Aquation of some transition metal acetates in acetic acid

Abstract

Solubilities of manganese(II), cobalt(II), nickel(II) and zinc(II) acetates in acetic acid were measured at 25°C to investigate the aquation of these metal acetates. The equilibrium of copper(II) acetate with water was studied by cryoscopy. Solubilities of manganese(II), cobalt(II), nickel(II) and zinc(II) acetates in anhydrous acetic acid are 0·0255, 0·0160, 0·0059 and 0·00121 molal, respectively. Equilibria of a metal acetate, M(OAc) 2, with water are written as follows: M( OAc) 2+H 2O ▪M( OAc) 2+(H 2O ) M( OAc) 2+H 2O ▪M( OAc) 2+(H 2O) 2 M( OAc) 2+H 2O ▪M( OAc) 2+(H 2O) 3 Overall formation constants of the aquated species, β M(OAc) 2(H 2O) and β M(OAc) 2(H 2O) 2 , were determined as 1·6 ± 0·2 and 1·7 ± 0·3 for manganese(II), 3·1 ± 0·2 and 6·3 ± 0·3 for zinc(II) acetate and 6·7 ± 0·3 and 11 ± 1 for cobalt(II) acetate, and only cobalt(II) acetate forms the triaquo species for which β Co(OAc) 2(H 2O) 3 = 28 ± 6. We have, however, no diaquo nickel(II) acetate, but only the monoaquo acetate for which β Ni(OAc) 2(H 2O) = 1·65 × 10 3. One of the two acetic acid molecules in the dimeric copper(II) acetate is substituted by a water molecule in acetic acid: Cu 2(OAc) 4(HOAc) 2→Cu 2(OAc) 4(H 2O)(HOAc) . The constant for this equilibrium was estimated to be 0·6 ± 0·3.