Paramagnetic Resonance in Transition-Metal Hexammine Complexes. I The Ni(II)(NH3)6 Complex Ion

Electron paramagnetic resonance studies have been carried out on the complex ion Ni(II)(NH₃)₆ in the host compounds Zn(NH₃)₆X₂ and Cd(NH₃)₆X₂, where X=Cl, Br, and I. The results support the conclusion that above a certain characteristic transition temperature ${\mathrm{(T}}_c)$ the ligand field in the hexammines is perfectly octahedral. The quenching of the rotation of the ammonia ligands at temperatures below $T_c$ was detected from the sudden transition of the isotropic Ni(II)(NH₃)₆ single line to three lines characteristic of an anisotropic spectrum. The anisotropic spectrum, which was not found in the previous work on concentrated hexammines, was very clear and the nonoctahedral part of the spin Hamiltonian was determined. The results agree qualitatively with the model of the hexammines proposed by Bates and Stevens, but quantitative agreement is not yet obtainable.