The Chemical Information Contained in the EXAFS Debye-Waller Factor: Covalency in Iron(III) Bis-diketonato Complexes

A new procedure to extract chemical information from EXAFS spectroscopy beyond the classic parameters, such as coordinating atoms, coordination numbers, and bond distances, by thorough analysis of the Debye-Waller-like factor is presented. In a comparative study of different iron(III) bis-diketonato complexes, which are highly relevant for the iron-catalyzed Michael addition reaction, the degree of covalency of the Fe-O bond between the iron center and the ligand is determined by a combined EXFAS, Raman, and UV/vis spectroscopic approach. A detailed description of the methodology is presented, and it will demonstrated that the comparative degree of covalency is correlated to the pK(a) value of the coordinating diketone. Moreover, it is shown that the distinct catalytic activity of the diketones can not be explained by differences of the covalency of the iron-ligand Fe-O bond.