Hapticity

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 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. Ferrocene: bis(η5-cyclopentadienyl)iron 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. The η-notation is encountered in many coordination compounds: Side-on bonding of molecules containing σ-bonds like H2: W(CO)3(PiPr3)2(η2-H2) Side-on bonded ligands containing multiple bonded atoms, e.g. ethylene in Zeise's salt or with fullerene, which is bonded through donation of the π-bonding electrons: K[PtCl3(η2-C2H4)].