Complexes featuring an unsupported Fe-Fe bond can give more direct insight into metal-metal bonding, but they are rare and typically feature low-spin Fe 0/I centers stabilized by π-acidic carbonyl ligands, for example the [Fe2(CO)8] 2-dianion (Figure ; A). To our knowledge, there are only two examples of isolated complexes with unsupported Fe-Fe bonds that are not diamagnetic. The first is a homoleptic Fe-Fe complex supported by macrocyclic [14]-l,3,8,10-tetraeneN4 (tim) ligands at each Fe site (B). Electron delocalization between the identical iron sites gave rise to an intermediate-spin S = 1 ground state. In contrast, the complex Ar′Fe-Fp (Ar′ = C6H3-2,6-(C6H3-2,6-i Pr2)2, Fp = FeCp(CO)2) (C) had two different iron sites: a slightly bent, two-coordinate high-spin iron(II) site and a low-spin iron(0) site in Fp. The sterically demanding terphenyl ligand protects a twocoordinate, high-spin Fe site. Despite a short Fe-Fe bond (2.393(1) Å), DFT calculations indicated that the metal-metal bond order was only 0.36, and it was classified as a dative bond from the Fp group to the two-coordinate iron. Inspired by this apparent "metalloligand" on a low-coordinate iron site, we report the synthesis and characterization of two complexes featuring unsupported Fe-Fe bonds. These show exceptionally large zero-field splitting which is sensitive to the details of the supporting bidentate ligand. Further, we use Mössbauer measurements to quantify the Fp "metalloligand" on a scale with more conventional ligands, giving insight for future design of multimetallic complexes. .3931(8) Å). The Fe-Fe bond in both complexes have a formal shortness ratio of 1.03-1.04 (sum of single-bond covalent radii Fe-Fe = 2.33 Å), and are significantly shorter than the Fe-Fe distance in metallic iron (2.48 Å). The short distance arises from the low coordination number of one iron atom in these complexes. Comparison of the solid state parameters of 1a and 1b shows that removal of the methyl groups on the ligand backbone leads to an increase in the N-Fe-N bond angle from 91.2(1)° (1a) to 95.1(1)° (1b). This is accompanied by a small decrease in the average N-Fe1- ). For clarity, only the staggered fit will be discussed here for both 1a and 1b. Modelling the 220 K Mössbauer spectrum as two staggered doublets led to isomer shift (δ) (and quadrupole splitting, |DEQ|) values of 0.03 mms -1 (|DEQ| = 1.80 mm s -1 ) and 0.57 mms -1 (|DEQ| = 0.90 mm s -1 ). The former doublet is similar to those found in CpFe 0 (CO)2 compounds, such as that in Ar′Fe-Fp (d 190 K = 0.08 mm s -1 , |DEQ| = 1.73 mm s -1 ) and has a smaller shift than Fe II compounds with similar coordination environments, such as CpFe II (CO)2Cl (d 220 K = 0.14 mm s -1 , |DEQ| = 1.83 mm s - 1 ), and smaller than the anion, K[CpFe 0 (CO)2] (d 80 K = 0.16 mm s -1 , |DEQ| = 2.07 mm s -1 ). The other doublet is comparable to values for high-spin three coordinate L c Fe II X species, for example L c FeCl (d 4.2 K = 0.74 mm s -1 , |DEQ| = 1.61 mm s -1 ) and L c FeCH3 (d 4.2 K = 0.48 mm s - 1 , |DEQ| = 1.74 mm s -1 ) (L c = 3,5-bis(2,6-diisopropylphenylimido)-2,6-dimethylheptyl). These data led to a working model for the complex that assigns the sites as being a low-spin Fe 0 (CpFe(CO)2) and a high-spin Fe II (LFe).