The iridium clusters [Ir-4(CO)(11)PPh2H], [Ir-4(CO)(8)(mu(3)-eta(2)-HCCPh)(mu-PPh2)(2)], [Ir-4(CO)(9)(mu(3)-eta(3)-Ph2PC(H)CPh)(mu-PPh2)] and [Ir-4(CO)(12)] were investigated as catalyst precursors for the selective hydrogenation of 1,5-cyclooctadiene (1,5-COD). The results showed that these clusters have a special catalytic behaviour producing high activity (average TON 2816) and high selectivity for the monohydrogenated product cyclooctene (COE) and the isomerisation products 1,3-COD and 1,4-COD, with almost complete suppression of the total hydrogenation reaction to cyclooctane (COA). When other iridium based catalyst precursors were used, however, e.g. Vaska's compound [IrClCO(PPh3)(2)], the dinuclear [Ir2Cl2(COE)(4)] and a heterogeneous catalyst composed of Ir metal supported on activated carbon, formation of the fully hydrogenated product COA was observed instead. The participation of the lr(4) clusters in the catalytic reaction rather than cluster fragments or highly dispersed Ir metallic particles is supported by experiments of light scattering of the cluster solutions after the reaction, addition of metallic Hg and filtration in celite followed by reuse of the solution and the celite for catalytic reactions. Reactions with cyclooctene, cyclohexene and 1,3-COD showed that the monomuclear precursor [IrClCO(PPh3)(2)] completely hydrogenates these substrates whereas all Ir-4 clusters exhibit very low activities. Based on experiments with Shapley's compound, [Ir-4(CO)(5)(C8H12)(2)(C8H10)], and literature results, an "anchor-type" interaction between 1,5-COD and the Ir-4 species is proposed, in which one double bond anchors the 1,5-COD molecule for the activation of the other double bond in a Ir-4-diolefin eta(2)-eta(2)-COD type activated species, which can lead to hydrogenation or isomerisation. (C) 2004 Elsevier B.V. All rights reserved.