THE RATES OF WATER EXCHANGE IN AI(III)-SALICYLATE AND AI(III)-SULFOSALICYLATE COMPLEXES - AS MODEL COMPOUNDS WITH FUNCTIONAL GROUPS OF HUMIC ACID

Abstract

Rate parameters are reported for exchange of hydration waters from the inner coordination sphere of Al(III)-sulfosalicylate [Al(sSal)+] and Al(III)-salicylate [Al(Sal)+] complexes to bulk solution as determined with 17O-NMR. The rate parameters for the Al(sSal)+ complex are: kex298 = (3.0 +/- 0.2) . 103 s-1, ΔH# = 37(+/-3) kJ mol-1, ΔS# = -54(+/-9) J mol-1 K-1; and for the Al(Sal)+ complex are: kex298 = 4.9(+/-0.3) . 103 s-1, ΔH# = 35(+/-3) kJ mol-1, ΔS# = -57(+/-11) J mol-1 K-1. These results, along with previous work, suggest that the lability of water molecules in bidentate carboxylate-phenolic complexes scales with the electron-donating properties of the ligand oxygens. Replacement of a coordinated carboxyl with a phenolic group in the ligand increases both the Lewis basicity and the value of kex298. A correlation between these parameters is proposed that can be used to predict rate coefficients for other bidentate aluminum complexes.