Hapticity

Number of contiguous atoms in a ligand that bond to the central atom in a coordination complex
the co ordination of a ligand to a central atom

In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η notation only applies when multiple atoms are coordinated (otherwise the κ notation is used). In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity (not hapticity), and the κ notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.

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 refer to ligands containing extended π systems or where agostic bonding is not obvious from the formula.

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: dichlorobis(η5 2,4 cyclopentadien 1 yl)titanium.

Molecules with polyhapto ligands are often fluxional, also known as stereochemically non rigid. Two classes of fluxionality 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 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 of organic chemistry, it is an example of a sigmatropic rearrangement. A related example is Bis(cyclooctatetraene)iron, in which the η4 and η6 C8H8 rings interconvert. Case 2, typically: complexes containing cyclic polyhapto ligands 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 torsion" explains, inter alia, why only one isomer can be isolated for Fe(η5 C5H4Br)2 since the torsional barrier is very low.