Inorganic chemistry

Metal–carbon compounds

Organometallic compounds place a metal in direct contact with carbon. Their bonds range from highly polar M–C σ bonds to multicentre metal–ligand interactions such as η² alkene, η⁵ cyclopentadienyl and metal carbonyl coordination.

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IntuitionIntuition: carbon can donate, accept or delocalise electrons onto metal

A metal–carbon bond is not one simple kind of bond. Organolithium reagents behave like strongly polarised carbanions, metal carbonyls receive π back-donation, and sandwich complexes spread bonding across many metal–carbon contacts.

Rotate ferrocene and compare the two parallel C₅H₅ rings. Iron is equally distant from all ten carbons in this eclipsed view.

SchoolSchool level: main families

Definition: Organometallic compound

A compound is usually called organometallic when a metal–carbon bond is present. The carbon may belong to an alkyl, aryl, carbonyl, carbene, alkene or aromatic ligand; the metal centre can be main-group, transition-metal, lanthanide or actinide.

Ligand classes
Class	Donation motif	Example
σ alkyl / aryl	2-electron donor bond	CH₃Li, PhMgBr
Carbonyl CO	σ donor + π acceptor	Ni(CO)₄
η⁵ cyclopentadienyl	6-electron delocalised donor	Fe(η⁵-C₅H₅)₂

Example: Distinguish polarity of M–C bonds

Which bond is more polarised toward carbon: C–Li or C–Fe in a typical organometallic complex?

Solution

C–Li, because lithium is much more electropositive. The carbon end carries substantial carbanion character, so organolithium reagents are strongly basic and nucleophilic.

UndergraduateUndergraduate: bonding and electron counting

The 18-electron rule treats a transition-metal complex like a goal of filling nine valence orbitals (5 nd + (n+1)s + 3(n+1)p). Ionic counting assigns ligand charges first; neutral counting treats radicals as one-electron donors. The two conventions should give the same total.

\text{metal valence electrons}+\sum\text{ligand donations}+\text{charge correction}

Example: Count electrons in ferrocene

Use ionic counting for Fe(η⁵-C₅H₅)₂.

Solution

Each Cp⁻ ligand donates six π electrons, so two Cp rings give 12. Fe²⁺ is d⁶, giving 18 total. This high formal count helps explain ferrocene’s stability.

Carbonyl complexes illustrate synergic bonding: CO donates a filled σ-type lone-pair orbital to the metal, while filled metal d orbitals back-donate into CO π* orbitals. Strong back-donation lowers the C–O stretching frequency in IR spectroscopy.

η notation counts contiguous carbons bound to the metal, not total ligand electrons. η²-ethylene uses two carbons of a C=C; η⁵-Cp uses all five ring carbons; η¹-alkyl uses only one carbon–metal σ contact. Hapticity can change by ring slippage or rearrangement.

AdvancedOrganometallic structure and reactivity

Metal–carbon bonding supports elementary organometallic steps: oxidative addition, reductive elimination, insertion and β-hydrogen elimination. Stability is not simply bond enthalpy; sterics, electron count, vacant sites and kinetic pathways often control whether a complex survives isolation.

The electronic picture differs across periodic regions: alkali organometallics are strongly polar and aggregate, late transition metals often follow 16/18-electron patterns, and early or f-block metals can be highly oxophilic and kinetically reactive. One rule does not describe every M–C bond.

References

The Organometallic Chemistry of the Transition Metals · R. H. Crabtree, 2014
Organometallics · C. Elschenbroich, 2006