The case of a μ₂-P aromatic phosphinine as a 4-electron donor forming σ- and π-three-center-two-electron bonds

Profound insight into the electronic structures of occasionally observed μ₂-P bridging phosphinines remains limited. In this work, we present the isolation and X-ray crystallographic characterization of a dimeric Rh(I) phosphinine complex exhibiting both η¹-P and μ₂-P phosphinine coordination modes. Variable temperature NMR analyses and DOSY spectrum measurement confirmed the presence of two types of fluxional phenomena in solution: η¹-P phosphinine bonding and dissociation, and η¹-P and μ₂-P equilibrium. DFT calculations in conjunction with single crystal X-ray diffraction studies suggest that the μ₂-P phosphinines donate four electrons via a σ-lone pair and a high-lying π-type electron pair, instead of two σ-lone pairs, forming σ- and π-three-center-two-electron bonds. The stronger π-type interactions lead to longer P-C bonds and larger negative coordination chemical shifts for μ₂-P phosphinines. However, the binding interactions of μ₂-P are thermodynamically weaker than those of η¹-P. Reactivity studies further confirm the labile nature of the μ₂-P phosphinine bonds, which could be easily converted to an η¹-P phosphinine.