31 zyxwvuts Inorganica Chimica Acta, 62 (1982) 31- 37 Binuclear Oxidative Addition-Reductive Elimination Reactions* JACK HALPERN Department of Chemistry, The University of Chicago, Chicago, Ill. 60637, U.S.A. Summary The mechanistic aspects of oxidative addition and reductive elimination reactions in systems involving binuclear complexes are discussed. A variety of mech- anisms are identified. In some cases the reactive species are mononuclear complexes. In others, bi- nuclear species are directly involved but the oxidative addition or reductive elimination step itself occurs at a single metal atom site. The systems examined fail to reveal distinctive reactivity patterns or roles for binuclear complexes in such oxidative addition or reductive elimination reactions bey ond those of thermodynamically stable reactant or product reser- voirs. Introduction The study of binuclear and polynuclear com- plexes, especially of transition metals, has attracted extensive interest in recent years [I]. Many such complexes, often exhibiting distinctive compositions and bonding arrangements, now have been synthe- sized and structurally characterized. At the same time, relatively little progress has been made toward the elucidation of the chemical behavior of such sys- tems and toward the realization of distinctive reactiv- ity patterns and catalytic properties on the part of polynuclear complexes, compared with their mono- nuclear counterparts [l] . This article addresses some fairly elementary as- pects of the chemistry of such systems, notably relating to oxidative addition [2] and reductive elimination reactions involving two metal centers, such as those depicted by eqns. l-3. The kinetic and mechanistic features of such reactions are dis- cussed with particular reference to the roles of ‘bi- nuclear’ vs. ‘mononuclear’ species and reactions steps. The clear recognition of the roles of binuclear complexes in such reactions tends to be complicated by the following considerations: *Based on a plenary lecture delivered at the Euchem Con- ference on ‘The Challenge of Polynuclear Inorganic Com- pounds’, Venice, Italy, September 1981. 0020-1693/82/0000-0000/$02.75 (a) The formation and dissociation of binuclear complexes often are fast compared with the reactions of interest. Hence, both mononuclear and binuclear complexes are present and accessible as reactants. The recognition of binuclear complexes as the initial reactants or final products does not necessarily iden- tify such species as being directly involved in the actual oxidative addition or reductive elimination step. (b) The oxidative addition-reductive elimination reactions typically observed in binuclear systems often are exhibited also by mononuclear complexes, so that the roles of binuclear complexes are not readi- ly identified by their distinctive chemistry. Indeed, very little distinctive chemistry, including chemistry relevant to catalysis, has thus far been identified for binuclear and polynuclear complexes. MsLa, (or 2ML) t Hz 2 2H-ML, (1) M2LZn (or 2MIJ + Oa 2 LM-O-O-ML, or LM-ML&),) (2) R-ML, + H-ML, + R-H + MzLzn (or 2ML) (3) Oxidative Addition of Hz The possible involvement of binuclear species or reactions in the catalytic activation of Hz was recog- nized early in the study of such systems [3, 41. Among the earliest samples of a homogeneously catalyzed hydrogenation reaction to be identified was the catalysis by copper(I) acetate in quinoline solution of the reduction by Ha of substrates such as copper(I1) or quinone (Q). The rate-law for these reactions, represented by eqn. 4, originally was inter- preted [3] in terms of catalysis by a binuclear (CI.?)~ complex according to eqn. 5 but this interpretation was subsequently questioned and an alternative mechanism involving a termolecular rate-determining step (eqn. 6) was favored [4]. Since the equilibrium for formation of the alleged binuclear complex (eqn. 7) presumably is rapidly established, the two alter- native interpretations are kinetically indistinguishable and the role, if any, of binuclear complexes in this system remains uncertain. 0 Elsevier Sequoia/Printed in Switzerland