“…Under 1-butene, two additional signals appear: site 3 ( g ∥ = 2.219, g ⊥ = 2.012) is attributed to Ni I , which coordinates to a π-system, since a similar shift to lower g -values has been found for Ni I interacting with π-electrons of unsaturated hydrocarbons. , It may arise from a Ni site that forms a π-complex with 1-butene. The g ⊥ value of signal 4 ( g ∥ = 2.233, g ⊥ = 2.090) is much larger, in comparison to those of signals 1–3, suggesting that signal 4 does not arise from a π-complex of Ni I with 1-butene but rather from a Ni I –(CH 2 ) 3 CH 3 complex formed by insertion of 1-butene into the Ni–H bond, which is commonly assumed to be the first step in the reaction cycle. − Signals 1 and 2 decrease with rising butene pressure, in favor of signals 3 and 4 (see Figure b, as well as Table S1), indicating that those Ni I species (Ni I ···Cp and/or Ni I ···COD) are converted to species 3 (Ni I ···π) and 4 (Ni I –(CH 2 ) 3 CH 3 ).…”