REACTIONS OF THE COORDINATIVELY UNSATURATED CLUSTER [OS3(MU-H)(CO)8(PH2PCH2P(PH)C6H4)] WITH ALKYNES RCCR (R = PH, C6H4ME, ME OR CF3) - CRYSTAL-STRUCTURES OF [OS3(CO)8(MU-3-ETA-2(PARALLEL-TO)-RC2R)(PH2PCH2PPH2)] (R = ME OR CF3) AND ELECTROCHEMICAL-BEHAVIOR OF TRIOSMIUM ALKYNE CLUSTERS

Treatment of the unsaturated triosmium cluster [Os3(mu-H)(CO)8{mu-Ph2PCH2P(Ph)C6H4}] 1 with diphenylacetylene leads to the formation of [Os3(CO)7(PhC2Ph)(Ph2PCH2PPh2)] 2a in which the alkyne is bonded in a mu3-eta2 (perpendicular-to) mode. Addition of CO to 2a gives [Os3(CO)7(mu-CO)(RC2R)(Ph2PCH2PPh2)] 3a (R = Ph) in which the alkyne is bonded in a mu3-eta2 (parallel-to) mode. Two-electron electrochemical reduction of 2a also causes this change in co-ordination mode of the alkyne ligand. Protonation of the electrochemically produced dianion [Os3(CO)7{mu3-eta2(parallel-to)PhC2Ph}(Ph2PCH2PPh2)]2- gives [Os3(mu-H)2(CO)7{mu3-eta2(parallel-to)PhC2Ph}(Ph2PCH2PPh2)] 4. The same product 4 has also been obtained by direct hydrogenation of 2a. A similar chemical and electrochemical behaviour is observed for the ditolylacetylene derivative [Os3(CO)7(p-MeC6H4C2C6H4Me-p)(Ph2PCH2PPh2)] 2b, where the two p-methyl substituents are diagnostic in H-1 NMR spectroscopy. With the alkynes RC=CR (R = Me or CF3), analogous complexes to 2 are not observed, instead complexes [Os3(CO)8{mu3-eta2(parallel-to)-RR}(Ph2PCH2PPh2)] 3c (R = Me) and 3d (R = CF3) are obtained. Single-crystal X-ray structure determinations of 3c and 3d have been performed.