Single crystal electronic spectra, electron spin resonance and magnetic data for some high spin tetragonal cobalt(II) complexes at cryogenic temperatures

Abstract

The electronic and esr spectra of a series of high spin cobalt(II) complexes of formula Co(EtNHCH 2CH 2NHEt) 2Z 2 (Z = Cl, Tr, NCS, NO 3) and Co(MeNHCH 2CH 2NHMe) 2(NO 3) 2 have been examined at R.T. and 10 K respectively. The single crystal polarised electronic spectra were analysed in terms of the Normalised Spherical Harmonic Hamiltonian and a good fit between observed and calculated band energies was obtained. All the complexes are trans, with the exception of Co(EtNHCH 2CH 2NHEt) 2(NO 3) 2 which is cis. The data are consistent with the presence of a 4E g ground term derived from the cubic 4T 1g term under the influence of a tetragonal distortion for all but one of the complexes. Orbital angular overlap model parameters were derived for this series. These were compared with similar data for cobalt(II) and nickel(II) in the literature, with special reference to the corresponding nickel(II) ethylenediamine complexes. The chief difference between the cobalt and nickel complexes, in an analogous pair of ethylenediamine complexes, is a significantly larger axial repulsion energy in the nickel case. This is attributed to the presence of an extra anti-bonding electron in nickel(II) versus cobalt(II). The esr and magnetic data are fully consistent with the tetragonal model developed for these complexes but do not provide any additional characterisation.