General Features of Oxidative Additions
Oxidative addition reactions usually involve a coordinatively unsaturated 16-electron metal complex or five-coordinate 18-electron species, and take the general from:
If the A and B ligands in the product are considered to be formally –1, then the metal center has increased its oxidation state by +2, and this is the origin of the name oxidative addition.
Oxidative reaction can occur when a metal complex behaves as both a Lewis acid and a Lewis base*.
Oxidative is facile if
❖ MyLx complex is coordinatively unsaturated. Examples: square planar 16VE complexes of d8 and d10 metals: RhI, IrI, Ni0, Pd0, PtII, and Pt0.
❖ The metal has an energetically accessible oxidation state My+2. Ni0 [pic]NiII PtII [pic] PtIV are facile but NiII [pic] NiIV .
In order for oxidative reaction to occur, vacant coordtination sites must be available. A six-coordinated complex is not a good candidate unless it losses ligands during the course of the reactions making available a site for interaction. A further requirement is that suitable orbitals must be available for bond formation. An 18-electron complex such as [Fe(CO)4]2- has only four ligands but addition of A[pic]B would require the use of antibonding orbitals, which of course is not energetically favorable.
Besides H2 many substrates undergo oxidative additions: HCl, Cl2 and other halogens and interhalogens, RCOOH, HsiR3, alkyl, aryl, vinyl, and benzylhalides, acyls RC(O)Cl and O2. Substrates with A [pic]B usually add to the metal with retention of an A[pic]B single bond. For example, aldehydes, ketones, alkenes and alkynes, particularay with electron withdrawing substituents, can undergo reactions which amount to an oxidative addition to the metal:
Of course the readiness of the metal center to react with potentially oxidative substrates depends on the nature of the metal and the other ligands. While electron-withdrawing ligands such as CO deactivates the metal, strong electron donor ligands such as Pme3 raise the energy of the metal centered non-bonding electron pairs (and so increase the metal basicity) which can lead to dramatic reactivity increases. Similarly, anionic complexes are more reactive than neutral ones.
The reaction mechanism depends on the nature of AB ligands and can be broken into three generally recognized categories as shown:
Ionic: AB [pic] A+ + B–
(L)nM + AB [pic] [ (L)nM….A…B ] [pic] (L)nM(A)(B)
Free Radical (Single electron transfer):
(L)nM + AB [pic] [(L)nM•]+ + [•AB]–
[•AB]– [pic] [•A] + B–
Possible radical termination steps:
[(L)nM•]+ + [•A] [pic] [ (L)nM—A] + [pic] (L)nM(A)(B) [(L)nM•]+ + B– [pic] [ (L)nM—Y] [pic] (L)nM(B)(A)
(L)nM + [•A] [pic] [(L)nM•]+ + A–
[(L)nM•]+ + AB [pic][ (L)nM—A] + [•B]
For species that are known to ionize, such as HI and HBr, the ionic mechanism is the most probable and will be especially favored in more polar solvents. The nucleophilic mechanism occurs with many organic halides and requires the availability of an unshared electron pair on the species,(L)nM . The radical electron-transfer mechanism is obvious candidate when XY is an oxidizing agent such as Cl2 and Br2.
Oxidative Addition of Organic Halides:
The available evidence indicates that oxidative addition of organic halides quite often proceeds by nucleophilic attack of the metal center on the halogen bearing carbon, as shown in the fig. The general example implies that addition of X–...
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