Question

Polydentate ligands can vary in the number of coordination positions they occupy. In each of the following, identify the polydentate ligand present and indicate the probable number of coordination positions it occupies: (a) \(\operatorname{Cr}(\mathrm{EDTA})^{-}\) (b) \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Br}_{2}\) (c) \(\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{3}\) (d) \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{O}\) -phen \()(\mathrm{CN})_{4}\right]\)

Short answer

(a) In the complex \(\operatorname{Cr}(\mathrm{EDTA})^{-}\), the polydentate ligand is EDTA (ethylenediaminetetraacetic acid) which is a hexadentate ligand, hence it occupies six coordination positions. (b) In the complex \(\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\mathrm{Br}_{2}\), the polydentate ligand is oxalate (\(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\)), which is a bidentate ligand, so it occupies two coordination positions. (c) In the complex \(\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right]\mathrm{Cl}_{3}\), the polydentate ligand is ethylenediamine (en), which is a bidentate ligand, and thus it occupies two coordination positions. (d) In the complex \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{O}\)-phen\((\mathrm{CN})_{4}\right]\), the polydentate ligand is O-phenanthroline (O-phen). This ligand is a bidentate ligand and therefore occupies two coordination positions.

Step-by-step solution

(a) Identify and analyze the EDTA ligand

In the given compound \(\operatorname{Cr}(\mathrm{EDTA})^{-}\), the polydentate ligand present is EDTA (ethylenediaminetetraacetic acid). The EDTA ligand possesses six donor atoms - two amine nitrogens and four carboxylate oxygens that can coordinate with the central metal ion. Therefore, EDTA is a hexadentate ligand.

(a) Determine the coordination positions of EDTA

As EDTA is a hexadentate ligand, it occupies six coordination positions in the complex \(\operatorname{Cr}(\mathrm{EDTA})^{-}\).

(b) Identify and analyze the oxalate ligand

In the given compound \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Br}_{2}\), the polydentate ligand present is oxalate \(\left(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\right)\). The oxalate ligand contains two donor atoms being the two carbonyl oxygens, so it is bidentate.

(b) Determine the coordination positions of the oxalate ligand

The oxalate ligand, as a bidentate ligand, occupies two coordination positions in the compound.

(c) Identify and analyze the ethylenediamine ligand

In the given compound \(\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{3}\), the polydentate ligand present is ethylenediamine (en). The ethylenediamine ligand has two donor atoms, both being amine nitrogens, and is therefore a bidentate ligand.

(c) Determine the coordination positions of the ethylenediamine ligand

As a bidentate ligand, ethylenediamine occupies two coordination positions in the compound.

(d) Identify and analyze the O-phenanthroline ligand

In the given compound \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{O}\) -phen \()(\mathrm{CN})_{4}\right]\), the polydentate ligand present is O-phenanthroline (O-phen). The O-phenanthroline ligand contains two donor atoms, being the two nitrogen atoms, so it is a bidentate ligand.

(d) Determine the coordination positions of the O-phenanthroline ligand

The O-phenanthroline ligand occupies two coordination positions in the complex, as it is a bidentate ligand.

Key concepts

Polydentate Ligands

In coordination chemistry, ligands are ions or molecules that can donate a pair of electrons to a metal atom or ion, forming a coordination complex. Polydentate ligands, also known as chelating agents, have multiple donor atoms capable of binding to the central metal ion at more than one coordination site.
This ability to connect at several points often creates ring structures and increases the stability of the resulting complex.
The number of sites where a polydentate ligand can bind to a central metal is known as the "denticity" of the ligand. Denticity is a fundamental attribute that can influence the geometry and stability of the coordination complex.

• Examples include ethylenediamine (en) with two donor atoms and oxalate ( ( ext{C}_2 ext{O}_4^{2-} ) ) with two donor oxygens.
• These ligands wrap around the central metal, effectively creating a strong bond due to the chelating effect.

Understanding polydentate ligands is crucial because it helps to predict how certain ligands will behave in forming coordination complexes.

Coordination Complexes

Coordination complexes consist of a central metal atom or ion surrounded by ligands. These ligands can be either neutral molecules or ions. The metal atom/ion and the ligands interact through coordinate covalent bonds, where the ligands donate electron pairs to the metal.
The metals are typically transition elements because they have empty d-orbitals that can accept electron pairs from the ligands.
Coordination numbers indicate how many ligand bonds are surrounding the metal center.

• The complex can exhibit a wide variety of structures, from linear to octahedral, depending on the number of ligands and the denticity.
• The chemical formula also provides insights into the stoichiometry and geometry of the compound. For instance, in ( ext{Cr}( ext{EDTA})^- ) , EDTA binds through its six coordinating atoms, leading to a stable octahedral structure.

These complexes are widely studied due to their numerous applications in catalysis, material sciences, and biochemistry.

Hexadentate Ligand

A hexadentate ligand has six donor atoms that can bind to a metal ion at once. This high level of denticity ensures a particularly stable complex due to the multiple bonds that are formed simultaneously.
Ethylenediaminetetraacetic acid (EDTA) is a classic example of a hexadentate ligand. It uses two nitrogen atoms and four oxygen atoms from carboxylate groups to coordinate with a metal ion.

• This ability to form six bonds gives EDTA exceptional chelating properties, making it very useful for sequestering metal ions in processes such as water purification and as an anticoagulant in medicine.
• In complexes like ( ext{Cr}( ext{EDTA})^- ) , EDTA ensures high stability, forming robust octahedral arrangements due to its hexadentate nature.

The chelating effect, accompanied by its ability to form multiple rings with the metal ion, enhances the overall stability of the compound.

Bidentate Ligand

Bidentate ligands have precisely two donor atoms that bind to a metal ion, making them an important class of polydentate ligands. These ligands form a five-membered or six-membered ring with the central metal, providing additional stability over monodentate ligands.
Common examples of bidentate ligands include ethylenediamine (en) and oxalate ( ( ext{C}_2 ext{O}_4^{2-} ) ). In the latter, two oxygen atoms coordinate the metal.

• This dual binding is beneficial because it can significantly enhance the thermodynamic stability of a complex.
• The coordination number for a bidentate ligand is usually two, but its ability to chelate gives the ligand a particular advantage in forming more stable metal complexes.

Understanding bidentate ligands is essential for predicting and controlling the characteristics of coordination complexes, as seen in compounds like ( ext{Ni}( ext{C}_2 ext{O}_4)( ext{H}_2 ext{O})_2 ext{Br}_2 ) and ( ext{Ru}( ext{en})( ext{NH}_3)_4 ) , where their bonding options determine the geometry and stability.