A series of new diphosphazane (PNP) ligands that contain 2,3,4,5-tetraethylphosphole or dibenzophosphole moieties has been synthesised. The new compounds have been screened for chromium-catalysed, selective ethene oligomerisation by in situ combination with CrCl3(thf)3 and methylaluminoxane (MAO). The ligands derived from 2,3,4,5-tetraethylphosphole produce highly active catalysts for ethene oligomerisation, which show excellent selectivity to C6 and C8 linear α-olefins. Complexes of the form [Cr(CO)4L] were synthesised and studied by IR spectroscopy and single-crystal XRD. Variable-temperature NMR spectroscopy was used to investigate restricted PN rotation in compounds with bulky nitrogen substituents.