Observation of back-donation in 3d metal cyanide complexes through N K absorption spectra

Abstract

Abstract N K and 3d atom L2,3 absorption spectra of hexacyano complexes in solid K4Fe(CN)6, Na4Fe(CN)6, K3Cr(CN)6, K3Mn(CN)6, K3Fe(CN)6, and K3Co(CN)6 have been measured by detecting the total electron yield. The N K spectra of the complexes are very similar and differ only in intensity of the lowest-energy absorption band. The intensity of this feature systematically decreases and its energy position systematically shifts to lower energies along the [Cr(CN)6]3−–[Mn(CN)6]3−–[Fe(CN)6]3− series. In the spectra of [Co(CN)6]3− and [Fe(CN)6]4− this band is lacking. A similar lowest-energy band is also observed in the 3d metal L2,3 absorption spectra of the complexes. These findings were qualitatively explained in terms of π-back-bonding in the formation of lower-energy unfilled electronic states in the 3d metal cyanide complexes. It was found that the lowest-energy band has a common origin in the N K and metal L2,3 spectra and it is associated with core electron transitions to the partly filled 2t2g MO of these complexes. The occurrence of this band in the N K absorption spectra can be treated as a direct experimental evidence for the appreciable π-back-bonding in the 3d metal cyanide complexes.