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

N K and 3d atom L absorption spectra of hexacyano complexes in solid K Fe(CN) , Na Fe(CN) , K Cr(CN) , 2,3 4 6 4 6 3 6 K Mn(CN) , K Fe(CN) , and K Co(CN) have been measured by detecting the total electron yield. The N K spectra of the 3 6 3 6 3 6 complexes are very similar and differ only in intensity of the lowest-energy absorption band. The intensity of this feature 32 systematically decreases and its energy position systematically shifts to lower energies along the [Cr(CN) ] – 6 32 32 32 42 [Mn(CN) ] –[Fe(CN) ] series. In the spectra of [Co(CN) ] and [Fe(CN) ] this band is lacking. A similar 6 6 6 6 lowest-energy band is also observed in the 3d metal L absorption spectra of the complexes. These findings were 2,3 qualitatively explained in terms of p-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 L spectra and it 2,3 is associated with core electron transitions to the partly filled 2t MO of these complexes. The occurrence of this band in the 2g N K absorption spectra can be treated as a direct experimental evidence for the appreciable p-back-bonding in the 3d metal cyanide complexes.  2001 Elsevier Science B.V. All rights reserved.