The equilibria of adduct formation between several nickel(II) tetraamine complexes and acetonitrile were determined from -40 to 80 °C in acetonitrile solution by the Evans NMR magnetic susceptibility method. The stability order for adduct formation of the paramagnetic complex in terms of the ligand was found to be: 2,2,2-tet, 3,3,3-tet > 3,2,3- tet > 2,3,2-tet > cyclam. This order parallels that found in previous studies in aqueous solution. However, in this study, enthalpic factors were found to be dominant whereas enthalpic and entropic factors have been reported to be comparable in magnitude in aqueous solution. Optical studies from 200-1500 nm were conducted on 0.01 M acetonitrile solutions of the complexes from 25-65 °C. Only small changes in the intensity and position of the optical bands were observed with temperature except for the 2,3,2-tet (468 nm), 3,2,3-tet (450 nm), and cyclam (460 nm) complexes. These indicated bands increase with increasing temperature, which can be explained by assuming a square-planar (diamagnetic)-octahedral (paramagnetic) equilibrium in agreement with the magnetic susceptibility data. Band assignments were made for the cis- and trans-octahedral isomers for each of the complexes. The order of cis-octahedral character for the complexes was found to be 2,2,2- tet, 3,3,3-tet > 2,3,2-tet > 3,2,3-tet > cyclam in agreement with previous studies in aqueous, DMSO, and DMF solutions. © 1986.