CONTACT SHIFT STUDIES OF NICKEL(II)-2,3-DIAMINOBUTANE COMPLEXES

The proton nmr contact shifts of the diastereoisomeric complexes [Ni(meso-bn)3]Cl2·2H2O and [Ni(d-bn)3]Cl2·2H2O (bn = 2,3-diaminobutane) have been investigated to ascertain more exactly what factors contribute to the difference in chemical shifts, required in principle, for diastereoisomers. This system is of particular interest because it can be investigated under conditions where there is no equilibrium between various species and the pseudocontact shift contributions are expected to be negligible. Thus, we can observe effects in the nmr spectra arising from the Fermi contact interaction for the diastereoisomers and thereby gain information about differences in the metal-ligand interaction. Only the methyl protons are detected in the nmr spectra of these complexes and within experimental error no differences exist in the contact shifts of the diastereoisomers. In view of sensitivity of the contact shift to bonding interactions the metal-ligand bond strengths must be of comparable magnitude in the two isomers and other factors are needed to account for the large differences in the stability constants of the diastereoisomers and reported chemical shift differences in many other diastereoisomers. Possible contributions to these phenomena are discussed. Spectral data are presented to substantiate our claim that these tris complexes undergo an octahedral ⟷ square-planar dissociation similar to that observed for “Lifschitz salts,” unless excess ligand is present. © 1969, American Chemical Society. All rights reserved.