Raman spectra of some complexes containing methyl‐, dimethyl‐ and trimethyl‐halophosphonium cations

Vibrational assignments for the skeletal modes of the solid‐state species (CH3)nPX5−n (X  Cl, Br; 1 ⩽ n ⩽ 3) have been made on the basis of their Raman Spectra. The results are consistent with ionic formulations (CH3)nPX4−n +X− and are compared with any previously available data. Two modifications of (CH3)2PCl2 +Cl− have been observed for the first time and their relative stabilities investigated. Raman spectra of the mixed halogen cationic compounds CH3PCl2Br+X−, CH3PClBr2 +X− and (CH3)2PClBr +X− are also reported and Lewis acid complexes of all the phosphonium cations show characteristic high‐frequency shifts in the PX stretching modes [relative to those in (CH3) RPX4−n +X−] which are rationalized in terms of anion‐cation interactions. CH3PCl4 has been observed in a molecular form and the transition to the ionic modification (at ca 160 K) has been monitored by variable temperature Raman spectroscopy.