Hyperfi ne I n teractions 64 (1990) 715- 720 715 MUON SPIN ROTATION (/~SR) IN A METAL-CLUSTER CARBONYL COMPOUND Paul H. DALLIN, Upali A. JAYASOORIYA * School ~1 Chemical Sciences. Universi O' of East Anglia, Norwich NR4 7TJ. U.K. Brian F. JOHNSON, Jack LEWIS University Chemical kahoratories, Unit,ersitv of Cambridge, Lensfield Roa~L Cambridge CB2 and IEI,I" U.K. Stephen F.J. COX, Christopher A. SCOTT Ruthe(/ord Appleton Laboratory, Chilton. Didcot. Oxon OXI I OQ)i; U.K. and Elisabeth MARSEGLIA Cavendish Laboratory, Madinglev Road, Camhridge CB3 0HE. U.K. ,tSR spectra from a metal carbonyl compound are reported for the first time. The metal cluster compound Cs2[OsmC(CO)24] shows a temperature dependent signal for implanted muons with the onset of motional narrowing at ca. 200 K. Also observed is a probable change in the width within the temperature range 40-140 K. A tentative assignment is provided for the muon sites. 1. Introduction The properties of implanted muons in bulk metals have proved useful in understanding the theoretically and industrially important problem of hydrogen in bulk metal systems [1]. The positive muon mimicks the properties of the proton very closely, and the low mass of the muon (mass ratio, mMu/m n -- 1/9) makes it an effective probe of quantum mechanical effects like tunneling. /,SR studies of organic compounds have also proved a fruitful area [2]. However, all attempts at observing a muon spin rotation signal from metal carbonyl compounds have up to now been unsuccessful. For the present investigation, we chose the Os m cluster system (fig. 1). This is sufficiently large to offer four tetrahedral sites to interstitial protons or muons: in this sense it resembles a tiny fragment of metal lattice so that a comparison with localisation and diffusion studies in bulk metals should be of interest. In addition, it offers surface or "bridging" sites. The Cs21Os10C(CO)24] compound was thought to be a favourable candidate for /,SR for several reasons. First, the ,':' J.C. Bahzer A.G. Scientific Publishing Company