Importance of photoelectron multiple scattering in the iron K-edge X-ray absorption spectra of spin-crossover complexes: Full multiple scattering calculations for several iron(II) trispyrazolylborate and trispyrazolylmethane complexes

Full multiple scattering calculations of the iron K-edge X-ray absorption near-edge spectra of the iron(II) low-spin complexes, Fe[HB(pz)(3)](2) and {Fe[HC(pz)(3)](2)}(BF4)(2) and high-spin complexes Fe[HB(3,5-(CH3)(2)pz)(3)](2) and {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}(BF4)(2). where pz is the pyrazole ring, have been carried out. Excellent agreement with the experimental spectra is achieved when a basic 33-atom cluster, corresponding to FeB2C18N12, is used for Fe[HB(pz:)(3)](2). when four fluorine atoms are added to form a 37-atom cluster for {Fe[HC(pz)(3)](2)}(BF4)(2), and when 12, methyl carbon atoms are added to form a 45-atom cluster for Fe[HB(3,5-(CH3)(2)pz)(3)](2) and {Fe[HC(3,5-(CH3)(2)-pz)(3)](2)}(BF4)(2). The full multiple scattering calculations with an increasing number of near-neighbor shells around the absorbing iron(II) ion reveal constructive and destructive contributions of subsequent shells upon the X-ray absorption spectral profiles. These calculations (1) permit an unambiguous assignment of specific absorption lines to scattering by specific near-neighbor shells, (2) reveal that medium-range scattering processes contribute significantly to the absorption profiles, and (3) permit a correlation between the shift in energy of the absorption edge upon going from a high-spin to a low-spin complex and the change in the distance of the first coordination shell around the absorbing iron(II) ion. The results of full multiple scattering calculations place constraints on the use of Natoli's rule in making assignments of absorption lines to near-neighbor shells.