The case of a μ-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 μ-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 η-P and μ-P phosphinine coordination modes. Variable temperature NMR analyses and DOSY spectrum measurement confirmed the presence of two types of fluxional phenomena in solution: η-P phosphinine bonding and dissociation, and η-P and μ-P equilibrium. DFT calculations in conjunction with single crystal X-ray diffraction studies suggest that the μ-P phosphinines donate four electrons 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 μ-P phosphinines. However, the binding interactions of μ-P are thermodynamically weaker than those of η-P. Reactivity studies further confirm the labile nature of the μ-P phosphinine bonds, which could be easily converted to an η-P phosphinine.