Infrared Absorption Studies of Aqueous Complex Ions. II. Cyanide Complexes of Cu (I) in Aqueous Solution

In the system CuCN–KCN–H2O, we have observed infrared absorption spectra of three distinct complex ions: Cu(CN)2−,ε=165±25 mole−1 liter cm−1 at 2125 cm−1;Cu(CN)3=,ε=1090±10 mole−1 liter cm−1 at 2094 cm−1;andCu(CN)4≡,ε=1657±15 mole−1 liter cm−1 at 2076 cm−1.At 29°C, the constants for the dissociation of Cu(CN)4≡ into Cu(CN)3=+CN— are: K4, 3c = 0.0076±0.0005 mole liter—1 [in terms of concentrations at 0.1—0.2f Cu(I)] and K4, 3a = 0.026 mole liter—1 (in terms of activities). Analogous values for the dissociation of Cu(CN)3= into Cu(CN)2—+CN— are K3, 2c = (2.44±0.36)×10—5 mole liter—1, and K3, 2a = 4.2×10—5. The above values are calculated from approximately 100 determinations. At 25°C, the corresponding values of these constants are approximately K4, 3c = 0.0057 and K3, 2c = 1.5×10—5. Using the activity constants determined in this paper and a value of 1×10—24 for the constant: [Cu+][CN—]2/[Cu(CN)2—] we derive the following values: [Cu+][CN—]3/[Cu(CN)3=] = 2.6×10—29 and [Cu+][CN—]4/[Cu(CN)4≡] = 5×10—31. The free energies of formation of the complex ions are thus: Cu(CN)2—, ΔFf0 = 58.5 kcal/mole; Cu(CN)3=, ΔFf0 = 91.8 kcal/mole; and Cu(CN)4≡, ΔFf0 = 129 kcal/mole.