viiib metal i e a metal selected from the group consisting of ruthenium rhodium palladium osmium iridium and platinum or a complex thereof the catalyst material also includes a bromide source this may be a quaternary ammonium or quaternary phosphonium bromide or a hexaalkylguanidinium bromide the guanidinium salts are often preferred they include the u2200 t bis pentaalkylguanidinium alkane salts salts in which the alkyl groups contain 2 6 carbon atoms and especially tetra n butylammonium bromide and hexaethylguanidinium bromide are particularly preferred other catalytic constituents are necessary in accordance with chaudhari et al the constituents include inorganic cocatalysts typically complexes of cobalt ii salts with organic compounds capable of forming complexes especially pentadentate complexes illustrative organic compounds of this type are nitrogen heterocyclic compounds including pyridines bipyridines terpyridines quinolines isoquinolines and biquinolines aliphatic polyamines such as ethylenediamine and tetraalkylethylenediamines crown ethers aromatic or aliphatic amine ethers such as cryptanes and schiff bases the especially preferred inorganic cocatalyst in many instances is a cobalt ii complex with bis 3 salicylalamino propylmethylamine organic cocatalysts may be present these cocatalysts include various terpyridine phenanthroline quinoline and isoquinoline compounds including 2 2 u2032 6 u2032 2 u2033 terpyridine 4 methylthio 2 2 u2032 6.