Catalyzed selective ethylene oligomerization for the production of short-chain linear α-olefins (LAOs) is a subject of paramount interest both from a industrial and academic viewpoint due to the importance of LAOs as feedstock in a great variety of processes ranging from copolymerization to the fabrication of detergents and plasticizer alcohols, only to name a few. Catalyst precursors comprising a chromium metal center and chelating bisphosphine ligands figure amongst the most efficient systems for the selective tri- and tetramerization of ethylene. P^N^P type ligands were synthesized in a Mannich-type reaction, and coordinated to chromium (III), affording novel octahedral complexes, which proved to be active in the oligomerization of ethylene upon activation with methylaluminoxane. It is demonstrated, that the coordination mode to the metal center, the basicity and the steric bulk of the ligand determine the selectivity of the catalyst towards the formation of a particular oligomer chain length. The experimental findings are rationalized by DFT calculations on the metalacyclic mechanism of the oligomerization reaction. It will be shown that in the case of the P^N^P ligand, growth of the metalacycle beyond a seven-membered ring is hindered depending on the steric bulk of the ligand and thus responsible for the selectivity towards 1-hexene, or inversely, the formation of 1-octene becomes possible.