Hapticity: Difference between revisions
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The Greek letter η (''eta'') is used to denote the number of atoms of a [[ligand]] that bind to a metal center. It is a key part of the nomenclature of [[organometallic chemistry]]. |
The Greek letter η (''eta'') is used to denote the number of atoms of a [[ligand]] that bind to a metal center. It is a key part of the nomenclature of [[organometallic chemistry]]. |
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The term is thus intended to indicate how a ligand, usually a hydrocarbon, is held by the metal. A superscripted number following the η denotes the number of atoms of a pi-delocalized ligand bound to the metal. For example, (η<sup>1</sup>-C<sub>3</sub>H<sub>5</sub>)Mn(CO)<sub>5</sub> means that the C<sub>3</sub>H<sub>5</sub> ligand ("allyl") is bound to Mn via one carbon atom. In contrast, the 18-electron rule anticipates that decarbonylation of that molecule would give the η<sup>3</sup> derivative, (η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Mn(CO)<sub>4</sub>.{{ref|Shriver&Atkins}} |
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==Examples== |
==Examples== |
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Revision as of 18:54, 23 July 2006
The Greek letter η (eta) is used to denote the number of atoms of a ligand that bind to a metal center. It is a key part of the nomenclature of organometallic chemistry.
The term is thus intended to indicate how a ligand, usually a hydrocarbon, is held by the metal. A superscripted number following the η denotes the number of atoms of a pi-delocalized ligand bound to the metal. For example, (η1-C3H5)Mn(CO)5 means that the C3H5 ligand ("allyl") is bound to Mn via one carbon atom. In contrast, the 18-electron rule anticipates that decarbonylation of that molecule would give the η3 derivative, (η3-C3H5)Mn(CO)4.[1]
Examples
- The molecule Fe(C5H5)2(CO)2 is usefully described as (η5-C5H5)Fe(η1-C5H5)(CO)2. This species contains the cyclopentadienyl ligand bonded in differing "hapticities."
- Reduction of the hexamethylbenzene complex [Ru(C6Me6)2]2+ by 2e gives the neutral derivative with the same formula, [Ru(C6Me6)2]. This transformation can be explained within the context of the 18 electron rule by specifying the hapticity:
- [Ru(η6-C6Me6)2]2+ + 2 e- → [Ru(η6-C6Me6)(η4-C6Me6)]
- Usually polyhapto ligands are π-delocalized hydrocarbons, but they also include heterocycles, as illustrated by the thiophene complex, Cr(η5-C4H4S)(CO)3. Even inorganic rings are described in this manner, a famous example is the hexamethyl borazine complex Cr(η6-B3N3Me6)(CO)3.
Electrons donated by pi-bonding organic ligands vs. hapticity
| Ligand | Electrons contributed (neutral counting) |
Electrons contributed (ionic counting) |
|---|---|---|
| η-1 Allyl | 1 | 2 |
| η-3 Allyl | 3 | 4 |
| η-2Butadiene | 2 | 2 |
| η-4Butadiene | 4 | 4 |
| η-1cyclopentadienyl | 1 | 2 |
| η-3cyclopentadienyl | 3 | 4 |
| η-5cyclopentadienyl | 5 | 6 |
| η-2Benzene | 2 | 2 |
| η-4Benzene | 4 | 4 |
| η-6Benzene | 6 | 6 |
Hapticity and fluctionality
Molecules with polyhapto ligands are often "fluctional", also known as stereochemically non-rigid. Two classes of fluctionality are prevalent for organometallic complexes of polyhapto ligands:
- Case 1, when the hapticity value is less than the number of sp2 carbon atoms. In such cases, the metal will migrate in from carbon to carbon, maintaining the same net hapticity. The η1-C5H5 ligand in (η5-C5H5)Fe( η1-C5H5)(CO)2 undergoes rearranges rapidly in solution such that Fe binds alternatingly to each carbon. This reaction is degenerate and, in the jargon or organic chemistry, the process is called a sigmatropic rearrangement.
- Case 2, complexes containing cyclc polyhapto ligand with maximized hapticity. Such ligands tend to rotate around the metal. A famous example is ferrocene, Fe(η5-C5H5)2, wherein the Cp rings rotate with a low energy barrier about an imaginary axis. This "ring whizzing" explains, inter alia, why only one isomer can be isolated for Fe(η5-C5H4Br)2. In this case, the motions are not necessarily degenerate, but they do have low energies of activation.
Hapticity vs. denticity
In some circles of inorganic chemistry, hapticity is distinguished from denticity. Hapticity usually applies to pi-bonded ligands such as alkenes, 1,3-dialkenes, and their protonated or deprotonated derivatives. Denticity is reserved for chelating ligands that are composed of individual binding atoms. In [Co(EDTA)]-, the EDTA4- is hexadentate and can be described using the notation [Co(κ6-EDTA)]-.
References
- ^ Shriver, D.; Atkins, P. (1999). Inorganic Chemistry, New York: W. H. Freeman.