THE STABILITY OF SOME COMPLEXES OF TRIVALENT COPPER

Abstract

New methods are given for the preparation of sodium copper (111) tellurate, Na9Cu(TeO6)2.16H2O, and sodium copper (111) periodate, Na7Cu(IO6)2.20H2O, previously reported by Malatesta. Additional analytical evidence is presented that these are really compounds of trivalent copper. The dissociation of the ions in alkaline aqueous solution was examined through the ability of the uncomplexed copper to catalyze the decomposition of sodium hypochlorite. Evidence is presented that the uncomplexed copper ion is Cu(OH)4−, and this gives both a 1-1 and a 1-2 complex with both tellurate and periodate. The 1-2 complexes appear to be Cu(HTeO6)2−7 and Cu(IO6)2−7 respectively; the 1-1 complexes are less stable. The equilibrium constants for the reversible dissociation of these ions into their simpler constituents in aqueous solution at 40 °C. are as follows: tellurate, 1-1 complex K1 = 1.8 × 10−6; 1-2 complex K2 = 1.1 × 10−11; periodate, 1-1 complex K1 = 3.4 × 10−5; 1-2 complex is K2 = 8.0 × 10−11. The smaller the constant the more stable the complex ion. In the case of the tellurate complex K2 is proportional to the square of the hydroxide ion concentration and these values are for 0.400 M sodium hydroxide; with periodate K2 is independent of the hydroxide concentration. Experiments at various temperatures give 20 kcal. as the heat of the reaction forming the 1-2 tellurate complex from the simple ions, and 71/2 kcal. for the 1-2 periodate complex. The stannate, stibnate, and selenate ions showed no signs of forming analogous compounds.