Reductive Elimination. An Inorganic Chemistry Topic

1. Submitted by
Shahdan Borbora
M.Sc. 4th Semester
Roll No. 11
REDUCTIVE ELIMINATION

2. An elementary step in which both the coordination number as
well as the oxidation state of the metal decreases while forming
a new covalent bond, therefore most often seen in higher
oxidation state
It is the microscopic reverse of oxidative addition
Introduction

3. General Information
Reductive elimination is more common in higher oxidation
states
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)

4. Factors Affecting Reductive Elimination
For Reductive Elimination to occur
The eliminating groups must be cis-oriented to each other
A high formal positive charge on the metal
The presence of bulky groups on the metal
An electronically stable organic product

5. Mononuclear Systems
Cis
Trans
This pathway is common for d8 metals Ni(II), Pd(II) and Au(III) and d6
metals Pt(IV), Ir(III) and Rh(III).
Mononuclear reductive elimination requires that the groups being
eliminated must be cis to one another on the metal center

6. Reductive elimination reactions are intramolecular and this can be seen
from the following example where no cross product is formed.

7. 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 seen with first row metals, which prefer
one unit change in oxidation state, but has been observed in
both second and third row metals.
Binuclear Systems

8. Reductive Elimination – Oh complexes
In octahedral complexes,
reductive elimination can be very
slow from the coordinatively
saturated center, and often,
reductive elimination only
proceeds via a dissociative
mechanism, where a ligand must
initially dissociate to make a five-
coordinate complex.
This complex adopts a distorted
trigonal bipyramidal structure
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 the square
planar four-coordinate complex.
The rate of reductive elimination is greatly
influenced by the geometry of the metal
complex

9. Reductive Elimination – Square
Planar Complexes
Reductive elimination of square planar complexes can progress
through a variety of mechanisms: dissociative, nondissociative, and
associative
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

10. For a nondissociative pathway, reductive elimination occurs from the
four-coordinate system to afford a two-coordinate complex. If the
eliminating ligands are trans to each other, the complex must first undergo
a trans to cis 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.

11. Effect of Added Ligands
Added ligands can inhibit, increase, or have no effect on the
rate of reductive elimination
Addition of a ligand induces the elimination
reaction. Here, the incoming phosphine creates
a fluxional 5-coordinated intermediate

12. Effect of solvent
Solvent leads to the formation of different elimination products
because prior ligand dissociation occurs in polar solvents and forms
coordinatively unsaturated intermediate that leads to final product

13. Oxidatively Induced Reductive
Elimination
Oxidizing a stable complex to an unstable oxidation state can induce a reductive
elimination, a process called oxidatively induced reductive elimination.
Rate is inhibited
by excess bipy
Homolytic cleavage rather than a
straight reductive elimination
Occurs via concerted
reductive elimination
FeII
FeIII
FeIV

14. Rates of Reductive Elimination
The rates of reductive elimination depend mainly on the thermodynamic
factor.
Usually fast, Reversible
Very fast, rarely reversible
Slow, Most likely non-reversible
If we consider that the DH-H = 104 kcal/mol and that the DM-H is 50-60 kcal/mol we see that these
are essentially balanced and there should be no thermodynamic preference for a dihydride
versus a reduced metal center.
But DR-H is typically 100 kcal/mol versus a metal alkyl bond strength of 30 to 40 kcal/mol. We see
that the thermodynamic situation is again approximately balanced with a slight preference for
the forward reaction.
DR-R is typically around 90 kcal/mol, so for two alkyl substituents, there is a strong
thermodynamic driving force for the reaction to go to the right. C-C bond activation is unusually
rare, but more examples continue to be found.

15. Application
Reductive elimination has found widespread application in
academia and industry, most notable being hydrogenation,
the Monsanto acetic acid process, hydroformylation and
cross coupling reactions. In many of these catalytic cycles,
reductive elimination is the product forming step and
regenerates the catalyst; however, in the Heck reactionand
Wacker process, reductive elimination is involved only in
catalyst regeneration

16. References
Basic Organometallic Chemistry, 2nd Edition, B D Gupta, A J Elias, 134-146,
2013, Universities Press(India) Pvt. Ltd.
Inorganic Chemistry, 3rd Edition, Gary L. Miessler, Donald A. Tarr, 525-526,
2015, Pearson India Education Services Pvt. Ltd.
Inorganic Chemistry, 4th Edition, James E. Huheey, Ellen A. Keiter, Richard L.
Keiter, Okhil K. Medhi, 637-642, 2013, Dorling Kindersley(India) Pvt. Ltd.
http://www.ilpi.com/organomet/reductive.html
https://en.wikipedia.org/wiki/Reductive_elimination
Gillie, A.; Stille, J. K. (1980). "Mechanisms of 1,1-Reductive Elimination from
Palladium". J. Am. Chem. Soc. 102: 4933. doi:10.1021/ja00535a018.
Gillie, Stille J. Am. Chem. Soc. 1980, 102, 4933.
Kochi et. al. Organometallics 1982, 1, 155
Komiya, Albright, Kochi, Hoffmann J. Am. Chem. Soc. 1976, 98, 7255

17. 17