $\mathrm{SCN}^{-}$ < $\mathrm{F}^{-}$ < $\mathrm{C}_{2}\mathrm{O}_{4}^{2-}$ < $\mathrm{CN}^{-}$. Chapter: Coordination Compounds.

The spin-only magnetic moment $\mu$ is given by $\mu = \sqrt{n(n+2)}\ BM$, where n is the number of unpaired electrons. For $\mathrm{Cr}^{2+}$, the electronic configuration is $3d^4$, with 4 unpaired electrons. Substituting n = 4, $\mu = \sqrt{4(4+2)} = \sqrt{24} \approx 4.90\ BM$. Chapter: Coordination Compounds.

Urea hydrolysis ultimately releases NH$_{3}$. Excess ammonia coordinates Cu$^{2+}$ to form the deep-blue tetraamminecopper(II) ion [Cu(NH$_{3}$)$_{4}$]$^{2+}$. Chapter: Coordination Compounds.