(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?

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.

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.

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.