SPECTROSCOPIC AND SUBSTITUTION KINETIC-STUDIES OF PENTACYANOFERRATE(II) COMPLEXES WITH SULFUR HETEROCYCLIC LIGANDS

The pentacyanoferrate(II) complexes of 1,4-thioxane, 1,4-dithiane and 1,3-dithiane have been prepared in ethanol-water solution and their so dium salts isolated. Microanalysis, UV-visible, infrared and Mossbauer spectroscopies, were used to characterize the complexes. Each complex has an absorption band in the visible region which is assigned to a d -d transition. Infrared measurements indicate that 1,4-thioxane, 1,4-d ithiane and 1,3-dithiane ligand are coordinated through the sulphur at om with the pentacyanoferrate(II) ion. The Mossbauer parameters, isome r shift and quadrupole splitting allowed the analysis of the sigma don or and pi acceptor ability of the ligands in these complexes. The kine tics of ligand exchange were studied by the standard spectrophotometri c technique for slow reactions, using pyridine as entering ligand. The existence of a rate saturation is interpreted as being consistent wit h a dissociative (D) mechanism. The rate constants of dissociation wer e found to be 5.7 x 10(-4) S-1, 5.6 x 10(-4) s-1 and 3.4 x 10(-4) s-1 for 1,4-thioxane, 1,4-dithiane and 1,3-dithiane, respectively. The app roximately constant values of DELTAG(double dagger) and the linear cor relation between DELTAH(double dagger) and DELTAS(double dagger) sugge st the existence of a sole mechanism for the studied reactions.