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

N K and 3d atom L-2,L-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 sp ectra of the complexes are very similar and differ only in intensity of the lowest-energy absorption band. The intensity of this feature systematicall y 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 s pectra of [Co(CN)(6)](3-) and [Fe(CN)(6)](4-) this band is lacking. A simil ar lowest-energy band is also observed in the 3d metal L-2,L-3 absorption s pectra of the complexes. These findings were qualitatively explained in ter ms of pi -back-bonding in the formation of lower-energy unfilled electronic states in the 3d metal cyanide complexes. It was found that the lowest-ene rgy band has a common origin in the N K and metal L-2,L-3 spectra and it is associated with core electron transitions to the partly filled 2t(2g) MO o f these complexes. The occurrence of this band in the N K absorption spectr a can be treated as a direct experimental evidence for the appreciable pi - back-bonding in the 3d metal cyanide complexes. (C) 2001 Elsevier Science B N. All rights reserved.