5.1. Werner’s Theory Coordination Compounds

Coordination Compounds are the backbone of modern inorganic and bio-inorganic chemistry and chemical industry.

In the previous Unit we learnt that the transition metals form a large number of complex compounds in which the metal atoms are bound to a number of anions or neutral molecules by sharing of electrons. In modern terminology such compounds are called coordination compounds.

Coordination compounds are essential in modern chemistry and are formed when transition metals bond with anions or neutral molecules. These bonds are significant because they allow us to explore complex chemical behaviors not seen in simpler compounds. Transition metals have unique properties that enable them to form various complex compounds by sharing their electrons with other atoms or molecules.

Consider how a chef combines different ingredients to create a unique dish. Just as ingredients can combine in various ways to produce different flavors, transition metals can bond with various anions and molecules to create complex coordination compounds with unique properties.

Alfred Werner (1866-1919), a Swiss chemist was the first to formulate his ideas about the structures of coordination compounds. He prepared and characterised a large number of coordination compounds and studied their physical and chemical behaviour by simple experimental techniques.

Alfred Werner was a pioneering chemist who laid the groundwork for our understanding of coordination compounds. By preparing and studying numerous compounds, he was able to observe how these compounds behave chemically and physically. His systematic approach allowed him to propose theories that explained the complex structures and bonding in coordination chemistry.

Imagine a detective solving a mystery by collecting evidence and making observations about different crime scenes. Just as the detective pieces together the clues to understand the bigger picture, Werner examined various coordination compounds to piece together the theory of coordination chemistry.

Werner proposed the concept of a primary valence and a secondary valence for a metal ion. Binary compounds such as CrCl3, CoCl2 or PdCl2 have primary valence of 3, 2 and 2 respectively.

Werner distinguished between primary and secondary valences in coordination compounds. The primary valence, often ionizable, is satisfied by negative ions, while the secondary valence refers to the stable bonds formed with neutral molecules or ions. This distinction helps explain how many ligands can be connected to a metal ion and dictates the overall structure of the coordination complex.

Think of primary and secondary valence like a family structure. The primary valence is like parents (negative ions) who can leave the house any time (they are ionizable), while the secondary valence is like stable family members (neutral molecules) who live in harmony within the home (the structure of the metal complex).

In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution.

In a famous experiment involving cobalt(III) chloride and ammonia, Werner observed different behaviors when excess silver nitrate was added. These patterns suggested that certain chloride ions were bonded differently than others, highlighting the complexity of bonding within coordination compounds. The results emphasized the differing interactions and stability of various ligands attached to a metal ion.

This situation is akin to uncovering secrets in a friendship. Not all friends are the same; some may share private information (like remaining in solution) while others might keep everything disclosed. Each interaction tells us more about the relationships between the friends (or ligands) in the 'social group' (or complex).

Werner proposed the following postulates: 1. In coordination compounds, metals show two types of linkages (valences)-primary and secondary. 2. The primary valences are normally ionisable and are satisfied by negative ions. 3. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.

Werner's main postulates assert that coordination compounds are characterized by two types of valences: primary and secondary. Primary valences tend to involve ionizable links formed with negative ions, while secondary valences involve stable non-ionizable links, typically with neutral molecules or negative ions. This secondary valence corresponds to what we know as the coordination number, which indicates how many particles are bonded to the central metal ion, forming the structure of the compound.

Imagine a busy office where the primary valences represent the employees who can freely leave the office (ionizable links), while the secondary valences are the core staff who must stay and work on projects (non-ionizable links), forming a solid team around the central manager (the metal ion).

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