The Pn–O stretch redshifts on coordination as the metal withdraws electron density through the O atom. The C–Oether crown stretch appears as a strong band near 1100 cm⁻¹. Toggle complexes to compare.
Key bands: Free Ph₃PO: P–O at ~1190 cm⁻¹. On coordination the band shifts down ~30–80 cm⁻¹ and merges with the C–Oether stretch near 1022 cm⁻¹. Free Ph₃AsO: As–O at ~890 cm⁻¹. Predicted shift on coordination: ~20–50 cm⁻¹ redshift. Bands marked with * are predicted/extrapolated from HSAB trends.