We report the synthesis, spectroscopy, and structural characterization of iron-alkyne and -alkene complexes of the type (LFe)-Fe-Me(ligand) [L-Me = bulky beta-diketiminate, ligand) HCCPh, EtCCEt, CH2CHPh, EtCHCHEt, HCC(p-C6H4OCH3), HCC(p-C6H4CF3)]. The neutral ligand exchanges rapidly at room temperature, and the equilibrium constants have been measured or estimated. The binding affinity toward the low-coordinate Fe follows the trend HCCPh > EtCCEt > CH2CHPh > EtCHCHEt similar to PPh3 > benzene. N-2. This trend is consistent with a model in which pi back-bonding from the formally Fe-I center is the dominant interaction in determining the relative binding affinities. In nitrogenase, alkynes are reduced while alkenes are unreactive, and this work suggests that the different binding affinities to low-coordinate Fe might explain the differential activity of the enzyme toward these two substrates.