Haptotropism in a Nickel Complex with a Neutral, π-Bridging cyclo-P 4 Ligand Analogous to Cyclobutadiene.

Dedicated to Professor Manfred Scheer on the occasion of his 65th birthday 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 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.