Potassium nonahydridorhenate(VII) 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 che ...

having the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a '' chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betw ...

K₂ReH₉. This colourless

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 quantiti ...

is soluble in water but only poorly soluble in most alcohols. The anion is a rare example of 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. Ma ...

bearing only

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...

ligands 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 elec ...

History

The study of

rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is ...

hydrides can be traced to the 1950s and included reports of the "rhenide" anion, supposedly Re⁻. These reports led to a series of investigations by A. P. Ginsberg and coworkers on the products from the reduction of

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 isoelectronic permanganate. The perrhenate anion is stable ove ...

. The ''rhenide'' anion, Re⁻, was based on the product of the reduction of

salts, such as the reduction of potassium perrhenate () by potassium metal. "Potassium rhenide" was shown to exist as a tetrahydrated complex, with the postulated chemical formula . This compound exhibits strongly reducing properties, and slowly yields hydrogen gas when dissolved in water. The lithium and thallous salts were also reported. Later research, however, indicates that the "rhenide" ion is actually a hydridorhenate complex. "Potassium rhenide" was shown to be in fact the nonahydridorhenate, , containing the anion in which the oxidation state of rhenium is actually +7. Other methods of reduction of perrhenate salts yield compounds containing other hydrido- complexes, including .

Structure, synthesis, and properties

is an unusual example of a nonacoordinated complex, its high

coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central io ...

being attributed to the small size of the hydride ligand and the high positive charge on the Re(VII) center. Its structure consists of a tricapped trigonal prism, as determined by neutron crystallography. The

diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...

sodium salt, like the analogous

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 spontaneou ...

compound, is prepared by treating an ethanol solution of sodium perrhenate, NaReO₄, with sodium metal. Via cation exchange, it can be converted to the corresponding

tetraethylammonium Tetraethylammonium (TEA), () or (Et4N+) is a quaternary ammonium cation consisting of four ethyl groups attached to a central nitrogen atom, and is positively charged. It is a counterion used in the research laboratory to prepare lipophilic salt ...

salt, (N(C₂H₅)₄)₂ReH₉. Isostructural with , consists of a trigonal prism with Re atom in the center and six hydrogen atoms at the corners. Three more hydrogen ligands define a triangle lying parallel to the base and crossing the prism in its center (see figure). Although those hydride ligands are not equivalent, their electronic structure is almost the same. The coordination number of 9 in this complex is the highest known for any rhenium complex. Recent DFT calculations on indicate that this dianion adopts the D_3h⇌C_4v⇌D_3h pathway in gas phase and solution, such interconversion originally proposed by Muetterties featuring a capped square antiprism structure as

transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...

has very low energy barrier. In solid, intramolecular motions of include (1) circle-dance mechanism (resembling Matisse's painting ''Dance (II)'') and (2) three-arm

turnstile A turnstile (also called a turnpike, gateline, baffle gate, automated gate, turn gate in some regions) is a form of gate which allows one person to pass at a time. A turnstile can be configured to enforce one-way human traffic. In addition, a t ...

rotation.

References

{{Rhenium compounds Metal hydrides Potassium compounds Rhenium compounds