Incoordination chemistry,hapticity is thecoordination of aligand to a metal center via an uninterrupted and contiguous series ofatoms.[1] The hapticity of a ligand is described with the Greek letterη ('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 theκ-notation is used). In addition, if the ligand coordinates through multiple atoms that arenot contiguous then this is considereddenticity[2] (not hapticity), and the κ-notation is used once again.[3] When naming complexes care should be taken not to confuse η withμ ('mu'), which relates tobridging ligands.[4][5]
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 byX-ray crystallography[6] where aniron atom is"sandwiched" between two parallelcyclopentadienyl rings.Cotton later proposed the termhapticity derived from the adjectival prefix hapto (from the Greekhaptein, to fasten, denoting contact or combination) placed before the name of the olefin,[7] 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 whereagostic bonding is not obvious from the formula.
Historically important compounds where the ligands are described with hapticity
Side-on bonded ligands containing multiple bonded atoms, e.g.ethylene inZeise's salt or withfullerene, which is bonded through donation of the π-bonding electrons:
Dioxygen in bis{(trispyrazolylborato)copper(II)}(μ-η2:η2-O2),
Note that with somebridging 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 (seehapticity vs. denticity).
The bonding of π-bonded species can be extended over several atoms, e.g. inallyl,butadiene ligands, but also incyclopentadienyl orbenzene rings can share their electrons.
Apparent violations of the18-electron rule sometimes are explicable in compounds 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 18-VE compound: [Ru(η6-C6Me6)(η4-C6Me6)].
Examples of polyhapto coordinated heterocyclic and inorganic rings: Cr(η5-C4H4S)(CO)3 contains thesulfur heterocyclethiophene and Cr(η6-B3N3Me6)(CO)3 contains a coordinated inorganic ring (B3N3 ring).
The hapticity of a ligand can change in the course of a reaction.[12] E.g. in a redox reaction:
Here one of the η6-benzene rings changes to a η4-benzene.
Similarly hapticity can change during a substitution reaction:
Here the η5-cyclopentadienyl changes to an η3-cyclopentadienyl, giving room on the metal for an extra 2-electron donating ligand 'L'. Removal of one molecule of CO and again donation of two more electrons by the cyclopentadienyl ligand restores the η5-cyclopentadienyl. The so-calledindenyl effect also describes changes in hapticity in a substitution reaction.
Hapticity must be distinguished fromdenticity. 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 of1,2-bis(diphenylphosphino)ethane (Ph2PCH2CH2PPh2), toNiCl2 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. intitanocene dichloride: dichlorobis(η5-2,4-cyclopentadien-1-yl)titanium.[13]
Molecules with polyhapto ligands are oftenfluxional, 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 isdegenerate and, in the jargon oforganic chemistry, it is an example of asigmatropic rearrangement.[citation needed] A related example isBis(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 isferrocene,[14] Fe(η5-C5H5)2, wherein the Cp rings rotate with a lowenergy barrier about theprincipal axis of the molecule that "skewers" each ring (seerotational 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.
^Jun Ho Shin; Brian M. Bridgewater; David G. Churchill; Mu-Hyun Baik; Richard A. Friesner; Gerard Parkin (2001). "An Experimental and Computational Analysis of the Formation of the Terminal Nitrido Complex (η3-Cp*)2Mo(N)(N3) by Elimination of N2 from Cp*2Mo(N3)2: The Barrier to Elimination Is Strongly Influenced by the exo versus endo Configuration of the Azide Ligand".J. Am. Chem. Soc.123 (41):10111–10112.doi:10.1021/ja011416v.
^Bunker, P.R. (1965). "The Vibrational Selection Rules and Torsional Barrier of Ferrocene".Molecular Physics.9 (3):247–255.doi:10.1080/00268976500100321.
2.png)
.png)
