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13C N.m.r. spectra of the compounds [Os,(CO),,-,(PEt,),](n = 1 or 2) indicate that carbonyl exchange occurs selectively along one edge of the Os, triangle at low temperatures. The process involves only six of the 12n carbonyl groups in [Os,(CO),,-,(PEt3),](n = 1 or 2). and a mechanism involving a double carbonyl bridge is proposed.Laboratory, Lensfield Road, Cambridge CB2 1 EW A CARBON-^^ n.m.r. study of isotopically enriched [Os,(CO),,] from ca. 70-180 "C showed that rapid intramolecular scrambling of the equatorial and axial carbonyl groups 0ccurs.l At least three mechanisms may be proposed for this process, two involving the formation of CO bridges and the third a polytopal rearrangement about individual osmium atoms. It is not possible to differentiate between the mechanisms on the evidence a t present available. We have now undertaken a variabletemperature 13C n.m.r. study of some phosphine-substituted trinuclear osmium carbonyls in order to obtain further information about the mechanisms of carbonyl exchange in cluster compounds, although it must be borne in mind that the introduction of the phosphine may give rise to a different mechanism from that occurring in the parent carbonyls. There is evidence that the introduction of tertiary phosphines into polynuclear carbonyl clusters induces the formation of CO bridges2 I t therefore seemed reasonable to suppose that phosphinesubstituted derivatives [0s,(C0)1~-~(PEt,),] would more readily undergo CO scrambling by a CO migration process.However, in addition, and more interestingly, we find that for the clusters [Os3(CO)1z-,(PEt,),] (n = 1 or 2) two distinct fluxional processes occur at different temperatures. This has been found to be the case. RESULTS AND DISCUSSIONAt -60 "C the 13C n.m.r. spectrum of the trisubstituted species [OS,(CO),(PE~,),] (in toluene) showed two 13C carbonyl peaks at 6 199.4 and 186.4 p.p.m. (downfield from SiMe,) with an intensity ratio of ca. 2 : 1. The lower-field peak was split, but this is not clearly resolved.

terised anionic species (see Scheme 2). Acidification leads to the isolation of the hydridocarbonyls, H,Os,(CO),,.The tetranuclear compounds were obtained in very small yields and are similar to the ' F. Calderszzo and F. L'Eplattenier, I m v g .

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The magnetic susceptibilities of four iron(ll1) and five ruthenium(ll1) compounds, all with the low-spin configuration P2,, have been measured in the range 80-300" K. The data are compared with the theory for a 2T,, ground term in the presence of a low-symmetry ligand-field component, and including electron delocalisation. The splitting of the orbital degeneracy of the term is estimated to be about 500 cm.-l in each of the iron compounds, but varies from 0 to 1000 cm.-l in the ruthenium compounds. The amount of t,, electron delocalisation is small in the iron complexes, and in all but one of the ruthenium complexes.THE effect of Iow-symmetry ligand fields, spin-orbit coupling, and electron delocalisation on the magnetic behaviour of transition-metal ions which have a 2T2u in Figure 1. As in previous Communications,l*2 A is defined as the orbital splitting of the 2T2s term by the axial component of the ligand field, in the absence of ground term in a cubic ligand-field has been reportedi an orbital singlet and an orbital doublet as illustrated