Electronic structure of cyanide and hexacyanoiron(II) anions using X-ray emission and X-ray photoelectron spectroscopies

Abstract

X-ray emission (X.e.) and X-ray photoelectron (X.P.) spectra have been recorded for sodium and potassium hexacyanoiron(I1) and also for potassium cyanide. Core-orbital photoelectron data enables the X-ray emission spectra and the valence-band photoelectron s ectrum to be aligned on a common energy scale. Peak coincidence in X.p. and X.e. spectra of the Fe complex identified the least tightly bound orbitals as predominately Fe 3d and showed that the group orbitals with the next highest ionisation energy had not only N 2 p and C 2 p (as in CN-) character but also Fe 3d and Fe 4p character due to covalent ligand-iron bond formation. Relative peak intensities in the X.P., and absences in the X.e., spectra indicated that the main peak in the valence-band region was composed of 2s orbitals (presumably both C and N) but that the ligand molecular orbitals giving rise to this peak played little part in bonding to iron. Comparisons of molecular-orbital ionisation energies and atomic-orbital coefficients with theoretical calculations showed only qualitative greement.

Fe L-Edge XAS Studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A Direct Probe of Back-Bonding

“…The VBCI analysis of the L-edge of K 3 [Fe(CN) 6 ] gives the π* orbital as having 14(2)% metal character. These values are in the same range as those predicted by calculations reported both here and elsewhere. ,,,, They also agree well with the more qualitative results of other experimental techniques, including X-ray crystallography, − absorption, and IR spectroscopy. ,,, We also note that there are other methods that could potentially quantify the amounts of back-bonding, including ligand K-edge XAS and XPS through shake-down satellites; however, these have not yet been developed sufficiently for this purpose. ,,

12 Effect of back-bonding on an L-edge: (A ) no back-bonding (pure Fe(II) low-spin (t 2g ) 6 ground state); (B ) π back-bonding in the ground state; and (C ) back-bonding in the ground and excited states.

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“…Many techniques have been applied to understand the nature of metal−ligand back-bonding, yet all are beset by complications when trying to separate the effects of σ donation from those of π back-donation. − The Fe L-edge is the consequence of a 2p → 3d transition. This transition is electric dipole allowed, so the intensity arises from metal d character in unfilled valence orbitals .…”

“…A function of the form absorption = [tan -1 ( k (energy − I 1 ) + π/2)(2/3)(1/π)] +[tan -1 ( k (energy − I 2 ) + π/2)(1/3)(1/π)], where k = 0.295 (obtained by experimental fit) , and I 2 = I 1 + 12.3 eV (due to spin−orbit coupling), was used to model the L 3 - and L 2 -edge jumps, as described previously . For the K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] data, the absolute energy of the arctangent was estimated on the basis of a combination of photoelectron spectroscopy (PES) data and a fit to the L-edge spectra. ,, The post-normalization L 3 intensities reported here were calculated as the intensity in the range 700−715 eV for [Fe(tacn) 2 ]Cl 2 , 703−718 eV for K 4 [Fe(CN) 6 ], 701−716 eV for [Fe(tacn) 3 ]Cl 3 , and 702.5−717.5 eV for K 3 [Fe(CN) 6 ], and the L 2 intensities for the normalized ranges were 715−730 eV for [Fe(tacn) 2 ]Cl 2 , 718−733 eV for K 4 [Fe(CN) 6 ], 716−731 eV for [Fe(tacn) 2 ]Cl 3 , and 717.5−732.5 eV for K 3 [Fe(CN) 6 ]. The different energy ranges were used to account for differences in spectral shift.…”

“…The model most widely invoked to describe such bonding interactions is that of Dewar, Chatt, and Duncanson, − in which σ donation and π acceptance are described as having a synergistic bonding effect (Figure ). Homoleptic octahedral complexes of transition metals have played a fundamental role in developing many ideas in coordination chemistry because their molecular orbital and bonding picture is simplified by symmetry. − Among the most widely investigated systems that display back-bonding are the iron(II) and iron(III) hexacyanides. −

1 Schematic illustration of the Dewar−Chatt−Duncanson model for back-bonding.

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“…Second, while σ donation and π back-donation have opposite effects on the C−N bond strength, their relative magnitudes cannot be uncoupled from the net vibrational frequency. So, as with absorption data, evidence for back-bonding is found from vibrational spectroscopy, but it does not quantify the relative contributions of σ donation and π back-donation. − Further, these techniques do not directly allow a comparison of the contributions of π back-bonding between two compounds with different ligand sets.…”