Problem 21 - Chemistry The Central Science

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 BH \(_{3}\) cannot?

Step-by-Step Solution

Answer

(a) Primary valence is the same as the oxidation number according to Werner's definition of valence. (b) The term used for the other type of valence is secondary valence or coordination number. (c) \(\mathrm{NH}_{3}\) can serve as a ligand because it has a lone pair of electrons on the nitrogen atom, while \(\mathrm{BH}_{3}\) cannot serve as a ligand since it lacks a lone pair of electrons on the boron atom.

Step 1: (a) Primary or secondary valence as oxidation number

Primary valence corresponds to the oxidation number in Werner's definition of valence. This reflects the charge on the central metal ion when all its ligands are removed.

Step 2: (b) Term for the other type of valence

The other type of valence, which is not the same as the oxidation number, is called secondary valence or coordination number. This refers to the number of ligand atoms directly bonded to the central metal ion in a coordination complex.

Step 3: (c) Ligand capabilities of \(\mathrm{NH}_{3}\) and \(\mathrm{BH}_{3}\)

\(\mathrm{NH}_{3}\) can serve as a ligand because it has a lone pair of electrons on the nitrogen atom, which can donate to a central metal ion to form a coordinate covalent bond. However, \(\mathrm{BH}_{3}\) cannot serve as a ligand as it does not have a lone pair of electrons on the boron atom to form a coordinate covalent bond with a central metal ion.

Key Concepts

Oxidation Number

The oxidation number in a coordination complex represents the charge of the central metal ion. It's like a helpful marker to keep track of what happens when ligands bond with metals. When you strip away all the ligands from the metal complex, what's left is the oxidation number of that metal.
For example, in Werner's Coordination Theory, primary valence refers to these oxidation numbers. This is because primary valence accounts for the ionic part of the complex. That means it directly reflects the charge that the central metal ion exhibits. Remembering this can help simplify how we look at complex structures!

Primary Valence

Primary valence is a crucial part of Werner's Coordination Theory. Simply put, it refers to the oxidation state or oxidation number of the central metal within a coordination compound.
But why does it matter? Well, primary valence informs us about how many ions or molecules are necessary to balance the charge of the metal ion. It's the metal's ability to exist in different charged forms.
Always remember: primary valence = oxidation number. This clear link can make understanding coordination chemistry more intuitive.

Secondary Valence

Secondary valence, also known as the coordination number, is all about how many ligands are bonded to the central metal ion. While the primary valence is about charge, secondary valence focuses on structure.
Werner's theory explains that secondary valence reveals how metallic complexes take shape. Essentially, it tells us the number of ligand atoms that cling to the central metal atom, forming the coordination sphere.
This coordination number affects the geometry of the complex—like how many 'friends' the metal has in the structure! Appreciating secondary valence gives insight into the three-dimensional arrangement of the coordination compounds.

Coordination Number

The coordination number is a key factor in defining the structure of coordination complexes. It tells us exactly how many ligand atoms are directly attached to the central metal ion.
Think of it as the number of direct connections the metal at the heart of the complex has! This count explains the geometry and stability of the complex, impacting properties like solubility and reactivity.
Common coordination numbers range from 2 to 9, usually reflecting the spatial arrangement—whether the ligands form a linear, square planar, or octahedral shape around the metal.

Ligand

Ligands are molecules or ions that donate electrons to the central metal ion, forming coordinate covalent bonds. They play an essential role in the stabilization of metal complexes.
For instance, ammonia ( NH_3 ) acts as a ligand because it has a lone pair of electrons that can form a bond with the metal. However, borane ( BH_3 ) cannot act as a ligand due to the absence of such a lone pair.
Different types of ligands, whether mono-, bi-, or polydentate based on their binding sites, determine the nature and properties of the coordination complex. Understanding ligands is vital for grasping the basics of coordination chemistry.