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Full proton and carbon assignments have been obtained for ((1,4,5-η)-cyclooctenyl)(1,1,1,5,5,5-hexafluoro-2,4-pentadionato)nickel(II) (1), a Keim type ethylene oligomerization catalyst, which is afforded by treatment of bis(1,5-cyclooctadiene)nickel with 1,1,1,5,5,5-hexafluoroacetylacetone. The larger metallacycle formed by nickel and the (1,4,5-η)-cyclooctenyl ligand in 1 is shown to be in a “nickela chair” conformation. The reaction of bis(1,5-cyclooctadiene)nickel with 1,1,1,5,5,5-hexafluoroacetylacetone-d 2 at low temperature shows a 70% incorporation of a deuterium atom anti to nickel at a β-carbon. This observation is incompatible with a hydride migration path for the formation of 1 but fully consistent with a direct protonation of an alkene coordinated to an electron-donating nickel.