Volume 184,number 5,6 CHEMICALPHYSICSLETTERS 4 October 1991 Multiple-scattering analysis of the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM of Os3 (CO) 1 2 carbonyl cluster X-ray absorption spectrum A. Filipponi zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Diparlimenfo di Fuica, Clniversrtb degS Studi dell’ dquila, Via Vetolo, 67010 Coppito - L’Aqurla, Italy A. Di Cicco Diparfimento di Matematlca e Fisica, Universit& degh Stud1 di Camermo, Via Madonna deile Carceri, 63032 Camerino (MC), Italy R. Zanoni, M. Bellatreccia Diparfimento di Chimica, Universitb di Roma “ La Sapienza’: Piazza/e Ado Moro 5, 00185 Rome, Italy V. Sessa Dipartimento di Science e Tecnologie Chimiche, II Universiiridi Roma “ Tor Vergata’: Via 0. Raimondo, 00173 Rome, Italy C. Dossi and R. Psaro Dipartimenfo di Chimica lnorganrca e Metallorganica and Centro CNR, Universitb dl Milano, Via G. Venezian 21, 20133 Milan, lta/y Received 4 June 1991; in final form 8 July 1991 A new theoretical approach has been used to analyze the X-ray absorption cross section of Os>(CO) I? carbonyl cluster. The method is based on a direct tit of the experimental cross sectton which makes use of theoretical signals associated with relevant atomic configurations in the molecule. In the case of the OS-C=O arrangement, it has been found that the commonly employed multiple-scattering series converges slowly at low k values; the calculation has, therefore, been performed using a more efficient continued-fraction algorithm that provides the “exact” signal summed to all orders. An excellent agreement in the range k= 2.7- 14.0 A-’ has been obtained, basically using X-ray diffraction results. Molecular metal clusters provide the highest size- selected way of investigating both size and adsorp- tion effects. Metal carbonyl clusters represent the largest known class of binary molecular clusters. Sev- eral different geometries have been reported: car- bony1 clusters with terminal, edge- or face-bridging CO ligands have been described. X-ray absorption spectroscopy (XAS) has been widely applied to pure and supported molecular clusters, particularly to metal carbonyl clusters [ I- 111. The advantages of performing data analysis by using the curved-wave formalism [ 121, have been pointed out in a study on several OS carbonyl clus- ters [ 1,2 ] where the sensitivity of XAS to locate non- bonded metal-metal distances was assessed. This analysis produced particularly satisfactory results in the case of larger clusters, where the number of metal neighbors exceeds the C=O contribution that cannot be reproduced appropriately by using a single-scat- tering (SS) theory. The necessity to include multi- ple-scattering (MS) effectsup to the triple-scattering order has been emphasized in a more recent paper [ 9 ] on cobalt carbonyl complexes. A detailed anal- ysis of the MS effects as a function of the Co-C=0 angle was presented, showing the possibility of dis- tinguishing between bridging and terminal carbonyl groups. A detailed description of the fitting proce- dure using the EXCURVE program and based on ef- 0009-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved. 485