Experimental NMRD Profiles for Some Low-Symmetry Ni(II) Complexes (S = 1) in Solution and Their Interpretation Using Slow-Motion Theory

Abstract

Experimental NMRD profiles for four Ni(II) complexes (S = I) in solution have been interpreted using slow-motion theory. Rhombicity in the zero-field splitting (ZFS) and noncoinciding static ZFS and dipole-dipole (DD) tensors are included in the model, which improves the physical picture in terms of the electronic structure and deformability of the complexes. In a previous study from our laboratory, Ni(dpm) 2 (aniline-d 5 ) 2 + 2 data were reported and analyzed using a model that assumed axially symmetric ZFS and coinciding static ZFS and DD tensors. These data are reinterpreted in the present article, which provides a nearly axially symmetric static ZFS. New experimental data on three aqueous solutions containing tetraaza complexes are also reported and interpreted. One of the systems, Ni([15]aneN 4 )(H 2 O) 2 + 2 , gives best-fit parameter values similar to those of Ni(dpm) 2 (aniline-d 5 ) 2 + 2 . These two systems have the two solvent molecules coordinated in axial positions. The second complex, Ni([12]aneN 4 )(H 2 O) 2 + 2 , differs substantially in that the water molecules are coordinated in the cis configuration and that the best fit was obtained using a highly rhombic ZFS. The third complex, Ni(tmc)(H 2 O) 2 + , is five-coordinated, which results in a rather large rhombicity. In all cases, the best-fit parameters are clearly outside of the Redfield limit, which means that simpler theories are of limited use. We have also found that the latter two systems differ very much from the two former systems in terms of electron-spin dynamics. The main reason lies in the difference in the relative magnitudes of the static and the fluctuating transient parts of the ZFS, and this feature has a great impact on the rhombicity effect.