Question

Name two coordination compounds which are important in biological systems.

Hint:

Tetrahedral splitting is small, so pairing usually does not occur. Hence tetrahedral complexes are generally high spin.

Answer 1

Concept:
Coordination compounds play an important role in biological systems. For example, haemoglobin contains iron and chlorophyll contains magnesium. Chelating ligands form ring structures with the central metal ion, making the complex more stable.

Step 1: Name two biologically important coordination compounds.
Two important coordination compounds in biological systems are: Haemoglobin and Chlorophyll Haemoglobin contains iron as the central metal ion and helps in oxygen transport. Chlorophyll contains magnesium as the central metal ion and helps in photosynthesis.

Step 2: Define chelate effect.
When a ligand binds to the same metal ion through two or more donor atoms, it forms a ring-like structure. Such ligands are called chelating ligands. The complex formed by chelating ligands is more stable than a similar complex formed by monodentate ligands. This extra stability is known as chelate effect.

Step 3: Give example of chelate effect.
Ethylenediamine is a bidentate ligand. \[ en=H_2NCH_2CH_2NH_2 \] It can bind through two nitrogen atoms and form a ring with the metal ion. Example: \[ [Ni(en)_3]^{2+} \] This complex is more stable because of chelate ring formation.

Step 4: Explain why low spin tetrahedral complexes are rarely formed.
In tetrahedral complexes, the splitting energy is small. \[ \Delta_t \lt \Delta_o \] The tetrahedral crystal field splitting is usually not large enough to overcome pairing energy. Therefore, electrons prefer to remain unpaired instead of pairing. Hence most tetrahedral complexes are high spin. Low spin tetrahedral complexes are rarely formed. Hence: Biologically important coordination compounds: haemoglobin and chlorophyll Chelate effect: extra stability due to ring formation by polydentate ligands \[ \text{Low spin tetrahedral complexes are rare because }\Delta_t\text{ is small.} \]