The mechanism of ethylene trimerisation and tetramerisation with chromium–diphosphinoamine (Cr–PNP)
catalysts has been studied by experimental and theoretical (DFT) methods. The effects of a pendant ether
donor (ortho-methoxyaryl ligand substitution) and of anion coordination to the active species have been
studied. In the former case, coordination of the ether donor to chromium favours 1-hexene by suppressing
formation of the bisIJethylene) chromacyclopentane intermediate which is postulated to be the major route
to 1-octene. The effect of anion coordination is similar and as the coordination strength increases, displacement
of the anion by a second ethylene ligand becomes more difficult, again favouring trimerisation
over tetramerisation. Hence, the experimentally observed effects of pendant donor coordination and
changes in anion coordination strength can be rationalised.