Question

Answer the following questions about the complexes \([NiCl_4]^{2-}\) and \([Ni(CN)_4]^{2-}\): \[ (i)\;\text{Write the hybridization involved in each case.} \] \[ (ii)\;\text{Which of them is the inner orbital complex and which one is the outer orbital complex?} \] \[ (iii)\;\text{Compare their magnetic behaviour.} \] \[ \text{Atomic number: }Ni=28 \]

Hint:

For \(Ni^{2+}\), weak field ligands like \(Cl^-\) give tetrahedral \(sp^3\) paramagnetic complexes, while strong field ligands like \(CN^-\) give square planar \(dsp^2\) diamagnetic complexes.

Answer 1

Concept:
Nickel has atomic number \(28\). \[ Ni:[Ar]3d^84s^2 \] In both complexes, nickel is in \(+2\) oxidation state. \[ Ni^{2+}:[Ar]3d^8 \] The nature of the ligand decides whether pairing of electrons will take place or not. \[ Cl^-=\text{weak field ligand} \] \[ CN^-=\text{strong field ligand} \]

Step 1: Analyse \([NiCl_4]^{2-}\).
In \([NiCl_4]^{2-}\), chlorine ligand is a weak field ligand. Weak field ligands do not cause pairing of electrons in the \(3d\)-orbitals. Therefore, nickel uses one \(4s\)-orbital and three \(4p\)-orbitals for hybridization. \[ \text{Hybridization}=sp^3 \] The geometry is tetrahedral. Since unpaired electrons remain present, the complex is paramagnetic.

Step 2: Analyse \([Ni(CN)_4]^{2-}\).
In \([Ni(CN)_4]^{2-}\), cyanide ligand is a strong field ligand. Strong field ligands cause pairing of electrons in \(3d\)-orbitals. After pairing, one inner \(3d\)-orbital becomes available for hybridization. Therefore, nickel uses: \[ 3d,\;4s,\;4p,\;4p \] orbitals. \[ \text{Hybridization}=dsp^2 \] The geometry is square planar. Since all electrons become paired, the complex is diamagnetic.

Step 3: Compare inner and outer orbital nature.
\[ [NiCl_4]^{2-} \] uses outer orbitals only, so it is an outer orbital complex. \[ [Ni(CN)_4]^{2-} \] uses inner \(3d\)-orbital, so it is an inner orbital complex. Hence: \[ [NiCl_4]^{2-}:sp^3,\;\text{outer orbital complex, paramagnetic} \] \[ [Ni(CN)_4]^{2-}:dsp^2,\;\text{inner orbital complex, diamagnetic} \]