The adduct formation of iron(III) chlorophyll a with phosphines or phosphites has been studied by spectroscopic methods. The red-shift in the Soret and Q bands caused by the axial coordination of phosphine or phosphite to iron(III) chlorophyll a is an evidence for the autoreduction of the central iron(III) ion. The 31P{1H} NMR spectrum of bis-adducts measured in the presence of excess phosphine or phosphite ligands gave a single peak in the down field range compared to that of the corresponding

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... free ligand. The ESR and X-ray photoelectron spectra have revealed that the central iron atom of the bis-adduct of iron chlorophyll a with phosphines or phosphites is divalent and in the low-spin state. The axial coordination of phosphine or phosphite influences the electronic configuration of the central iron atom and the macrocyclic chlorine ligand to induce the autoreduction of the central iron(III) ion.