What is a ligand in complex ion formation?

A ligand is a molecule or ion that acts as a Lewis base, donating a pair of electrons to a central metal ion. Ligands can be neutral molecules like ammonia or water, or ions such as cyanide or hydroxide. They surround the central metal ion and form coordinate bonds, creating a complex ion.

Why do transition metals readily form complex ions?

Transition metal ions form complex ions readily because they have high charge density and empty d orbitals available to accept electron pairs from ligands. These characteristics make them strong Lewis acids, enabling them to attract and bind ligands effectively, creating stable coordination complexes.

How does the formation constant relate to complex ion stability?

The formation constant (Kf) is the equilibrium constant for the reaction between a metal ion and ligands to form a complex ion. A larger Kf value indicates a more stable complex ion, meaning the reaction strongly favors product formation. The dissociation constant (Kd) is the mathematical inverse, Kd = Kf−1.

How does complex ion formation affect the solubility of sparingly soluble salts?

Complex ion formation increases solubility by removing free metal ions from solution. When a strong ligand binds metal ions, it depletes the equilibrium concentration of free ions, driving the solubility equilibrium toward dissolution. This allows more solid salt to dissolve to maintain equilibrium.

What happens when ammonia is added to a silver chloride solution?

When ammonia is added to silver chloride solution, the ammonia ligands bind to silver ions, forming the complex ion [Ag(NH3)2]+. This removes free silver ions from solution, shifting the solubility equilibrium and allowing more silver chloride to dissolve, increasing overall solubility.

How can you calculate equilibrium ion concentrations in a complex ion system?

Use an ICE table to track initial concentrations, changes, and equilibrium concentrations of all species. Account for stoichiometry—if one metal ion reacts with multiple ligands, multiply the metal ion change by the ligand coefficient. Substitute equilibrium values into the Kf expression to solve for unknown concentrations.

Why are metal ions always hydrated in aqueous solutions?

Metal ions are always hydrated because water molecules act as Lewis bases, donating electron pairs to metal ions, which function as Lewis acids. The water molecules surround the metal ion through coordinate bonding, forming a hydrated metal ion complex in solution.

16.12

Formation of Complex Ions

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Metal ions are always hydrated in aqueous solutions. The water molecules act as Lewis bases, sharing their lone pair of electrons with the metal ions, which behave as Lewis acids.

When a Lewis base stronger than water is added, it displaces the water molecules and surrounds the central metal ion, forming a complex ion. The molecule or ion acting as the Lewis base is called a ligand.

In hexammine cobalt(III) chloride, hexammine cobalt is a complex ion, where the 6 ammonia molecules are the ligands that octahedrally surround the central cobalt ion.

Because transition metal ions have a high charge density and empty d orbitals to accommodate shared electrons, they are particularly prone to forming complex ions.

The equilibrium constant for the reaction between the metal ion and the ligand is called the formation constant, K_f. The larger the value of K_f, the more stable the complex ion.

Forming such stable complex ions often increases the solubility of sparingly soluble metal salts.

Consider silver sulfide, which exists in solution in an equilibrium of aqueous ions and undissolved solid.

If silver sulfide is added to sodium cyanide solution, the silver ions combine with cyanide to form the complex ion dicyanoargentate.

If 0.20 moles of silver sulfide is added to one liter of a 0.90 M sodium cyanide solution, the equilibrium concentration of silver ions, x, can be calculated from an ICE table.

The initial concentrations of silver, cyanide, and dicyanoargentate ions are 0.20 M, 0.90 M, and 0, respectively.

Because of the high K_f, and the much higher concentration of cyanide compared to silver ions, essentially all the silver ions are converted to dicyanoargentate ions.

One aqueous silver ion reacts with 2 cyanide ions to form dicyanoargentate. So, the change in molar concentration of cyanide ions will be 2 × 0.20, or 0.40 M.

Thus, at equilibrium the concentration of dicyanoargentate ions can be assumed to be the same as the initial concentration of silver, while cyanide ion concentration would be 0.90 − 0.40 M, or 0.50 M.

Substituting these values into the expression for K_f yields 0.2 molar divided by x times 0.5 squared. When the expression is solved for x, the resulting concentration is 8.0 × 10⁻²² M.

The very small equilibrium concentration of silver ions indicates that the formation of complex ions depletes free silver ions from the solution.

This drives the silver sulfide solubility equilibrium towards the ions, allowing more solid to dissolve.

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A type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transit…