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

Dedicated to Professor Manfred Scheer on the occasion of his 65th birthday The reaction of (1)Ni(η² -cod), 2, incorporating a chelating bis(N-heterocyclic carbene) 1, with P₄ in pentane yielded the dinuclear complex [(2)Ni]₂ (μ₂ ,η²  : η² -P₄ ), 3, formally featuring a cyclobutadiene-like, neutral, rectangular, π-bridging P₄ -ring. In toluene, the butterfly-shaped complex [(1)Ni]₂ (μ₂ ,η²  : η² -P₂ ), 4, with a formally neutral P₂ -unit was obtained from 2 and either P₄ or 3. Computational studies showed that a haptotropic rearrangement involving two isomers of the μ₂ ,η²  : η² -P₄ coordination mode and a low-energy μ₂ ,η⁴  : η⁴ -P₄ 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 P₇ (SiMe₃ )₃ , forming complex 5 with a norbornane-like P₇ ligand, was also observed.