A survey has been made to identify those metal ions that will displace hydrogen ions from benzene with formation of arylmetal complexes. Certain stable complexes were isolated while the formation of thermally unstable complexes was detected by the appearance of biphenyl. An isotopic tracer method was used to seek hydrolytically unstable complexes. Gold(rll), mercury(ll), and thallium(lll) react with benzene in aqueous acid solution while gold(lll). rnercury(ii), palladium(ll), platinum(ll), and thallium(tll) react in acetic acid solution. Reactivity is associated with ' class b ' character. A more detailed study of the reaction of diacetatopalladium with benzene and toluene in 0.5N-perchloric acid in acetic acid was made. It appears to be an electrophilic substitution that yields arylpalladium complexes. These decompose by a concerted mechanism to give biaryls and either palladium metal or a univalent palladium complex depending on temperature. An explosive purple complex, [Pd(C,H,) (H,O)CIO,], has been isolated. The ultraviolet absorption of this compound at 380 and 550 mp has been used to determine its rate of appearance in solution during the reaction of benzene with diacetatopalladium in perchloric acid-acetic acid at 50". The kinetic data are consistent with a second-order reaction between benzene and monomeric palladium(ll) (k2 = 4.6 x 1 0-4 I. mole-lsec.-l) followed by fast decomposition. For deuteriobenzene, a large primary isotope effect (kH/kD = 5.0) was found.THE potentialities of organometallic reactions for reactions would be found. We now report a study of the industrial catalytic syntheses are important. There direct formation of metal-carbon bonds and especially have been efforts to elucidate the chemistry of an element palladium-carbon bonds in aromatic systems ; later such as nickel or palladium in the hope that useful A. Aguilo in ' Advances in ~~~~~~~ Chemistry,' ed.

The acetoxylations of Pb(OAc), and Pd(OAc), have been examined and extended to include benzene, toluene, xylenes, and hexamethylbenzene as substrates. Acetic acid was used as solvent with various concentrations of acetate ion. The reactions in strongly basic solution are inhibited by oxygen and for these a radical-chain reaction scheme has been suggested in which Pb"' and Pd' are chain-carrying intermediates. Careful exclusion of oxygen allows the reactions of Pb(OAc), to take place at a much lower temperature than has been reported hitherto. Under conditions of inhibition by oxygen or in acid solution other reaction pathways become dominant. A brief comparison is made with the acetoxylations of cerium(1v). manganese(ll1). and mercury(l1).I N Part I some electrophilic substitutions of a group of ' class b ' metal ions were described. A corollary of this electrophilic character is the production of oxidised species in addition to those that follow upon simple metal-for-hydrogen displacement. This paper is concerned particularly with the acetoxylations of diacet at o palladium and tetra-acet at olead and with methylations due to oxidation of the acetate ion.The oxidations of Pd(OAc), are of fairly recent discovery3p4 and are important because the ready reoxidation of palladium metal allows fully catalytic syntheses even on an industrial ~c a l e . ~~~The acetoxylations of Pd(OAc), show a close relationship to those of Pb(OAc),' and therefore these have been studied together. Undoubtedly an important common feature of these oxidising agents is the low stability of the next lowest oxidation state. Any mechanistic scheme must take account of this fact and in the present case complexes derived from odd valencies appear to take part in a chain reaction that is very sensitive to the presence of oxygen as a scavenger.8 By contrast cerium(1v) ammonium acetate and triacetatomanganese are reduced to a valency one unit lower which is stable and nonoxidising. The acetoxylations of these reagents are not chain processes. In each case the facts can be accounted for by assuming electron transfer between aromatic substrate and oxidising ion, as suggested by Andrulis et aZ.9 for Mn(OAc),.The observed similarity in the reactivity of Pb(OAc), and Pd(OAc), in acetic acid is especially marked for the side-chain reactions of the methyl group as in toluene. The course of each reaction is diverted by perchloric acid, yielding other products, and both reactions are almost completely inhibited by oxygen. However, lead(1v) readily oxidises acetate ions to a radical that decomposes to carbon dioxide together with methyl radicals. The latter undergo substitutions and abstractions that are not found for Pd(OAc),. The free