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❑ Decomposition Pathways: Transition metals σ-hydrocarbyls, due to their coordinative
unsaturation are not kinetically stable and this origins several facile pathways for their
decomposition and strong reactivity. These paths can be classified as;
(1) migration of a substituent (generally hydrogen) from a σ−bonded hydrocarbyl
to the metal,
(2) intermolecular reductive elimination and
(3) homolytic fission of M-C bond.
1. Migration of a substituent from a σ−bonded hydrocarbyl to the metal: It occurs via 3
types of mechanism:
i. β−elimination: The most facile route for decomposition, preferred by transition metal
alkyls is through the so called hydride transfer accompanied with alkene elimination.
This type of reaction involves transfer of one hydrogen atom from the second or
β−carbon atom of the alkyl chain to the metal, increasing its coordination number.
Resulting intermediate hydridoalkene complex tends to lose alkene upon formation of
metal-hydride, which in some cases may decompose to metal and hydrogen too.
ii. α−elimination:
α-Elimination pathways are much less observed for transition metal hydrocarbyls and
main examples involve metal-methyls.
However, these are of considerable synthetic utility in main group hydrocarbyls and
involve the transfer of a substituent 'R' from the α−carbon atom to the metal.
➢ Example: TaMe5, WMe6 and ReMe6 are unstable and decompose explosively with the
formation of methane via four-centred transition state.
➢ This plays a key role towards synthesis of alkylidene complexes.
➢ Several metal benzyls and neopentyls, especially of titanium and tantalum are known to
decompose by this mechanism
iii. γ−elimination: It has been observed for some main group hydrocarbyls, but it’s rare
in transition metal chemistry
2. Intermolecular reductive elimination: It is reverse of oxidative addition reaction.
Both the oxidation state and coordination number of the central metal are reduced by
two units. Hence, this pathway would be confined to metals with stable oxidation states
differing by two units, which is more common for the later transition metals.
3. Homolytic fission of M-C bond:
These are common pathways for decomposition of main group alkyls, but several
transition metal alkyls also follow it.
Zr(CH2Ph)4 → Zr + 2PhCH2CH2Ph

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DECOMPOSITION PATHWAYS of TM-alkyl complexes.pdf

  • 1. ❑ Decomposition Pathways: Transition metals σ-hydrocarbyls, due to their coordinative unsaturation are not kinetically stable and this origins several facile pathways for their decomposition and strong reactivity. These paths can be classified as; (1) migration of a substituent (generally hydrogen) from a σ−bonded hydrocarbyl to the metal, (2) intermolecular reductive elimination and (3) homolytic fission of M-C bond.
  • 2. 1. Migration of a substituent from a σ−bonded hydrocarbyl to the metal: It occurs via 3 types of mechanism: i. β−elimination: The most facile route for decomposition, preferred by transition metal alkyls is through the so called hydride transfer accompanied with alkene elimination. This type of reaction involves transfer of one hydrogen atom from the second or β−carbon atom of the alkyl chain to the metal, increasing its coordination number. Resulting intermediate hydridoalkene complex tends to lose alkene upon formation of metal-hydride, which in some cases may decompose to metal and hydrogen too.
  • 3. ii. α−elimination: α-Elimination pathways are much less observed for transition metal hydrocarbyls and main examples involve metal-methyls. However, these are of considerable synthetic utility in main group hydrocarbyls and involve the transfer of a substituent 'R' from the α−carbon atom to the metal.
  • 4. ➢ Example: TaMe5, WMe6 and ReMe6 are unstable and decompose explosively with the formation of methane via four-centred transition state. ➢ This plays a key role towards synthesis of alkylidene complexes. ➢ Several metal benzyls and neopentyls, especially of titanium and tantalum are known to decompose by this mechanism
  • 5. iii. γ−elimination: It has been observed for some main group hydrocarbyls, but it’s rare in transition metal chemistry 2. Intermolecular reductive elimination: It is reverse of oxidative addition reaction. Both the oxidation state and coordination number of the central metal are reduced by two units. Hence, this pathway would be confined to metals with stable oxidation states differing by two units, which is more common for the later transition metals.
  • 6. 3. Homolytic fission of M-C bond: These are common pathways for decomposition of main group alkyls, but several transition metal alkyls also follow it. Zr(CH2Ph)4 → Zr + 2PhCH2CH2Ph