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

(i) Coordination compounds important in biological systems

Concept: Many naturally occurring biological molecules are coordination compounds in which a metal ion is coordinated to various ligands. These compounds perform vital biological functions such as oxygen transport, photosynthesis and enzyme catalysis.

Examples:

• Haemoglobin It is an iron-containing coordination compound present in red blood cells. The central metal ion is: \[ Fe^{2+} \] It is responsible for transportation of oxygen from lungs to body tissues.

• Chlorophyll It is a magnesium-containing coordination compound present in green plants. The central metal ion is: \[ Mg^{2+} \] It plays a vital role in photosynthesis.
\[ \boxed{\text{Haemoglobin and Chlorophyll}} \]

(ii) Chelate effect

Definition: The enhanced stability of complexes containing chelating ligands compared to analogous complexes containing monodentate ligands is known as the chelate effect. Chelating ligands possess two or more donor atoms and form ring structures with the central metal ion. These rings greatly increase the thermodynamic stability of the complex.

Example: Ethane-1,2-diamine (en) is a bidentate ligand. \[ [Ni(en)_3]^{2+} \] is much more stable than \[ [Ni(NH_3)_6]^{2+} \] because en forms chelate rings around the metal ion. \[ \boxed{ \text{Greater stability due to ring formation is called chelate effect.} } \]

(iii) Why are low spin tetrahedral complexes rarely formed ?

Concept: In tetrahedral complexes, the crystal field splitting energy \[ \Delta_t \] is relatively small. \[ \Delta_t = \frac{4}{9}\Delta_o \] where \[ \Delta_o \] is the octahedral splitting energy.

Explanation: Since \[ \Delta_t \] is small, the energy required for electron pairing is usually greater than the crystal field splitting energy. Therefore electrons prefer to remain unpaired rather than pair up. As a result, tetrahedral complexes are generally high-spin complexes. Hence low-spin tetrahedral complexes are rarely formed. \[ \boxed{ \Delta_t \lt \text{Pairing Energy} } \] Therefore electron pairing does not occur.