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 trispyrazolylmethanecomplexes

Full multiple scattering calculations of the iron K-edge X-ray absorption n ear-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 ri ng, have been carried out. Excellent agreement with the experimental spectr a 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 3 7-atom cluster for {Fe[HC(pz)(3)](2)}(BF4)(2), and when 12, methyl carbon a toms are added to form a 45-atom cluster for Fe[HB(3,5-(CH3)(2)pz)(3)](2) a nd {Fe[HC(3,5-(CH3)(2)-pz)(3)](2)}(BF4)(2). The full multiple scattering ca lculations with an increasing number of near-neighbor shells around the abs orbing iron(II) ion reveal constructive and destructive contributions of su bsequent shells upon the X-ray absorption spectral profiles. These calculat ions (1) permit an unambiguous assignment of specific absorption lines to s cattering by specific near-neighbor shells, (2) reveal that medium-range sc attering processes contribute significantly to the absorption profiles, and (3) permit a correlation between the shift in energy of the absorption edg e upon going from a high-spin to a low-spin complex and the change in the d istance of the first coordination shell around the absorbing iron(II) ion. The results of full multiple scattering calculations place constraints on t he use of Natoli's rule in making assignments of absorption lines to near-n eighbor shells.