Explain the bonding in coordination compounds in terms of Werner’s postulates.
Alfred Werner was the first to formulate his ideas about the structures of coordination compounds. He proposed the concept of primary valence and secondary valence for a metal ion in a coordination compound.
Primary valence refers to the oxidation state of the metal ion. For example, binary compounds such as CrCl₃, CoCl₂ and PdCl₂ have primary valences of 3, 2 and 2 respectively. In modern language, these linkages are recognised as ionisable (ionic) bonds.
Secondary valence refers to the coordination number, which determines the number of ligand atoms attached to the metal ion. These linkages are recognised as non-ionisable (covalent) bonds in modern chemistry.
Werner postulated that a metal atom/ion uses two types of linkages in a coordination compound. Using the property of isomerism, Werner predicted the geometrical shapes of a large number of coordination entities.
However, Werner's theory could not explain why only certain elements form coordination compounds, why bonds have directional properties, and why coordination compounds have characteristic magnetic and optical properties.
Explanation
The answer covers all key aspects of Werner's postulates from the textbook: (1) the concept of primary and secondary valences, (2) examples of primary valences in binary compounds, (3) modern interpretation of these linkages as ionisable and non-ionisable bonds, (4) Werner's use of isomerism to predict geometrical shapes, and (5) limitations of his theory. The answer follows the 5-mark format with 5 paragraphs and uses bold for key terms as required.