Question

Discuss the chelate effect with an example. Explain the nature of bonding in \( [\text{Fe}(\text{CN})_6]^{4-} \) on the basis of valence bond theory. (2+2=4)

Hint:

Chelate effect = extra stability from ring-forming polydentate ligands (entropy driven), e.g. \( [Ni(en)_3]^{2+} \). For \( [Fe(CN)_6]^{4-} \): Fe is \( +2 \) (\( 3d^6 \)), strong-field \( CN^- \) pairs the electrons giving \( d^2sp^3 \), octahedral, diamagnetic.

Answer 1

Step 1: Chelate effect.
When a polydentate (bidentate or higher) ligand binds to a central metal ion through two or more donor atoms, it forms a closed ring containing the metal. Such ring-forming ligands are called chelating ligands and the rings are chelate rings. The chelate effect is the observation that a complex containing chelate rings is much more stable than a comparable complex formed by an equal number of similar monodentate ligands.

Step 2: Example and reason.
\( [\text{Ni}(\text{en})_3]^{2+} \) (en = ethylenediamine, bidentate) is far more stable than \( [\text{Ni}(\text{NH}_3)_6]^{2+} \), even though both bind six N atoms to Ni. The extra stability is mainly an entropy effect: when one chelating molecule replaces two monodentate ligands, the number of free particles in solution increases, so \( \Delta S \) is positive and \( \Delta G = \Delta H - T\Delta S \) becomes more negative.

Step 3: Oxidation state and electron configuration in \( [\text{Fe}(\text{CN})_6]^{4-} \).
Let the oxidation state of Fe be x: \( x + 6(-1) = -4 \Rightarrow x = +2 \). Iron (Z = 26) is \( [\text{Ar}]3d^6 4s^2 \); \( \text{Fe}^{2+} \) is \( [\text{Ar}]3d^6 \), giving 6 electrons distributed as \( t_{2g} \) type with 4 unpaired electrons in the free ion.

Step 4: Effect of the strong field ligand and hybridisation.
\( \text{CN}^- \) is a strong field ligand, so it forces the 3d electrons to pair up. The six 3d electrons occupy only three d orbitals (all paired), leaving two inner 3d orbitals empty. These two empty 3d orbitals combine with one 4s and three 4p orbitals to give \( d^2sp^3 \) hybridisation.

Step 5: Geometry and magnetic nature.
The six \( d^2sp^3 \) hybrid orbitals accept lone pairs from six \( \text{CN}^- \) ligands, giving an octahedral, inner-orbital (low spin) complex. As all electrons are paired, \( [\text{Fe}(\text{CN})_6]^{4-} \) is diamagnetic.
\[ \boxed{\text{Octahedral, } d^2sp^3, \text{ low spin, diamagnetic}} \]