The invention provides a mixed heteroatomic ligand for an oligomerization of olefins catalyst, which ligand includes at least three heteroatoms, of which at least one heteroatom is sulfur and at least two heteroatoms are not the same. The invention also provides a multidentate mixed heteroatomic ligand for an oligomerization of olefins catalyst, which ligand includes at least three heteroatoms of which at least one is a sulfur atom. The ligand may also contain, in addition to sulfur, at least one nitrogen or phosphorous heteroatom.