The binuclear nickel(II) complex Ni₂Cl₄(cyclo-tetraphos) (cyclo-tetraphos = cis,trans,cis-1,2,3,4-tetrakis-(diphenylphosphino)cyclobutane) forms, upon activation with MAO, an effective and robust catalyst for the oligomerisation of ethylene to Schulz–Flory distributions of C₄–C₁₄ olefins with a mechanism where both chain transfer and chain propagation are first order in ethylene.