Mechanism of reaction of the binuclear dimer of [Fe(CN)5(OH2)]3– with uncharged ligands in aqueous solution

Abstract

The kinetics of reactions of the binuclear dimer of [Fe(CN)₅(OH₂)]^3– with pyridine (py), nitrosobenzene (PhNO), 3CN-py, and 4CN-py, respectively, have been measured at pH 5.5–6.0. With a 20-fold, or larger, excess of the complexing ligand (no extra salt added) the average k_obs. at 25 °C for all reagents used is 0.024 ± 0.001 s^–1. The activation parameters for the reaction of the dimer with 3CN-py are ΔH^‡= 70.3 ± 3.0 kJ mol^–1 and ΔS^‡= 29.7 ± 4.0 J K^–1 mol^–1. Ratios of rate constants (competition ratios) for the reactions of various ligand pairs (Y^l and Y²) with [Fe(CN)₅(OH₂)]^3– and its binuclear dimer have been determined. The competition ratios at 25 °C with Y¹,Y² defined as (a) py,PhNO, (b) 3CN-py,4CN-py, (c) 4CN-py,py, (d) 4CN-py,PhNO, and (e) 3CN-py,PhNO are: for the binuclear dimer, (a) 1.35, (b) 1.04, (c) 1.61, (d) 2.17, (e) 2.21; for [Fe(CN)₅(OH₂)]^3–, (a) 1.32, (b) 1.01, (c) 1.66, (d) 2.15, (e) 2.19. Almost equal competition ratios for identical pairs Y¹,Y² for the dimer and [Fe(CN)₅(OH₂)]^3– suggest that the aqua-complex is the only intermediate in the replacements involving the binuclear complex, and that the binuclear complex is singly bridged, having the structure [(NC)₅Fe(µ-NC)Fe-(CN)₄(OH₂)]^6–.