Prussian Blue Analogue CsFe[Cr(CN)₆] as a Matrix for the Fe(II) Spin-Crossover

The origin of the intriguing spin-transition behavior of the Prussian blue analogue cesium iron hexacyanochromate CsFe[Cr(CN)₆] has been investigated by means of correlated ab initio CASPT2 calculations. Using the smallest transiting core [Fe(NC)₆]^4-, the relative importance of the local ligand field and the Madelung field generated by the rest of the crystal was estimated. It is shown that in the presence of a frozen-charge environment, the high-spin state lies lower in energy than the low-spin state, thus excluding the possibility of observing a spin transition. In contrast, the charge reorganization in the environment evaluated from unrestricted periodic Hartree−Fock calculations creates a prerequisite for the spin-transition phenomenon. The influence of the disorder in the cesium ions' positions on the spin transition has been examined as a possible stabilizing factor of the low-spin state of [Fe(NC)₆]^4-. It is concluded that this experimentally observed disorder cannot account solely for the unprecedented behavior of the CsFe[Cr(CN)₆] compound.