Problem 21 - Chemistry: The Central Science in SI Units, Global Edition

Question

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

Step-by-Step Solution

Answer

(a) The primary valence corresponds to the oxidation number in Werner's definition. (b) The other type of valence is secondary valence, also known as coordination number. (c) \(\mathrm{NH}_{3}\) can serve as a ligand because it has a lone pair of electrons on the nitrogen atom that can donate to a central metal ion, whereas \(\mathrm{BH}_{3}\) cannot, as it has an empty p-orbital and lacks a lone pair to donate.

Step 1: Determine the type of valence corresponds to oxidation number

According to Werner's theory, a transition metal complex can have two types of valence: primary valence and secondary valence. Primary valence corresponds to the oxidation number of the central metal atom. It is the number of ions or atoms directly bonded to the metal, exhibiting ionic character.

Step 2: Identify the term for the other type of valence

The other type of valence in Werner's theory is secondary valence, also known as coordination number. Secondary valence corresponds to the number of coordinate covalent bonds formed between the central metal ion and the ligands. This type of valence is responsible for the geometrical structure of the complex.

Step 3: Explain ligand properties of NH3 and BH3

Ammonia \(\mathrm{NH}_{3}\) can act as a ligand because it has a lone pair of electrons on the nitrogen atom that can donate to the central metal ion in a complex, forming a coordinate covalent bond. On the other hand, borane \(\mathrm{BH}_{3}\) cannot act as a ligand because boron has an empty p-orbital instead of a filled orbital with a lone pair that can be donated to a metal ion. Hence, \(\mathrm{BH}_{3}\) does not have the necessary electron donor property to form a coordinate covalent bond with a central metal ion.

Key Concepts

Werner's Theory

Werner's Theory provides a foundational framework in coordination chemistry. It explains how complex compounds are formed by transition metals and how they interact with surrounding ligands, which are molecules or ions that donate a pair of electrons to the metal center.
At the heart of Werner's Theory are two types of valence associated with metal ions in a complex:

• Primary Valence: This is equivalent to the oxidation number of the central metal ion. It represents the number of ions bonded to the metal, typically through ionic bonds. These bonds are charge-based interactions.
• Secondary Valence: Also referred to as the coordination number, this describes the total number of coordinate covalent bonds the central metal ion forms with its ligands. These are formed when ligands donate electron pairs to the metal, creating stable complexes.

Werner's innovative approach allowed for a greater understanding of complex ion compositions and spatial arrangements. It laid the groundwork for the development of crystal field theory and other coordination chemistry principles.

Oxidation Number

Oxidation number, or oxidation state, is a concept used to indicate the degree of oxidation (or loss of electrons) of an atom in a chemical compound. In the context of Werner's Theory, it is directly related to the primary valence of a transition metal complex.
Here are key points regarding oxidation numbers:

• The oxidation number provides insight into the charge of a central metal ion within a complex.
• It is crucial for predicting the types of chemical reactions and stability of complex ions.
• In coordination compounds, oxidation numbers help in determining the formula and nomenclature.

By understanding the oxidation number, chemists can deduce how a complex will behave in chemical reactions and how strongly it will interact with its ligands.

Ligands

Ligands are molecules or ions that bind to a central metal atom or ion to form a coordination complex. They play a crucial role in the stability and reactivity of these complexes.
Key attributes of ligands include:

• They donate one or more pairs of electrons to the metal ion, forming coordinate covalent bonds.
• Common examples of ligands are water ( H 2 O ), ammonia ( N H 3 ), and chloride ions (Cl^-).
• Ligands can be categorized based on the number of donor atoms, such as monodentate (one donor atom) or polydentate (multiple donor atoms).

Ammonia ( N H 3 ) serves as a good example, as it has a lone pair of electrons on its nitrogen atom that can be donated to form a coordinate covalent bond. Understanding ligands' properties helps scientists design metal complexes for various applications in catalysis, material science, and medicine.

Coordinate Covalent Bond

A coordinate covalent bond, also known as a dative covalent bond, is a type of chemical bond where both electrons in the bond come from the same atom, typically a ligand in coordination chemistry.
This bond is different from a typical covalent bond where each atom contributes one electron to the bond:

• In a coordinate covalent bond, the ligand provides a lone pair of electrons to the empty orbital of a metal ion.
• This bond is essential for the formation of stable coordination complexes.
• Even though coordinate covalent bonds initially form through this unique donor mechanism, they often possess similar strength and properties as standard covalent bonds.

Such bonds are pivotal in coordination chemistry, allowing for intricate metal-ligand formations that determine the structural geometry and function of the resulting complex. For instance, while ammonia ( N H 3 ) can donate electrons to form these bonds, borane ( B H 3 ) cannot because it lacks available electron pairs for donation.