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

Abstract

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 (Tc) the ligand field in the hexammines is perfectly octahedral. The quenching of the rotation of the ammonia ligands at temperatures below Tc 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.

Artificial Molecular Rotors

“…Rotations in [Ni(NH 3 ) 6 ]X 2 crystals (X = Cl, Br, I) have been studied by EPR, − and observed line-width variations have been explained in terms of correlated rotations of the NH 3 groups . Trapp and Shyr proved this by looking at the EPR spectra of [Ni(NH 3 ) 6 ]X 2 in [Zn(NH 3 ) 6 ]X 2 and [Cd(NH 3 ) 6 ]X 2 hosts (where X = Cl, Br, I). Above the critical temperature ( T c ), the complexes existed in a perfectly octahedral environment, and below the T c , the motion is frozen.…”

Hexammine Nickel Complex: Cooperative Rotation of Six NH3Groups

“…4), hence, they are of the order of Zeeman splitting in EPR. Typical splttings of the electronic multiplets of Ni 2 + in crystal field are 104 cm-1 and the zerofield splitting in nickel hexammines is about 1 cm -1 [12][13][14].…”

Ligand field design enables quantum manipulation of spins in Ni2+ complexes

“…In the orbitally non-degenerate case, this can be achieved via incorporation of strong eld ligands, as D is inversely proportional to the energetic separation of excited states from the ground state. 25 Previously, the only examples of commercial frequency [26][27][28][29][30][31][32][33] which are exceptionally host-and counterion-dependent, due to the weak eld monodentate ligands and the exible primary coordination sphere. As a result, for a nickel coordination complex with the same coordinating ligands, D can vary by an order of magnitude as a function of counterion.…”