Metal alkyls and metal-carbon sigma bonds

Stability is kinetic, not thermodynamic

Transition-metal–carbon bond enthalpies are typically comparable to main-group M–C bonds, and they generally increase down a triad (Hf–C stronger than Ti–C), the opposite of main-group trends. M–C bonds are also stronger for sp^2 and sp carbons (aryl, vinyl, alkynyl) than for sp^3 alkyls. Yet \mathrm{TiMe_4} decomposes well below room temperature. The instability is kinetic: low-barrier decomposition routes exist.

Decomposition routes

• β-Hydride elimination: needs a β-H, a vacant cis site, a syn-coplanar M–C–C–H arrangement, and gives a stable alkene plus M–H.
• α-Hydrogen abstraction: gives alkylidenes (M=CHR), common for crowded high-valent early metals.
• Reductive elimination of alkanes from alkyl hydrides or dialkyls.
• Homolysis, especially for weak bonds or radical-stabilising groups.

Designing stable alkyls

1. No β-hydrogens: methyl, benzyl, neopentyl CH_2CMe_3, (trimethylsilyl)methyl CH_2SiMe_3, perfluoroalkyls. \mathrm{Ti(CH_2CMe_3)_4} can be distilled, whereas \mathrm{TiEt_4} decomposes far below room temperature. 2. No vacant cis site: coordinatively saturated 18-e species such as \mathrm{CpFe(CO)_2Et}, or adding chelating phosphines to block sites. 3. β-H that cannot reach the metal: bulky tBu groups, aryls and vinyls (the M–C–C angle near 120° holds β-H away), bridgehead groups such as 1-norbornyl where the resulting alkene would violate Bredt's rule. 4. Metallacycles: small rings such as metallacyclopentanes cannot easily achieve the syn-coplanar geometry.

Synthesis

• Transmetallation (most common): \mathrm{MCl_n + n\,RLi} or RMgX → \mathrm{MR_n} + salt; milder reagents (AlR_3, ZnR_2, SnR_4) for sensitive metals.
• Nucleophilic attack of metal anions on alkyl halides: \mathrm{[CpFe(CO)_2]^- + MeI \rightarrow CpFe(CO)_2Me + I^-}.
• Oxidative addition of R–X to low-valent metals.
• Insertion of alkenes into M–H bonds.

Agostic interactions

In electron-deficient complexes a C–H bond can donate its σ electrons to an empty metal orbital, forming a three-centre two-electron M···H–C interaction. Evidence: short M···H distances, a reduced ^1J_{\mathrm{CH}} coupling (around 75–100 Hz instead of about 125 Hz), low-frequency \nu_{\mathrm{CH}} and upfield ^1H shifts. β-Agostic structures resemble a frozen point on the path to β-hydride elimination; α-agostic interactions assist alkene insertion in polymerisation catalysts.

Metal alkyls are central to modern catalysis, and controlling their fate is a major design problem.

Polymerisation. In Ziegler–Natta and single-site catalysts, chain growth is repeated alkene insertion into M–alkyl bonds; chain termination by β-H elimination or β-H transfer to monomer sets molecular weight. Late-metal (Ni, Pd) α-diimine catalysts exploit rapid β-H elimination/reinsertion ("chain walking") to make branched polyethylene from ethene alone. α-Agostic assistance, first proposed to explain isotope effects in insertion, is supported by isotopic perturbation studies.

Alkyl cross-coupling. Using C(sp^3) electrophiles and nucleophiles was long hindered by slow oxidative addition and fast β-H elimination. Nickel catalysts with radical OA, bulky ligands that accelerate reductive elimination, and photoredox/Ni dual catalysis now couple secondary and tertiary alkyls, often enantioconvergently.

C–H activation and σ-complexes. Alkane σ-complexes, in which an intact C–H bond binds the metal, have been characterised in solution by low-temperature NMR and in single crystals by solid-state methods. They are intermediates on the path to oxidative addition or σ-bond metathesis, and studying them informs alkane functionalisation and dehydrogenation catalysis with pincer iridium complexes.

Bioorganometallics. Coenzyme B_{12}'s Co–C bond homolyses on enzyme binding to generate an adenosyl radical; methylcobalamin transfers methyl groups heterolytically. How the protein lowers the Co–C bond dissociation energy is studied by spectroscopy and QM/MM.

Analysis. Agostic bonds are judged with neutron diffraction, ^1J_{\mathrm{CH}} and isotopic perturbation of equilibrium; computationally, QTAIM and NBO analyses quantify the interaction, though agreement on borderline cases (for example "anagostic" contacts in d^8 complexes) remains debated.