Class 12 NEET Chemistry Coordination Compounds

Concept:The geometry of a complex depends on the coordination number (C.N.), the oxidation state/electronic configuration of the metal and the strength of the ligand field.Complexes/ions:A: $[\mathrm{Pt}(\mathrm{Cl})_2(\mathrm{NH}_3)_2]$B: $[\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_3$C: $[\mathrm{NiCl}_4]^{2-}$D: $[\mathrm{Fe}(\mathrm{CO})_5]$Explanation:For A: Pt is in the $+2$ state with a $5d^8$ configuration and C.N. $=4$.Since $\mathrm{Pt}^{2+}$ is a heavy $5d$ metal, the crystal field splitting is very large, so pairing occurs and the hybridisation is $dsp^2$, giving a square planar shape (both cis and trans isomers are square planar).Hence A matches III (square planar).For B: Co is in the $+3$ state ($3d^6$) with C.N. $=6$ in $[\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_3$.$\mathrm{NH}_3$ is a strong field ligand, so the $3d$ electrons pair up and the hybridisation is $d^2sp^3$, giving an octahedral shape.Hence B matches I (octahedral).For C: Ni is in the $+2$ state ($3d^8$) with C.N. $=4$ in $[\mathrm{NiCl}_4]^{2-}$.$\mathrm{Cl}^-$ is a weak field ligand, so no pairing of $3d$ electrons occurs and the hybridisation is $sp^3$, giving a tetrahedral shape (paramagnetic with 2 unpaired electrons).Hence C matches IV (tetrahedral).For D: Fe is in the zero oxidation state with C.N. $=5$ in $[\mathrm{Fe}(\mathrm{CO})_5]$ ($\mathrm{Fe}$ effectively has a $3d^8$ configuration after the $4s$ electrons are used).CO is a strong field ligand and the hybridisation is $dsp^3$, giving a trigonal bipyramidal shape.Hence D matches II (trigonal bipyramidal).Shortcut:For C.N. $=4$: a $d^8$ metal with a strong ligand field (e.g. $\mathrm{Ni}^{2+}$ with $\mathrm{CN}^-$, or any $\mathrm{Pt}^{2+}$ complex) is square planar, whereas a weak ligand field (e.g. $\mathrm{Ni}^{2+}$ with $\mathrm{Cl}^-$) gives a tetrahedral shape.For C.N. $=5$, $dsp^3$ hybridisation gives a trigonal bipyramidal shape, and for C.N. $=6$, $d^2sp^3$ hybridisation gives an octahedral shape.Answer:A-III, B-I, C-IV, D-II, i.e. the option A-III, B-I, C-IV, D-II.