Question

Name two coordination compounds which are important in biological systems. (ii) What is meant by chelate effect ? Give an example. (iii) Why are low spin tetrahedral complexes rarely formed ?

Hint:

Almost all tetrahedral complexes are high-spin because tetrahedral crystal field splitting is too small to force electron pairing.

Answer 1

Step 1: Coordination compounds in biological systems.
Haemoglobin (central metal $Fe^{2+}$, transports $O_2$ from lungs to body tissues) and Chlorophyll (central metal $Mg^{2+}$, essential for photosynthesis in green plants) are two important examples.
Step 2: Chelate effect with example.
The enhanced stability of complexes formed by polydentate chelating ligands over analogous monodentate ligand complexes is the chelate effect. Example: $[Ni(en)_3]^{2+}$ (en = ethane-1,2-diamine forms 5-membered chelate rings) is significantly more stable than $[Ni(NH_3)_6]^{2+}$ because ring formation releases more solvent molecules, increasing entropy.
Step 3: Why low-spin tetrahedral complexes are rarely formed.
In tetrahedral geometry, crystal field splitting $\Delta_t = \frac{4}{9}\Delta_o$, which is only about 44% of octahedral splitting. This small $\Delta_t$ is almost always less than the pairing energy, so electrons prefer to remain unpaired (high-spin configuration). Low-spin tetrahedral complexes would require $\Delta_t$ to exceed pairing energy, which almost never happens in practice.