Problem 9 - 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 corresponds to the ionic bonds between the central metal ion and negative counter-ions, while secondary valence refers to the coordinate covalent bonds between the central metal ion and ligands. These are now called 'oxidation number' and 'coordination number.' (b) NH3 can serve as a ligand because its nitrogen atom has a lone pair of electrons that it can donate to form a coordinate covalent bond with the central metal ion. However, BH3 lacks a lone pair of electrons on its boron atom, rendering it unable to form coordinate covalent bonds and act as a ligand.

Step 1: Part (a): Primary and Secondary Valence Concepts

Werner's concepts of primary valence and secondary valence were introduced to explain the bonding and structure of coordination compounds. Primary valence refers to the ionic bonds formed between the central metal ion and negative counter-ions, while secondary valence refers to the coordinate covalent bonds formed between the central metal ion and ligands. Nowadays, the concepts of primary valence and secondary valence are better represented by the terms 'oxidation number' and 'coordination number,' respectively.

Step 2: Part (b): NH3 and BH3 as Ligands

To act as a ligand, a molecule must have a lone pair of electrons that it can donate to form a coordinate covalent bond with the central metal ion. In the case of NH3 (ammonia), the nitrogen atom has a lone pair of electrons that it can donate, making it an effective ligand. The chemical formula for ammonia can be written as \(\mathrm{N}(\mathrm{H}_{3})\), clearly showing the lone pair of electrons on the nitrogen. On the other hand, the BH3 (borane) molecule has a boron atom that lacks a lone pair of electrons. Instead, the boron atom forms bonds with three hydrogen atoms, giving it a full valence shell but no lone pair of electrons that could act as a donor. Thus, BH3 cannot form coordinate covalent bonds with a central metal ion, and it is not a ligand.