Borderline hydrides
Encyclopedia
Borderline hydrides typically refer to hydride
s formed of hydrogen and elements
of the periodic table
in group 11
and group 12
and indium
(In) and thallium
(Tl). These compounds have properties intermediate between covalent hydrides and saline hydrides. Hydride
s are chemical compounds that contain a metal
and hydrogen
acting as a negative ion
.
and covalent bond
types. A specific examples of a borderline hydride CuH2, cupric hydride, that appears as a spongy reddish-brown substance is a moderate reducing agent
. It will catalytically oxidize hypophosphorous acid
to phosphorous acid
at room temperature, and it gives off hydrogen gas when subjected to heat.
ZnH2 is also a solid at room temperature that breaks down at 90°C, but even left alone decomposes over several days to zinc metal and hydrogen gas. Hydrogen telluride
(H2Te) and hydrogen selenide
(H2Se) are both borderline hydrides of high volatility that produce strong, unpleasant odors.
For instance, cupric hydride is formed by reacting copper sulphate and hypophosphorous acid at about 70°C, forming a yellow precipitate that soon turns red-brown.
Zinc hydride, ZnH2 can be formed by the reduction of either a zinc halide
or dimethylzinc
.
, indicating that a single coordination point on the metal atom bonds to two contiguous atoms from another molecule, in this case H2. A well-known example of this is from the first such molecule to be synthesized with a coordinated hydrogen ligand (dihydrogen complex
): W(CO)3(PPri3)2(η2-H2).
Classic dihydrides containing the dihydride M-(H)2 ligands are typically found as a tautomer
with the non-classical dihydrogen complexes containing a M-(η2-H2) group.
Borderline hydrides exist with a bond character somewhere between the classical and non-classical hydrides. Those that are thermally unstable exhibit stretching frequencies νHH greater than 2150 cm1 as a result of poor electron donation from the metal center. An electron dense metal center will yield hydride with a νHH less than 2060 cm1, while anything between is considered to be in the borderline region. Kubas, et al. state that a stretching frequency of 2090 cm1 is within the bounds of stable H2 complexes while 2060 cm1 is right on the borderline between dihydrogen and dihydrides.
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
s formed of hydrogen and elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
of the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
in group 11
Group 11 element
A Group 11 element is one in the series of elements in group 11 in the periodic table, consisting of transition metals which are the traditional coinage metals of copper , silver , and gold...
and group 12
Group 12 element
A group 12 element is one of the elements in group 12 in the periodic table. This includes zinc , cadmium and mercury . The further inclusion of copernicium in group 12 is supported by recent experiments on individual Cn atoms...
and indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
(In) and thallium
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
(Tl). These compounds have properties intermediate between covalent hydrides and saline hydrides. Hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
s are chemical compounds that contain a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
acting as a negative ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
.
Properties
Borderline hydrides exhibit bonding characteristics between ionicIonic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
and 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....
types. A specific examples of a borderline hydride CuH2, cupric hydride, that appears as a spongy reddish-brown substance is a moderate reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
. It will catalytically oxidize hypophosphorous acid
Hypophosphorous acid
Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H3PO2. Inorganic chemists refer to the free acid by this name , or the acceptable name of phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols...
to phosphorous acid
Phosphorous acid
Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic , not triprotic as might be suggested by this formula. Phosphorous acid is as an intermediate in the preparation of other phosphorus compounds.-Nomenclature and tautomerism:H3PO3 is more clearly described with...
at room temperature, and it gives off hydrogen gas when subjected to heat.
ZnH2 is also a solid at room temperature that breaks down at 90°C, but even left alone decomposes over several days to zinc metal and hydrogen gas. Hydrogen telluride
Hydrogen telluride
Hydrogen telluride is the inorganic compound with the formula H2Te. The simplest hydride of tellurium, it is rarely encountered because of it decomposes rapidly to the constituent elements. Most compounds with Te-H bonds are unstable with respect to loss of H2. H2Te is chemically and structurally...
(H2Te) and hydrogen selenide
Hydrogen selenide
Hydrogen selenide is the inorganic compound with the formula H2Se. It is the simplest and virtually the only hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit: 0.05 ppm over an 8 hour period...
(H2Se) are both borderline hydrides of high volatility that produce strong, unpleasant odors.
Examples
- CuH2 copper hydrideCopper hydrideCopper hydride is a binary compound of hydrogen and copper. The Cu-H bond is weak and therefore the compound when formed is unstable. The reaction of copper sulfate and sodium hypophosphite with some sulfuric acid gives a red-colored precipitate. By heating the precipitate it decomposes to...
- ZnH2 zinc hydrideZinc hydrideZinc hydride is a chemical compound of zinc and hydrogen, ZnH2, which is used as a reducing agent in organic synthesis. First reported in 1947, it is a white crystalline powder when freshly made which turns grey if left at room temperature for a few days, presumably due to the decompostion to...
- TlH3 thallium hydrideThallium hydrideThallium Hydride is a compound composed of Thallium and Hydrogen atoms . Thallium is the heaviest member of the Group 13 metals, the stability of group 13 metal hydrides decreases with increasing periodic number. This is commonly attributed to poor overlap of the valence metal orbitals with that...
- H2Po polonium hydrideHydrogen polonideHydrogen polonide is a chemical compound with the formula H2Po. It is a more covalent compound than most metal hydrides because polonium is more like a nonmetal...
- H2Te hydrogen tellurideHydrogen tellurideHydrogen telluride is the inorganic compound with the formula H2Te. The simplest hydride of tellurium, it is rarely encountered because of it decomposes rapidly to the constituent elements. Most compounds with Te-H bonds are unstable with respect to loss of H2. H2Te is chemically and structurally...
- H2Se hydrogen selenideHydrogen selenideHydrogen selenide is the inorganic compound with the formula H2Se. It is the simplest and virtually the only hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit: 0.05 ppm over an 8 hour period...
Synthesis
Borderline hydrides are most commonly formed via the acidification or reduction of metal salts.For instance, cupric hydride is formed by reacting copper sulphate and hypophosphorous acid at about 70°C, forming a yellow precipitate that soon turns red-brown.
Zinc hydride, ZnH2 can be formed by the reduction of either a zinc halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
or dimethylzinc
Dimethylzinc
Dimethylzinc, also known as Zinc methyl, DMZ, or DMZn is a colorless mobile liquid Zn2, formed by the action of methyl iodide on zinc at elevated temperature or on zinc sodium alloy....
.
Alternative Definition
A more recent definition of borderline hydrides refers to hydrides that exist between classic and non-classic dihydrides. The classic form is the dihydride M(H)2 configuration, where the metal is bound to two free hydrogen atoms. The non-classic form contains two hydrogen atoms bound to a central metal atom with an η2-H2 hapticityHapticity
The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
, indicating that a single coordination point on the metal atom bonds to two contiguous atoms from another molecule, in this case H2. A well-known example of this is from the first such molecule to be synthesized with a coordinated hydrogen ligand (dihydrogen complex
Dihydrogen complex
Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. The prototypical complex is W32. This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported...
): W(CO)3(PPri3)2(η2-H2).
Classic dihydrides containing the dihydride M-(H)2 ligands are typically found as a tautomer
Tautomer
Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...
with the non-classical dihydrogen complexes containing a M-(η2-H2) group.
Borderline hydrides exist with a bond character somewhere between the classical and non-classical hydrides. Those that are thermally unstable exhibit stretching frequencies νHH greater than 2150 cm1 as a result of poor electron donation from the metal center. An electron dense metal center will yield hydride with a νHH less than 2060 cm1, while anything between is considered to be in the borderline region. Kubas, et al. state that a stretching frequency of 2090 cm1 is within the bounds of stable H2 complexes while 2060 cm1 is right on the borderline between dihydrogen and dihydrides.