Giant Ising-Type Magnetic Anisotropy in Trigonal Bipyramidal Ni(II) Complexes: Experiment and Theory

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni­(II) complexes, [Ni­(Me6tren)­Cl]­(ClO4) (1) and [Ni­(Me6tren)­Br]­(Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental Dexpt value (energy difference between the Ms = ± 1 and Ms = 0 components of the ground spin state S = 1) estimated to be between −120 and −180 cm–1. The theoretical study shows that, for an ideally trigonal Ni­(II) complex, the orbital degeneracy leads to a first-order spin–orbit coupling that results in a splitting of the Ms = ± 1 and Ms = 0 components of approximately −600 cm–1. Despite the Jahn–Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin–orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical Dtheor between −100 and −200 cm–1) is obtained.