Due to the relatively small contribution of the guest water to the total electron density (3%) and peak broadening resulting from the small crystallite sizes, it was not possible to localise the guest water from the the Fourier difference map generated from Rietveld refinement of Cr[Bi(SCN) 6 • H 2 O. Calculation of the geometric voids in the empty framework structure did however reveal a single void capable of containing a water molecule [Fig. ]. Rietveld refinement of the framework structure including an oxygen atom situated in that void with its occupancy fixed to the value derived from TGA measurements produced a small improvement in the quality of fit compared to the empty framework. The limitations of the powder diffraction data prevented us from deriving further information about the location of the guest water molecule. This hydrate can therefore be considered to be an intermediate member between the full perovskites NH ]. The rigidifying effect of guests has been noted in both inorganic and molecular framework materials, where even weakly interacting guests can reduce the material's flexibility. The maximum thermal expansion seen in Cr[Bi(SCN) 6 ], of 1.7 % over 250 • C range, is smaller than both the volume change on water uptake (2.3 %) and particularly the volume change relative to the cubic structure aristotype (54 %). This suggests that octahedral tilt transitions to higher symmetry structures are unlikely to occur in this family on heating, even if framework decomposition can be avoided. to absorb strongly, both in the solid state and in concentrated solutions of bismuth thiocyanate (λ max = 472(3) nm), due to LMCT to the empty Bi 6p states. The observed absorption maximum Fe[Bi(SCN) 6 ] occurs at significantly lower energies (λ max = 740(20) nm) than observed for Fe 3+ thiocyanate in solution (λ max = 480(20) nm) indicating that interactions between metal sites are important. This difference between solution and solid-state spectra has also been reported for simple salts of the hexakisisothiocyanato iron(III) complexes, implying that this difference is not solely due to the presence of bismuth. To understand better the optoelectronic properties of these materials we therefore carried out density-functional theory (DFT) calculations on Fe[Bi(SCN) 6 ] and Cr[Bi(SCN) 6 ]. We calculated the electronic band structures using spin-polarised GGA as implemented by CASTEP on geometry-optimised structures, and then used OptaDOS to determine from these results the optical band gap and the electronic density of states, including the atom and angular momentum projected density of states. The calculated optical band gaps were significantly smaller than those observed experimentally 20(5) eV), as is commonly observed in DFT calculations due to the self-interaction error, which is particularly pronounced for the contracted d orbitals. We therefore repeated the calculations of the electronic structure including a Hubbard U which we systematically varied. We found that introducing this additional term into the Hamilitonian did not perturb qualitative features of the electronic structure aside from the energy of the transition metal d states, but did allow us to reproduce the experimentally observed band gaps for both Cr[Bi(SCN) 6 ] (U = 2.5 eV, E calc. density of states suggests that transition is primarily LMCT in character [Fig. )]. The narrow width of the calculated conduction and valence bands for both compounds indicates that that the near band-edge states are highly localised, and so the effective masses of holes and electrons will be very large [Fig. )]. This is also borne out by examination of the Kohn-Sham orbitals at the valence band maximum, which are localised and confined to the ligands [Fig. ]. The lack of extensive delocalisation suggests that these frameworks will be more suited to applications such as photocatalytic rather than those that rely on bulk electron transport. Photoluminescence measurements on both single crystal and powder samples showed negligible fluorescence, suggesting that there is are a high concentration of optical defects, borne out by the significant band-tails observed in both absorption and reflectance measurements.