Question

For each of the following polydentate ligands, determine (i) the maximum number of coordination sites that the ligand can occupy on a single metal ion and (ii) the number and type of donor atoms in the ligand: (a) ethylenediamine (en), (b) bipyridine (bipy), (c) the oxalate anion \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right)\), (d) the \(2-\) ion of the porphine molecule (Figure 24.8); (e) [EDTA]^{4- } .

Short answer

In summary, for each polydentate ligand, (a) Ethylenediamine (en): maximum coordination sites: 2, donor atoms: 2 nitrogen; (b) Bipyridine (bipy): maximum coordination sites: 2, donor atoms: 2 nitrogen; (c) Oxalate anion \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right)\): maximum coordination sites: 2, donor atoms: 2 oxygen; (d) 2\(^-\) ion of the porphine molecule: maximum coordination sites: 4, donor atoms: 4 nitrogen; (e) [EDTA]\(^{4-}\): maximum coordination sites: 6, donor atoms: 2 nitrogen, 4 oxygen.

Step-by-step solution

(a) Ethylenediamine (en)

Ethylenediamine (en) is an organic compound with the formula H\(_{2}\)NCH\(_{2}\)CH\(_{2}\)NH\(_{2}\). In this molecule, there are two nitrogen atoms which can act as donor atoms. So, the maximum number of coordination sites is 2, and there are 2 nitrogen donor atoms.

(b) Bipyridine (bipy)

Bipyridine (bipy) is a bidentate ligand with the formula C\(_{10}\)H\(_{8}\)N\(_{2}\). It consists of two pyridine rings connected by a single bond. There are two nitrogen atoms in the bipyridine molecule, which can act as donor atoms. So, the maximum number of coordination sites is 2, and there are 2 nitrogen donor atoms.

(c) Oxalate anion

The oxalate anion has the formula \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right)\). In this ion, there are two oxygen atoms with lone pairs of electrons that can form coordinate bonds with metal ions. So, the maximum number of coordination sites is 2, and there are 2 oxygen donor atoms.

(d) 2\(^-\) ion of the porphine molecule

The porphine molecule is a large macrocyclic compound with four nitrogen atoms that are available for coordination with metal ions. So, the maximum number of coordination sites is 4, and there are 4 nitrogen donor atoms.

(e) [EDTA]\(^{4-}\)

Ethylenediaminetetraacetic acid (EDTA) is a hexadentate ligand, meaning it can form up to six coordinate bonds with a metal ion. It has 2 nitrogen donor atoms and 4 oxygen donor atoms. So, the maximum number of coordination sites is 6, and there are a total of 6 donor atoms (2 nitrogen, and 4 oxygen atoms).

Key concepts

Polydentate Ligands

Polydentate ligands are fascinating components of coordination chemistry. These ligands, also known as chelating agents, can attach to a central metal atom or ion at multiple points. This ability to bind at multiple sites is due to the presence of two or more donor atoms within the ligand, which donates electrons to the metal ion. The degree to which a ligand can bind to a metal ion is described by its *denticity*. The term *dentate* relates to the number of donor atoms through which the ligand can attach to the metal ion.
For example:

• *Bidentate ligands* attach at two sites.
• *Tridentate ligands* attach at three sites.
• *Hexadentate ligands* like EDTA can attach at six sites.

Polydentate ligands often form more stable complexes compared to monodentate ligands because they create rings with the metal ions, which provides extra stability through a phenomenon called the chelate effect.

Coordination Sites

Understanding coordination sites is key to grasping how complex metal-ligand structures form. Coordination sites refer to the specific positions on a metal ion where ligands can bind. A metal ion typically has an *octahedral*, *tetrahedral*, or *square planar* arrangement of coordination sites, depending on the number of ligands and their spatial arrangement around the metal ion.
In coordination chemistry, the coordination number of a central metal atom defines the number of ligand attachments it can have. For instance:

• A coordination number of 6 often results in an octahedral coordination geometry.
• A coordination number of 4 might lead to tetrahedral or square planar geometries.

Polydentate ligands can occupy multiple coordination sites, and this feature is why they are so efficient in forming stable complexes.

Donor Atoms

In the realm of coordination chemistry, donor atoms are the ones that provide a pair of electrons to form a coordinate bond with a central metal ion. These atoms are found in the ligands and are critical for the binding process. Common donor atoms include:

• *Nitrogen* atoms found in amine and pyridine groups.
• *Oxygen* atoms, which are frequently found in carboxylate or hydroxy groups.

The presence of multiple donor atoms allows chelating ligands to form stable complexes with metal ions. Each donor atom in a ligand contributes to the ligand's denticity, enabling it to coordinate through several sites. Donor atoms add stability to complexes through the formation of closed rings due to the chelate effect.

Ethylenediamine (en)

Ethylenediamine, commonly abbreviated as *en*, is a simple bidentate ligand, which means it can attach to a metal ion at two coordination sites. The chemical formula of ethylenediamine is \[H_2NCH_2CH_2NH_2\]This compound contains two nitrogen atoms, each acting as a donor atom. These nitrogen atoms carry lone pairs of electrons that they can share with a metal ion, allowing ethylenediamine to form two coordinate bonds. The ability to bind at two sites makes it effective in creating chelate complexes, which are more stable than those formed by monodentate ligands.

EDTA

Ethylenediaminetetraacetic acid, known as EDTA, is a powerful hexadentate ligand in coordination chemistry. This ligand can connect to a metal ion through six points, making it highly effective for forming stable complexes. EDTA contains:

• Two nitrogen donor atoms.
• Four oxygen donor atoms.

The overall structure allows EDTA to wrap around a metal ion tightly, using all six donor atoms to create a very stable complex due to the multiple contact points. With its versatility and strength, EDTA is widely used in various applications, from medicine as a chelating agent to analytical chemistry for complexometric titrations.