The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H(2)ase, active site has prompted revisits of the classic organometallic chemistry involving the FeFe bond and bridging ligands, particularly of the (mu-SCH2XCH2S)[Fe(CO)(3)](2) and (mu-SCH2XCH2S)[Fe(CO)(2)L](2) (X = CH2, NH; L = PMe3, CN, and NHCs (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN) and electrophilic (nitrosonium, NO+) ligand substitution on the diiron dithiolate framework of the (mu-pdt)[Fe(CO)(2)NHC][Fe(CO)(3)] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H(2)ase active site. While CO substitution by PMe3, followed by reaction with NO+, produces (mu-pdt)(mu-CO)[Fe(NHC)(NO)][Fe(CO)(2)PMe3](+), the alternate order of reagent addition produces the structural isomer (mu-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)(3)](+), illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (mu-pdt)[Fe(CO)(2)CN][Fe(CO)(3)](-), (mu-pdt)[Fe(CO)(3)](2), and (mu-pdt)[Fe(CO)(2)NO][Fe(CO)(3)](+), provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.