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?

Short answer

(a) A monodentate ligand binds to a central metal atom/ion in a coordination complex through one donor atom, while a bidentate ligand binds through two donor atoms. (b) Three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex. (c) A tridentate ligand can bind to a central metal atom/ion through three donor atoms, meaning its structure enables these atoms to form simultaneous bonds with the metal ion, potentially creating a more stable complex.

Step-by-step solution

(a) Definition of Monodentate and Bidentate Ligands

A monodentate ligand is a species that can bind to a central metal atom/ion in a coordination complex through only one donor atom, while a bidentate ligand can bind to a central metal atom/ion through two donor atoms. Usually, bidentate ligands have a flexible structure that allows both donor atoms to attach simultaneously to the metal center.

(b) Number of Bidentate Ligands for a Six-Coordinate Complex

A six-coordinate complex is a coordination complex in which the central metal atom/ion is surrounded by six ligand species. Since each bidentate ligand can attach to the metal center using two donor atoms, we would need: \[\frac{6 \text{ coordination sites}}{2 \text{ donor atoms per bidentate ligand}} = 3 \text{ bidentate ligands}\] So, three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex.

(c) Information about a Tridentate Ligand Molecule

A tridentate ligand is a molecule that can bind to a central metal atom/ion in a coordination complex through three donor atoms. This means that the molecule has a structure that enables the three donor atoms to form bonds with the central metal ion simultaneously. The molecule can occupy three coordination sites on the central metal ion, potentially creating a more stable complex compared to some monodentate or bidentate ligands, due to the chelate effect.

Key concepts

Monodentate Ligands

In coordination chemistry, understanding the types of ligands is crucial for grasping how compounds form and behave. A monodentate ligand binds to a central metal atom or ion through a single donor atom. Imagine it as a single handshake. These ligands are simple and can only coordinate once with the metal center. Examples of monodentate ligands include water (\(\text{H}_2\text{O}\)), ammonia (\(\text{NH}_3\)), and chloride ions (\(\text{Cl}^-\)).

Monodentate ligands are often small and straightforward, and they allow for flexibility in terms of how they arrange themselves around the metal. They form a simpler and often weaker bond compared to polydentate ligands. The term "mono" signifies that there is only one point of attachment available from the ligand to the metal center.

Bidentate Ligands

Bidentate ligands are fascinating because they can bind to a central metal ion using two donor atoms. This is akin to both hands shaking hands with two sections of a single central entity. Bidentate ligands usually have a flexible structure that allows both of its donor atoms to coordinate simultaneously with the metal center.

A common example of a bidentate ligand is ethylenediamine (\(\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2\)), where the nitrogen atoms act as the two donors. Since bidentate ligands can form two bonds, they are very effective at forming stable complexes. The chelate effect—where rings are formed during coordination, increasing the stability of complexes—plays a significant role here. Essentially, having two bonds makes it harder for the ligand to detach from the metal, contributing to the robustness of the coordination complex. For a coordination sphere of a six-coordinate complex, typically, three bidentate ligands are needed to satisfy the six donor sites.

Tridentate Ligands

Tridentate ligands take complexity a step further, engaging three donor atoms to bind to the central metal. It's like a group of three coordinating skills wrapping around the metal ion. An example of a tridentate ligand is terpyridine, where three nitrogen atoms are responsible for binding.

These ligands afford even greater stabilization to the coordination complex through the formation of multiple chelate rings, involving three donor atoms. The more donor atoms that can simultaneously interact with a metal, the stronger and more stable the complex generally becomes. Through the chelate effect, tridentate ligands help form complexes that may exhibit different chemical behaviors compared to those formed by monodentate or bidentate ligands.

Tridentate ligands not only improve the stability of complexes, but also influence the geometry and overall dynamic of the metal center, often leading to unique chemical properties and applications in industrial and biochemical processes.