Ligand effects on the kinetics of the reversible binding of NO to selected aminocarboxylato complexes of iron(II) in aqueous solution

Rate constants for the formation and dissociation of Fe^II(L)NO (L = aminocarboxylato) have been determined using stopped-flow, temperature jump, flash photolysis, and pulse radiolysis techniques. For a series of ligands the formation rate constants vary between 1.6 × 10⁶ and 2.4 × 10⁸ M^-1 s^-1, whereas the dissociation rate constants vary between 0.11 and 3.2 × 10³ s^-1 at 25°C. These rate constants result in stability constants (K_NO = k₁/k_d) ranging from 5.0 × 10² to 1.1 × 10⁷ M^-1, which are in good agreement with values of K_NO determined by a combined spectrophotometric and potentiometric technique. The results are discussed with reference to available literature data, and interpreted in terms of a generalized reaction mechanism.