Potassium octachlorodirhenate(III) is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the formula K₂ Re₂ Cl₈. This dark blue

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

is well known as an early example of a compound featuring

quadruple bond A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the m ...

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.

Synthesis and reactions

Soviet chemists first reported K₂ e₂Cl₈in 1954, but it was not until 1964 that

Cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor perce ...

and Harris characterized the compound as featuring a short Re–Re bond, the first of its kind discovered. The results of this classic study subsequently led to new work into other metals capable of forming metal–metal bonds, such as

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...

, and

technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...

. A high-yield synthesis of the tetrabutylammonium salt involves treating the

perrhenate The perrhenate ion is the anion with the formula , or a compound containing this ion. The perrhenate anion is tetrahedral, being similar in size and shape to perchlorate and the valence isoelectronicity, isoelectronic permanganate. The perrhenate a ...

salt with

benzoyl chloride Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It is ...

followed by HCl: :2 ''n''-C₄H₉)₄N.html" ;"title="n-Butyl">''n''-C₄H₉)₄N">n-Butyl">''n''-C₄H₉)₄N ReO₄ + 8 C₆H₅COCl → ''n''-C₄H₉)₄Nsub>2 e₂Cl₈+ organic products : Octachlorodirhenate(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 An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

e₂Br₈sup>2−, which can easily be converted into other dirhenium species.

Structure and bonding

In the e₂Cl₈sup>2−, the Re–Re

bond distance In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...

is 2.24 Å, the Re–Re–Cl

bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemical ...

s are 104°, and the Cl–Re–Cl angles are 87°. The chloride ligands are fully

eclipsed In chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X–A–B–Y is 0°. Such a conformation can exist in any open chain, ...

. 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 e₂Cl₈sup>2− anion has a weak

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

character. With the configuration, Re(III) is well suited to engage in quadruple bonding. Electrons are allocated to give the configuration σ²π⁴δ², resulting in a

bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...

of 4 between the rhenium centers. The brilliant color of the e₂Cl₈sup>2− arises from the δ→δ* electronic transition.

References

{{Rhenium compounds Rhenium compounds Chlorides Metal halides Chloro complexes Potassium compounds Chlorometallates