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 $$ \left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}+2 \mathrm{py} \rightleftharpoons\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}+\mathrm{bipy} $$ would you predict the equilibrium constant to be larger or smaller than one?

Pyridine (\(\mathrm{C}_{5} \mathrm{H}_{5}\mathrm{N}\)) is an aromatic compound containing a nitrogen atom with a lone pair of electrons in its structure. A monodentate ligand can bond with the central metal atom/ion through only one donor atom, while a bidentate ligand can bond with the central metal atom/ion via two donor atoms. In the case of pyridine, there is only one nitrogen atom with a lone pair of electrons that can act as a donor atom, so pyridine is a monodentate ligand. Answer: (a) Pyridine acts as a monodentate ligand.

We are given the equilibrium reaction: $$ \left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}+2 \mathrm{py} \rightleftharpoons\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}+\mathrm{bipy} $$ The reaction involves the substitution of a bidentate ligand, bipyridine (\(\mathrm{bipy}\)) by two monodentate ligands, pyridine (\(\mathrm{py}\)) in the complex \(\left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}\). The product is the complex \(\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}\) and bipyridine.

To predict whether the equilibrium constant will be larger or smaller than one, we need to consider the thermodynamic stability of the complexes and the preferential binding of ligands. Chelate Effect is an observation that bidentate ligands, which can form a chelate with a metal ion, bind more tightly than comparable monodentate ligands. This behavior is mostly due to the entropy effect, which favors the binding of a single bidentate ligand over two monodentate ligands. In this case, the complex \(\left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}\) is more stable and has a higher thermodynamic preference due to the presence of the bidentate ligand (bipyridine) as compared to the complex \(\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}\), which only has monodentate ligands (pyridine). As a result, we can predict that the equilibrium constant for this reaction will be smaller than one. Answer: (b) The equilibrium constant would be smaller than one.