Question

(a) What is themeaning of the term coordination number as it applies to metal complexes? (b) Generally speaking, what structural feature characterizes substances that can serve as ligands in metal complexes? Give an example of a ligand that is neutral and one that is negatively charged. (c) Would you expect ligands that are positively charged to be common? Explain. (d) What type of chemical bonding is characteristic of coordination compounds? Illustrate with the compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\)

Short answer

The coordination number refers to the number of ligands directly bonded to a central metal ion in a coordination complex, forming coordinate covalent bonds. Ligands possess electron donor atoms with lone pairs and can be neutral (e.g., ammonia, NH3), negatively charged (e.g., chloride ion, Cl-), or, uncommonly, positively charged. Coordination compounds exhibit coordinate covalent bonding, as seen in the compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\), which has a coordination number of 6 and displays coordinate covalent bonding between Co and NH3 ligands.

Step-by-step solution

1. Definition of Coordination Number

Coordination number refers to the number of ligand atoms that are directly bonded to the central metal ion in a coordination complex. These ligands donate lone pair of electrons to the metal ion, forming coordinate covalent bonds.

2. Structural Features of Ligands

Ligands are molecules or ions that can donate lone pairs of electrons to a metal ion, forming a coordinate covalent bond. The structural feature that characterizes these substances is the presence of one or more electron donor atoms with a lone pair of electrons. These donor atoms can be oxygen, nitrogen, sulfur, and/or halogens.

3. Examples of Neutral and Negatively Charged Ligands

Neutral ligands do not carry any charge. An example of a neutral ligand is ammonia (NH3), which has a lone pair of electrons on nitrogen that can form a bond with a metal ion. A negatively charged ligand has a negative charge on the donor atom. An example of a negatively charged ligand is chloride ion (Cl-), which has a lone pair of electrons on chlorine that can form a bond with a metal ion.

4. Positively Charged Ligands

Generally, positively charged ligands are not common. This is because most metal ions have positive charges, and the formation of coordinate covalent bonds between two positively charged species would be unfavorable due to electrostatic repulsion. Ligands typically have negative or neutral charge to be attracted to the positively charged metal ions.

5. Chemical Bonding in Coordination Compounds

Coordination compounds exhibit coordinate covalent bonding. In this type of bonding, a ligand donates a lone pair of electrons to the central metal ion, forming a bond. This is different from ionic bonding (transfer of electrons) or covalent bonding (equal sharing of electrons).

6. Application to \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\)

In the compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\), the cobalt (Co) is the central metal ion and it is surrounded by six ammonia (NH3) ligands, which are neutral. The coordination number for this compound is 6, as there are six ammonia ligands bonded to the cobalt ion. The overall charge of the complex is +3, as the cobalt has a charge of +3 and each ammonium ligand is neutral. There are three chloride ions (Cl-) outside the complex which are negatively charged and balance the charge of the complex. The chemical bonding in this compound is characterized by coordinate covalent bonding between the cobalt ion and ammonia ligands.

Key concepts

Coordination Number

In the context of coordination chemistry, the coordination number is a crucial concept that helps to define the structure of metal complexes. It refers to the total number of ligand atoms that are directly bonded to the central metal ion in a complex. These ligands provide lone pairs of electrons to form bonds known as coordinate covalent bonds with the metal ion.
The coordination number essentially tells us how many ligand molecules are attached to a metal ion through these bonds. For instance, in the coordination complex of \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\), the coordination number is 6, indicating that six ammonia molecules are coordinating with the cobalt ion.
This concept is vital because the coordination number can significantly influence the geometry and, therefore, the properties of the metal complex.
Different metals and different ligands can result in different coordination numbers, highlighting the diversity of structures within coordination compounds.

Ligands

Ligands are entities that attach to central metal atoms to form coordination complexes through coordinate covalent bonds. They play a pivotal role as electron donors in these compounds.
A structural characteristic common to all ligands is the presence of at least one donor atom that possesses a lone pair of electrons. This lone pair is what they donate to the metal ion, allowing the formation of a coordinate bond.

• Neutral ligands do not possess any net charge. A classic example is ammonia (NH3), where the nitrogen atom provides the lone pair of electrons.
• Negatively charged ligands carry a negative charge, such as the chloride ion (Cl-), which uses its lone pair from the chlorine atom to form bonds.

Unlike neutral or negatively charged ligands, positively charged ligands are rare due to the electrostatic repulsion they would have with metal ions, which are often positively charged themselves.
Understanding ligands is key to grasping how diverse coordination compounds can be and how their properties can vary.

Coordinate Covalent Bond

Coordinate covalent bonds form a fundamental aspect of coordination chemistry. These bonds occur when a ligand donates a lone pair of electrons to a metal ion.
In \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\mathrm{Cl}_{3}\), each ammonia molecule donates a lone pair of electrons from nitrogen to the central cobalt ion, resulting in a coordinate covalent bond.
This type of bonding is distinct from traditional covalent bonds, where electron pairs are shared equally. It's also different from ionic bonds, where electrons are transferred entirely from one atom to another.
Coordinate covalent bonding allows metal ions to accept multiple lone pairs from different ligands, thereby forming complex structures. The nature of these bonds significantly affects the stability and reactivity of coordination compounds.

Chemical Bonding in Coordination Compounds

Chemical bonding in coordination compounds is predominantly characterized by coordinate covalent bonds. This form of bonding is essential in the formation and structure of these compounds.
In coordinate covalent bonding, each ligand offers a lone pair of electrons to bind with the metal ion, such as in the compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\mathrm{Cl}_{3}\), where cobalt forms bonds with six ammonia ligands, leading to a stable complex.

• The central metal ion acts as an electron-pair acceptor.
• The ligands act as electron-pair donors.

This bonding is critical because it dictates not just the immediate structure but also many properties such as color, magnetism, and reactivity of the compound.
Understanding the nature of chemical bonding in coordination compounds helps predict how these interesting and often colorful compounds will behave under different conditions.