Origin of the Magnetic Anisotropy in Heptacoordinate NiII and CoII Complexes

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear NiII and CoII complexes were investigated by a combination of experiment and ab initio calculations. The zero‐field splitting (ZFS) parameters D of [Ni(H2DAPBH)(H2O)2](NO3)2⋅2 H2O (1) and [Co(H2DAPBH)(H2O)(NO3)](NO3) [2; H2DAPBH=2,6‐diacetylpyridine bis‐ (benzoyl hydrazone)] were determined by means of magnetization measurements and high‐field high‐frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the NiII complex indicates stabilization of the highest MS value of the S=1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for CoII indicates stabilization of the MS=±1/2 sublevels of the S=3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the NiII complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the dxy and ${{\rm{d}}_{x^2 - y^2 } }$ orbitals owing to the pseudo‐pentagonal‐bipyramidal symmetry of the complex. For CoII, all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex. The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear NiII and CoII complexes [Ni(DAPBH)(H2O)2](NO3)2⋅2 H2O (1) and [Co(DAPBH)(H2O)(NO3)](NO3) [2; H2DAPBH=2,6‐diacetylpyridine bis(benzoyl hydrazone)] were investigated experimentally and theoretically. The zero‐field splitting (ZFS) parameters were estimated by means of magnetization measurements and EPR spectroscopy, and then ab initio calculations were carried out to rationalize the origin of the experimentally observed anisotropy and develop simple rules to tune the ZFS through small modifications of the complexes.