Stopped-flow mechanistic study of bromide substitution by an organonitrile at an iron(II) phosphinic centre; a π-electron driven process

Article abstract of DOI:10.1016/s0020-1693(96)05240-1

The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr₂(depe)₂] (depe=Et₂PCH₂CH₂PEt₂) by the organonitrile N≡CCH₂C₆H₄OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br^-), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH₂C₆H₄OMe-4)(depe) ₂]⁺ and trans-[Fe(NCCH₂C₆H₄OMe-4) ₂(depe)₂]²⁺, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe)₂]⁺ (k₁=1.52±0.02 s^-1) and [Fe(NCR)(depe)₂]²⁺ (k₃=0.063±0.008 s^-1) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br^- for such metal centres has also been investigated and a stronger inhibiting effect of added Br^- is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands.