Synthesis, structural, and E.S.R. studies of some low-spin Co(II) complexes with tetradentate N2S2 and N2Se2 Schiff-bases

Single crystal and powder E.S.R. studies of low-spin N,N?-ethylene bis(thioacetylacetoneiminato)Co(II) N,N?-ethylenebis(selenoacetylacetone-iminato)Co(II), and some related compounds, diluted in the corresponding Ni(II) and Zn(II) complexes are reported. The Co(II) complexes and their Ni(II) analogues are all isomorphous. The crystal and molecular structure of N,N?-ethylenebis(thioacetylacetoneiminato)Ni(II) is given. The crystals are orthorhombic, space group P212121, with unit cell dimensions a = 7·297 (2), b = 12·051 (4), c = 15·887 (3) Å, and Z = 4. The structure has been determined from diffractometer data by the heavy atom method and refined by least-squares to R = 0·0425. The coordination geometry at the nickel atom is square planar, the chromophore being NiN2S2, with the N and S atoms in a cis arrangement. The complex is completely planar, approximately normal to the a-axis, with the Ni-S vectors almost parallel to the b- and c-axes. The determination of the ground state of the cobalt ion has been attempted. It is shown that the unpaired electron is in an essentially |yz > or |z 2 > orbital, depending on which host crystal, Ni(II) or Zn(II) respectively, is used. This result is, probably, the consequence of small axial perturbations, which are present in the zinc complexes. In agreement with this, excited quartet states shift significantly to lower energies on going from a strictly square planar geometry, nickel hosts, to some form of pentacoordination, zinc hosts. The assignment of the ground state is supported by the behaviour of the isotropic part of the metal hyperfine splitting.