Metal ion catalysed acid hydrolysis of acidopentaammine cobalt(III) complexes: Kinetics and mechanism of Al(III), Ga(III) and In(III) catalysed hydrolysis of malonatopentaamminecobalt(III) complex

Abstract

Al(III), Ga(III) and In(III) have been found to catalyse the aquation reaction of the malonatopentaamminecobalt(III) complex. The catalytic activities of these metal ions have been ascribed to the formation of the reactive species, (NH 3) 5CoCO 2CH 2CO 2M 4+. At 60°C and 0·3M ionic strength (adjusted with NaClO 4), the equilibrium constant ( K′ 2) of the reaction, ( NH 3) 5 CoCO 2 CH 2 H 2+ + M 3− ⇌ ( NH 3) 3 CoCO 2 CH 2 CO 2 M 4− + H 4− and the rate constant ( k 3) of aquation of the binuclear species, ( NH 3) 5 CoCO 2 CH 2 CO 2 M 4+ + H 2 O → ( NH 3) 5 CoOCH 2 3+ + CH 2( CO 2) 2 M 4− are found to be 0·334(±0·036), 1·01(±0·07) × 10 −5 sec −1 for Al(III); 0·55(±0·02), 1·01(±0·10) × 10 −5 sec −1 for Ga(III); and 0·178(±0·086), 0·635(±0·157) × 10 −5 sec −1 for In(III) respectively. The species, [(NH 3) 5CoCO 2CH 2CO 2Ga 2OH] 6+, has also been found to be involved as the catalytically active labile intermediate in the Ga(III) ion catalysed aquation of the cobalt(III) substrate. The activation parameters for the metal ion catalysed path of aquation of the malonato complex have been determined. CoO bond fission is tentatively suggested to be the rate determining step in the k 3 path.