Interaction of Glycyl – Glycine with Hydroxopentaaquarhodium(Iii) Ion in Aqueous Medium: Kinetic and Mechanistic Studies

The kinetics of interaction between glycyl-glycine (L-L'H) and [Rh(H 2 O) 5 OH] 2+ [Rh(H₂O)₅OH] ²⁺ have been studied spectrophotometrically in aqueous medium as a function of [Rh(H₂O)₅OH] ²⁺], [glycyl - glycine], pH and temperature at constant ionic strength. At pH 4.3, the interaction with glycyl-glycine shows two parallel paths: both processes are ligand-dependent. The rate constant for the processes are: k₁ ∼ 10 ⁻³ s ⁻¹ and k₂∼10⁻⁵ s⁻³. The activation parameters calculated from Eyring plots are: ΔH₁^≠ - 26.7 ± 0.3 kj mol⁻¹, ΔS₁^≠ =-207+1 JK⁻¹ mol⁻¹, ΔH₂^≠ = 92.4 ± 1.3 kJ mol⁻¹ and ΔS₂^≠ = −42±4 JK⁻¹ mol⁻¹. The ligand glycyl-glycine, with different donor atoms (N and O), attacks two rhodium(III) ions (Rh(III) being a borderline acid), with different reactivity. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for both interaction processes. From the temperature dependence of the outer-sphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH₁^∘ = 21.8 ± 1.9 kJ mol⁻¹, ΔS₁^∘ = 124± 6 JK⁻¹ mol⁻¹, ΔH₂^∘ = 12.7 ±0.5 kj mol⁻¹ and ΔS₂^∘ 89±2 JK⁻¹ mol″⁻¹ which give a negative AG° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer- sphere association complex for both the parallel paths. The product of the reaction has been characterized from IR and ESI-mass spectroscopic analysis.