Problem 17 - 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?

Step-by-Step Solution

Answer

(a) Monodentate ligands have only one atom available to bind to a single position on a central metal atom/ion, whereas bidentate ligands have two atoms available to bind simultaneously to two different positions. (b) Three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex. (c) A tridentate ligand can simultaneously bind through three distinct donor atoms to three different coordination sites on the central metal atom/ion and can occupy half of the coordination sphere for a six-coordinate complex.

Step 1: (a) Definition of Monodentate and Bidentate Ligands

Monodentate ligands are ligands that have only one atom (usually nitrogen, oxygen, or sulfur) available to bind to a single position on a central metal atom or ion. Examples of monodentate ligands include ammonia (NH₃), water (H₂O), and cyanide (CN⁻). Bidentate ligands, on the other hand, have two atoms (donor atoms) available to bind simultaneously to two different positions on a central metal atom or ion. These donor atoms are typically linked by a chain of atoms. Common examples of bidentate ligands include ethylenediamine (en), which contains two nitrogen (N) donor atoms, and oxalate ion (C₂O₄²⁻), which contains two oxygen (O) donor atoms.

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

A six-coordinate complex has six vacant positions around the central metal ion or atom available for ligands to bind. Considering that a bidentate ligand can simultaneously link to two separate positions on the central metal atom/ion, we can divide the total number of coordination sites (6) by the number of sites that one bidentate ligand occupies (2): Number of bidentate ligands needed = 6 / 2 = 3 Therefore, three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex.

Step 3: (c) What is Known About a Tridentate Ligand?

Based on the description of a tridentate ligand, we know that this molecule can simultaneously bind through three distinct donor atoms to three different coordination sites on the central metal atom or ion. These three donor atoms may consist of the same or different chemical elements (such as nitrogen, oxygen, or sulfur) and are connected by a chain of atoms within the ligand. A tridentate ligand can occupy half of the coordination sphere for a six-coordinate complex.