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

Abstract

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 ; and Cu ( 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: K 4, 3 c = 0.0076±0.0005 mole liter—1 [in terms of concentrations at 0.1—0.2f Cu(I)] and K 4, 3 a = 0.026 mole liter—1 (in terms of activities). Analogous values for the dissociation of Cu(CN)3 = into Cu(CN)2 —+CN— are K 3, 2 c = (2.44±0.36)×10—5 mole liter—1, and K 3, 2 a = 4.2×10—5. The above values are calculated from approximately 100 determinations. At 25°C, the corresponding values of these constants are approximately K 4, 3 c = 0.0057 and K 3, 2 c = 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 —, ΔFf 0 = 58.5 kcal/mole; Cu(CN)3 =, ΔFf 0 = 91.8 kcal/mole; and Cu(CN)4 ≡, ΔFf 0 = 129 kcal/mole.