Haptotropism in a Nickel Complex with a Neutral, π‐Bridging cyclo‐P4 Ligand Analogous to Cyclobutadiene

The reaction of ( 1 )Ni(η 2 -cod), 2 , incorporating a chelating bis( N -heterocyclic carbene) 1 , with P 4 in pentane yielded the dinuclear complex [( 2 )Ni] 2 (μ 2 ,η 2 :η 2 -P 4 ), 3 , formally featuring a cyclobutadiene-like, neutral, rectangular, π-bridging P 4 -ring. In toluene, the butterfly-shaped complex [( 1 )Ni] 2 (μ 2 ,η 2 :η 2 -P 2 ), 4 , with a formally neutral P 2 -unit was obtained from 2 and either P 4 or 3 . Computational studies showed that a low energy barrier haptotropic rearrangement involving two isomers of the μ 2 ,η 2 :η 2 -P 4 coordination mode and a low energy μ 2 ,η 4 :η 4 -P 4 coordination mode, as previously predicted for related nickel cyclobutadiene complexes, could explain the coalescence observed in the low temperature NMR spectra of 3 . The insertion of the ( 1 )Ni fragment into a P–P bond of P 7 (SiMe 3 ) 3 , forming complex 5 with a norbornane-like P 7 ligand, was also observed.