Alkalide
Encyclopedia
An alkalide is a chemical compound
in which alkali metal
s are anions (that is, they bear a negative charge). Such species are notable because alkali metals were previously thought to appear in salt
s only as cations. Alkalide compounds have also been synthesized containing a cation of the alkaline earth metal barium
.
for this ionic compound
. The traditional explanation for this phenomenon is that the loss of one electron
from elemental sodium to produce a cation with a single positive charge produces a stable closed-shell electron configuration
. Sodium was thought to always form singly charged cations until the discovery of alkalides and the same arguments apply to the remainder of the alkali metals.
−, K
−, Rb
−, and Cs
−. These species are called sodide or natride, potasside or kalide, rubidide, and caeside, respectively. “Lithides”, compounds containing Li
−, are not currently known. The known alkalides, first discovered in the 1970s, are of theoretical interest due to their unusual stoichiometry
and low ionization potential
s. Alkalide species are chemically related to the electride
s, salts containing trapped electrons as the "anions".
)]+Na−. This salt contains both Na+ and Na−. The cryptand isolates and stabilizes the Na+, preventing its reduction by the Na−. Dimers of cationic and anionic sodium have also been observed, as has an H+Na− salt known as "inverse sodium hydride
".
Normally, alkalides are thermally labile due to the high reactivity of the alkalide anion, which is theoretically able to break most covalent bond
s including the C–O bonds in a typical cryptand
. The introduction of a special cryptand ligand containing amines instead of ether linkages has allowed the isolation of kalide and natrides that are stable at room temperature.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
in which alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
s are anions (that is, they bear a negative charge). Such species are notable because alkali metals were previously thought to appear in salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s only as cations. Alkalide compounds have also been synthesized containing a cation of the alkaline earth metal barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
.
"Normal" chemistry: the case of Na+
Alkali metals are well known to form salts. Table salt, or sodium chloride Na+Cl−, illustrates the usual role of an alkali metal such as sodium: its positive charge is balanced by a negatively charged ion in the empirical formulaEmpirical formula
In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
for this ionic compound
Ionic compound
In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...
. The traditional explanation for this phenomenon is that the loss of one electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
from elemental sodium to produce a cation with a single positive charge produces a stable closed-shell electron configuration
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
. Sodium was thought to always form singly charged cations until the discovery of alkalides and the same arguments apply to the remainder of the alkali metals.
Scope of alkalides
Known alkalides include NaSodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
−, K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
−, Rb
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
−, and Cs
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
−. These species are called sodide or natride, potasside or kalide, rubidide, and caeside, respectively. “Lithides”, compounds containing Li
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
−, are not currently known. The known alkalides, first discovered in the 1970s, are of theoretical interest due to their unusual stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
and low ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
s. Alkalide species are chemically related to the electride
Electride
An electride is an ionic compound in which an electron is the anion. The first electrides to be studied in depth were solutions of alkali metals in ammonia. When sodium metal dissolves in ammonia, the result is a blue solution consisting of [Na6]+ and solvated electrons. Such solutions are powerful...
s, salts containing trapped electrons as the "anions".
Examples
A typical alkalide is the sodium natride salt [Na(2,2,2-cryptCryptand
Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers,...
)]+Na−. This salt contains both Na+ and Na−. The cryptand isolates and stabilizes the Na+, preventing its reduction by the Na−. Dimers of cationic and anionic sodium have also been observed, as has an H+Na− salt known as "inverse sodium hydride
Sodium hydride
Sodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
".
Normally, alkalides are thermally labile due to the high reactivity of the alkalide anion, which is theoretically able to break most covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s including the C–O bonds in a typical cryptand
Cryptand
Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers,...
. The introduction of a special cryptand ligand containing amines instead of ether linkages has allowed the isolation of kalide and natrides that are stable at room temperature.