chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, hexol is a

cation 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 ...

with formula ⁶⁺ — a

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

consisting of four

cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

cations in

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

+3, twelve

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

molecules , and six hydroxy

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 conven ...

s , with a net charge of +6. The hydroxy groups act as bridges between the central cobalt atom and the other three, which carry the ammonia

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s. Salts of hexol, such as the sulfate (SO₄)₃(H₂O)_x, are of historical significance as the first synthetic non-carbon-containing

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

compounds.

Preparation

Salts of hexol were first described by Jørgensen, although it was Werner who recognized its structure. The cation is prepared by heating a solution containing the ''cis''-diaquotetramminecobalt(III) cation o(NH₃)₄(H₂O)₂sup>3+ with a dilute base: :4 o(NH₃)₄(H₂O)₂sup>3+ + 2 HO⁻ → ⁶⁺ + 4 NH₄⁺ + 4 H₂O

Hexol sulfate

Starting with the sulfate and using

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

as the base, depending on the conditions, one obtains the 9-hydrate, the 6-hydrate, or the 4-hydrate of hexol sulfate. These salts form dark brownish-violet or black tabular crystals, with low solubility in water. When treated with concentrated

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

, hexol sulfate converts to ''cis''-diaquotetramminecobalt(III) sulfate. In boiling dilute

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

, hexol sulfate further degrades with evolution of oxygen and nitrogen.

Optical properties

The hexol cation exists as two optical isomers that are mirror images of each other, depending on the arrangement of the bonds between the central cobalt atom and the three bidentate peripheral units o(NH₃)₄(HO)₂ It belongs to the D

point group In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...

. The nature of chirality can be compared to that of the ferrioxalate anion . In a historic set of experiments, a salt of hexol with an optically active anion — specifically, its D-(+)-bromo camphorsulfonate – was resolved into separate salts of the two cation isomers by fractional crystallisation. A more efficient resolution involves the bis(tartrato)diantimonate(III) anion. The hexol hexacation has a high specific rotation of 2640°.

"Second hexol"

Werner also described a second achiral hexol (a minor byproduct from the production of Fremy's salt) that he incorrectly identified as a linear tetramer. The second hexol is hexanuclear (contains six cobalt centres in each ion), not tetranuclear. Its point group is C, and its formula is , whereas that of hexol is . :

References

{{reflist

External links

* Hexol Molecule of the Month September 199
Website

Cobalt(III) compounds Stereochemistry Ammine complexes Cobalt complexes