NMR Spectroscopy in an Advanced Inorganic Lab: Structural Analysis of Diamagnetic and Paramagnetic Ni(II) Schiff Base Complexes

In a two-step synthesis starting with Ni(CH[subscript 3]COO)[subscript 2] ·4H[subscript 2]O and salicylaldehyde, students prepare Nethyl and N-isopropyl Ni(II) Schiff base (imine) complexes. After isolation and recrystallization, the products are characterized by room-temperature magnetic susceptibility measurements, FTIR spectroscopy, and [superscript 1]H, [superscript 13]C{[superscript 1]H}, and [superscript 13]C DEPT-135 or [superscript 13]C APT NMR spectroscopy. The results are analyzed by considering the steric requirements of the ethyl and isopropyl groups and the subsequent effect on the preferred geometries, either the diamagnetic square-planar geometry, as expected for a Ni(II) d[superscript 8] complex, or the paramagnetic tetrahedral geometry. Students are challenged to explain why the spectra for these similar complexes are so different and what this difference reveals about steric requirements in determining geometry.