The cluster [PPN][HFe3(CO)(9)SO2] reacts with CH3OSO3CF3 to give [HFe3(CO)(9)SO2CH3], which was characterized by infrared and NMR spectroscopy and X-ray crystallography. The methyl group binds to the exo oxygen of the coordinated SO2 ligand in [PPN][HFe3(CO)(9)SO2]. Extended Huckel calculations reveal that SO2-CH3 interactions occur through an SO2 orbital that is not involved in bonding to the metals. Crystallographic data : space group P ($) over bar 1, a = 8.622(3) Angstrom, b = 14.046(3) Angstrom, c = 15.289(3) Angstrom, alpha = 69.11(2)degrees, beta = 77.18(2)degrees, gamma = 74.74(2)degrees.