Zero-Field Splitting in a Series of Structurally Related Mononuclear NiII-Bispidine Complexes

The synthesis, single-crystal X-ray structures, electronic absorption spectra, and magnetic properties of six Ni-II complexes with a tetradentate (L-1) and three pentadentate (L-2, L-3, L-4) bispidine ligands (3,7-diazabicyclo[3.3:1]nonane derivatives), Ni(L-1-H2O)(OH2)(2)](PF6)(2), [Ni(L-1 center dot H2O)(O2NO)]NO3, [Ni(L-1 center dot H2O)(OOCCH3)]PF6, [Ni(L-2 center dot H2O)NCMe](PF6)(2), [Ni(L-3 center dot H2O)OH2)(PF6)(2), and [Ni(L-4 center dot H2O)NCMe] (PF6)(2) are reported. The Ni-donor bonding to pyridine and tertiary amine groups and oxygen- or nitrogen-bound coligands, completing the octahedral coordination sphere of Ni-II, is analyzed using a combination of ab initio electronic structure calculations (complete active space self-consistent field, CASSCF, followed by N-electron valence perturbation theory, NEVPT2) and angular overlap ligand field analysis. Magnetic properties are rationalized with an analysis of the magnetic anisotropy in terms of zero-field splitting and g-tensor parameters, obtained from first principles, and their correlation with the N-II-donor bonding parameters from the ligand field analysis of the ab initio results. A two-dimensional spectrochemical series of the ligands considered, according to their sigma and pi bonding to Ni-II, is also derived.