Question

Pyridine \(\left(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\right)\), abbreviated py, is the molecule (a) Why is pyridine referred to as a monodentate ligand? (b) For the equilibrium reaction $$ \left[\mathrm{Ru}(\mathrm{py})_{4}(\mathrm{bipy})\right]^{2+}+2 \mathrm{py} \rightleftharpoons\left[\mathrm{Ru}(\mathrm{py})_{6}\right]^{2+}+\text { bipy } $$ what would you predict for the magnitude of the equilibrium constant? Explain your answer.

Short answer

(a) Pyridine is referred to as a monodentate ligand because it has only one donor atom (the nitrogen atom) and forms a single coordination bond with the metal ion. (b) The equilibrium constant for the given reaction would favor the formation of the product with bidentate ligand ([Ru(py)$_4$(bipy)]$^{2+}$), meaning its value would be less than 1. This prediction is based on the fact that bidentate ligands, such as bipyridine, form stronger bonds with metal ions than monodentate ligands, like pyridine, due to the chelate effect.

Step-by-step solution

(a) Monodentate ligand definition

To answer part (a), we first need to know the definition of a monodentate ligand. A monodentate ligand is a molecule that forms a coordination bond with a central metal ion through only one donor atom.

(a) Pyridine's donor atom

Pyridine, C5H5N, has a nitrogen atom with a lone pair of electrons that can form a coordination bond with a metal ion.

(a) Monodentate nature of pyridine

Since pyridine has only one donor atom (i.e., the nitrogen atom) and forms a single coordination bond with the metal ion, it can be referred to as a monodentate ligand.

(b) Comparing ligand strength

To predict the magnitude of the equilibrium constant for the given reaction, we can consider the strength of the ligands involved, namely pyridine (py) and bipyridine (bipy). In general, bidentate ligands, such as bipy, tend to form stronger bonds with metal ions than monodentate ligands.

(b) Analyzing the reaction

In the given equilibrium reaction, we can see that two pyridine molecules are exchanging places with one bipyridine molecule on the ruthenium complex.

(b) Predicting the equilibrium constant magnitude

Since bidentate ligands form stronger bonds than monodentate ligands, we can predict that the equilibrium constant for this reaction would favor the formation of the product with bidentate ligand, i.e., [Ru(py)\(_4\)(bipy)]\(^{2+}\). This would mean the equilibrium constant has a value less than 1.

(b) Explanation

The main reason behind this prediction is that bidentate ligands have stronger coordinating abilities than monodentate ligands like pyridine. This is due to the chelate effect, where bidentate ligands form a more stable complex by occupying two adjacent coordination sites on the metal ion, resulting in the formation of a ring structure. This increased stability favors the formation of the complex with the bidentate ligand, making the equilibrium constant smaller than 1.

Key concepts

Ligand Strength

Understanding the concept of ligand strength is crucial when predicting the behavior of complexes in chemical reactions. Ligands are ions or molecules that donate at least one pair of electrons to a central metal atom or ion to form a coordination bond, thus achieving a stable electronic configuration. Ligand strength refers to the ligand's ability to form strong, stable bonds with this central metal atom.

In the context of the pyridine (py) and bipyridine (bipy) equilibrium reaction, ligand strength plays a pivotal role. Pyridine, being a monodentate ligand, binds through a single point of attachment. Bipyridine, on the other hand, is a bidentate ligand, which means it can attach through two points, generally leading to stronger binding due to the chelate effect. This difference in binding points affects the equilibrium of the reaction, generally favoring the formation of complexes with bidentate ligands over those with monodentate ligands.

When thinking about ligand strength, imagine how a Velcro strap (bidentate ligand) will hold more securely than a single button (monodentate ligand) on a jacket. This analogy can help explain why complexes with bidentate ligands are often more stable.

Coordination Bond

A coordination bond, also known as a dative covalent bond, is formed when one atom donates both of the electrons for the bond. In coordination chemistry, this is typically seen when a ligand donates its pair of electrons to a central metal atom or ion. The strength of this bond has a significant impact on the stability of the resulting complex and is influenced by several factors, including the identity of the metal, the ligand itself, and how the ligand binds to the metal ion.

In our case with pyridine, the nitrogen atom within the ligand has a lone pair of electrons that it donates to a metal ion, like ruthenium, to form this bond. The way pyridine binds, using only its lone nitrogen, reflects its nature as a monodentate ligand. This single point of attachment, while significant, usually results in a less stable bond compared to ligands that form multiple coordination bonds.

Equilibrium Constant

The equilibrium constant, denoted as K, is a quantitative measure of the composition of a reaction mixture at equilibrium. It reflects the relative concentrations of products to reactants and is determined by the specific reaction at a given temperature. An equilibrium constant greater than 1 indicates that products are favored at equilibrium, while a value less than 1 suggests that reactants are favored.

Applying this knowledge to our ruthenium complex reaction, we predict that the equilibrium constant would be less than 1 since the forward reaction leads to a complex with monodentate ligands (pyridine) rather than the more stable bidentate ligands (bipy). This is due to the stronger coordination bond and greater stability of the bipy-containing complex, which implies that the reverse reaction to form this complex is favored.

Chelate Effect

The chelate effect describes the enhanced binding affinity and stability of chelating ligands, which are ligands that can form two or more coordination bonds to a single metal ion. This occurs because the formation of multiple bonds results in the creation of one or more five- or six-membered rings, which are particularly stable due to their low strain and strong interaction with the central metal.

As an example, bipyridine is a chelating ligand that can create a more stable complex with a metal ion than pyridine, which only forms a single coordination bond. The chelate effect significantly influences the equilibrium constant of reactions involving metal complexes, as seen in our ruthenium complex scenario. The equilibrium is likely to shift towards the species with the chelating ligands, in this case, the complex containing bipyridine, because of the increased stability afforded by the chelate effect.

Bidentate Ligands

Bidentate ligands, such as bipyridine, can form two bonds to a central metal atom or ion. These ligands are important in coordination chemistry due to their ability to increase the stability of metal complexes. They achieve this by forming chelates—ring structures that securely bind the metal center more so than monodentate ligands, which attach at just a single point.

The two points of electron donation in bidentate ligands allow them to occupy two coordination sites on the metal ion simultaneously. This forms a more stable complex and is the basis behind the chelate effect. In molecular terms, think of bidentate ligands as a pair of hands grabbing onto a metal ion, while monodentate ligands are akin to just a single hand. The 'two-handed grip' of the bidentate ligand results in a more secure and stable connection with the metal ion.