While extended X - ray absorption fi ne structure (EXAFS) in general does
not give direct information about the geometry of ligand atoms about the
metal center, it is possible to relate specifi c X - ray absorption edge behaviors
to geometry for nickel(II) compounds, for instance. In the examples discussed
by Scott in Figure 13 of reference 4 , Ni(II) complexes with square - planar ( D 4 h )
symmetry give rise to a K edge spectra with a large characteristic pre - edge
peak about 5 eV below the main edge. The peak is assigned to a 1 s → 4 pz
transition with simultaneous ligand→ metal change transfer (LMCT) to a
Ni(II) 3 d orbital. Because neither octahedral nor tetrahedral Ni(II) complexes
show this peak (but have their own characteristic spectral behavior), one can,
in combination with information about ligands and bond lengths from EXAFS,
assign metal complex geometry. Figure 13 of reference 4 also shows another
typical behavior: Soft ligands result in lower edge energy. Information about
the oxidation state of a metal ion can also be gathered from edge and pre - edge
behavior. Cu(I) two - and three - coordinate complexes (tetrahedral Cu(I) com-
plexes do not show the behavior) exhibit a pre - edge peak that disappears upon
oxidation to Cu(II), allowing for quantitative determination of Cu(I) content
in these systems.^5
3.2.2 Descriptive Examples,
Quantitative information about the fi rst coordination sphere structure depends
on analysis of EXAFS data. From analytical data or knowledge of common
ligands in metalloenzymes (N, O, S, Se) one can decide which ligands are likely
to be present in the coordination sphere. An example discussed by Scott^4 tests
the hypothesis of a Cu(II) – S bond being present in the compounds shown in
Figure 3.3.
Fourier transformation of Cu EXAFS data gathered on the Cu(MPG)
complex reveals two separate peaks representing shells at distances of 1.9 and
2.3 Å. When tested for Ns (coordination number), metal – ligand distance ( Ras )
and Debye – Waller parameter difference ( Δσas^2 ) followed by comparison to
known model compounds, results show that the presence of both a Cu – (N,O)
Figure 3.3 Cu(II) complex with potential chelating ligand mercaptopropionylglycine
(MPG).
S
H 3 C
N
O
O
O
OH 2
[Cu(MPG)(H 2 O)]-
Cu
XAS AND EXAFS 81
