Dissolution of FeBr3 in MeO-py with stirring for 2.5 hours, followed by filtration and vapour diffusion with acetone results in the formation of orange plate-like crystals in 2 weeks. Crystals of [Fe III 8O6(µ- OH)6(MeO-py)12Br2]Br4•3H2O•2MeO-py (1• 3H2O•2MeO-py; Figure ) are in a monoclinic crystal system and structure solution was performed in the space group C2/c (Table ; Figure ). The asymmetric unit contains half the formula. The metallic skeleton of 1 describes a hexagonal bipyramid, in which a ring of six octahedral Fe III ions (Fe1-3 and symmetry equivalent, s.e.) is capped top and bottom by a tetrahedral Fe III ion (Fe4 and s.e.). The tetrahedral Fe III ions are linked to the [Fe6] ring through three µ3-O 2-ions (O10, O20, O30 and s.e.), which further bridge two octahedral Fe ions around the inner rim of the wheel. The outer rim is bridged by six µ-OH -ions (O1H, O2H, O3H and s.e.). The coordination sphere of the tetrahedral Fe ion is completed by the presence of a terminal Br -ion (Br3 and s.e.), and those of the octahedral Fe ions by two MeO-py molecules. Fe-O-Fe bond angles fall in the ranges Fe(tet)-O-Fe(oct), 122.62-123.42°, and Fe(oct)-O-Fe(oct), 95.25-98.38°; note that the former are very much bigger than the latter. The Br -counter anions (Br1, Br2 and s.e.) are H-bonded to the µ-OH ions (Br…O, 3.199-3.227 Å), as are the two MeO-py and three H2O molecules of crystallization (Br…O, 3.365-3.674 Å; N…O, 2.847 Å; Figure ). Closest inter-cluster interactions are between neighbouring MeO-py molecules at C/O…C/O distances ≥ 3.45 Å (Figure ). The structural similarity between 1 and [Fe17] can be seen in Figure (where btaH = 1,2,3-benzotriazole) and [Gd12Fe14O12(OH)18(tea)6(CH3COO)16(H2O)8] (where H3tea = triethanolamine), whose Fe metallic skeletons both describe hexacapped hexagonal bipyramids, albeit with the Fe ions all being octahedral. Magnetic measurements of 1 reveal strong antiferromagnetic interactions between the Fe III ions. The experimental dc susceptibility data (T = 2-300 K, B = 0.1 T) for 1 are plotted in Figure as the χT product versus T, where χ is the molar magnetic susceptibility, T is the temperature, and B the field. The value of χT at T = 300 K is ~38 cm 3 K mol -1 , larger than that expected for the sum of the Curie constants for eight Fe III (S = 5/2) ions with gFe = 2.00 (35 cm 3 K mol -1 ). As the temperature decreases, the magnitude of χT increases rapidly, reaching a maximum value of ~57 cm 3 K mol -1 at T = 16 K, where it then plateaus before decreasing slightly to a value of ~55 cm 3 K mol -1 at T = 2 K. The data are clearly indicative of a ferrimagnetic system, with the maximum in χT suggesting a ground state spin value of S = 10. This is corroborated by magnetisation data (T = 2-7 K, B = 0.5 -7 T; Figure ) which rise rapidly with increasing field strength and saturate just below M = 20 µB. Given the large discrepancy in the Fe(tet)-O-Fe(oct) and and Fe(oct)-O-Fe(oct) bond angles, the data suggest a strong antiferromagnetic interaction between the tetrahedral and octahedral Fe ions, analogous to that seen for [Fe17], 8,13 and consistent with published magneto-structural studies of O-bridged Fe III compounds. The magnetic data can be simulated using exact diagonalisation 18 and an isotropic spin-Hamiltonian 𝐻 ̂= -2 ∑ 𝐽 𝑖𝑗 𝑖<𝑗 𝑠 ⃗ ̂𝑖 • 𝑠 ⃗ ̂𝑗 with a coupling scheme that assumes just two independent exchange interactions, Jring and Jcap, describing the interaction between the octahedral ions in the [Fe6] wheel and between the tetrahedral and octahedral Fe ions, respectively (Figure ). This affords Jring = +4.475 cm -1 and Jcap = -17.376 cm -1 with g = 2.00. This results in a spin ground state, S = 10.