Problem 32 - Chemistry: The Central Science

Question

Pyridine \(\left(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\right),\) abbreviated py, is the molecule (a) Would you expect pyridine to act as a monodentate or bidentate ligand? (b) For the equilibrium reaction $$\left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}+2 \mathrm{py} \Longrightarrow\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}+\mathrm{bipy}$$ would you predict the equilibrium constant to be larger or smaller than one?

Step-by-Step Solution

Answer

In summary, pyridine acts as a monodentate ligand due to having only one nitrogen atom with a lone pair of electrons for bonding. For the given equilibrium reaction, the equilibrium constant is predicted to be smaller than one because the initial complex with a bidentate ligand is more stable due to the chelate effect.

Step 1: Determine if Pyridine is Monodentate or Bidentate

Pyridine has the chemical formula \(\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N}\). The nitrogen atom in the pyridine molecule has a lone pair of electrons, which it can donate to a transition metal to form a coordinate covalent bond, making it a ligand. Let's analyze if pyridine can act as a monodentate or bidentate ligand. A monodentate ligand has only one donor atom that bonds to the metal, while a bidentate ligand has two donor atoms that can bond to the metal. From the pyridine molecule, we can see that there is only one nitrogen atom acting as a donor site. Other carbon atoms in the ring do not have a lone pair of electrons to donate. Therefore, pyridine serves as a monodentate ligand.

Step 2: Predict the Equilibrium Constant for the Given Reaction

The given equilibrium reaction is: $$\left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}+2 \mathrm{py} \Longrightarrow\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}+\mathrm{bipy}$$ In this reaction, \([\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})]^{2+}\) is losing a bipyridine (bipy) ligand and forming a complex with six pyridine (py) molecules. The reaction involves the substitution of bipy by two py ligands. This substitution is a sign of chelate effect, which states that metal complexes with multidentate ligands (such as bipy) are more stable than those with similar monodentate ligands (such as py). The increased stability is due to the entropic advantage of releasing fewer particles when a chelating ligand binds to the metal compared to monodentate ligands. Since the chelate effect causes the initial complex to be more stable than the product complex containing only monodentate ligands, the equilibrium constant for the reaction will be smaller than one. As a result, we would predict the equilibrium constant to be smaller than one.