Problem 29 - Chemistry: The Central Science

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) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}(o-\mathrm{phen})\right] \mathrm{Cl}_{3}\) (b) $\left[\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right] \

Step-by-Step Solution

Answer

In the given coordination complexes: (a) The polydentate ligand present is \(o-\mathrm{phen}\) (ortho-phenanthroline), which is a bidentate ligand with two nitrogen donor atoms. It occupies two coordination positions on the central metal ion (Co). (b) The polydentate ligand present is \(\mathrm{C}_{2}\mathrm{O}_{4}\) (oxalate ion), which is a bidentate ligand with two oxygen donor atoms. It occupies two coordination positions on the central metal ion (Cr).

Step 1: (a) Identifying the polydentate ligand in \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}(o-\mathrm{phen})\right]\mathrm{Cl}_{3}\)

In this complex, we have the following ligands: \(\mathrm{NH}_{3}\) and \(o-\mathrm{phen}\). The polydentate ligand is \(o-\mathrm{phen}\), which stands for ortho-phenanthroline, a bidentate ligand with two nitrogen atoms acting as donor atoms.

Step 2: (a) Determining the denticity of \(o-\mathrm{phen}\)

Since \(o-\mathrm{phen}\) has two nitrogen donor atoms, it is a bidentate ligand, and thus occupies two coordination positions on the central metal ion (Co).

Step 3: (b) Identifying the polydentate ligand in \(\left[\mathrm{Cr}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]\)

In this complex, we have the following ligands: \(\mathrm{C}_{2}\mathrm{O}_{4}\) and \(\mathrm{H}_{2}\mathrm{O}\). The polydentate ligand is \(\mathrm{C}_{2}\mathrm{O}_{4}\), which stands for oxalate ion, a bidentate ligand with two oxygen atoms acting as donor atoms.

Step 4: (b) Determining the denticity of \(\mathrm{C}_{2}\mathrm{O}_{4}\)

Since \(\mathrm{C}_{2}\mathrm{O}_{4}\) has two oxygen donor atoms, it is a bidentate ligand, and thus occupies two coordination positions on the central metal ion (Cr).

Key Concepts

Coordination Chemistry

Coordination chemistry refers to the study of compounds formed between metal ions and various organic or inorganic ligands. These interactions give rise to complex species known as coordination compounds or metal complexes. In such complexes, the central metal ion binds with ligands through coordinate covalent bonds, where the ligands donate a pair of electrons to the metal. Coordination compounds are characterized by their coordination number, which is the number of ligand donor atoms attached to the metal ion. These compounds play crucial roles in a broad range of areas, including biochemistry, materials science, and industrial processes.

When learning about coordination chemistry, it's essential to understand the terminology used to describe the structures and geometry of coordination complexes. Polydentate ligands, also called chelating agents, can attach to a metal ion at multiple points. The number of bonds a ligand forms with a central ion is called its denticity, and these interactions can significantly influence the stability and properties of the resulting metal complex.

Bidentate Ligands

Bidentate ligands are a type of polydentate ligand that can form two bonds with a single metal ion. In the context of coordination chemistry, these types of ligands have two donor atoms which can be oxygen, nitrogen, sulfur, or other elements capable of sharing electrons with a metal ion. Bidentate ligands create a ring-like structure with the metal ion at the binding points, known as chelation, which often increases the stability of the metal complex.

Ions or molecules such as ethylenediamine (en) and oxalate \(\mathrm{C}_{2}\mathrm{O}_{4}\) are classic examples of bidentate ligands. Chelation therapy, which is the medical use of chelating agents to bind and remove heavy metals from the body, relies heavily on the strong bonding produced by bidentate ligands. Understanding the specific geometric arrangements and the number of attachment points of bidentate ligands is crucial for predicting the shapes and reactivity of coordination compounds they form.

Metal Ion Complexes

Metal ion complexes consist of a central metal ion surrounded by ligands. The formation of these complexes involves the metal ion accepting electron pairs from the ligands to form coordinate covalent bonds. The overall charge, geometry, and reactivity of metal ion complexes depend on the nature of the metal ion, the type of ligands involved, and the ligands' denticities.

In the given exercise, the crucial understanding is that ligands like \(\mathrm{NH}_{3}\) are monodentate, forming a single bond with the metal ion, whereas others, such as ortho-phenanthroline (o-phen) and oxalate, are bidentate, wrapping around the metal ion to form two bonds. These multiple bonds often make bidentate ligand-based complexes more stable than those with monodentate ligands. Learning to identify and count coordination sites, and recognizing the type of ligands bound to a metal ion allows one to systematically evaluate and predict the properties of coordination compounds such as their magnetic behavior, color, and potential reactivity in chemical reactions.