5.9. Exercises

Explain the bonding in coordination compounds in terms of Werner’s postulates.

Werner's postulates regarding coordination compounds focus on the roles of metals and ligands in forming these compounds. He categorized metal bonds into primary and secondary valences. The primary valence is the number of ionizable bonds that the metal can form with negative ions, while the secondary valence is the total number of ligands attached directly to the metal center. This framework allows us to understand that coordination compounds consist of metal ions surrounded by ligands in specific geometries, which can be predicted by the secondary valence proposed by Werner.

Think of a coordination compound like a flower bouquet. The metal center represents the vase, and the ligands are the flowers. Each type of flower can represent different ligands (like ammonia or chloride), and together they form a beautiful arrangement (the coordination compound) that cannot be easily separated without disturbing the whole bouquet.

FeSO4 solution mixed with (NH4)2SO4 solution in 1:1 molar ratio gives the test of Fe ion but CuSO4 solution mixed with aqueous ammonia in 1:4 molar ratio does not give the test of Cu ion. Explain why?

When FeSO4 is mixed with (NH4)2SO4, the iron ions can remain ionized in solution, allowing them to be detected as Fe²⁺ ions. In contrast, when CuSO4 is mixed with excess ammonia, the copper ions form a complex with ammonia (like [Cu(NH3)4]²⁺), which does not release free Cu²⁺ ions into the solution for detection. Thus, the color change indicating the presence of Cu ions does not occur.

Imagine trying to find a hidden treasure. In one scenario, you can see the treasure chest (like the Fe ions), while in another scenario, the chest is locked up and requires a key (the ammonia ligand) to access the coins inside (the Cu ions), making it hard to see or test for them directly.

Explain with two examples each of the following: coordination entity, ligand, coordination number, coordination polyhedron, homoleptic and heteroleptic.

Coordination entities can be defined as complexes where a central metal atom is bonded to surrounding ligands. For example, [Cu(NH3)4]²⁺ is a coordination entity formed by copper as the central atom and ammonia as ligands. Ligands are ions or molecules bonded to the central atom, such as Cl⁻ in [CoCl4]²⁻. The coordination number refers to the number of ligands attached to the central atom—like 4 in the case of [Ni(CO)4]. A coordination polyhedron refers to the geometrical arrangement of these ligands around the central atom, which can be tetrahedral or octahedral. Homoleptic complexes contain only one kind of ligand, like [Cu(NH3)6]²⁺, while heteroleptic complexes have multiple kinds of ligands, like [Co(NH3)4Cl2].

Think of coordination compounds like a city. The central metal atom is the city center, the ligands are different modes of transportation (buses, bicycles, cars) bringing people to and from the center. Each type of transportation can represent a different ligand type, and the total number of transportation modes (like buses and cars) surrounding the center indicates the coordination number, while the layout of roads represents the coordination polyhedron.

What is meant by unidentate, didentate and ambidentate ligands? Give two examples for each.

Unidentate ligands attach to the metal at one point, like Cl⁻ or NH3, which each bond through a single atom. Didentate ligands bind at two points, such as ethylenediamine (en) or oxalate (C2O4²⁻). Ambidentate ligands can bind through two different atoms; for example, thiocyanate (SCN⁻) can bind through either the sulfur or nitrogen atom. Understanding these terms helps in predicting the structure and bonding of coordination compounds.

Imagine you are at a party with different types of friends. Unidentate friends can only hold your hand (one bond), didentate friends can link arms from both sides (two bonds), while ambidentate friends can grab your elbow with one arm and hold your hand with the other, making them flexible in how they connect!

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