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E4: Complex Ion Formation Constants

A stability constant is an equilibrium constant for the formation of a complex in solution that measures the strength of the interaction between the ligands and metal that form the complex. The formation of complexes between a metal ion, M, and ligands, L, is really a ligand exchange (i.e., substitution reaction) with metal ions in aqueous solutions present as aqua-ligated ions.

Halo Ligands \(K_f\)
Al3+ + 6 F\(\rightleftharpoons\) [AlF6]3- 2.5 x 104
Al3+ + 4 F\(\rightleftharpoons\)  [AlF4]-1 2.0 x 108
 Be2+ + 4 F-  \(\rightleftharpoons\)  [BeF4]2- 1.3 x 1013
Sn4+ + 6 F-  \(\rightleftharpoons\)  [SnF6]2- 1.0 x 1025
Cu+ + 2 Cl\(\rightleftharpoons\)  [CuCl2]-1 3.0 x 105
Ag+ + 2 Cl\(\rightleftharpoons\)  [AgCl2]-1 1.8 x 105
Pb2+ + 4 Cl\(\rightleftharpoons\)  [PbCl4]2- 2.5 x 1015
Zn2+ + 4 Cl\(\rightleftharpoons\)  [ZnCl4]2- 1.6 
Hg2+ + 4 Cl-  \(\rightleftharpoons\)  [HgCl4]2- 5.0 x 1015
Cu+ + 2 Br-  \(\rightleftharpoons\)  [CuBr2]-1 8.0 x 105
Ag+ + 2 Br-  \(\rightleftharpoons\)  [AgBr2]-1 1.0 x 1011
Hg2+ + 4 Br-  \(\rightleftharpoons\)  [HgBr4]2- 3.0 x 104
Cu+ + 2 I-  \(\rightleftharpoons\)  [CuI2]-1 8.0 x 108
Ag+ + 2 I-  \(\rightleftharpoons\)  [AgI2]-1 1.0 x 1011
Pb2+ + 4 I-  \(\rightleftharpoons\)  [PbI4]2- 3.0 x 104
Hg2+ + 4 I-  \(\rightleftharpoons\)  [HgI4]2- 1.9 x 1030
   
Ammine Ligands \(K_f\)
Ag+ + 2 NH3  \(\rightleftharpoons\)    [Ag(NH3)2]+ 1.6 x 107
Zn2+ + 4 NH3  \(\rightleftharpoons\)   [Zn(NH3)4]2+ 7.8 x 108
Cu2+ + 4 NH3  \(\rightleftharpoons\)  [Cu(NH3)4]2+ 1.1 x 1013
Hg2+ + 4 NH3  \(\rightleftharpoons\)  [Hg(NH3)4]2+ 1.8 x 1019
Co2+ + 6 NH3  \(\rightleftharpoons\)  [Co(NH3)6]2+ 5.0 x 104
Co3+ + 6 NH3  \(\rightleftharpoons\)  [Co(NH3)6]3+ 4.6 x 1033
Cd2+ + 6 NH3  \(\rightleftharpoons\)  [Cd(NH3)6]2+ 2.6 x 105
Ni2+ + 6 NH3  \(\rightleftharpoons\)   [Ni(NH3)6]2+ 2.0 x 108
Cyanide Ligands \(K_f\)
Fe2+ + 6 CN-  \(\rightleftharpoons\)  [Fe(CN)6]4- 1.0 x 1024
Fe3+ + 6 CN-  \(\rightleftharpoons\)  [Fe(CN)6]3- 1.0 x 1031
Ag+ + 2 CN-  \(\rightleftharpoons\)  [Ag(CN)2]-1 5.3 x 1018
Cu+ + 2 CN-  \(\rightleftharpoons\)  [Cu(CN)2]-1 1.0 x 1016
Cd2+ + 4 CN-  \(\rightleftharpoons\)  [Cd(CN)4]2- 7.7 x 1016
Au+ + 2 CN-  \(\rightleftharpoons\)  [Au(CN)2]-1 2.0 x 1038
   
Other monodentate ligands \(K_f\)
Ag+ + 2 CH3NH2  \(\rightleftharpoons\)  [Ag(CH3NH2)2]+1 7.8 x 106
Cd2+ + 4 SCN-  \(\rightleftharpoons\)  [Cd(SCN)4]2- 1.0 x 103
Cu2+ 2 SCN-  \(\rightleftharpoons\)  [Cu(SCN)2 5.6 x 103
Fe3+ 3 SCN-  \(\rightleftharpoons\)  [Fe(SCN)3] 2.0 x 106
Hg2+ 4 SCN-  \(\rightleftharpoons\)  [Hg(SCN)4]2- 5.0 x 1021
Cu2+ 4 OH-  \(\rightleftharpoons\)  [Cu(OH)4]2- 1.3 x 1016
Zn2+ 4 OH-  \(\rightleftharpoons\)  [Zn(OH)4]2- 2.0 x 1020
   
Bidentate ligands
(en = ethylenediamine )
           \(K_f\)            
Mn2+ + 3 en  \(\rightleftharpoons\)   [Mn(en)3]2+ 6.5 x 105
Fe2+ + 3 en  \(\rightleftharpoons\)  [Fe(en)3]2+ 5.2 x 109
Co2+ + 3 en  \(\rightleftharpoons\)  [Co(en)3]2+ 1.3 x 1014
Co3+ + 3 en  \(\rightleftharpoons\)  [Co(en)3]3+ 4.8 x 1048
Ni2+ + 3 en  \(\rightleftharpoons\)  [Ni(en)3]2+ 4.1 x 1017
Cu2+ + 2 en  \(\rightleftharpoons\)  [Cu(en)2]2+ 3.5 x 1019
Co2+ + 3 C2O42-  \(\rightleftharpoons\)  [Co(C2O4)3]4- 4.5 x 106
Fe3+ + 3 C2O42-  \(\rightleftharpoons\)  [Fe(C2O4)3]3- 3.3 x 1020