Steric effects and five-co-ordination in first-row transition-metal compounds. Compounds with a linear tetra-amine ligand

Abstract

Compounds of Mn^II, Co^II, Ni^II, Cu^II and Zn^II of the type M(chelate)X₂[where chelate =NN′-di-(3-aminopropyl)piperazine; X = one of a variety of anions] are reported. Square-pyramidal five-co-ordinate cations in the compounds [M(chelate)X]X and [M(chelate)X]Y are characterised for the following: where M = Co^II, X = Br and I; where M = Ni^II, X = Cl, Br and possibly NCS; and where M = Cu^II, X = Cl, Br, I, NCS, and Y = ClO₄, PF₆. In addition, a five-co-ordinate species Cu(chelate)Cl₂ in which the ligand is only terdentate with a free NH₂, also is characterised. Four co-ordinate planar [MN₄]²⁺ species also are characterised for Ni^II, Cu^II and perhaps Co^II. No significant structural data is available for the Mn^II, Fe^II, or Zn^II species. The structural assignments for the solid compounds rely on X-ray data (powder-pattern correlations with a known three-dimensional structure), i.r. and electronic spectra, and magnetic measurements. Structural studies of solutions in non-aqueous solvents also are reported. These rely on electronic spectra. Specific ion-pairing limits the use of conductivity data. The origin of the five-co-ordinate structures in these compounds is discussed in terms of the peculiar steric effects of the ligand, which is the precursor of a class of homologous ligands, each of which is expected to give five-co-ordinate compounds of these metals.