The syntheses, interconversions, and spectroscopic properties of a set of iron carbonyl clusters containing an interstitial carbide are reported. This includes the low temperature X-ray structures of the six-iron clusters (Y)(2)[Fe-6(mu(6)-C)(mu(2)-CO)(4)(CO)(12)] (1a-c; where Y = NMe4, NEt4, PPh4); the five-iron cluster [Fe-5(mu 5-C)(CO)(15)] (3); and the novel formulation of the five-iron cluster (NMe4)(2)[Fe-5(mu 5-C)(mu 2-CO)(CO)(13)] (4). Also included in this set is the novel charge-neutral cluster, [Fe-6(mu 6-C)(CO)(18)] (2), for which we were unable to obtain a crystallographic structure. As synthetic proof for the identity of 2, we performed a closed loop of interconversions within a family of crystallographically defined species (1, 3, and 4): [Fe-6](2-) -> [Fe-6](0) -> [Fe-5](0) -> [Fe-5](2-) -> [Fe-6](2-). The structural, spectroscopic, and electronic properties of this missing link cluster 2 were investigated by IR, Raman, XPS, and Mossbauer spectroscopiesas well as by DFT calculations. A single -> CO feature (1965 cm(-1)) in the IR spectrum of 2, as well as a prominent Raman feature (symm = 1550 cm(-1)), are consistent with the presence of terminal carbonyls and a {(mu 6-C)Fe-6} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p3/2 feature (707.4 eV) as compared to that of 1 (705.5 eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF6) under CO atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by Mssbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with d = 0.076 and 0.064 mm s(-1). The high-field Mssbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with d = -0.18, -0.11, and +0.41 mm s(-1). This indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field Mossbauer spectra for 2 were simulated assuming these two groups and a diamagnetic ground state. Taken together, the Mossbauer results and DFT calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized pentagonal bipyramidal structure for 2, the Fe(II)-C-carbide distances are compressed (similar to 1.84 angstrom), while the Fe(0)-C-carbide distances are elongated (similar to 2.05 angstrom). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of nitrogenase, which may in turn stimulate N-2 reduction.