On the electronic structure ofN,N′-ethylenebis-(acetylacetonatiminato)Co(II), Co(II)acacen III. Comparison of ENDOR results with semiempirical calculations

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Abstract
The magnetic parameters (electron g-, hyperfine and quadrupole tensors) of Co(acacen) in Ni(acacen). ½H20, which have been derived from a single crystal E.P.R./ENDOR study, are interpreted using the results of model calculations at EHMO, INDO and Xα level. A theory for the calculation of the magnetic parameters from the MO data for low symmetry systems with low-lying excited states is given. Within this framework, the non-coaxiality of the g-, cobalt hyperfine and quadrupole tensors is explained. The hyperfine tensors of the ligand nuclei are described qualitatively by the MO calculations, a quantitative explanation, however, is not possible. This is, among other reasons, due to the neglect of polarization effects (EHMO, Xα) and of spin-orbit coupling contributions (INDO, Xα). Their importance is illustrated by the negative spin densities found in carbon 2pz orbitals and by the large deviation of the g principal values from the free electron g e, respectively. Problems like asymmetric hyperfine tensors for low symmetry systems and the derivation of structural information from ENDOR results are briefly commented on. The nitrogen quadrupole tensors are interpreted in terms of the Townes-Dailey theory.