Advanced Inorganic Chemistry/Metal-Alkyl Complexes
Metal-alkyl complexes were initially thought to be unstable because of the weak metal-carbon bonds, making them difficult to synthesize. In fact, the problem pertains to their kinetic stability. Metal alkyls generally undergo decomposition pathways of low activation barriers. In regards to simple alkyls, they are sigma donors in which they can donate 1 or 2 electrons to the metal .
The metal carbon bond (M-C) bond is composed of a positive, charged metal and negative, charged carbon. As shown in Figure 1, as the electronegativity of metals increase, its reactivity decreases in contrast. To elaborate, the alkyl ligand's reactivity is inversely associated with the metal center's electronegativity.
In regards to the carbon's hybridization, sp-hybridized ligands are the least nucleophilic. The figure to the right elaborates on the nucleophilic order of the ligands.
1. Nucleophilic attacks with frequently used reagents such as R-Li or Pb-(R)4 is common for alkyl complexes (example displayed in figure 1). These nucleophiles range from strong to weak, as stronger nucleophiles can sometimes yield an unwanted reduction. The hard-soft principles of Pearson elaborates and explains the interactions of alkylation. Metals with strong leaving groups are more likely to undergo a nucleophilic attack, as the alkyl ligand transfers from a metal to another metal.
2. Electrophilic attack can also occur when the anionic metal complex is strong enough of a nucleophile to attack an alkyl and acyl halides in an electrophilic attack (figure 3). For this occurrence, the metal needs to have a readily available lone pair and open coordination site. The negative charge is shifted to the electrophile's leaving group while the overall charge of the complex increased by 1 .
3. Oxidative addition is another method of synthesis, and there are multiple mechanisms that exist (figure 4). A representative example of oxidative addition is M + X-Y -> M-X-Y, where X-Y is cleaved and there is a new formation of ligands (-X and -Y) on the metal. The oxidation state of the metal is then increased and the total electron of the complex is increased by 2.
4. Migratory insertion is another method to synthesize alkyl complexes, resulting in an addition across pi bonds, as shown in figure 5. Certain alkyl complexes can be synthesized this way. For instance, the perfluoroalkyl complex is stable and so, fluoroalkenes insertion is generally favorable compared to elimination (take note of the figure on the right).
Most metal alkyls generally are vulnerable to decomposition pathways with low activation barriers. The most common pathway is beta-hydride elimination followed by reductive elimination being the second, most common pathway.
Beta-hydride elimination, as shown in figure 6, is the transfer of a hydrogen atom from the β ligand to the metal center of the complex. To prevent it from occurring, alkyls must not contain β-hydrogens, an orientation where the β positioned hydrogen cannot access the metal, and yield an unstable alkene. To make β elimination possible :
- The β-carbon must contain a hydrogen.
- The metal-carbon and carbon-hydrogen bond must be in a syn-coplanar orientation.
- There must exist an open coordination site.
- The metal must have 16 e-'s or less and is at least d2.
Reductive elimination, as shown in figure 7, involves eliminating a molecule from a transition metal complex where the metal is reduced via 2 e-'s. Elimination occurs in the cis orientation with groups being eliminated. An unstable oxidative state can encourage the elimination to proceed . In details, the alkyl ligand interacts with a second type of ligand on the metal, in which the metal is consequently reduced by 2 units, a total electron count reduction by 2. Reductive elimination is generally favorable thermodynamically when X = H. However, reductive elimination is not favorable when X = halogen.
Stable Alkyl ComplexesEdit
To prevent beta-hydride elimination from occurring and to stabilize desired species, one can avoid this by increasing to 18 e-'s. A transition metal-alkyl complex with 18 e-'s dictates that all the orbitals are in their full capacity, and there is no interaction that can occur with the beta-hydrogen and transition metal center. Another way is to utilize stable alkyl ligands that do not contain beta-hydrogens and cannot undergo elimination. Such alkyls without beta-hydrogens are WMe6, Ti(CH2Ph)4, and C2F5Mn(CO)5 . The orientation of ligands can also be used to block beta-hydrogen elimination by orienting the alkyl in such a way that prevents the metal center from being accessed by the beta-hydrogen due to steric hindrance (figure 8). Such examples of bulky alkyls in which the beta-hydrogen cannot access the metal due to the orientation or bulkiness of the ligand are PdPh2L2, Cr(CMe3)4, and Cr(CHMe3)4 . A third method is to use alkyls that would yield an unstable alkene as the product.
There is importance to metal-alkyl studies, as it is significant in the organometallic and catalytic systems and context. Before metal alkyls were fully studied and analyzed, there was speculation between thermodynamics and kinetic instability of metal-C sigma bonds, determining that it was indeed kinetics instability that made it difficult for organometallic chemists to isolate metal alkyls. Regardless, there are various, significant applications to metal alkyl, in which can be elaborated upon in Applications of Metal Carbonyl Anions in the Synthesis of Unusual Organometallic Compounds and used in Strem Chemicals company for chemical vapor deposition, atomic layer deposition, etc (defined in their website). In essence, it is useful to make comparisons between various metal alkyl complexes and draw predictions pertaining to their characteristics and behavior.
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