The present disclosure relates to a catalyst system comprising i) (a) an N2-phosphinyl bicyclic amidine chromium salt or (b) a chromium salt and an N2-phosphinyl bicyclic amidine and ii) an organoaluminum compound. The present disclosure also relate to a process comprising: a) contacting i) ethylene; ii) a catalyst system comprising (a) an N2-phosphinyl bicyclic amidine chromium salt complex or (b) a chromium salt and an N2-phosphinyl bicyclic amidine; ii) an organoaluminum compound, and iii) optionally an organic reaction medium; and b) forming an oligomer product in a reaction zone.