Recent experimental and theoretical studies of the nonlinear dynamic phenomena: electrochemical oscillations and multistability occurring during the electrode processes of thiocyanate and azide complexes of nickel(II) at mercury electrodes, are reviewed. These processes exhibit, in their current- potential (I-E) characteristics, the region of the negative differential resistance (NDR) which, under appropriate conditions, is a cause for spontaneous oscillations and/or multiplication of the steady-states. For the electroreduction of thiocyanate complexes of nickel(II) both oscillations (at the steady-state and streaming electrodes) and multi-stability (at streaming electrodes) were reported. The digital simulation of bistability confirmed the electrochemical mechanism underlying this phenomenon. Application of the linear stability analysis led to the derivation of general stability conditions for the processes at the streaming electrodes and to the theoretical bifurcation diagram for the Ni(II)-SCN^- electroreduction, well concordant with the experimental one. For the electroreduction of the azide complexes of nickel, bistability and (rarely experimentally observed) tristability were reported. The source of tristability is the existence of two regions of NDR in the I-E characteristics of the Ni(II)-N₃^- reduction, caused by the reduction of the central ion, followed, at more negative potentials, by the parallel reduction of the azide ligand to ammonia, controlled by the amount of H⁺ added. Linear stability analysis led to the construction of the bifurcation diagrams, well concordant with the experimental ones. These results constitute the contribution to the nonlinear dynamic behaviour of the electrochemical systems with NDR characteristics and prove that the streaming electrode is a powerful electrode type for such studies.