Problem 18 - Chemistry: The Central Science

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 \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{\underline{\phantom{xx}}}^{2-}\right)\), (d) the \(2-\) ion of the porphine molecule (Figure 24.8); (e) [EDTA] \(^{4-}\).

Step-by-Step Solution

Answer

(a) Ethylenediamine (en) has 2 coordination sites, with 2 nitrogen donor atoms. (b) Bipyridine (bipy) has 2 coordination sites, with 2 nitrogen donor atoms. (c) The oxalate anion \(\left(\mathrm{C}_{2}\mathrm{O}_{4}{\underline{\phantom{xx}}}^{2-}\right)\) has 2 coordination sites, with 2 oxygen donor atoms. (d) The \(2-\) ion of the porphine molecule has 4 coordination sites, with 4 nitrogen donor atoms. (e) [EDTA] \(^{4-}\) has 6 coordination sites, with 2 nitrogen and 4 oxygen donor atoms.

Step 1: (a) Ethylenediamine (en)

Ethylenediamine (en) is a bidentate ligand with the formula \(\mathrm{H}_{2}\mathrm{NCH}_{2}\mathrm{CH}_{2}\mathrm{NH}_{2}\). It has two donor atoms, both of them being nitrogen atoms with a lone pair of electrons. The maximum number of coordination sites it can occupy on a single metal ion is 2.

Step 2: (b) Bipyridine (bipy)

Bipyridine (bipy) is a bidentate ligand with two pyridine rings connected at their 2,2'-positions. The two donor atoms are nitrogen atoms on each pyridine ring, each having a lone pair of electrons. The maximum number of coordination sites it can occupy on a single metal ion is 2.

Step 3: (c) The oxalate anion \(\left(\mathrm{C}_{2}\mathrm{O}_{4}{ }^{2-}\right)\)

The oxalate anion has the formula \(\mathrm{C}_{2}\mathrm{O}_{4}{\underline{\phantom{xx}}}^{2-}\) and it is a dianionic ligand. It has two donor atoms, both of them being oxygen atoms forming two electron pairs when bonded to a metal ion. The maximum number of coordination sites it can occupy on a single metal ion is 2.

Step 4: (d) The \(2-\) ion of the porphine molecule (Figure 24.8)

The porphine molecule's \(2-\) ion is capable of binding to a metal ion through its four nitrogen atoms. Each of these nitrogen atoms has a lone pair of electrons, making this ligand a tetradentate ligand. The maximum number of coordination sites it can occupy on a single metal ion is 4.

Step 5: (e) [EDTA] \(^{4-}\)

EDTA (\(\mathrm{H}_{4}\mathrm{Y}\)) is a hexadentate ligand capable of binding to a metal ion through six donor atoms. These donor atoms include two nitrogen atoms and four oxygen atoms, each having a lone pair of electrons. The [EDTA] \(^{4-}\) ligand can form six bonds with a metal ion, and the maximum number of coordination sites it can occupy on a single metal ion is 6.

Key Concepts

Polydentate Ligands

Polydentate ligands, often referred to as chelating agents, are molecules that can form multiple bonds to a single metal ion. These multidentate ligands have several donor atoms, which provide electrons to form these connections with the metal centers. The number of bonds that a ligand can form with a metal ion is an essential characteristic of these molecules and is described by terms such as bidentate (two bonding sites), tridentate (three), tetradentate (four), pentadentate (five), and hexadentate (six).

For example, ethylenediamine (en) is a bidentate ligand, meaning it has two points of attachment. Each nitrogen atom in the molecule can donate a pair of electrons to the metal ion, creating a stable ring-like structure. This process is often compared to the way a claw ('chela' in Greek) would hold onto an object, hence the term 'chelating' is used to describe this binding action. Polydentate ligands are significant because they form very stable complexes with metal ions, which in many biological systems, industrial applications, and purification processes.

Donor Atoms

Donor atoms are the cornerstone of coordination chemistry, as they are the parts of ligands that provide electron pairs for bonding with metal ions. These atoms typically contain lone pairs that can be donated to vacant orbitals on a metal ion, establishing what is called a coordinate covalent bond. The identity of the donor atom is fundamental because it influences the strength and the type of bond formed. Common donor atoms include nitrogen, oxygen, sulfur, and phosphorus.

In our examples, ethylenediamine and bipyridine possess nitrogen donor atoms, while oxalate and EDTA have both oxygen and nitrogen donor atoms. Nitrogen is a common donor atom, found in many biological ligands, because the lone pair on nitrogen is readily available to bond with metals. Oxygen, on the other hand, is often found in anionic ligands like oxalate, where the negative charge increases the donor atom's ability to share its electrons with a metal center, solidifying the formation of the complex.

Metal Ion Complexes

Metal ion complexes, or coordination compounds, consist of a central metal atom or ion surrounded by molecules or ions called ligands. The number of bonds that can be formed between a metal ion and its ligands is known as the coordination number, which depends on the size, charge, and electron configuration of the metal as well as the size and electronic structure of the ligands.

For instance, the versatility of EDTA, a hexadentate ligand, allows it to occupy six coordination sites on a single metal ion, making it highly effective at sequestering metal ions. This property is why it's frequently used in medical treatments to bind and remove excess or toxic metals from the body. Complexes formed by these processes have applications ranging from catalysts and electronic devices to dyes and pharmaceuticals, underscoring the importance of metal-ligand interactions in the field of coordination chemistry.