The mechanism of displacement of dihydrogen and dinitrogen from iron, ruthenium and osmium hydrides and implications for models of nitrogenase action

Abstract

The substitution of dihydrogen in complexes [FeH(H₂)(phosphine)_x]⁺ [phosphine = R₂PCH₂CH₂PR₂ (R = Et or Me) or P(CH₂CH₂PR′₂)₃ (R′ = Me or Ph)] by ligands L (MeCN, PhCN, or Cl^–) has been shown to be first order in the concentration of complex and zero order in the concentration of L, in both acetone and thf. Activation parameters have been determined, and the mechanism of substitution is proposed to involve rate-determining loss of H₂ from the parent complexes and subsequent rapid co-ordination of L. This mechanism differs from that recently proposed for an analogous complex of Ph₂PCH₂CH₂PPh₂, and the reasons for this are discussed. Less thorough studies of some related dinitrogen complexes, and of some homologous complexes of Ru and Os, are consistent with a similar loss of dinitrogen or dihydrogen being rate determining.