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In chemistry, hexol is a
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Cobalt compounds Stereochemistry Coordination complexes
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 conve ...
with formula 6+ — a coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
consisting of four cobalt
Cobalt is a chemical element with the 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. The free element, ...
cations in oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...
+3, twelve ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
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 conve ...
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 ( 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 ele ...
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 (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from i ...
compounds.
Preparation
Salts of hexol were first described byJørgensen
Jorgensen or Joergensen (original spelling: Jørgensen ) is a common Danish-Norwegian patronymic surname meaning "son of Jørgen" (Danish version of the Greek Γεώργιος (geōrgios), cf. English George). Jørgensen is the tenth most common ...
, 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 usingammonium 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, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...
, hexol sulfate converts to ''cis''-diaquotetramminecobalt(III) sulfate. In boiling dilute sulfuric acid, hexol sulfate further degrades with evolution of oxygen and nitrogen.
Optical properties
The hexol cation exists as twooptical isomer
In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are d ...
s 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 mathematical group of symmetry operations ( isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every ...
. 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-(+)-bromocamphor
Camphor () is a waxy, colorless solid with a strong aroma. It is classified as a terpenoid and a cyclic ketone. It is found in the wood of the camphor laurel ('' Cinnamomum camphora''), a large evergreen tree found in East Asia; and in the k ...
sulfonate – 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
In chemistry, specific rotation ( '') is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sa ...
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
{{reflistExternal links
* Hexol Molecule of the Month September 199Website
Cobalt compounds Stereochemistry Coordination complexes