STOPPED-FLOW MECHANISTIC STUDY OF BROMIDE SUBSTITUTION BY AN ORGANONITRILE AT AN IRON(II) PHOSPHINIC CENTER - A PI-ELECTRON DRIVEN PROCESS

The kinetics of the displacement reactions of the bromide ligands of t rans-[FeBr2(depe)(2)] (depe = Et(2)PCH(2)CH(2)PEt(2)) by the organonit rile N = CCH(2)C(6)H(4)OMe-4, in tetrahydrofuran (either in the absenc e or in the presence of added Br-), to give the corresponding mono- an d dinitrile complexes trans-[FeBr(NCCH(2)C(6)H(4)OMe-4)(depe)(2)](+) a nd trans-[Fe(NCCH(2)C(6)H(4)OMe-4)(2)(depe)(2)](2+), have been investi gated by stopped-flow spectrophotometry. The substitution reaction occ urs by a mechanism involving rate-limiting dissociation of bromo ligan ds to form the unsaturated intermediates [FeBr(depe)(2)](+) (k(1) = 1. 52+/-0.02 s(-1)) and [Fe(NCR)(depe)(2)](2+) (k(3) = 0.063+/-0.008 s(-1 )) which add the nitrile ligand to form those nitrile complexes. The c ompetition between the nitrile and Br- for such metal centres has also been investigated and a stronger inhibiting effect of added Br- is ob served for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of pi-electroni c effects of these unsaturated metal centres and of the bromide and ni trile ligands.