Reductive elimination is anelementary step inorganometallic chemistry in which theoxidation state of the metal center decreases while forming a newcovalent bond between twoligands. It is themicroscopic reverse ofoxidative addition, and is often the product-forming step in many catalytic processes. Since oxidative addition and reductive elimination are reverse reactions, the same mechanisms apply for both processes, and the product equilibrium depends on the thermodynamics of both directions.[1][2]
Reductive elimination is often seen in higher oxidation states, and can involve a two-electron change at a single metal center (mononuclear) or a one-electron change at each of two metal centers (binuclear, dinuclear, or bimetallic).[1][2]
For mononuclear reductive elimination, the oxidation state of the metal decreases by two, while thed-electron count of the metal increases by two. This pathway is common for d8 metals Ni(II), Pd(II), and Au(III) and d6 metals Pt(IV), Pd(IV), Ir(III), and Rh(III). Additionally, mononuclear reductive elimination requires that the groups being eliminated must be cis to one another on the metal center.[3]
For binuclear reductive elimination, the oxidation state of each metal decreases by one, while the d-electron count of each metal increases by one. This type of reactivity is generally seen with first row metals, which prefer a one-unit change in oxidation state, but has been observed in both second and third row metals.[4]
As with oxidative addition, several mechanisms are possible with reductive elimination. The prominent mechanism is aconcerted pathway, meaning that it is a nonpolar, three-centeredtransition state with retention ofstereochemistry. In addition, anSN2 mechanism, which proceeds with inversion of stereochemistry, or aradical mechanism, which proceeds with obliteration of stereochemistry, are other possible pathways for reductive elimination.[1]
The rate of reductive elimination is greatly influenced by the geometry of the metal complex. Inoctahedral complexes, reductive elimination can be very slow from the coordinatively saturated center; and often, reductive elimination only proceeds via adissociative mechanism, where a ligand must initially dissociate to make a five-coordinate complex. This complex adopts a Y-type distortedtrigonal bipyramidal structure where a π-donor ligand is at the basal position and the two groups to be eliminated are brought very close together. After elimination, a T-shaped three-coordinate complex is formed, which will associate with a ligand to form thesquare planar four-coordinate complex.[5]
Reductive elimination of square planar complexes can progress through a variety of mechanisms:dissociative, nondissociative, andassociative. Similar to octahedral complexes, a dissociative mechanism for square planar complexes initiates with loss of a ligand, generating a three-coordinate intermediate that undergoes reductive elimination to produce a one-coordinate metal complex. For a nondissociative pathway, reductive elimination occurs from the four-coordinate system to afford a two-coordinate complex. If the eliminating ligands aretrans to each other, the complex must first undergo atrans tocis isomerization before eliminating. In an associative mechanism, a ligand must initially associate with the four-coordinate metal complex to generate a five-coordinate complex that undergoes reductive elimination synonymous to the dissociation mechanism for octahedral complexes.[6][7]
Reductive elimination is sensitive to a variety of factors including: (1) metal identity and electron density, (2) sterics, (3) participating ligands, (4)coordination number, (5)geometry, and (6)photolysis/oxidation. Additionally, because reductive elimination and oxidative addition are reverse reactions, any sterics or electronics that enhance the rate of reductive elimination must thermodynamically hinder the rate of oxidative addition.[2]
First-row metal complexes tend to undergo reductive elimination faster than second-row metal complexes, which tend to be faster than third-row metal complexes. This is due to bond strength, with metal-ligand bonds in first-row complexes being weaker than metal-ligand bonds in third-row complexes. Additionally, electron-poor metal centers undergo reductive elimination faster than electron-rich metal centers since the resulting metal would gain electron density upon reductive elimination.[8]
Reductive elimination generally occurs more rapidly from a more sterically hindered metal center because the steric encumbrance is alleviated upon reductive elimination. Additionally, wide ligandbite angles generally accelerate reductive elimination because the sterics force the eliminating groups closer together, which allows for moreorbital overlap.[9]
Kinetics for reductive elimination are hard to predict, but reactions that involvehydrides are particularly fast due to effects of orbital overlap in the transition state.[10]
Reductive elimination occurs more rapidly for complexes of three- or five-coordinate metal centers than for four- or six-coordinate metal centers. For even coordination number complexes, reductive elimination leads to an intermediate with a strongly metal-ligandantibonding orbital. When reductive elimination occurs from odd coordination number complexes, the resulting intermediate occupies anonbonding molecular orbital.[11]
Reductive elimination generally occurs faster for complexes whose structures resemble the product.[2]
Reductive elimination can be induced by oxidizing the metal center to a higher oxidation state via light or an oxidant.[12]
Reductive elimination has found widespread application in academia and industry, most notable beinghydrogenation,[13] theMonsanto acetic acid process,[14]hydroformylation,[15] andcross-coupling reactions.[16] In many of these catalytic cycles, reductive elimination is the product forming step and regenerates the catalyst; however, in theHeck reaction[17] andWacker process,[18] reductive elimination is involved only in catalyst regeneration, as the products in these reactions are formed viaβ–hydride elimination.
