Multiplet Effects in X-ray Absorption - Inorganic Chemistry and ...

F. de Groot / Coordination Chemistry Reviews 249 (2005) 31–63 57Fig. 24. Essential features of the Ni–X–Fe 4 S 4 structures that have beenproposed for CODH (reprinted with permission from [61], copyright 2000American Chemical Society).Hitchcock’s group is well known for their core levelEELS studies, in particular on organic molecules [57]. Wewould like to mention their study on iron-carbonyl andiron–ferrocene complexes in the early nineties [58,59]. Atthat time the multiplet model was just being developed andcharge transfer not yet included. In addition, multiplet theory(in its simplest form) is best suited to describe oxidesand halides. In particular the ferrocene have little resemblanceto the ligand field multiplet spectra, as they wouldneed a much better description of the covalence effects andof -(back)bonding. These effects are not yet included intothe multiplet model, though some progress is being madeas will be discussed below.The group of Cramer applied 2p XAS including its MCDeffect to metallo-enzymes and related model compounds[60–63]. The MCD experiments are discussed in the reviewof Cramer, elsewhere in this issue. As an example, the2p XAS spectra of Carbon monoxide dehydrogenase fromClostridium thermoaceticum (Ct, CODH) and Rhodospirillumrubrum (Rr, CODH) will be discussed [61] (Fig. 24).The electronic structure of the nickel site in this enzymeis studied in comparison with a large number of modelcompounds. The 2p XAS spectra of the model compoundsare analyzed taking two parameters as measure (1) thecenter-of-gravity of the L 3 edge and (2) the branching ratio.The branching ratio is defined as the integrated intensity ofthe L 3 edge divided by the integrated intensity of the whole2p XAS spectrum, i.e. the L 3 and L 2 edge added. The valueof the branching ratio without multiplets is 2/3, given bythe degeneracy of the L 3 and L 2 edges. Because the multipleteffects are of the same order of magnitude as the 2pspin–orbit coupling the branching ratios are modified bymultiplet effects, and the details of the effect are stronglyinfluenced by the spin state of the metal. This makes thebranching ratio to a tool to determine the spin state oftransition metals, as can be seen from Fig. 25.From Fig. 25, it can be observed that the early transitionmetal ions have branching ratios that are strongly deviatingFig. 25. The branching ratio of transition metal ions (given by theirformal number of 3d-electrons) as predicted from crystal field multiplettheory. High-spin states are given by the solid line; low-spin is givenby the dashed line. This picture will be modified by lower symmetriesand charge transfer (reprinted with permission from [12], copyright 1994Elsevier Science).from 2/3 and that low-spin compounds have relatively largeL 2 edges. MnO and other 3d 5 compounds have the highestbranching ratio of approximately 0.85.Fig. 26 shows the analysis of the nickel model compoundsfor their energy position and branching ratio. It canbe seen that the various valences and spin-states can be dis-Fig. 26. Two-dimensional correlation of branching ratio vs. L 3 centroidposition for a set of Ni model compounds with different oxidation stateand spin states. Key: Ni I filled circles; low-spin Ni II inverted triangles;high-spin Ni II upright triangles; low-spin Ni III black diamonds; high-spinNi III open circle; Ni IV square. Ellipses are drawn only to guide the eye(reprinted with permission from [61], copyright 2000 American ChemicalSociety).