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

Triphenylhalophosphonium halides, Ph3PX2, form crystals comprising bridged linear cations [Ph3P-X-X-X-PPh3](+) where the X-3 bridge is shortened from 6.56 angstrom in Cl-Cl-Cl to 6.37 angstrom in the Br-Br-Br system. It is proposed that this structure is stabilised by five-centre/six-electron (5c-6e) hypervalent interactions.