Upon coordinating P₄ to electron poor cyclopentadienyl-

iron cations, the average P-P bond distances

shrink and the respective P₄ breathing mode in the

Raman spectra (600 cm-¹, P₄, free) is blueshifted by

>40 cm-¹ in [CpFe(CO)(L)(h¹-P₄)]⁺ cations (L=CO or

PPh₃). Analysis suggests that this corresponds to an umpolung

of the bonding from more phosphidic in the

known, electron-rich systems to more phosphonium-like

in the reported electron-poor versions. This may open

new functionalization pathways for white phosphorus P₄.