Studies on the magnetic crossover in five-co-ordinate complexes of iron(II), cobalt(II), and nickel(II). Part I

Abstract

Pentaco-ordinate complexes of the type M(pnp)X₂ or M(pnp)XY are described; M = Fe^II, Co^II, or Ni^II; X = Cl, Br, or I; Y = NCS; pnp is the new ligand 2,6-bis-(2-diphenylphosphinoethyl)pyridine. Distorted trigonalbipyramidal structures are assigned on the basis of physical properties. Magnetic susceptibility measurements (80–400 K) classify the compounds as (a) high-spin, (b) low-spin, or (c) existing in a high-spin ⇌ low-spin equilibrium. The latter are the first examples of spin isomerism in pentaco-ordinate d⁶, d⁷, and d⁸ systems. Analysis of the magnetic susceptibility vs. temperature data reveals that the alteration in spin is accompained by other changes in the molecule and/or lattice. The results are discussed in relation to the factors governing the spin multiplicity of five-co-ordinate compounds.

In solution the compounds dissociate to a variable extent into tetrahedral or planar [M(pnp)X]⁺ complex ions. The extent of dissociation is greatest for M = Ni^II and X = Cl, and least for X = I. The ligand pnp appears to impart a class ‘b’ character to the metal.