CIS-1,2-BIS(DIPHENYLPHOSPHINO)ETHENE NICKEL(0) COMPLEXES

We have studied the ligand properties of cis-Ph2PCH=CHPPh2 at nickel(0). Several homoleptic nickel(0) alkene complexes react with the twofold molecular quantity of cis-Ph2PCH=CHPPh2 to afford the red crystalline homoleptic 1:2 complex Ni(cis-Ph2PCH=CHPPh2)2 (1).The reactions of Ni(cdt) or Ni(C2H4)3 with equimolar amounts of cis-Ph2PCH=CHPPh2 yield orange-yellow crystals of the homoleptic 1:1 complex {(mu-cis-Ph2PCH=CHPPh2)Ni)n (2). Sparingly soluble 2 is presumably oligomeric with the (PCH=CHP)Ni moiety pi-bonded to another nickel(0) center. The pi bridges are rather stable and are only cleaved by strong pi acceptors. For example, 2 reacts with CO to yield the known carbonyl complex (cis-Ph2PCH=CHPPh2)Ni(CO)2 (3). In contrast to the above synthesis of 2, the 1:1 reaction of Ni(cod), with cis-Ph2PCH=CHPPh2 affords orange-red crystals of the alkene complex (cis-Ph2PCH=CHPPh2)Ni(cod)-THF (4) in moderate yield. Ni(1,5-hexadiene), reacts readily with cis-Ph2PCH=CHPPh, (1:1) to give yellow (cis-Ph2PCH=CHPPh2)Ni(rac-eta2,eta2-C6H10) (5, 77%), which exhibits a fluxional diene coordination. That these thermally labile alkene complexes are formed at all, is ascribed to the chelate effect of the dienes. From the reaction of 2, 4 or 5 with ethyne, yellow microcrystals of (cis-Ph2PCH=CHPPh2)Ni(C2H2).Et2O (6) are obtained.