Formation Constants (Kf) for Complex Ions
Formation (stability) constants Kf at 25 °C for common complex ions, each with its formation equilibrium. Large Kf values explain why complexation dissolves sparingly soluble salts: couple the dissolution with the formation equilibrium (K = Ksp × Kf) to compute solubility in the presence of a complexing agent.
| Complex Ion | Formation Equilibrium | Kf (25 °C) | log Kf |
|---|---|---|---|
| [AlF6]3- | Al3+ + 6 F- ⇌ [AlF6]3- | 7 × 1019 | 19.8 |
| [Cd(NH3)4]2+ | Cd2+ + 4 NH3 ⇌ [Cd(NH3)4]2+ | 1.3 × 107 | 7.1 |
| [Cd(CN)4]2- | Cd2+ + 4 CN- ⇌ [Cd(CN)4]2- | 3 × 1018 | 18.5 |
| [Co(NH3)6]2+ | Co2+ + 6 NH3 ⇌ [Co(NH3)6]2+ | 1.3 × 105 | 5.1 |
| [Co(NH3)6]3+ | Co3+ + 6 NH3 ⇌ [Co(NH3)6]3+ | 2.3 × 1033 | 33.4 |
| [Cu(CN)2]- | Cu+ + 2 CN- ⇌ [Cu(CN)2]- | 1 × 1016 | 16.0 |
| [Cu(NH3)4]2+ | Cu2+ + 4 NH3 ⇌ [Cu(NH3)4]2+ | 1.7 × 1013 | 13.2 |
| [Fe(CN)6]4- | Fe2+ + 6 CN- ⇌ [Fe(CN)6]4- | 1.5 × 1035 | 35.2 |
| [Fe(CN)6]3- | Fe3+ + 6 CN- ⇌ [Fe(CN)6]3- | 2 × 1043 | 43.3 |
| [Fe(SCN)6]3- | Fe3+ + 6 SCN- ⇌ [Fe(SCN)6]3- | 3.2 × 103 | 3.5 |
| [HgCl4]2- | Hg2+ + 4 Cl- ⇌ [HgCl4]2- | 1.1 × 1016 | 16.0 |
| [Ni(NH3)6]2+ | Ni2+ + 6 NH3 ⇌ [Ni(NH3)6]2+ | 2 × 108 | 8.3 |
| [AgCl2]- | Ag+ + 2 Cl- ⇌ [AgCl2]- | 1.8 × 105 | 5.3 |
| [Ag(CN)2]- | Ag+ + 2 CN- ⇌ [Ag(CN)2]- | 1 × 1021 | 21.0 |
| [Ag(NH3)2]+ | Ag+ + 2 NH3 ⇌ [Ag(NH3)2]+ | 1.7 × 107 | 7.2 |
| [Zn(CN)4]2- | Zn2+ + 4 CN- ⇌ [Zn(CN)4]2- | 2.1 × 1019 | 19.3 |
| [Zn(OH)4]2- | Zn2+ + 4 OH- ⇌ [Zn(OH)4]2- | 2 × 1015 | 15.3 |
| [Fe(SCN)]2+ | Fe3+ + SCN- ⇌ [Fe(SCN)]2+ | 8.9 × 102 | 2.9 |
| [Ag(SCN)4]3- | Ag+ + 4 SCN- ⇌ [Ag(SCN)4]3- | 1.2 × 1010 | 10.1 |
| [PbI4]2- | Pb2+ + 4 I- ⇌ [PbI4]2- | 3 × 104 | 4.5 |
| [PtCl4]2- | Pt2+ + 4 Cl- ⇌ [PtCl4]2- | 1 × 1016 | 16.0 |
| [Cu(CN)4]2- | Cu2+ + 4 CN- ⇌ [Cu(CN)4]2- | 1 × 1025 | 25.0 |
| [Co(SCN)4]2- | Co2+ + 4 SCN- ⇌ [Co(SCN)4]2- | 1 × 103 | 3.0 |
Important Notes
- Kf is the equilibrium constant for FORMING the complex from the free metal ion and ligands; its inverse (1/Kf) is the dissociation constant Kd.
- Very large Kf values (10¹⁰ and beyond) mean essentially complete complexation when excess ligand is present — the free metal-ion concentration becomes vanishingly small.
- To dissolve a precipitate with a complexing agent, couple the equilibria: AgCl(s) + 2 NH₃ ⇌ [Ag(NH₃)₂]⁺ + Cl⁻ has K = Ksp × Kf.
- ChemWhiz problems that need a Kf supply the value in the problem statement; this table is the general reference.
Source: OpenStax Chemistry 2e, Appendix K — Formation Constants for Complex Ions (CC BY 4.0)