Hydrogen bonding or transient coordination of pendant group: Cause for lability in ligand substitution reaction of aminopolycarboxylatoruthenium(III) complexes

Abstract

The rate of substitution of water molecule in [LRu^III(H₂O)l complexes [L= ethylenediaminetetraacetate (EDTA) anion and N-hydroxyethylethylenediaminetriacetate (HEDTRA) anion] with CI^- ion has been studied spectrophotometrically in aqueous medium at 30°C. The rates of substitutions of CI^- in [LRuCI] with thiocyanate (SCN^-) have also been studied as a function of SCN^- concentration in the temperature range 30-50°C. In both the cases, ruthenium(III)-EDTA is found to be more reactive than the corresponding HEDTRA complex. Activation parameters <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H^≠ </em>and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>S^≠ </em>for the substitution of [LRu^IIICI]^- complexes with SCN^- are determined and a suitable mechanism is proposed. The reactivity of [LRu^IIICI]^- towards SCN^- substitution is discussed in terms of the transient coordination of the uncoordinated - COOH or -CH₂ - OH pendant group which assists in the elimination of leaving group and ligation of substituting nucleophiles.