Spectroscopic and substitution kinetic studies of pentacyanoferrate(II) complexes with sulphur heterocyclic ligands

Add time:09/25/2019    Source:sciencedirect.com

The pentacyanoferrate(II) complexes of 1,4-thioxane, 1,4-dithiane and 1,3-dithiane have been prepared in ethanol-water solution and their sodium salts isolated. Microanalysis, UV-visible, infrared and Mo¨ssbauer spectroscopies, were used to characterize the complexes. Each complex has an absorption band in the visible region which is assigned to ad-d transition. Infrared measurements indicate that 1,4-thioxane, 1,4-dithiane and 1,3-dithiane ligand are coordinated through the sulphur atom with the pentacyanoferrate(II) ion. The Mo¨ssbauer parameters, isomer shift and quadrupole splitting allowed the analysis of the σ donor and π acceptor ability of the ligands in these complexes. The kinetics of ligand exchange were studied by the standard spectrophotometric technique for slow reactions, using pyridine as entering ligand. The existence of a rate saturation is interpreted as being consistent with a dissociative (D) mechanism. The rate constants of dissociation were found to be 5.7 × 10−4 s−1, 5.6 × 10−4 s−1 and 3.4 × 10−4 s−1 for 1,4-thioxane, 1,4-dithiane and 1,3-dithiane, respectively. The approximately constant values of ΔG‡ and the linear correlation between ΔH‡ and ΔS‡ suggest the existence of a sole mechanism for the studied reactions.

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