Question

a) Using Werner's definition of valence, which property is the same as oxidation number, primary valence or secondary walence? (b) What term do we normally use for the other type of valence? (c) Why can the \(\mathrm{NH}_{3}\) molecule serve as a ligand but the \(\mathrm{BH}_{3}\) molecule cannot?

Short answer

(a) Primary valence is the same as oxidation number, as it refers to the number of electrons involved in bonding. (b) The term we normally use for secondary valence is "coordination number." (c) $\mathrm{NH}_{3}$ can serve as a ligand because its nitrogen atom has a lone pair of electrons that can be donated to a central atom or ion. In contrast, $\mathrm{BH}_{3}$ cannot serve as a ligand because its central boron atom does not have any lone pairs of electrons to donate.

Step-by-step solution

Question (a) - Primary or Secondary Valence?

To determine which property, primary or secondary valence, is the same as the oxidation number, we need to understand the definition of both primary and secondary valence. Primary valence (or ionization potential) refers to the number of electrons that an atom loses, gains, or shares in a chemical bond, and is used as an indicator of the oxidation state of the element. Secondary valence (also known as coordination number) is the number of neighboring atoms or ions that are directly bonded to the central atom or ion in a complex. Since oxidation number is related to the number of electrons involved in bonding, primary valence is the same as the oxidation number.

Question (b) - Term for Other Type of Valence

Since primary valence is the same as oxidation number, we need to identify the term for the other type of valence, which is secondary valence. The term we normally use for secondary valence is "coordination number."

Question (c) - Why NH3 Can Serve as Ligand but BH3 Cannot?

To understand why NH3 can serve as a ligand while BH3 cannot, we need to first know that a ligand is a molecule or ion that donates a pair of electrons to a central atom or ion in a complex. In the NH3 molecule, the nitrogen atom has a lone pair of electrons, which can be donated to the central atom or ion in a complex. This process of donating an electron pair allows NH3 to act as a ligand. On the other hand, the BH3 molecule has boron as its central atom. Boron has only three valence electrons, which means that it forms three covalent bonds and does not have any lone pairs of electrons to donate. As a result, BH3 cannot act as a ligand.

Key concepts

Oxidation Number

An essential concept in chemistry is the oxidation number, which helps us understand how electrons are distributed in compounds and reactions. It is a theoretical number assigned to an atom in a molecule that represents the number of electrons it has gained, lost, or appears to share when bonding with other atoms.

For example, in a molecule of water (H2O), hydrogen has an oxidation number of +1 because it has effectively 'donated' one electron to oxygen. Oxygen, on the other hand, has an oxidation number of -2, as it 'gathers' two electrons, one from each hydrogen atom. These numbers assist in determining the electron configuration of atoms in molecules and also play a crucial role in balancing chemical equations.

Understanding oxidation numbers is vital as they are also indicative of an element's oxidation state—which can hint at its reactivity, the type of compound it will form, and how it will interact with other substances in a reaction.

Coordination Number

The coordination number in chemistry is indicative of the number of ligand atoms that surround and are bonded to a central metal atom within a coordination complex. This number is significant as it gives insights into the structural arrangement and the type of bonds present in the complex.

Take, for example, the hexaaquaion [Fe(H2O)6]²⁺, where the Fe²⁺ ion is surrounded by six water molecules. In this case, the coordination number of Fe²⁺ is 6. This shows a direct coordination relationship between the iron ion and the water molecules, explaining the formation of complex geometries like octahedral, tetrahedral, or square planar.

The coordination number can also affect the physical properties of complexes, such as color, magnetic behavior, and solubility, hence it's important to accurately determine it when analyzing coordination compounds.

Ligand Properties

Ligands are molecules or ions that have the ability to donate a pair of electrons to a metal ion to form a coordination complex. The properties of ligands are critical as they determine the nature of the coordination bond and the overall stability of the complex that forms.

One key property of ligands is their electron-donating ability, often attributed to the presence of lone pairs. Ligands like ammonia (NH3) possess lone pairs on the nitrogen atom that can be used to form bonds with metal ions. In contrast, a molecule like borane (BH3) lacks such lone pairs and cannot serve as a ligand.

Ligand Strength and Chelation

The strength of a ligand is defined by its ability to induce crystal field splitting in metal ions—strong field ligands, like CN⁻, result in a large split, while weak field ligands, like F⁻, result in a smaller split. Additionally, some ligands can form more than one coordinate bond with the same metal ion, a property known as chelation, which greatly increases complex stability.

Ligands play a pivotal role in the field of coordination chemistry and are utilized in various applications such as catalysis, material science, and medicine.