Electronic effects of metal hexacyanoferrates: An XPS and FTIR study

Abstract

A series four metal hexacyanoferrates, with a general formula of KxMy[Fe(CN)6]z·qH2O, with x, y, z and q representing stoichiometric numbers and M = Fe (1), Co (2), Ni (3) and Cu (4), were prepared by a simple co-precipitation reaction. The yields of 1–4 were found to be a function of the Pauling scale electronegativity, σM, the order of increasing yield of the metal hexacyanoferrates were found to be 1 (M = Fe, 26% yield, σM = 1.83) < 2 (M = Co, 43% yield, σM = 1.88) < 4 (M = Cu, 85% yield, σM = 1.90) < 3 (M = Ni, 94% yield, σM = 1.91).Multiple peaks of the cyano-group stretching frequencies, v(C≡N), were observed in the 1900–2200 cm−1 area of the infrared spectroscopy of 1–4. Each stretching frequency at the different wavenumbers represents a different possible combination of the different oxidation states e.g. FeII-C≡N-FeII, FeIII-C≡N-FeII, FeIII-C≡N-FeIII and FeII-C≡N-FeIII. Infrared spectroscopy also confirmed that the prepared metal hexacyanoferrates exhibited interstitial sites where water molecules or potassium ions could be trapped.X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of each metal as well as the different oxidation states in which they occur. XPS was very useful in calculating the ratio between the metals as well as the ratio of each oxidation state of the different metals in 1–4. These ratios were then used to derive the stoichiometry of each metal hexacyanoferrate. All compounds contained FeII and FeIII which delivered 2p3/2 photoelectron lines at ca. 708 eV for FeII and 710 eV for FeIII. Secondary (satellite) peaks were also found in all compounds at a few eV higher than the main 2p photoelectron lines that were ascribed to the charge transfer that exists between the iron and the CN ligand. The XPS spectra of all non-iron metals also showed charge transfer peaks at a few eV higher than the main 2p photoelectron lines.