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 \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right),(\mathrm{d})\) the \(2-\) ion of the porphine molecule (Figure 23.13 ); (e) [EDTA] \(]\) -

Short answer

In summary: (a) Ethylenediamine (en) can occupy 2 coordination sites with 2 nitrogen donor atoms. (b) Bipyridine (bipy) can occupy 2 coordination sites with 2 nitrogen donor atoms. (c) Oxalate anion can occupy 2 coordination sites with 2 oxygen donor atoms. (d) Porphine 2- ion can occupy 4 coordination sites with 4 nitrogen donor atoms. (e) [EDTA]^- can occupy 6 coordination sites with 2 nitrogen donor atoms and 4 oxygen donor atoms.

Step-by-step solution

(a) Ethylenediamine (en)

For ethylenediamine (en), the chemical formula is \(NH_2CH_2CH_2NH_2\). It has two nitrogen donor atoms, each with one lone pair of electrons that can form coordinate bonds with a metal ion. Therefore, the maximum number of coordination sites that the ligand can occupy on a single metal ion is 2. There are 2 nitrogen donor atoms in the ligand.

(b) Bipyridine (bipy)

Bipyridine (bipy) has the chemical formula \(C_{10}H_8N_2\), and it consists of two pyridine rings connected by a single carbon-carbon bond. Each pyridine ring has a nitrogen atom with a lone pair of electrons that can form a coordination bond with a metal ion. Therefore, the maximum number of coordination sites that the ligand can occupy on a single metal ion is 2, and there are 2 nitrogen donor atoms in the ligand.

(c) Oxalate anion

The oxalate anion has the chemical formula \(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\). There are two oxygen atoms with lone pairs of electrons that can form coordinate bonds with a metal ion. Therefore, the maximum number of coordination sites that the ligand can occupy on a single metal ion is 2. There are 2 oxygen donor atoms in the ligand.

(d) Porphine 2- ion

The porphine 2- ion is a large and complex molecule with multiple potential donor atoms. The four nitrogen atoms located in the center of the porphine ring each have lone pairs of electrons that can form coordinate bonds with a metal ion. Therefore, the maximum number of coordination sites that the ligand can occupy on a single metal ion is 4. There are 4 nitrogen donor atoms in the ligand.

(e) [EDTA]

[EDTA]^- is the ethylenediaminetetraacetate ion and has a chemical formula of \(\mathrm{C}_{10} \mathrm{H}_{12} \mathrm{N}_{2} \mathrm{O}_{8}{ }^{2-}\). It has a total of 6 donor atoms: 2 nitrogen atoms and 4 oxygen atoms. The nitrogen donor atoms come from the two amino groups, and the oxygen donor atoms come from the four carboxylate groups. Therefore, the maximum number of coordination sites that the ligand can occupy on a single metal ion is 6. There are 2 nitrogen donor atoms and 4 oxygen donor atoms in the ligand.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of chemistry that focuses on the study of compounds featuring metal atoms or ions, which are surrounded by other chemical species known as ligands. These ligands are molecules or ions that can donate a pair of electrons to form a coordinate covalent bond with the metal, creating a coordination complex.

Each complex features a central metal ion or atom, which can vary in its oxidation state, size, and other chemical properties, and the ligands, which can bind to the metal in various numbers and arrangements. The region around the metal where the ligands are directly attached is called the coordination sphere.

A polydentate ligand, also known as a multidentate or chelating ligand, is capable of attaching to a metal ion or atom through multiple binding sites. This involves donation of electron pairs from the ligand to the metal, resulting in the formation of several coordinate covalent bonds within a single ligand-metal interaction. This multidentate bonding leads to the creation of highly stable complexes compared to those formed with monodentate ligands, which attach at a single point.

Donor Atoms

Donor atoms are the atoms within a ligand that actually bind to the central metal atom or ion in coordination complexes. They are the sources of the electron pairs that are donated to the metal center to form coordinate bonds. The most common donor atoms include nitrogen, oxygen, and sulfur, which have lone pairs of electrons that make them effective at binding to metal ions.

The nature of the donor atoms, including their size, electronegativity, and the electron-donating ability, can greatly influence the stability and properties of the resulting coordination complex. For instance, nitrogen donor atoms, as found in ethylenediamine and bipyridine, tend to form very strong bonds with metal ions, particularly those belonging to the first transition series. In contrast, oxygen donors, such as those in the oxalate anion, may form somewhat weaker bonds dependent on the metal but still result in significant stabilization of the complex.

Chelation

Chelation is a specific type of coordination where a single ligand occupies multiple coordination sites on a metal ion. This occurs because the ligand possesses more than one donor atom, each capable of donating a pair of electrons to the metal. The term chelation derives from the Greek word 'chele', which means claw, referring to the way these ligands appear to grasp the central metal ion.

Chelating ligands, such as ethylenediamine (en) or the ethylenediaminetetraacetate ion (EDTA), significantly increase the stability of a metal complex through the 'chelate effect'. This is primarily because chelate rings are formed during the coordination process, reducing the possibility of the ligand being replaced and thus enhancing the overall stability of the complex.

EDTA, a hexadentate ligand, forms up to six bonds with a single metal ion, making it one of the most potent chelators used in coordination chemistry. Its ability to tightly bind metal ions has made it incredibly useful in a variety of applications, such as water softening, as a chelating agent in medical treatments, and even in the remediation of heavy metal contaminated environments.