Magnetic cross-over in six-co-ordinate iron (II) complexes of cis-1,2-bis(diphenylphosphino)ethylene

Abstract

Reaction of iron(II) halides and pseudohalides with cis-1,2-bis(diphenylphosphino)ethylene (dppen) yields the complexes [Fe(dppen)₂X₂](X = Cl, Br, NCS, or N₃) and [Fe(dppen)Br₂]. Physical properties indicate that the [Fe(dppen)₂X₂] complexes have six-co-ordinate, probably trans, structures. Magnetic susceptibilities and Mössbauer spectra classify the complexes as having S= 0 (X = NCS or N₃) or S= 2 (X = Br) ground states, or as existing in a temperature-dependent high-spin ⇌ low-spin equilibrium (X = Cl). The Mössbauer parameters of the complexes are discussed in terms of the bonding properties of dppen. The properties of [Fe(dppen)Br₂], and of the dominant species occurring in solutions of [Fe(dppen)₂X₂](X = Cl or Br), are consistent with tetrahedral structures. The product of reaction of iron(III) chloride with dppen is the complex salt [Fe(dppen)₂Cl₂][FeCl₄] in which the cation contains the metal in the S=½ state.