Agostic complex
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.
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 H
2 itself, illustrated in W(CO)
3(PPr
i3)
2H
2.
[Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001] 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. 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-di
hydrogen complexes, in which both (
low valent) ruthenium-dihydrogen complexes and the (
high valent) ruthenium-di
hydrides can be observed.
* W(CO)
3(PPr
i3)
2H
2.
* W(CH3)6 [Kaupp, M., "The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry", (1996) J. Am. Chem. Soc., 118 (12), 3018 -3024 ]* 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.
