Potassium octachlorodirhenate

Potassium octachlorodirhenate

Identifiers
CAS Number	13841-78-6
Properties
Chemical formula	K2Re2Cl8
Appearance	blue solid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium octachloridodirhenate(III) is an inorganic compound with the formula K₂Re₂Cl₈. This dark blue salt is well known as an early example of a compound featuring quadruple bond between its metal centers. Although the compound has no practical value, its characterization was significant in opening a new field of research into complexes with quadruple bonds.^[1]

Synthesis and reactions

Soviet chemists first reported K₂[Re₂Cl₈] in 1954, but it was not until 1964 that a different study correctly characterized the compound as featuring a short Re-Re bond, the first of its kind discovered.^[2] The results of this classic study subsequently led to new work into other metals capable of forming metal bonds, such as Cr, Mo, W, and Tc.^[3][4][5]

A high-yield synthesis of the tetrabutylammonium salt involves treating the perrhenate salt with benzoyl chloride followed by HCl:

2 [(n-C₄H₉)₄N]ReO₄ + 8 C₆H₅COCl → [(n-C₄H₉)₄N]₂[Re₂Cl₈] + organic products

Structure of the anion Re₂Cl₈²⁻.

Octachloridodirhenate(III) is a precursor to other complexes with multiply bonded rhenium centers as the quadruple bond is quite stable and is often maintained in ligand-substitution reactions. For example, upon treatment with concentrated HBr, the complex forms the analogous anion [Re₂Br₈]²⁻, which can easily be converted into other dirhenium species.^[6]

Structure and bonding

In the [Re₂Cl₈]²⁻, the Re-Re bond distance is 2.24 Å, the Re-Re-Cl bond angles are 104°, and the Cl-Re-Cl angles are 87°. The chloride ligands are fully eclipsed. Although this geometry results in repulsive interactions between the chloride ions,, this conformation allows for maximum δ-δ overlap between the Re(III) centers, a factor which overrides the unfavorable chloride repulsions. The [Re₂Cl₈]²⁻ anion has weak electrophilic character.^[7][1] With the configuration, Re(III) is well suited to engage in quadruple bonding, Electrons are allocated to give the configuration σ², π⁴, δ², resulting in a bond order of 4 between the Re centers. The brilliant color of the [Re₂Cl₈]²⁻ arises from the δ→δ* electronic transition.^[6]

References

1. Cotton, F. A.; Walton, R. A. “Multiple Bonds Between Metal Atoms” Oxford (Oxford): 1993. ISBN 0-19-855649-7.
2. Cotton, F. A.; Harris, C. B. Inorg. Chem., 1965, 4 (3), 330-333. doi:10.1021/ic50025a015
3. Hao, S.; Gamboratta, S.; Bensimon, C. J. Amer. Chem. Soc. 1992, 114, 3556. doi:10.1021/ja00035a061
4. Cotton, F.A.; Eglin, J.L.; Hong, B.; James, C.A. J. Amer. Chem. Soc. 1992, 114, 4915. doi:10.1021/ja00038a074}
5. Cotton, F.A.; Chen, H.; Daniels, L.M.; Feng, X. J. Amer. Chem. Soc. 1992, 114, 8980. doi:10.1021/ja00049a031
6. Barder, T.J.; Walton, R.A. Inorg. Synth. 1985, 23, 116-118. doi:10.1002/9780470132548.ch22
7. Shtemenko, A. V.; Kozhura, O. V.; Pasenko, A. A.; Domasevitch, K. V. Polyhedron, 2003, 22(12), 4191-4196. doi:10.1016/S0277-5387(03)00288-2