An Investigation of Prussian Blue Analogues by Mc)ssbauer Spectroscopy and Magnetic Susceptibility

The Miissbauer spectra and magnetic susceptibilities have been obtained for a series of Prussian Blue analogues of general formula MjAIMB(CN)&.mHzO where MA and MB are transition metal ions, j and k vary with the oxidation states of MA and MB and m typically has values from 8 to 14. The compounds were prepared from the hexacyano acids or with large quaternary ammonium counterions and are therefore not contaminated with alkali cations. In each analogue, A or B is iron and the second metal is Mn, Cu, Co, Cr or Ru. In each case it was possible to assign the site (A or B), oxidation state and spin state to each transition metal ion. This group of compounds are all class II mixed valence species from their colours, but do not show evidence of linkage isomerism or redox changes compared to the starting materials. The Miissbauer linewidths are consistent with the Ludi model of Prussian Blue. The structure of Prussian Blue, Fe,[Fe(CN)& x H,O, prototype of a series of polymeric cyanides, has been the object of intensive investigations.’ Historically, it was the first documented synthetic coordination complex and its unexpected deep blue colour has triggered many investigations. Many of these studies have attempted to discern whether there were discrete oxidation states or valence equivalency for the structurally different iron atoms. The bulk of information accumulated by X-ray diffraction,2-4 infrared spectroscopy,s*6 magnetic susceptibility,5,7*8 electronic structure,’ Miissbauer studies,‘&i6 photoelectron spectroscopy,‘7*‘8 and neutron diffraction studies19 support the assignment of discrete oxidation states as