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A series of four-coordinate Ni(lI) complexes derived from ,ß-msaturated ß-ketoamines and of the general formulation Ni[RyCOCH(NHR)Ra]2 have been synthesized. Two groups of complexes having Ra = Ry = CH¡ and Ra = CH•., Ry = CeH5 were thoroughly investigated by spectral, magnetic, and proton resonance measurements.For both groups it has been demonstrated that a planar ± tetrahedral equilibrium exists whose position is strongly dependent on the steric nature of the R group. As in several other systems, the presence of the configurational equilibrium allows favorable relaxation conditions such that well-defined isotropic proton contact shifts have been measured and analyzed. It is concluded that the spin imbalance exists in the lowest filled ligand -orbital. For both series it was found that when R = sec-alkyl the complexes are ~/00% tetrahedral at room temperature, but when R = aryl in the first series and n-alkyl in the second series the equilibrium is strongly shifted toward the planar form.In these cases AF, AH, and AS values for the structural change were determined from the contact shifts. The detection of a small amount of molecular association of complexes having R = aryl was demonstrated from contact shift measurement.A brief comparison of solution equilibria and -bonding in ß-ketoamine and salicylaldimine nickellfl) complexes is given.