Palladium carboxylates were hydrogenated in the presence of tertiary phosphines (P/Pd = 1 1) under ambient conditions. Phenyl-substituted phosphines gave benzene and palladium clusters with P-Pd covalent bonds, e.g. [Pd5(PPh)2(PPh3)2], while the products from more basic trialkylphosphines proved to be palladium hydride clusters. The former systems were active hydrogenation catalysts for various alkynes and conjugated dienes, and the latter catalysed the hydrogenation of 1-alkenes as well. Only traces of alkanes could be detected as long as alkynes or dienes were present, and internal alkynes gave cis-alkenes selectively.
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