The present disclosure relates to a catalyst system comprising i) (a) a bicyclic 2-[(phosphinyl)aminyl] cyclic imine chromium salt or (b) a chromium salt and a bicyclic 2-[(phosphinyl)aminyl] cyclic imine and ii) an organoaluminum compound. The present disclosure also relates to a process comprising: a) contacting i) ethylene; ii) a catalyst system comprising (a) a 2-[(phosphinyl)aminyl] cyclic imine chromium salt complex or (b) a chromium salt and a bicyclic 2-[(phosphinyl)aminyl] cyclic imine; ii) an organoaluminum compound, and iii) optionally an organic reaction medium; and b) forming an oligomer product in a reaction zone.