Selective synthetic routes to isomeric diiron dithiolate complexes containing the (EtO)(2)PN(Me)P(OEt)(2) (PNP) ligand in an unsymmetrical chelating role, for example, (mu-pdt)[Fe(CO)(3)][Fe(CO)(kappa(2)-PNP)] (3) and as a symmetrically bridging ligand in (mu-pdt)(mu-PNP)[Fe(CO)(2)](2) (4), have been developed. 3 was converted to 4 in 75% yield after extensive reflux in toluene. The reactions of 3 with PMe3 and P(OEt)(3) afforded bis-monodentate P-donor complexes (mu-pdt)[Fe(CO)(2)PR3][Fe(CO)(2)(PNP)] (PR3 = PMe3 5; P(OEt)(3), 7), respectively, which are formed via an associative PMe3 coordination reaction followed by an intramolecular CO-migration process from the Fe(CO)3 to the Fe(CO)(PNP) unit with concomitant opening of the Fe-PNP chelate ring. The PNP-monodentate complexes 5 and 7 were converted to a trisubstituted diiron complex (mu-pdt)(mu-PNP)[Fe(CO)PR3][Fe(CO)(2)] (PR3 = PMe3, 6; P(OEt)(3), 8) on release of 1 equiv CO when refluxing in toluene. Variable-temperature P-31 NMR spectra show that trisubstituted diiron complexes each exist as two configuration isomers in solution. All diiron dithiolate complexes obtained were characterized by MS, IR, NMR spectroscopy, elemental analysis, and X-ray diffraction studies.