The case of a μ 2 -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 2 -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 1 -P and μ 2 -P phosphinine coordination modes. Variable temperature NMR analyses and DOSY spectrum measurement confirmed the presence of two types of fluxional phenomena in solution: η 1 -P phosphinine bonding and dissociation, and η 1 -P and μ 2 -P equilibrium. DFT calculations in conjunction with single crystal X-ray diffraction studies suggest that the μ 2 -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 μ 2 -P phosphinines. However, the binding interactions of μ 2 -P are thermodynamically weaker than those of η 1 -P. Reactivity studies further confirm the labile nature of the μ 2 -P phosphinine bonds, which could be easily converted to an η 1 -P phosphinine.