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

Step 1: Part (i): Two coordination compounds important in biological systems.
(a) Haemoglobin: a coordination compound of iron(II) ($Fe^{2+}$) with a porphyrin ligand (haem group). It is present in red blood cells and carries oxygen from the lungs to body tissues. (b) Chlorophyll: a coordination compound of magnesium ($Mg^{2+}$) with a porphyrin ligand. It is the green photosynthetic pigment in plants, responsible for capturing light energy.
Step 2: Part (ii): Define the chelate effect.
The chelate effect refers to the extra thermodynamic stability that a chelate (complex formed by a polydentate ligand) has compared to an analogous complex formed by monodentate ligands with the same donor atoms. Chelating ligands form rings with the metal, and these ring structures are extra-stable.
Step 3: Give an example of the chelate effect.
$[Cu(en)_2]^{2+}$ (two bidentate $en$ ligands, 4 N donors) is much more stable than $[Cu(NH_3)_4]^{2+}$ (four monodentate $NH_3$ ligands, 4 N donors), even though both have 4 nitrogen donor atoms coordinated to $Cu^{2+}$. The extra stability of the chelated complex is the chelate effect.
Step 4: Explain the chelate effect thermodynamically.
When a polydentate ligand replaces several monodentate ligands, there is a net increase in the number of free species in solution (the displaced monodentate ligands are released). This increases entropy ($\Delta S > 0$), making $\Delta G = \Delta H - T\Delta S$ more negative and the reaction more spontaneous. The chelated complex is therefore more stable.
Step 5: Part (iii): Why low spin tetrahedral complexes are rarely formed.
The crystal field splitting energy in a tetrahedral field is: \[ \Delta_t = \frac{4}{9}\Delta_o \] Since $\Delta_t$ is only about 44% of $\Delta_o$ (the octahedral splitting), the tetrahedral field is much weaker. For low spin to occur, $\Delta_t$ must exceed the pairing energy $P$. Since $\Delta_t$ is so small (even for strong field ligands), it almost never overcomes the pairing energy. Electrons remain unpaired (high spin) in tetrahedral complexes even with strong field ligands.
Step 6: Summarise all three answers.
(i) Haemoglobin ($Fe^{2+}$, oxygen transport) and Chlorophyll ($Mg^{2+}$, photosynthesis). (ii) Chelate effect: extra stability from entropy gain when polydentate ligands form rings; example: $[Cu(en)_2]^{2+}$ > $[Cu(NH_3)_4]^{2+}$. (iii) $\Delta_t = \frac{4}{9}\Delta_o$ is too small to exceed pairing energy, so low spin tetrahedral complexes are rare. \[ \boxed{\Delta_t = \frac{4}{9}\Delta_o \ll P \Rightarrow \text{low spin tetrahedral complexes rarely form}} \]