Problem 21 - Chemistry The Central Science

Question

(a) What is the difference between Werner's concepts of primary valence and secondary valence? What terms do we now use for these concepts? (b) Why can the \(\mathrm{NH}_{3}\) molecule serve as a ligand but the \(\mathrm{BH}_{3}\) molecule cannot?

Step-by-Step Solution

Answer

(a) Werner's primary valence refers to the oxidation state of a central metal ion, while secondary valence refers to the coordination number, i.e., the number of coordinating ligands. Nowadays, we use the terms oxidation state and coordination number. (b) \(\mathrm{NH}_{3}\) can serve as a ligand because it has a lone pair of electrons on the nitrogen atom, which can form a coordinate covalent bond with a central metal ion. In contrast, \(\mathrm{BH}_{3}\) cannot act as a ligand because boron has no available lone pair of electrons to form such bond.

Step 1: Understand Primary and Secondary Valences

Werner's coordination theory introduced the concepts of primary valence and secondary valence to explain the behavior of complex compounds. Primary valence refers to the number of negative ions required to satisfy the charge of a central metal ion in a coordination compound, and it represents the oxidation state of the central atom (metal). Secondary valence, on the other hand, refers to the number of ligands (neutral, cationic, or anionic) that coordinate to the central metal ion. It represents the coordination number of the complex.

Step 2: Current Terms for Primary and Secondary Valences

Nowadays, the primary valence concept is usually called ionic or oxidation state, while the secondary valence concept is often referred to as the coordination number, which represents the number of coordinating ligands.

Step 3: Discuss why NH3 can act as a ligand and BH3 can't

A ligand is a molecule or ion that can donate an electron pair to a central metal ion in a coordination compound. In the case of the \(\mathrm{NH}_{3}\) molecule (ammonia), there is a lone pair of electrons on the nitrogen atom which can participate in the formation of a coordinate covalent bond with a central metal ion. This electron donation makes \(\mathrm{NH}_{3}\) a good ligand. As for the \(\mathrm{BH}_{3}\) molecule (borane), there are three single covalent bonds between the boron atom and the three hydrogen atoms. Boron has only three valence electrons, which means all of its valence electrons are shared with the hydrogen atoms in the bonds, and there is no remaining lone pair on the boron atom to participate in the formation of a coordinate covalent bond with a central metal ion. Hence, a \(\mathrm{BH}_{3}\) molecule cannot act as a ligand.

Key Concepts

Primary Valence

Primary valence describes the oxidation state of the central metal atom in a coordination compound. In simpler terms, it is the number of electrons a metal atom needs to either lose or gain to achieve a stable electronic configuration. The concept was first introduced by Alfred Werner, a pioneer in the field of coordination chemistry.

This valence is also referred to as the ionic valence because it determines the charge of the metal ion in the compound. The primary valence corresponds to the metal's ability to form stable ionic bonds with other ions, such as chloride ions (e.g., in CoCl₃).

Understanding primary valence helps in predicting the chemical behavior and reactivity of coordination compounds as it directly relates to the compound's electrical properties.

Secondary Valence

Secondary valence refers to the coordination number of the central metal ion in a coordination compound. This represents the total number of ligand attachments to the metal. Ligands are ions or molecules that donate a pair of electrons to the metal to form a complex.

Werner described this valence as the secondary valence because it reflects the spatial arrangement of the ligands around the metal ion rather than the electrochemical properties. Nowadays, secondary valence is synonymous with the term 'coordination number'. This number gives insight into the geometry and structure of the complex, such as octahedral, tetrahedral, or square planar shapes.

Recognizing the secondary valence helps chemists understand stereochemical aspects of coordination compounds, which influence properties such as color, magnetism, and reactivity.

Ligands

Ligands are essential players in the formation of coordination compounds. These are ions or molecules with one or more pairs of electrons to share with the central metal ion, thus forming coordinate covalent bonds.

The ability of a molecule to act as a ligand primarily depends on the presence of lone pairs of electrons. For example:

• In ammonia (\( \mathrm{NH}_3 \)), the nitrogen atom has a lone pair that it can donate to form a bond with a metal ion, making it an effective ligand.
• Borane (\( \mathrm{BH}_3 \)), however, does not possess lone electron pairs, disqualifying it as a potential ligand.

Some ligands can bind through multiple sites and are termed "polydentate" or "chelate ligands". The versatility of ligands affects the stability and properties of the resulting complex.

Oxidation State

The oxidation state in a coordination compound indicates the charge that the central metal atom would have if all the bonding were purely ionic. It is equivalent to the primary valence of the metal ion and reveals how electrons are shared in the compound.

In isolation, the oxidation state of a metal can be found by accounting for the charges of the ligands and the overall charge of the complex. For example, in a complex such as \( \text{CoCl}_3 \cdot 6 \text{NH}_3 \), cobalt's oxidation state is (+3), since three negative chloride ions balance the positive charge required by cobalt.

Understanding oxidation states assists in predicting the reaction behavior and stability of coordination compounds in various chemical environments.

Coordination Number

Coordination number refers to how many "places" on a metal ion are occupied by ligands in a coordination compound. It ties closely to secondary valence and essentially gives details about the topology of the compound.

Common coordination numbers include:

• Six, as seen in an octahedral geometry.
• Four, typically resulting in either a tetrahedral or square planar geometry.
• Two, often leading to a linear arrangement.

This number not only impacts the compound's geometry but also influences its chemical reactions and physical properties, affecting characteristics like solubility, color, and magnetic behavior. Knowing the coordination number aids in understanding how coordination complexes interact with their environment and their potential applications in fields like catalysis and material science.