Geometry optimised structures and associated spectral properties were calculated using density-functional theory (DFT), using the structure of Fe[Bi(SCN) 6 ] determined from single crystal diffraction as a starting model. Calculations were performed using the plane wave CASTEP DFT code, and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was used with Vanderbilt ultrasoft pseudopotentials. A basis set containing plane waves with energies of up to 800 eV and a Monkhorst-Pack (MP) grid corresponding to a Brillouin zone (BZ) sampling grid finer than 2π× 0.03 Å -1 was used. Geometry optimisations were performed with spin polarisation and spectral calculations were additionally performed with DFT+U. To determine the influence of the value of U on the electronic structure, separate calculations were performed with U = 0 and 2.5-5.5 eV for the Cr d orbitals and U = 0 and 4.0-6.0 eV for the Fe d orbitals in 0.5 eV steps. Changing U values had little impact on band occupancy and the overall structure of the bands, however by changing the energy of the metal d orbitals it was possible to find values of U that reproduced the experimentally observed band gaps, U = 2.5 eV and U = 4.5 eV were chosen for the Cr and Fe d-orbitals respectively. These values are broadly consistent with U values found in previous studies. Density of states for both materials was calculated using OptaDOS using adaptive broadening. Additional information on the calculations, and particularly the variation in U can be found in the supplementary information. ] it was possible to grow diffraction quality single crystals, from which we were able to solve their structures. These structures were used as the starting points for Rietveld refinement of synchrotron powder diffraction data of Cr III [Bi(SCN) 6 ], confirming that despite the differences in solubility, which we ascribe to the kinetic inertness of Cr III complexes, all three compounds adopt isotypical structures consisting of M III N 6 and Bi III S 6 octahedra linked by thiocyanate ligands into a 3D network with pcu topology [Fig. )]. This structure is both analogous to the cyanide Prussian blues structure and can also be described as a double perovskite structure with vacancies occupying the A-site.