Hapticity

The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to the metal. In general the η-notation is only used when there is more than one atom coordinated (otherwise the κ-notation is used, see also hapticity vs. denticity). Hapticity of a ligand can change to accommodate the 18-electron rule.

Examples
The η-notation is encountered in many coordination compounds:
 * Side-on bonding of molecules containing σ-bonds like H 2 :
 * W(CO)3(PiPr3)2(η2-H2)
 * Side-on bonded ligands containing multiple bonded atoms, e.g. ethylene in Zeise's salt, which is bonded through donation of the π-bonding electrons:
 * K[PtCl3(η2-C2H4)].H2O
 * Related complexes containing bridging π-ligands:
 * (μ-η2:η2-C2H2)Co2(CO)6 and (Cp*2Sm)2(μ-η2:η2- N2)
 * Dioxygen in bis{(trispyrazolylborato)copper(II)}(μ-η2:η2-O2),
 * Note that with some bridging ligands, an alternative bridging mode is observed, e.g. κ1,κ1, like in (Me3SiCH2)3V(μ-N2-κ1(N),κ1(N'))V(CH2SiMe3)3 contains a bridging dinitrogen molecule, where the molecule is end-on coordinated to the two metal centers (see hapticity vs. denticity).


 * The bonding of π-bonded species can be extended over several atoms, e.g. in allyl, butadiene ligands, but also in cyclopentadienyl or benzene rings can share their electrons.
 * Apparent violations of the 18-electron rule sometimes are explicable in with unusual hapticities:
 * The 18-VE complex (η5-C5H5)Fe(η1-C5H5)(CO)2 contains one η5 bonded cyclopentadienyl, and one η1 bonded cyclopentadienyl.
 * Reduction of the 18-VE compound [Ru(η6-C6Me6)2]2+ (where both aromatic rings are bonded in an η6-coordination), results in another 18VE compound: [Ru(η6-C6Me6)(η4-C6Me6)].
 * Examples of polyhapto coordinated heterocyclic and inorganic rings: Cr(η5-C4H4S)(CO)3 contains the sulfur heterocycle thiophene and Cr(η6-B3N3Me6)(CO)3 contains a coordinated inorganic ring (B3N3 ring).

Hapticity vs. denticity
Hapticity must be distinguished from denticity. Polydentate ligands coordinate via multiple coordination sites within the ligand. In this case the coordinating atoms are identified using the κ-notation, as for example seen in coordination of 1,2-bis(diphenylphosphino)ethane (Ph2PCH2CH2PPh2), to NiCl2 as dichloro[ethane-1,2-diylbis(diphenylphosphane)-κ2P]nickel(II). If the coordinating atoms are contiguous (connected to each other), the η-notation is used, as e.g. in titanocene dichloride: dicholorobis(η5-2,4-cyclopentadien-1-yl)titanium.

Hapticity and fluxionality
Molecules with polyhapto ligands are often "fluxional", also known as stereochemically non-rigid. Two classes of fluctionality are prevalent for organometallic complexes of polyhapto ligands:
 * Case 1, typically: when the hapticity value is less than the number of sp2 carbon atoms. In such situations, the metal will often migrate in from carbon to carbon, maintaining the same net hapticity. The η1-C5H5 ligand in (η5-C5H5)Fe( η1-C5H5)(CO)2 rearranges rapidly in solution such that Fe binds alternatingly to each carbon atom in the η1-C5H5 ligand.  This reaction is degenerate and, in the jargon or organic chemistry, it is an example of a sigmatropic rearrangement.
 * Case 2, typically: complexes containing cyclc polyhapto ligand with maximized hapticity. Such ligands tend to rotate.  A famous example is ferrocene, Fe(η5-C5H5)2, wherein the Cp rings rotate with a low energy barrier about the principal axis of the molecule that "skewers" each ring (see rotational symmetry).  This "ring whizzing" explains, inter alia, why only one isomer can be isolated for Fe(η5-C5H4Br)2.  In this case, the rotamers are not necessarily degenerate, but the rotational barriers have low energies of activation.

History
The need for additional nomenclature for organometallic compounds became apparent in the mid-1950s when Dunitz, Orgel, and Rich described the structure of the "sandwich complex ferrocene by X-ray crystallography where an iron atom is "sandwiched" between two parallel cyclopentadienyl rings. Cotton later proposed the term hapticity derived from the adjectival prefix hapto (from the Greek haptein, to fasten, denoting contact or combination) placed before the name of the olefin, where the Greek letter η (eta) is used to denote the number of contiguous atoms of a ligand that bind to a metal center. The term is usually employed to describe ligands containing extended π-systems or where agostic bonding is not obvious from the formula.

Historically important compounds where the ligands are described with hapticity

 * Ferrocene
 * Uranocene - bis(η8-1,3,5,7-cyclooctatetraene)uranium
 * W(CO)3(PPri3)2(η2-H2 ) - the first compound to be synthesized with a dihydrogen ligand.
 * IrCl(CO)[P(C6H5)3]2(η2-O2) - the dioxygen derivative which forms reversibly upon oxygenation of Vaska's complex.