Trimerisation and oligomerisation of olefins using a chromium based catalyst

The invention provides a olefin oligomerisation catalyst system that includes: a multidentate mixed heteroatomic ligand, which ligand includes three donor heteroatoms of which one donor heteroatom is sulfur, one donor heteroatom is nitrogen and one donor heteroatom is a phosphorous atom and wherein the ligand is selected such that none of the donor heteroatoms are directly bonded to any of the other donor heteroatoms; and chromium; wherein the ligand is of the following type: R 1 A(R 2 BR 3 R 4 )(R 5 CR 6 ) wherein R 1 , R 3 , R 4 and R 6 may be hydrogen or independently be selected from the groups consisting of alkyl, aryl, aryloxy, halogen, nitro, alkoxycarbonyl, carbonyloxy, alkoxy, aminocarbonyl, carbonylamino, dialkylamino, or derivatives thereof, or aryl substituted with any of these substituents; R 2 and R 5 may be the same or different and are C 1 to C 15 hydrocarbyls; A is nitrogen; B is phosphorous; and C is sulfur. The invention also provides a process for the oligomerisation of olefins including the step of contacting the olefins with such a catalyst system.