Electronic Structure of Three-Coordinate Fe^II and Co^II -Diketiminate Complexes

The beta-diketiminate supporting group, [ArNCRCHCRNAr](-), stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnac(tBu))ML, where M = Fe-II, Co-II and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two Fe-II complexes (S = 2) have also been previously characterized by M & ouml;ssbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the Co-II complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The Co-II complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two Fe-II complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 +/- 1 cm(-1) for both. The two Co-II complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm(-1) while CoCH3 has negative D = -49 cm(-1). Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.