Hapticity
In
History
The need for additional nomenclature for organometallic compounds became apparent in the mid-1950s when Dunitz,
Historically important compounds where the ligands are described with hapticity
- 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.[8][9]
- IrCl(CO)[P(C6H5)3]2(η2-O2): the dioxygen derivative which forms reversibly upon oxygenation of Vaska's complex.
Examples
The η-notation is encountered in many coordination compounds:
- Side-on bonding of molecules containing σ-bonds like H2:
- Side-on bonded ligands containing multiple bonded atoms, e.g. fullerene, which is bonded through donation of the π-bonding electrons:
- K[PtCl3(η2-C2H4)].H2O
- Related complexes containing bridging π-ligands:
- (μ-η2:η2-
- Dioxygenin 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 rings can share their electrons.
- Apparent violations of the 18-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 the sulfur heterocycle thiophene and Cr(η6-B3N3Me6)(CO)3 contains a coordinated inorganic ring (B3N3 ring).
Electrons donated by "π-ligands" versus hapticity
Ligand | Electrons contributed (neutral counting) |
Electrons contributed (ionic counting) |
---|---|---|
η1- allyl |
1 | 2 |
η3- allyl cyclopropenyl |
3 | 4 |
η2-butadiene | 2 | 2 |
η4-butadiene | 4 | 4 |
η1-cyclopentadienyl | 1 | 2 |
η3-cyclopentadienyl | 3 | 4 |
η5-cyclopentadienyl pentadienyl cyclohexadienyl |
5 | 6 |
η2-benzene | 2 | 2 |
η4-benzene | 4 | 4 |
η6-benzene | 6 | 6 |
η7-cycloheptatrienyl | 7 | 6 or 10 |
η8- cyclooctatetraenyl |
8 | 10 |
Changes in hapticity
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-called
Hapticity vs. denticity
Hapticity must be distinguished from
Hapticity and fluxionality
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 sigmatropic rearrangement.[citation needed] 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 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.
References
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- IUPAC. 2004. p. 16.
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