28.5.4—Ligand exchanges

When two ligands compete for a metal ion, the larger formation constant corresponds to the more thermodynamically favoured complex under comparable conditions.

Write the competing formation or exchange equation and combine Kstab expressions if needed. The resulting equilibrium constant tells which complex is favoured, while the ratio also depends on ligand concentrations.

If [M(NH₃)₄]²⁺ has a larger formation constant than [M(H₂O)₆]²⁺, adding enough ammonia can shift hydrated metal ions towards the ammine complex.

Do not infer that the largest Kstab always gives the largest concentration: a ligand present in tiny concentration may lose the competition.