Phosphine-modified thioester derivatives are shown to serve as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls, which replicate key structural features of the active site of the hydrogenase Hmd. The reaction of Ph2PC6H4C(O)SPh and sources of Fe(0) generates both Fe(SPh)(Ph2PC6H4CO)(CO)(3) (1) and the diferrous diacyl Fe-2(SPh)(2)(CO)(3)(Ph2PC6H4CO)(2), which carbonylates to give 1. For the extremely bulky arylthioester Ph2PC6H4C(O)SC6H3-2,6-(2,4,6-trimethylphenyl)(2), oxidative addition is arrested and the Fe(0) adduct of the phosphine is obtained. Complex 1 reacts with cyanide to give Et4N[Fe(SPh)(Ph2PC6H4CO)(CN)(CO)(2)] (Et4N[2]). C-13 and P-31 NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2](-) in nu(CN) and nu(CO) regions very closely matches that for Hmd(CN). XANES and EXAFS measurements also indicate close structural and electronic similarity of Et4N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH2NC, but the adduct is more labile than Et4N[2]. Tricarbonyl 1 was found to reversibly protonate to give a thermally labile derivative, IR measurements of which indicate that the acyl and thiolate ligands are probably not protonated in Hmd.