錯体イオンの形成

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.

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.

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.

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.

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.

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.

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.