The reaction of NapP 2 S 4 (1; Nap = naphthalene-1,8-diyl) with chlorine gas gave [Nap(PCl 2 )(PCl 4 )] (2), displaying a rare σ 4 P-σ 6 P bonding interaction. An X-ray structure analysis confirmed the PCl 5 -like, P−P bond containing phosphoniumphosphoride structure of 2 in the crystal, which was also found in solution at low temperature. At ambient and higher temperatures, dynamic behaviour on the NMR time-scale was observed, which was assigned to interchange of the ionic phosphonium-phosphoride form 2 and the molecular bis(phosphorane) Nap(PCl 3 ) 2 form 3, rather than to the ionic phosphonium salt-phosphorane form [Nap(PCl 3 )(PCl 2 )][Cl] 4. Electronic structure calculations were performed at the