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We report the reactivity, structures and spectroscopic characterization of reactions of phosphine-based ligands (mono-, di- and tri-dentate) with iron-carbide carbonyl clusters. Historically, the archetype of this cluster class, namely [Fe 6 (μ 6 -C)(μ 2 -CO) 4 (CO) 12 ] 2- , can be prepared on a gram-scale but is resistant to simple ligand substitution reactions. This limitation has precluded the relevance of iron-carbide clusters relating to organometallics, catalysis and the nitrogenase active site cluster. Herein, we aimed to derive a simple and reliable method to accomplish CO → L (where L = phosphine or other general ligands) substitution reactions without harsh reagents or multi-step synthetic strategies. Ultimately, our goal was ligand-based chelation of an Fe n (μ n -C) core to achieve more synthetic control over multi-iron-carbide motifs relevant to the nitrogenase active site. We report that the key intermediate is the PSEPT-non-conforming cluster [Fe 6 (μ 6 -C)(CO) 16 ] (2: 84 electrons), which can be generated in situ by the outer-sphere oxidation of [Fe 6 (μ 6 -C)(CO) 16 ] 2- (1: closo , 86 electrons) with 2 equiv. of [Fc]PF 6 . The reaction of 2 with excess PPh 3 generates a singly substituted neutral cluster [Fe 5 (μ 5 -C)(CO) 14 PPh 3 ] (4), similar to the reported reactivity of the substitutionally active cluster [Fe 5 (μ 5 -C)(CO) 15 ] with monodentate phosphines (Cooke & Mays, 1990). In contrast, the reaction of 2 with flexible, bidentate phosphines (DPPE and DPPP) generates a wide range of unisolable products. However, the rigid bidentate phosphine bis(diphenylphosphino)benzene (bdpb) disproportionates the cluster into non-ligated Fe 3 -carbide anions paired with a bdpb-supported Fe(ii) cation, which co-crystallize in [Fe 3 (μ 3 -CH)(μ 3 -CO)(CO) 9 ] 2 [Fe(MeCN) 2 (bdpb) 2 ] (6). A successful reaction of 2 with the tripodal ligand Triphos generates the first multi-iron-chelated, authentic carbide cluster of the formula [Fe 4 (μ 4 -C)(κ3-Triphos)(CO) 10 ] (9). DFT analysis of the key (oxidized) intermediate 2 suggests that its (μ 6 -C)Fe 6 framework remains fully intact but is distorted into an axially compressed, 'ruffled' octahedron distinct from the parent closo cluster 1. Oxidation of the cluster in non-coordinating solvent allows for the isolation and crystallization of the CO-saturated, intact closo -analogue [Fe 6 (μ 6 -C)(CO) 17 ] (3), indicating that the intact (μ 6 -C)Fe 6 motif is retained during initial oxidation with [Fc]PF 6 . Overall, we demonstrate that redox modulation beneficially 'bends' Wade-Mingo's rules via the generation of electron-starved (non-PSEPT) intermediates, which are the key intermediates in promoting facile CO → L substitution reactions in iron-carbide-carbonyl clusters., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)