The invention provides a mixed heteroatomic ligand for an oligomerisation of olefins catalyst, which ligand includes at least three heteroatoms, of which at least one heteroatom is nitrogen and at least two heteroatoms are not the same. The invention also provides a multidentate mixed heroatomic ligand for an oligomerization of olefins catalyst, which ligand includes at least three heteroatoms. At least one heteroatom may be nitrogen and at least 2 heteroatoms may not be the same.