Metal Alkyl Complexes

Introduction

Transition metal alkyl complexes play a critical role in a variety of important organometallic reactions such as olefin polymerization and hydroformylation. Early attempts to synthesize these complexes were unsuccessful, so it was originally thought that such species were inherently unstable due to weak metal-carbon bonds. In fact, the issue is not their thermodynamic stability (M-C bond dissociation energies are typically 40-60 kcal/mol with 20-70 kcal/mol being a practical range), but their kinetic stability. More on this below.

Structure and Bonding

Simple alkyls are simple sigma donors, that can be considered to donate one or two electrons to the metal center depending on which electron counting formalism you use.

For simple metal alkyls, the M-R bond distance is typically 190 to 220 pm. This is approximately the sum of the covalent radii of carbon and metal, r_C = 77 pm and r_M ~120 pm. Realize that the first row transition metals are smaller, so any M-X bond distance will usually be smaller by 10-20 pm or so.

Alkyls can bridge two metal centers, something that is well known from aluminum-alkyl chemistry. For example, consider the condensed phase structure of these Al-alkyls (see Oliver et. al. Organometallics 1982, 1, 1307):

For Al₂Me₆, we see sharp resonances for bridging and terminal Me groups at -75 degrees C in the ¹H NMR, but these coalesce and are one average signal at room temperature. This indicates a very low barrier to interconversion of the two groups. Interestingly, the dimer-monomer equilibrium contains only 0.0047% monomer at 20 °C!

Bridging alkyls are also known for other metals such as lanthanum and zirconium (see Waymouth, Santarsiero, Grubbs J. Am. Chem. Soc. 1984, 106, 4050-4051):

In this structure, the bridging Me group is nearly planar (C is only 8 pm out of the plane defined by the three methyl hydrogens vs. 30 pm for a typical sp³ carbon). The Me bridge is slightly asymmetric (Zr1-C = 259.9(7) and Zr2-C = 245.6(7) pm ) and the Zr-C-Zr angle is 147.8(3) degrees. This non-linearity was ascribed to better orbital overlap on the basis of MO calculations. The J_CH = 136 Hz, indicating a fair amount of sp² character.

Note that while drawing five bonds to carbon makes organic chemists shudder, an MO diagram featuring 3-center-2-electron bonds easily explains the bonding in these complexes.

Another type of unusual alkyl group involves agostic interactions in which part of the alkyl substituent coordinates to the metal in addition to the M-C bond.

Some of the most common routes include:

1. Metathetical exchange using a carbon nucleophile (R^-). Common reagents are RLi, RMgX (or R₂Mg), ZnR₂, AlR₃, BR₃, and PbR₄. This spans a range from strong to very weak as strong nucleophiles can sometimes result in undesired reduction.

   Much of this alkylation chemistry can be understood with Pearson's "hard-soft" principles. Here are just a few examples of nucleophilic routes; notice that some of the homoleptic alkyls are rather unstable. This is because they have low d-counts, are susceptible to alpha- and/or beta-hydride elimination, and lack good pi-donating ligands to stabilize their high oxidation states:

   

   Beware of reactions where the hard-soft interactions are not so clear as these can represent equilibria instead of complete reactions

   MCl + AlR₃ M-R + AlR₂Cl.

2. Metal-centered nucleophiles (i.e. using R⁺ as a reagent) Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example:

   NaFp + RX Fp-R + NaX     [Fp = Cp(CO)₂Fe]

   One could propose an S_N² (associative mechanism) for this reaction, but a single electron transfer mechanism (SET) could also be postulated.

   Whitesides and coworkers examined the above reaction using a stererochemical probe to show that this reaction proceeds with complete inversion of stereochemistry, consistent with an S_N² mechanism. See J. Am. Chem. Soc. 1974 , 96, 2814. However, the nature of the nucleophile is important, and under certain circumstances this can occur through the SET mechanism.

3. Oxidative Addition. This requires a covalently unsaturated, low-valent complex (16 e- or less). A classic example:

   

4. Insertion. To form an alkyl, this usually involves an olefin insertion. The simplest generic example is the insertion of ethylene into an M-X bond, i.e.

   M-X + CH₂CH₂ M-CH₂CH₂-X

   Insertions such as this are involved in the hydrozirconation reaction.

5. Deinsertion. This is not a highly common method. The reaction usually involves desinsertion of carbon monoxide from an intermediate acyl complex:

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Metal alkyls can decompose by several different routes:

1. Beta-hydride elimination. This is the most important decomposition mode for metal alkyls. Please read the beta-hydride section of this hypertext for more detail on this very important reaction.
2. Alpha-hydride elimination. When beta-elimination is not possible, alpha elimination may become important. In a few rare cases, the rate of alpha elimination can be greater than or equal to the rate of beta elimination. Delta eliminations are also possible.
3. Reductive elimination. This is often a key step in catalytic reactions such as the Monsanto acetic acid process.
4. Homolytic cleavage. Under most circumstances, homolytic cleavage is not a problem. Most of these rare cases involve first row transition metals with facile redox couples such as Co(III)-Co(II), Cu(I)-Cu(0) and Ti(IV)-Ti(III), but not all such complexes are unstable. Here is one example:

   

   Homolytic cleavage can also be induced by an oxidation reaction that leads to an unstable oxidation state:

   

Empirical order of stabilities

The factors discussed above all influence the stability of metal alkyl complexes. We can come up with some experimental observations about the relative stabilities of different alkyl ligands:

1. 1-norbornyl > benzyl > trimethylsilyl > neopentyl > Ph ~ Me >> Et (1^o R) > 2, 3^o R

   

   Notice that norbornyl not only has a difficult time approaching the metal center, but that the olefin that would be generated would be highly strained (and violate Bent's rule).

2. Fluoroalkyl > alkyl (i.e. -C_nF_2n+1 > -C_nH_2n+1)

   CF bonds are very strong (120-130 kcal/mol vs. 98-104 kcal/mol for alkyl C-H).

3. Chelating (metallacycles) > nonchelating (acyclic)

   

   The dialkyl shown on the left decomposes at 110 ^oC with k_dec = 1.0 s^-1. In contrast, the metallacycle has k = 5.3 x 10^-3 s^-1. The beta-hydrogen has a close approach to the metal in the dialkyl case, but not the metallacycle.

4. 3rd row > 2nd row > 1st row transition metals.

   

5. Strong electron-donating ligands increase stability.

   

   The carbonyl ligand reduces electron density on the metal through pi-backbonding, in contrast to the phosphine ligand, which is a good sigma donor.