Substitution of acetonitrile co-ordinated to iron(II) by trimethyl phosphite: a kinetic study of the formation and disappearance of [Fe(NCMe)4{P(OMe)3}2]2+

Abstract

The red soluble species formed by mixing solutions of [Fe(NCMe)₆]²⁺ and trimethyl phosphite in acetonitrile at room temperature has been identified as low-spin [Fe(NCMe)₄{P(OMe)₃}₂]²⁺ from a stopped-flow spectrophotometric kinetic study. An interchange mechanism, occurring via an outer-sphere complex, is proposed for its formation, the relatively slow interchange step, k_i= 1.03 ± 0.10 s^–1 at 297 ± 0.1 K, being a consequence of steric control by P(OMe)₃. Subsequent reactions which involve the formation of [Fe(NCMe)_6–n{P(OMe)₃}_n]²⁺(n= 3 or 4) are far slower. Formation of the trisubstituted cation is first order in [P(OMe)₃].