The More Elegant Route

Studies concerned with the problem of how the binuclear products of Eqs. (4) and (5) are formed led to an alternative synthetic route which proved to be of general importance. As already mentioned, the first step in the reaction of CsH5M(all) complexes with L (a donor molecule) undoubtedly involves formation of the mononuclear complexes C5H5M(all)L, some of which for both Pd and Pt can be isolated and characterized (13,14). These primary products react with a second molecule of L to give either directly MLj and CsHs(all) or a further intermediate C5H5M(all)L2 [e.g., for L = P(OR)3 (9)] which in most cases is very shortlived and gives MLj by loss of allylcyclopentadiene. In the presence of L, the 14-electron complex MI produces ML, or ML4 (depending on the 

The more elegant route—which we call for simplicity the method of [1 + l]-addition —may also be used to prepare other binuclear complexes containing either two allyl groups, or one allyl group and one halogen in the bridging positions. The results so far obtained are summarized in Eqs. (9) and (10). Compounds 45-47 are the first allyl-transition metal complexes in which not one metal atom but a dimetal unit is coordinated sandwich-like by two i73-bonded allylic ligands (75, 79). 

Compound Pd(P-t-Bu3)2, which is too inert to react with C5H5Pd(2-MeC3H4) or Pd(2-MeC3H4)2 to give a binuclear product, reacts with [(2-MeC3H4)PdCl]2 to form 50. In this stable compound the two metal atoms are well protected by the bulky tri-/er/-butylphosphine ligands. 

The preparation of the compounds (2-RC3H4)(MeCOO)Pd2L2 was achieved by the reaction of the acetate-bridged palladium(II) complexes [(2-RC3H4)Pd]20t-MeCOO)2 with PdL (L = P-/-Pr3). The analogous binuclear /x-benzoate and /x-trifluoroacetate (Pd—Pd) compounds are accessible by the same route (26). 

The intermediate diene complexes 67 and 68 containing dihapto-bonded (monodentate) diolefinic ligands are fairly labile, and are therefore preferred sources of the fragment C5H5Pd(OCOMe) which forms the stable binuclear product 69 on treatment with Pd(P-/-Pr3)2 (26). The final step in the synthesis of 69 may thus be seen as a modification of the [1 + l]-addi-tion route. 

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