Interaction of cis-(amine)-bis-(ethylene diamine) cobalt(III) salicylates with Fe(III), Al(III) and Cu(II): Stability constants of [(NH 3)(en) 2CoCO 2C 6H 3( X)OM] ( n+1)+ species

Abstract

The cis-(amine)-bis-(ethylene diamine) cobalt(III) salicylates, [(NH 3)(en) 2CoCO 2C 6H 3( X)OH] 2+ ( X = H, 5-Br, 5-SO 3 −, 5-NO 2 and 3-NO 2), have been synthesised. The association equilibria involving the deprotonated form of these complexes and Fe(III), Al(III) and Cu(II) have been studied spectrophotometrically in aqueous medium. The p K OH of the salicylate complexes and the stability constants of the binuclear species. [(NH 3)-(en) 2CoCO 2C 6H 3( X)OM] ( n+1)+ , are reported at 29·5(±0·2)°C and μ = 1·0 M (adjusted with NaClO 4). For a given cobalt(III) substrate, the stability constants of the binuclear species ( K ML ) decrease in the sequence: Fe(III)>Al Al(III)>Cu(II). Both p K OH and log K ML values decrease in the sequence: H > 5-Br > 5-SO 3 − > 5-NO 2 > 3-NO 2. Log K ML vs p K OH plots are linear with slopes close to unity for all the three metal ions. The metal ions in their binuclear complexes are presumed to be chelated by the phenate oxygen and the carboxyl group bound to the Co(III) centre.