X-RAY-ABSORPTION SPECTROSCOPIC STUDIES ON THE IRIDIUM(III) COMPLEXES

The iridium(III) complexes, 1-3, which are the key intermediates for a variety of well established reaction pathways for the metal-catalyzed reactions of alkynes such as oligomerization or polymerization, have been characterized by X-ray absorption spectroscopic analysis. Accordi ng to the Ir L-III-edge X-ray absorption spectra for those complexes, the white lines for all the complexes represent a single peak, which s uggests that the iridium(III) ions in the complexes are in a low spin state. From the least square fittings to XANES (X-ray absorption near edge structure) spectra, it has been found that the white Line positio n for complex 3 is shifted to a lower energy side, and its area is sma ller compared to the others, indicating that the ligand to metal charg e transfer for the former becomes more significant than the latter. Ac cording to EXAFS (extended X-ray absorption fine structure) analyses, the spectral shapes clearly show the structural difference in the comp lexes between 1 or 2 and 3, which is well consistent with XANES result s. The bond distances, d(Ir-C) and d(Ir-N), determined from EXAFS anal yses, are reciprocally proportional to the IR frequencies, nu(C = O) a nd nu(N = C), Of the corresponding Ligands, indicating that the bonds, Ir-C and Ir-N, are competing with the bonds, C = O and N = C. (C) 199 8 Elsevier Science B.V. All rights reserved.