Direct evidence of a multicentre halogen bond: unexpected contraction of the P–XXX–P fragment in triphenylphosphine dihalides

Abstract

Triphenylhalophosphonium halides, Ph₃PX₂, form crystals comprising bridged linear cations [Ph₃P–X–X–X–PPh₃]⁺ where the X₃ bridge is shortened from 6.56 Å in Cl–Cl–Cl to 6.37 Å in the Br–Br–Br system. It is proposed that this structure is stabilised by five-centre/six-electron (5c–6e) hypervalent interactions.