Sydnone Complexes of Pentacyanoferrate(II)

Abstract

AbstractThe 3‐methylsydnone (CH3Sd) and 3‐phenylsydnone (PhSd) complexes of pentacyanoferrate(II) were prepared in aqueous solution by the reaction of Fe(CN)5OH23− with excess ligand. Metal to ligand charge transfer transitions were observed for both complexes at 442 nm (CH3Sd) and 494 nm (PhSd), respectively. The reduction potentials were measured with the Ef values of 0.65 and 0.67 V (vs. NHE) for CH3Sd and PhSd, respectively, at 25 °C, μ = 0.10 M LiClO4 and pH = 5. The kinetics of the substitution reaction were also investigated, and kf and kd values (25 °C, μ = 0.10 M LiClO4,pH = 5) are (5.4 ± 0.2) × 10−2M−1s−1 and (1.4 + 0.1) × 10−5s−1 (CH3Sd), (8.8 ±0.1) × 10−2M−1s−1 and (4.7 ±0.1) × 10−5s−1 (PhSd). The kf values for both sydnone complexes are approximately four orders of magnitude smaller than other pentacyanoferrate(II) complexes with neutral ligands. The difference may arise from the mesoionic character of the sydnone compounds rather than via a different reaction mechanism. Spectroscopic, kinetic and electrochemical results all suggest that the sydnone ring shows a better π‐backbonding stabilization with Fe(CN)53− moiety than that of the pyridine ring.