Agostic complex

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Agostic complexes are coordination compounds where the σ bond of a ligand serves as the electron pair donor.

One can view agostic interactions as the "third tier" of metal-ligand bonding:

Tier 1: To form classical Lewis acid-base adducts, ligands utilize nonbonded pairs of electrons. Werner chemistry is based on this type of interaction between a Lewis-acidic metal and a Lewis-basic ligand.

Tier 2: With the advent of organometallic chemistry, it became apparent that alkenes could serve as ligands via donation of electrons in π bonds. The classic example is Zeise's salt, in which ethylene is a ligand on a platinum atom.

Tier 3: Molecules lacking both lone pairs and π-bonds also interact with metals (and other Lewis acids), forming isolable complexes in some cases. The interaction entails "sharing" of the electrons in a σ bond with the metal. Such interactions can be described as 3-center, 2-electron bonds, familiar in the chemistry of boron hydrides, aluminium alkyls, and carbocations.

1 Agostic bonding
2 Examples
3 Applications
4 References

[edit] Agostic bonding

The term "agostic bonding" is usually reserved to describe the complexation of the C-H bond of a hydrocarbon to a metal, however, the concept has been extended to include other ligands. The most famous extension is to the complexation of H₂ itself, illustrated in W(CO)₃(PPrⁱ₃)₂H₂.^[1] Silanes are also well-known to form such complexes.

It is widely suspected that agostic bonding describes the interaction of hydrocarbons with metals and non-metals alike.^{[citation needed]} For these ligands, which in general do not possess a 'normal' Lewis basic site, to undergo (catalytic or stoichiometric) activation, the first interaction is an agostic interaction of the metal centre with one of the ligands σ-bonds, followed by a (heterolytic or homolytic) cleavage of the σ-bond (oxidative addition). A subtle example can be seen with some ruthenium-dihydrogen complexes, in which both (low valent) ruthenium-dihydrogen complexes and the (high valent) ruthenium-dihydrides can be observed.^{[citation needed]}

[edit] Examples

W(CO)₃(PPrⁱ₃)₂H₂.^[1]

[edit] Applications

With the increased availability of natural gas as a feedstock, there is growing interest in the exploitation of agostic interactions between methane and metals.

Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. One prominent example is Ziegler-Natta catalysis, where the highly electrophilic metal center has agostic interactions with the growing polymer chain.^{[citation needed]}