X-RAY-ABSORPTION PRE-EDGE STUDIES OF HIGH-SPIN IRON(II) COMPLEXES

Fe K-edge X-ray absorption spectroscopy is utilized to study a series of 22 synthetic high-spin iron(II) complexes. The 1s --> 3d pre-edge peak of each complex is quantitated and compared with the others in order to explore its correction with the coordination number and symmetry of the iron center. Like the high-spin iron(III) complexes (Roe,A.L,; Schneider,D.J.; Mayer,R.J.; Pyrz,J.W. Que,L.,Jr. J. Am. Chem. Soc. 1984, 106, 1676-1681), the iron(ll) complexes can be grouped on the basis of their normalized pre-edge peak intensities: the six-coordinate complexes have pre-edge areas from 4 to 6 units, the five-coordinate from 8 to 13 units, and the tetrahedral 16 to 21 units. Three six-coordinate ''iron(II)'' nitrosyl complexes examined have pre-edge areas comparable to those of normal iron(II) five-coordinate complexes due to their highly distorted geometry. The information obtained here can be used to deter;niiie the coordination number of the high-spin iron(lI) centers in iron(II)-containing proteins and other model complexes and to complement analyses based on extended X-ray absorption fine structure (EXAFS).