Haptotropism in a Nickel Complex with a Neutral, pi-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(eta(2)-cod), 2, incorporating a chelating bis(N-heterocyclic carbene) 1, with P-4 in pentane yielded the dinuclear complex [(2)Ni](2)(mu(2),eta(2) : eta(2)-P-4), 3, formally featuring a cyclobutadiene-like, neutral, rectangular, pi-bridging P-4-ring. In toluene, the butterfly-shaped complex [(1)Ni](2)(mu(2),eta(2) : eta(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 mu(2),eta(2) : eta(2)-P-4 coordination mode and a low-energy mu(2),eta(4) : eta(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(SiMe3)(3), forming complex 5 with a norbornane-like P-7 ligand, was also observed.