PART 12(B): REDUCTIVE ELIMINATION IN ORGANOMETALLIC COMPOUNDS FOR CSIR NET/GATE/IIT JAM

At time 38.22, in problem 8 there is more cis combination 1,3, thus 5 cis combination should give 20% probability of H elimination.
Reductive elimination is the microscopic reverse of oxidative addition. It is literally oxidative addition run in reverse oxidative addition backwards. During reductive elimination, the electrons in the M–X bond head toward ligand Y, and the electrons in M–Y head to the metal. The eliminating ligands are always X-type! On the whole, the oxidation state of the metal decreases by two units, two new open coordination sites become available, and an X–Y bond forms. What does the change in oxidation state suggest about changes in electron density at the metal? As suggested by the name “reductive,” the metal gains electrons. The ligands lose electrons as the new X–Y bond cannot possibly be polarized to both X and Y, as the original M–X and M–Y bonds were. As with oxidative addition, several mechanisms are possible with reductive elimination. The prominent mechanism is a concerted pathway, meaning that it is a nonpolar, three-centered transition state with retention of stereochemistry. In addition, an SN2 mechanism, which proceeds with inversion of stereochemistry, or a radical mechanism, which proceeds with obliteration of stereochemistry, are other possible pathways for reductive elimination.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.
Metal identity and electron density:
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.
Sterics: Reductive elimination generally occurs more rapidly from a more sterically hindered metal center because the steric encumbrance is alleviated upon reductive elimination. Additionally, wide ligand bite angles generally accelerate reductive elimination because the sterics force the eliminating groups closer together, which allows for more orbital overlap.

Participating ligands: Kinetics for reductive elimination are hard to predict, but reactions that involve hydrides are particularly fast due to effects of orbital overlap in the transition state.
Coordination number: 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-ligand antibonding orbital. When reductive elimination occurs from odd coordination number complexes, the resulting intermediate occupies a nonbonding molecular orbital.
Geometry: Reductive elimination generally occurs faster for complexes whose structures resemble the product.
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