Polyhedron Vol. 8, No. 6, pp. 801404, 1989 Printed in Great Britain 0277-5387/89 $3.00 + .Xl 0 I989 Pergamon Press plc zyxwvutsr A PRESSURE TUNING SPECTROSCOPIC STUDY OF THE SULPHIDO-METAL CLUSTERS, M&-S),(PEt&+ (M=Fe, n=2; M=Co, n=l) JEFFERY L. COFFER, JOHN R. SHAPLEY* and HARRY G. DRICKAMER* School of Chemical Sciences, Department of Physics, and Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, IL 61801, U.S.A. zyxwvutsrqponmlkjihg (Received 11 August 1988 ; accepted after revision 13 October 1988) Abstract-The effect of pressure on the visible absorption spectra of the sulphido-metal clusters, M6(p3-S)S(PEt3)n6+ (M = Fe, n = 2; M = Co, n = 1) has been measured in both crystalline and polymethylmethacrylate environments up to 171 kbar. The broad absorption at ca 15.4 kK (650 nm) for each cluster has been deconvoluted into three separate peaks. In the case of the iron cluster, tentative assignments of the transitions are made on the basis of their shifts with pressure in conjunction with a previously-reported SCF-Xcr calculation. The cobalt cluster shows a much weaker pressure dependence, which is attributed to weaker metal-metal interactions and more delocalized ligand character in the frontier orbitals. Extensive interest in the chemistry of sulphido metal clusters has developed in recent years, due mainly to their significance as models for hetero- geneous metal-sulphide catalysts’ or the active sites of certain redox proteins.2 In particular, Ghi- lardi and co-workers have succeeded in the synthesis and structural characterization of octahedral, para- magnetic clusters of the type M&-S)B(PEt&+ (M = Fe, n = 2 ; M = Co, n = 1, 0). 3,4 These clus- ters are specifically noteworthy due to their struc- tural similarity to clusters present in the bulk metal sulphides known as the Chevrel phases, some of which become superconducting upon the appli- cation of modest pressures. 5 The electronic structure of these molecular clus- ters is an active issue. For Fe&3-S)8(PEt3)i+, mag- netic susceptibility and EPR measurements suggest a ground state electron configuration with six unpaired electrons.3 This is at odds with a SCF-Xa calculation that indicates only four unpaired spins, with the highest occupied level predominantly Fe-Fe bonding. The analogous cobalt cluster, with seven additional valence electrons, is also para- magnetic4 but details of its magnetic properties or electronic structure are presently unknown. In an effort to cast light on the nature of the bonding in these systems, a study of the effects of pressure on the visible electronic spectra of M6 *Authors to whom correspondence. should be addressed. (p3-S)g(PEt);+ (M = Fe, n = 2 ; M = Co, n = 1) was undertaken. Pressure tuning spectroscopy (PTS) has been demonstrated to be a powerful means of probing the electronic structure of various molecules, including metal clusters.6 Changes in static pressure may effect dramatic changes in the electronic energy levels, with such changes highly dependent upon the nature of the orbital inter- actions in the molecule. EXPERIMENTAL [M,(~3,-S),(PEt,),I[B(C,H,),1, (M = Fe, n = 2 ; M = Co, n = 1) were prepared by literature methods. 3,4 Crystalline samples of each were exam- ined using Nujol as a pressure transmitting medium. Both compounds were also studied in a polymeric environment by dissolution in poly- methylmethacrylate (PMMA). Each sample was loaded into a diamond anvil cell (DAC). All runs were reproduced at a minimum of two separate loads. The ruby fluorescence method was utilized for pressure calibration. 7The light source employed was an Oriel tungsten halogen lamp, dispersed by a Kratos l/4 m monochromator using either a 1000 or 450 blaze grating. Light in the visible region was detected by EMI 9558 or 9658 photomultiplier tubes, along with an Ortec photon counting system. Further details concerning the optical system may be found elsewhere. * 801