“…Moreover, in the IR spectra of 1a and 1b, PAC (C 6 H 5 ) bond stretching vibrations were observed in the 1106 and 1103 cm À1 , respectively. In addition, PAC (five membered ring) stretching vibration which is also sensitive to the coordination, is shifted to lower frequency with respect to the free phosphorus ylide ligand [39], suggesting some removal of the electron density of the PAC bond [41].…”