Binary compounds of hydrogen are binary chemical compounds containing just hydrogen and one other chemical element. By convention all binary hydrogen compounds are called hydrides even when the hydrogen atom in it is not an anion. [1] [2] [3] [4] These hydrogen compounds can be grouped into several types.
Binary hydrogen compounds in group 1 are the ionic hydrides (also called saline hydrides) wherein hydrogen is bound electrostatically. Because hydrogen is located somewhat centrally in an electronegative sense, it is necessary for the counterion to be exceptionally electropositive for the hydride to possibly be accurately described as truly behaving ionic. Therefore, this category of hydrides contains only a few members.
Hydrides in group 2 are polymeric covalent hydrides. In these, hydrogen forms bridging covalent bonds, usually possessing mediocre degrees of ionic character, which make them difficult to be accurately described as either covalent or ionic. The one exception is beryllium hydride, which has definitively covalent properties.
Hydrides in the transition metals and lanthanides are also typically polymeric covalent hydrides. However, they usually possess only weak degrees of ionic character. Usually, these hydrides rapidly decompose into their component elements at ambient conditions. The results consist of metallic matrices with dissolved, often stoichiometric or near so, concentrations of hydrogen, ranging from negligible to substantial. Such a solid can be thought of as a solid solution and is alternately termed a metallic- or interstitial hydride. These decomposed solids are identifiable by their ability to conduct electricity and their magnetic properties (the presence of hydrogen is coupled with the delocalisation of the valence electrons of the metal), and their lowered density compared to the metal. Both the saline hydrides and the polymeric covalent hydrides typically react strongly with water and air.
It is possible to produce a metallic hydride without requiring decomposition as a necessary step. If a sample of bulk metal is subjected to any one of numerous hydrogen absorption techniques, the characteristics, such as luster and hardness of the metal is often retained to a large degree. Bulk actinoid hydrides are only known in this form. The affinity for hydrogen for most of the d-block elements are low. Therefore, elements in this block do not form hydrides (the hydride gap) under standard temperature and pressure with the notable exception of palladium. [5] Palladium can absorb up to 900 times its own volume of hydrogen and is therefore actively researched in the field hydrogen storage.
Elements in group 13 to 17 (p-block) form covalent hydrides (or nonmetal hydrides). In group 12 zinc hydride is a common chemical reagent but cadmium hydride and mercury hydride are very unstable and esoteric. In group 13 boron hydrides exist as a highly reactive monomer BH3, as an adduct for example ammonia borane or as dimeric diborane and as a whole group of BH cluster compounds. Alane (AlH3) is a polymer. Gallium exists as the dimer digallane. Indium hydride is only stable below −90 °C (−130 °F). Not much is known about the final group 13 hydride, thallium hydride.
Due to the total number of possible binary saturated compounds with carbon of the type CnH2n+2 being very large, there are many group 14 hydrides. Going down the group the number of binary silicon compounds (silanes) is small (straight or branched but rarely cyclic) for example disilane and trisilane. For germanium only 5 linear chain binary compounds are known as gases or volatile liquids. Examples are n-pentagermane, isopentagermane and neopentagermane. Of tin only the distannane is known. Plumbane is an unstable gas.
The hydrogen halides, hydrogen chalcogenides and pnictogen hydrides also form compounds with hydrogen, whose lightest members show many anomalous properties due to hydrogen bonding.
Non-classical hydrides are those in which extra hydrogen molecules are coordinated as a ligand on the central atoms. These are very unstable but some have been shown to exist.
Polyhydrides or superhydrides are compounds in which the number of hydrogen atoms exceed the valency of the combining atom. These may only be stable under extreme pressure, but may be high temperature superconductors, such as H3S, superconducting at up to 203 K. Polyhydrides are actively studied with the hope of discovering a room temperature superconductor.
The relative stability of binary hydrogen compounds and alloys at standard temperature and pressure can be inferred from their standard enthalpy of formation values. [6]
H2 0 | He | ||||||||||||||||
LiH −91 | BeH2 negative | BH3 41 | CH4 −74.8 | NH3 −46.8 | H2O −243 | HF −272 | Ne | ||||||||||
NaH −57 | MgH2 −75 | AlH3 −46 | SiH4 31 | PH3 5.4 | H2S −20.7 | HCl −93 | Ar | ||||||||||
KH −58 | CaH2 −174 | ScH2 | TiH2 | VH | CrH | Mn | FeH, FeH2 | Co | Ni | CuH | ZnH2 | GaH3 | GeH4 92 | AsH3 67 | H2Se 30 | HBr −36.5 | Kr |
RbH −47 | SrH2 −177 | YH2 | ZrH2 | NbH | Mo | Tc | Ru | Rh | PdH | Ag | CdH2 | InH3 | SnH4 163 | SbH3 146 | H2Te 100 | HI 26.6 | Xe |
CsH −50 | BaH2 −172 | LuH2 | HfH2 | TaH | W | Re | Os | Ir | Pt | Au | Hg | Tl | PbH4 252 | BiH3 247 | H2Po 167 | HAt positive | Rn |
Fr | Ra | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | Og |
↓ | |||||||||||||||||
LaH2 | CeH2 | PrH2 | NdH2 | PmH2 | SmH2 | EuH2 | GdH2 | TbH2 | DyH2 | HoH2 | ErH2 | TmH2 | YbH2 | ||||
Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No |
Covalent hydrides | metallic hydrides |
Ionic hydrides | Intermediate hydrides |
Do not exist | Not assessed |
The isolation of monomeric molecular hydrides usually require extremely mild conditions, which are partial pressure and cryogenic temperature. The reason for this is threefold - firstly, most molecular hydrides are thermodynamically unstable toward decomposition into their elements; secondly, many molecular hydrides are also thermodynamically unstable toward polymerisation; and thirdly, most molecular hydrides are also kinetically unstable toward these types of reactions due to low activation energy barriers.
Instability toward decomposition is generally attributable to poor contribution from the orbitals of the heavier elements to the molecular bonding orbitals. Instability toward polymerisation is a consequence of the electron-deficiency of the monomers relative to the polymers. Relativistic effects play an important role in determining the energy levels of molecular orbitals formed by the heavier elements. As a consequence, these molecular hydrides are commonly less electron-deficient than otherwise expected. For example, based on its position in the 12th column of the periodic table alone, mercury(II) hydride would be expected to be rather deficient. However, it is in fact satiated, with the monomeric form being much more energetically favourable than any oligomeric form.
The table below shows the monomeric hydride for each element that is closest to, but not surpassing its heuristic valence. A heuristic valence is the valence of an element that strictly obeys the octet, duodectet, and sexdectet valence rules. Elements may be prevented from reaching their heuristic valence by various steric and electronic effects. In the case of chromium, for example, stearic hindrance ensures that both the octahedral and trigonal prismatic molecular geometries for CrH
6 are thermodynamically unstable to rearranging to a Kubas complex structural isomer.
Where available, both the enthalpy of formation for each monomer and the enthalpy of formation for the hydride in its standard state is shown (in brackets) to give a rough indication of which monomers tend to undergo aggregation to lower enthalpic states. For example, monomeric lithium hydride has an enthalpy of formation of 139 kJ mol−1, whereas solid lithium hydride has an enthalpy of −91 kJ mol−1. This means that it is energetically favourable for a mole of monomeric LiH to aggregate into the ionic solid, losing 230 kJ as a consequence. Aggregation can occur as a chemical association, such as polymerisation, or it can occur as an electrostatic association, such as the formation of hydrogen-bonding in water.
1 | 2 | 3 | 4 | 5 | 6 | 5 | 4 | 3 | 2 | 1 | 2 | 3 | 4 | 3 | 2 | 1 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
H 20 | |||||||||||||||||
LiH [7] 139 (−91) | BeH 2 [8] 123 | BH 3 [9] 107 (41) | CH 4 −75 | NH 3 −46 | H 2O −242 (−286) | HF −273 | |||||||||||
NaH [10] 140 (−56) | MgH 2 142 (−76) | AlH 3 [11] 123 (−46) | SiH 4 34 | PH 3 5 | H 2S −21 | HCl −92 | |||||||||||
KH 132 (−58) | CaH 2 192 (−174) | ScH 3 | TiH 4 | VH 2 [12] | CrH 2 [13] | MnH 2 [14] (−12) | FeH 2 [15] 324 | CoH 2 [16] | NiH 2 [17] 168 | CuH [18] 278 (28) | ZnH 2 [19] 162 | GaH 3 [20] 151 | GeH 4 92 | AsH 3 67 | H 2Se 30 | HBr −36 | |
RbH 132 (−47) | SrH 2201 (−177) | YH 3 | ZrH 4 | NbH 4 [12] | MoH 6 [21] | Tc | RuH 2 [15] | RhH 2 [22] | PdH [23] 361 | AgH [18] 288 | CdH 2 [19] 183 | InH 3 [24] 222 | SnH 4 163 | SbH 3 146 | H 2Te 100 | HI 27 | |
CsH 119 (−50) | BaH 2213 (−177) | LuH 3 | HfH 4 | TaH 4 [12] | WH 6 [25] 586 | ReH 4 [14] | Os | Ir | PtH 2 [26] | AuH [18] 295 | HgH 2 [27] 101 | TlH 3 [28] 293 | PbH 4 252 | BiH 3 247 | H 2Po 167 | HAt 88 | |
Fr | Ra | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | |
↓ | |||||||||||||||||
3 | 4 | 5 | 6 | 7 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 2 | ||||
LaH 3 | CeH 4 | PrH 3 | NdH 4 | Pm | SmH 4 | EuH 3 [29] | GdH 3 | TbH 3 | DyH 4 | HoH 3 | ErH 2 | TmH | YbH 2 | ||||
Ac | ThH 4 [30] | Pa | UH 4 [31] | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No |
Monomeric covalent | Oligomeric covalent | ||
Polymeric covalent | Ionic | ||
Polymeric delocalised covalent | |||
Unknown solid structure | Not assessed |
This table includes the thermally unstable dihydrogen complexes for the sake of completeness. As with the above table, only the complexes with the most complete valence is shown, to the negligence of the most stable complex.
A molecular hydride may be able to bind to hydrogen molecules acting as a ligand. The complexes are termed non-classical covalent hydrides. These complexes contain more hydrogen than the classical covalent hydrides, but are only stable at very low temperatures. They may be isolated in inert gas matrix, or as a cryogenic gas. Others have only been predicted using computational chemistry.
8 | 18 | 8 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
LiH(H 2) 2 [7] | Be | BH 3(H 2) | |||||||||||
Na | MgH 2(H 2) n [32] | AlH 3(H 2) | |||||||||||
K | Ca [33] | ScH 3(H 2) 6 [34] [35] | TiH 2(H 2) [36] | VH 2(H 2) [12] | CrH2(H2)2 [37] | Mn | FeH 2(H 2) 3 [38] | CoH(H 2) | Ni(H 2) 4 | CuH(H2) | ZnH 2(H 2) | GaH 3(H 2) | |
Rb | Sr [33] | YH 2(H 2) 3 | Zr | NbH 4(H 2) 4 [39] | Mo | Tc | RuH 2(H 2) 4 [40] | RhH3(H2) | Pd(H 2) 3 | AgH(H2) | CdH(H 2) | InH 3(H 2) [41] | |
Cs | Ba [33] | Lu | Hf | TaH 4(H 2) 4 [12] | WH 4(H 2) 4 [42] | Re | Os | Ir | PtH(H 2) | AuH 3(H 2) | Hg | Tl | |
Fr | Ra | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | |
↓ | |||||||||||||
32 | 18 | ||||||||||||
LaH 2(H 2) 2 | CeH 2(H 2) | PrH 2(H 2) 2 | Nd | Pm | Sm | Eu | GdH 2(H 2) | Tb | Dy | Ho | Er | Tm | Yb |
Ac | ThH4(H2)4 [43] | Pa | UH 4(H 2) 6 [31] | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No |
Assessed[ by whom? ] | Not assessed |
Hydrogen has a highly variable solubility in the elements. When the continuous phase of the solution is a metal, it is called a metallic hydride or interstitial hydride, on account of the position of the hydrogen within the crystal structure of the metal. In solution, hydrogen can occur in either the atomic or molecular form. For some elements, when hydrogen content exceeds its solubility, the excess precipitates out as a stoichiometric compound. The table below shows the solubility of hydrogen in each element as a molar ratio at 25 °C (77 °F) and 100 kPa.
He | |||||||||||||||||
LiH <1×10−4 [nb 1] [44] | Be | B | C | N | O | F | Ne | ||||||||||
NaH <8×10−7 [nb 2] [45] | MgH <0.010 [nb 3] [46] | AlH <2.5×10−8 [nb 4] [47] | Si | P | S | Cl | Ar | ||||||||||
KH <<0.01 [nb 5] [48] | CaH <<0.01 [nb 6] [49] | ScH ≥1.86 [nb 7] [50] | TiH 2.00 [nb 8] [51] | VH 1.00 [nb 9] [52] | Cr | MnH <5×10−6 [nb 10] [53] | FeH 3×10−8 [54] | Co | NiH 3×10−5 [55] | CuH <1×10−7 [nb 11] [56] | ZnH <3×10−7 [nb 12] [57] | Ga | Ge | As | Se | Br | Kr |
RbH <<0.01 [nb 13] [58] | Sr | YH ≥2.85 [nb 14] [59] | ZrH ≥1.70 [nb 15] [60] | NbH 1.1 [nb 16] [61] | Mo | Tc | Ru | Rh | PdH 0.724 [62] | AgH 3.84×10−14 [63] | Cd | In | Sn | Sb | Te | I | Xe |
CsH <<0.01 [nb 17] [64] | Ba | Lu | Hf | TaH 0.79 [nb 18] [65] | W | Re | Os | Ir | Pt | AuH 3.06×10−9 [62] | HgH 5×10−7 [66] | Tl | Pb | Bi | Po | At | Rn |
Fr | Ra | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | Og |
↓ | |||||||||||||||||
LaH ≥2.03 [nb 19] [67] | CeH ≥2.5 [nb 20] [68] | Pr | Nd | Pm | SmH 3.00 [69] | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | ||||
Ac | Th | Pa | UH ≥3.00 [nb 21] [70] | Np | Pu | Am | Cm | Bk | Cf | Es | FM | Md | No |
Miscible | Undetermined |
Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles are embedded. The guest is said to be isolated within the host matrix. Initially the term matrix-isolation was used to describe the placing of a chemical species in any unreactive material, often polymers or resins, but more recently has referred specifically to gases in low-temperature solids. A typical matrix isolation experiment involves a guest sample being diluted in the gas phase with the host material, usually a noble gas or nitrogen. This mixture is then deposited on a window that is cooled to below the melting point of the host gas. The sample may then be studied using various spectroscopic procedures.
Aluminium hydride is an inorganic compound with the formula AlH3. Alane and its derivatives are part of a family of common reducing reagents in organic synthesis based around group 13 hydrides. In solution—typically in etherial solvents such tetrahydrofuran or diethyl ether—aluminium hydride forms complexes with Lewis bases, and reacts selectively with particular organic functional groups, and although it is not a reagent of choice, it can react with carbon-carbon multiple bonds. Given its density, and with hydrogen content on the order of 10% by weight, some forms of alane are, as of 2016, active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles. As of 2006 it was noted that further research was required to identify an efficient, economical way to reverse the process, regenerating alane from spent aluminium product.
Plumbane is an inorganic chemical compound with the chemical formula PbH4. It is a colorless gas. It is a metal hydride and group 14 hydride composed of lead and hydrogen. Plumbane is not well characterized or well known, and it is thermodynamically unstable with respect to the loss of a hydrogen atom. Derivatives of plumbane include lead tetrafluoride, PbF4, and tetraethyllead, (CH3CH2)4Pb.
Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.
Zinc hydride is an inorganic compound with the chemical formula ZnH2. It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the binary first row transition metal hydrides. A variety of coordination compounds containing Zn–H bonds are used as reducing agents, however ZnH2 itself has no common applications.
Scandium trihydride is an unstable molecular chemical compound with the chemical formula ScH3. It has been formed as one of a number of other molecular scandium hydride products at low temperature using laser ablation and identified by infrared spectroscopy. Scandium trihydride has recently been the subject of Dirac–Hartree–Fock relativistic calculation studies, which investigate the stabilities, geometries, and relative energies of hydrides of the formula MH3, MH2, or MH.
Cadmium hydride is an inorganic compound with the chemical formula (CdH
2)
n. It is a solid, known only as a thermally unstable, insoluble white powder.
Indium trihydride is an inorganic compound with the chemical formula. It has been observed in matrix isolation and laser ablation experiments. Gas phase stability has been predicted. The infrared spectrum was obtained in the gas phase by laser ablation of indium in presence of hydrogen gas InH3 is of no practical importance.
Thallane is an inorganic compound with the empirical chemical formula TlH3. It has not yet been obtained in bulk, hence its bulk properties remain unknown. However, molecular thallane has been isolated in solid gas matrices. Thallane is mainly produced for academic purposes.
Titanium(IV) hydride is an inorganic compound with the empirical chemical formula TiH
4. It has not yet been obtained in bulk, hence its bulk properties remain unknown. However, molecular titanium(IV) hydride has been isolated in solid gas matrices. The molecular form is a colourless gas, and very unstable toward thermal decomposition. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.
An iron hydride is a chemical system which contains iron and hydrogen in some associated form.
Mercury(II) hydride is an inorganic compound with the chemical formula HgH
2. It is both thermodynamically and kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it known as a white, crystalline solid, which is kinetically stable at temperatures below −125 °C (−193 °F), which was synthesised for the first time in 1951.
Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (CrH)
n. It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular chromium(I) hydride with the formula CrH has been isolated in solid gas matrices. The molecular hydride is very reactive. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.
Chromium(II) hydride, systematically named chromium dihydride and poly(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (CrH2)n. Although it is thermodynamically unstable toward decomposition at ambient temperatures, it is kinetically metastable.
Uranium(IV) hydride is a chemical compound with the chemical formula UH4, a metal hydride composed of uranium and hydrogen.
Iron(I) hydride, systematically named iron hydride and poly(hydridoiron) is a solid inorganic compound with the chemical formula (FeH)
n (also written ([FeH])
n or FeH). It is both thermodynamically and kinetically unstable toward decomposition at ambient temperature, and as such, little is known about its bulk properties.
Iron(II) hydride, systematically named iron dihydride and poly(dihydridoiron) is solid inorganic compound with the chemical formula (FeH
2)
n (also written ([FeH
2])n or FeH
2). ). It is kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it is known as a black, amorphous powder, which was synthesised for the first time in 2014.
Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.
Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the CSIRO.
Dialane is an unstable compound of aluminium and hydrogen with formula Al2H6. Dialane is unstable in that it reacts with itself to form a polymer, aluminium hydride. Isolated molecules can be stabilised and studied in solid hydrogen.