complex ion formation

Meaning: A complex ion forms when a central metal cation, often a transition metal, binds ligands: molecules or anions with lone pairs, such as NH₃, H₂O, CN⁻ or OH⁻. Each bond is coordinate covalent, with the ligand donating both electrons, so the metal is a Lewis acid and the ligand a Lewis base. The number of ligand bonds is the coordination number. Formation constants are usually large, so the equilibrium lies far toward the complex.

Equation: Ag⁺ + 2NH₃ ⇌ Ag(NH₃)₂⁺, Kf = [Ag(NH₃)₂⁺] / ([Ag⁺][NH₃]²), where brackets are equilibrium concentrations in mol/L and Kf, the formation constant, is treated as unitless and depends on temperature.

On the MCAT: A sparingly soluble salt such as AgCl is placed in NH₃(aq). Predict higher solubility: the complex removes free Ag⁺, so Le Chatelier's principle shifts dissolution to the right. Adding the two reactions multiplies their constants: K = Ksp × Kf. Amphoteric hydroxides such as Al(OH)₃ dissolving in excess OH⁻ follow the same logic.

Common confusion: Complex formation does not change Ksp, which depends on temperature. The salt dissolves more because free metal ion is tied up in the complex, keeping the ion product below Ksp. A large Kf means a stable complex, not a less soluble salt.