# 16.12: Formation Constants

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The following table provides $$K_i$$ and $$\beta_i$$ values for selected metal–ligand complexes, arranged by the ligand. All values are from Martell, A. E.; Smith, R. M. Critical Stability Constants, Vols. 1–4. Plenum Press: New York, 1976. Unless otherwise stated, values are for 25 oC and zero ionic strength. Those values in brackets are considered less reliable.

Acetate

$$\ce{CH3COO-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Mg2+ 1.27
Ca2+ 1.18
Ba2+ 1.07
Mn2+ 1.40
Fe2+ 1.40
Co2+ 1.46
Ni2+ 1.43
Cu2+ 2.22 1.41
Ag+ 0.73 –0.09
Zn2+ 1.57
Cd2+ 1.93 1.22 –0.89
Pb2+ 2.68 1.40

Ammonia

$$\ce{NH3}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Ag+ 3.31 3.91
Co2+ (T = 20 °C) 1.99 1.51 0.93 0.64 0.06 –0.73
Ni2+ 2.72 2.17 1.66 1.12 0.67 –0.03
Cu2+ 4.04 3.43 2.80 1.48
Zn2+ 2.21 2.29 2.36 2.03
Cd2+ 2.55 2.01 1.34 0.84

Chloride

$$\ce{Cl-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Cu2+ 0.40
Fe3+ 1.48 0.65

Ag+ ($$\mu = 5.0 \text{ M}$$)

3.70 1.92 0.78 –0.3
Zn2+ 0.43 0.18 –0.11 –0.3
Cd2+ 1.98 1.62 –0.2 –0.7
Pb2+ 1.59 0.21 –0.1 –0.3

Cyanide

$$\ce{CN-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Fe2+ 35.4 ($$\beta_6$$)
Fe3+ 43.6 ($$\beta_6$$)
Ag+ 20.48 ($$\beta_2$$) 0.92
Zn2+ 11.07 ($$\beta_2$$) 4.98 3.57
Cd2+ 6.01 5.11 4.53 2.27
Hg2+ 17.00 15.75 3.56 2.66
Ni2+ 30.22 ($$\beta_4$$)

Ethylenediamine

$$\ce{H2NCH2CH2NH2}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Ni2+ 7.38 6.18 4.11
Cu2+ 10.48 9.07
Ag+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 4.700 3.00
Zn2+ 5.66 4.98 3.25
Cd2+ 5.41 4.50 2.78

EDTA log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Mg2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 8.79
Ca2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 10.69
Ba2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 7.86
Bi3+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 27.8
Co2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 16.31
Ni2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 18.62
Cu2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 18.80
Cr3+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) [23.4]
Fe3+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 25.1
Ag+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 7.32
Zn2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 16.50
Cd2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 16.46
Hg2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 21.7
Pb2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 18.04
Al3+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 16.3

Fluoride

$$\ce{F-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Al3+ 6.11 5.01 3.88 3.0 1.4 0.4

Hydroxide

$$\ce{OH-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Al3+ 9.01 [9.69] [8.3] 6.0
Co2+ 4.3 4.1 1.3 0.5
Fe2+ 4.5 ]2.9] 2,6 –0.4
Fe3+ 11.81 10.5 12.1
Ni2+ 4.1 3.9 3.
Pb2+ 6.3 4.6 3.0
Zn2+ 5.0 [6.1] 2.5 [1.2]

Iodide

$$\ce{I-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Ag+ 6.58 [5.12] [1.4]
Cd2+ (T = 18 °C) 2.28 1.64 1.08 1.0
Pb2+ 1.92 1.28 0.7 0.6

Nitriloacetate log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Mg2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 5.41
Ca2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 6.41
Ba2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 4.82
Mn2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 7.44
Fe2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 8.33
Co2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 10.38
Ni2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 11.53
Cu2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 12.96
Fe3+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 15.9
Zn2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 10.67
Cd2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 9.83
Pb2+ (T = 20 °C, $$\mu = 0.1 \text{ M}$$) 11.39

Oxalate

$$\ce{C2O4^{2-}}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Ca2+ ($$\mu = 1 \text{ M}$$) 1.66 1.03
Fe2+ ($$\mu = 1 \text{ M}$$) 3.05 2.10
Co2+ 4.72 2.28
Ni2+ 5.16
Cu2+ 6.23 4.04
Fe3+ ($$\mu = 0.5 \text{ M}$$) 7.53 6.11 4.83
Zn2+ 4.87 2.78

1,10-phenanthroline log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Fe2+ 20.7 ($$\beta_3$$)
Mn2+ ($$\mu = 0.1 \text{ M}$$) 4.0 3.3 3.0
Cu2+ ($$\mu = 0.1 \text{ M}$$) 7.08 6.64 6.08
Ni2+ 8.6 8.1 7.6
Fe3+ 13.8 ($$\beta_3$$)
Ag+ ($$\mu = 0.1 \text{ M}$$) 5.02 7.04
Zn2+ 6.2 [5.9] [5.2]

Thiosulfate

$$\ce{S2O3^{2-}}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Ag+ (T = 20 °C) 8.82 4.85 0.53

Thiocyanate

$$\ce{SCN-}$$

log $$K_1$$ log $$K_2$$ log $$K_3$$ log $$K_4$$ log $$K_5$$ log $$K_6$$
Mn2+ 1.23
Fe2+ 1.31
Co2+ 1.71
Ni2+ 1.76
Cu2+ 2.33
Fe3+ 3.02
Ag+ 4.8 3.43 1.27 0.2
Zn2+ 1.33 0.58 0.09 –0.4
Cd2+ 1.89 0.89 0.02 –0.5
Hg2+ 17.26 ($$\beta_2$$) 2.71 1.83

This page titled 16.12: Formation Constants is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey.