Problem 27 - Chemistry: The Central Science

Question

(a) What is the difference between a monodentate ligand and a bidentate ligand? (b) How many bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex? (c) You are told that a certain molecule can serve as a tridentate ligand. Based on this statement, what do you know about the molecule? mathrm{Br}$

Step-by-Step Solution

Answer

(a) Monodentate ligands bind to the central metal atom through one site (one donor atom), while bidentate ligands bind through two sites (two donor atoms). Bidentate ligands occupy two coordination sites and may form chelate rings. (b) To fill the coordination sphere of a six-coordinate complex, three bidentate ligands are needed, as each ligand occupies two coordination sites (3 x 2 = 6). (c) A tridentate ligand binds to the central metal atom via three sites (three donor atoms), meaning the molecule has three atoms with lone pairs that can form coordinate covalent bonds with the metal atom. It occupies three coordination sites and may form multidentate chelate rings.

Step 1: (a) Monodentate vs Bidentate Ligands

Monodentate ligands are those that bind to the central metal atom of a complex through only one site (one donor atom). In contrast, bidentate ligands are those that bind to the central metal atom via two sites (two donor atoms). Bidentate ligands occupy two coordination sites in a complex and may form chelate rings with the metal atom.

Step 2: (b) Bidentate Ligands for Six-Coordinate Complex

A six-coordinate complex has a total of six coordination sites to be filled. Since a bidentate ligand occupies two coordination sites, to fill the entire coordination sphere of a six-coordinate complex, you would need three bidentate ligands. This is because 3 bidentate ligands × 2 coordination sites per ligand = 6 coordination sites.

Step 3: (c) Information about Tridentate Ligand

A tridentate ligand is one that binds to the central metal atom via three sites (three donor atoms). This means that the molecule has three atoms with lone pairs of electrons, which can form coordinate covalent bonds with the central metal atom. These three donor atoms could be the same type (e.g., three oxygen atoms) or different types (e.g., two oxygen atoms and one nitrogen atom). The tridentate ligand occupies three coordination sites in a complex and may form multidentate chelate rings with the central metal atom.

Key Concepts

Monodentate Ligands

Monodentate ligands, also known as "one-toothed" ligands, attach to a central metal atom or ion through a single point of contact. This contact point is typically a lone pair of electrons donated by the ligand's donor atom. The simplicity of monodentate ligands means they occupy just one place in the coordination sphere of a metal complex, making them quite versatile.
They are often small molecules or ions like water (H₂O), ammonia (NH₃), or chloride ions (Cl⁻).

• They allow for straightforward and predictable complex formation.
• Their single bond limits their ability to stabilize a metal center against dissociation, compared to multi-dentate ligands.

Understanding how monodentate ligands function is crucial for comprehending more complex ligand behaviors.

Bidentate Ligands

Bidentate ligands, meaning "two-toothed," have the ability to bind to a metal center through two separate donor atoms. This capability allows them to form what are known as "chelate rings" with the central metal atom.
Chelation significantly increases the stability of the complex.

• Each bidentate ligand occupies two coordination sites within the metal's coordination sphere.
• Common examples include ethylenediamine (en) and oxalate ion (C₂O₄²⁻).

A key aspect of bidentate ligands is their efficiency; for example, to fully saturate a six-coordinate complex, you would need just three bidentate ligands. This feature makes them invaluable in catalytic processes and applications requiring robust complex stability.

Tridentate Ligands

Tridentate ligands, also named "three-toothed" ligands, can attach to a metal center via three donor atoms. This attribute allows for even more stable and complex structures, often forming multidentate chelate rings.
The three points of attachment allow for increased structural rigidity and potentially more diverse and unique coordination geometries.

• These ligands will take up three coordination sites.
• Their donor atoms can be of the same type (such as solely nitrogen atoms) or comprise different elements, creating variety in their bonding.
• Examples of tridentate ligands include terpyridine and diethylenetriamine.

Understanding tridentate ligands is crucial for grasping the complexities and broader implications of coordination chemistry, as they play a significant role in fields like bioinorganic chemistry and industrial catalysis.