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Binary compounds of hydrogen

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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.

The periodic table of the stable binary hydrides

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 Fe 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 HfH2 TaH W Re Os Ir Pt Au Hg Tl PbH4 252 BiH3 247 H2Po 167 HAt positive Rn
Fr Ra 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 LuH2
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Binary compounds of hydrogen
Covalent hydrides metallic hydrides
Ionic hydrides Intermediate hydrides
Do not exist Not assessed

Molecular hydrides

Most monomeric hydrides are isolable only under extreme conditions (i.e. at cryogenic temperatures, and often embedded in a rare gas matrix). This is generally attributable to poor contribution of the atomic orbitals of the respective atoms with the s-orbital of hydrogen; and to the low activation enthalpies of autopolymerisation reactions, which electron-deficient monomers are prone to undergo. Relativistic effects interfere with the energy levels of molecular orbitals in the lowest periods of the periodic table. Hence, the degree of electron-deficiency of a monomer may be estimated incorrectly, if this is not taken into account. For example, based on it's 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, being energetically more stable in monomeric form.

The table below shows the monomeric hydride for each element, which 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 other valence rules. Elements may be prevented from reaching their heuristic valence for various electronic discrepancies. In chromium's case, stearic hindrance ensures that both the octahedral and trigonal prismatic molecular geometries for CrH
6
are metastable, with a very low energy barrier for conversion to a Kubas complex.

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.

Classical hydrides

Classical hydrides
1 2 3 4 5 6 5 4 3 2 1 2 3 4 3 2 1
H
2
0
LiH[7] 139
(−91)
BeH
2
[8] 123
BH
3
[9] 107
(41)
CH
4
−75
NH
3
−46
H
2
O
−242
(−286)
HF −273
NaH[10] 140
(−56)
MgH
2
142
(−76)
AlH
3
[11] 123
(−46)
SiH
4
34
PH
3
5
H
2
S
−21
HCl −92
KH 132
(−58)
CaH
2
192
(−174)
ScH
3
TiH
4
VH
2
[12]
CrH
2
[13]
MnH
2
[14]
FeH
2
[15] 324
CoH
2
[16]
NiH
2
[17]
CuH[18] 278
(28)
ZnH
2
[19] 162
GaH
3
[20] 151
GeH
4
92
AsH
3
67
H
2
Se
30
HBr −36
RbH 132
(−47)
SrH
2
201
(−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
2
Te
100
HI 27
CsH 119
(−50)
BaH
2
213
(−177)
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
2
Po
167
HAt 88
Fr Ra 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 3
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
LuH
3
Ac ThH
4
[30]
Pa UH
4
[31]
Np Pu Am Cm Bk Cf Es Fm Md No Lr
Legend
Monomeric covalent hydride Oligomeric covalent hydride
Polymeric covalent hydride Ionic hydride
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.

Non-classical covalent hydrides

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 crogenic gas. Others have only been predicted using computational chemistry.

Non-classical covalent hydrides
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 Mn FeH
2
(H
2
)
3
[37]
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
Mo Tc RuH
2
(H
2
)
4
[38]
RhH3(H2) Pd(H
2
)
3
AgH(H2) CdH(H
2
)
InH
3
(H
2
)
[39]
Cs Ba[33] Hf TaH
4
(H
2
)
4
WH
4
(H
2
)
4
Re Os Ir PtH(H
2
)
AuH
3
(H
2
)
Hg Tl
Fr Ra 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 Lu
Ac ThH4(H2)4 Pa UH
4
(H
2
)
6
[31]
Np Pu Am Cm Bk Cf Es Fm Md No Lr
Legend
Assessed[by whom?] Not assessed

Hydrogen solutions

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][40]
Be B C N O F Ne
NaH
<8×10−7

[nb 2][41]
MgH
<0.010

[nb 3][42]
AlH
<2.5×10−8

[nb 4][43]
Si P S Cl Ar
KH
<<0.01

[nb 5][44]
CaH
<<0.01

[nb 6][45]
ScH
≥1.86

[nb 7][46]
TiH
2.00

[nb 8][47]
VH
1.00

[nb 9][48]
Cr MnH
<5×10−6

[nb 10][49]
FeH
3×10−8

[50]
Co NiH
3×10−5

[51]
CuH
<1×10−7

[nb 11][52]
ZnH
<3×10−7

[nb 12][53]
Ga Ge As Se Br Kr
RbH
<<0.01

[nb 13][54]
Sr YH
≥2.85

[nb 14][55]
ZrH
≥1.70

[nb 15][56]
NbH
1.1

[nb 16][57]
Mo Tc Ru Rh PdH
0.724

[58]
AgH
3.84×10−14

[59]
Cd In Sn Sb Te I Xe
CsH
<<0.01

[nb 17][60]
Ba Hf TaH
0.79

[nb 18][61]
W Re Os Ir Pt AuH
3.06×10−9

[58]
HgH
5×10−7

[62]
Tl Pb Bi Po At Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
LaH
≥2.03

[nb 19][63]
CeH
≥2.5

[nb 20][64]
Pr Nd Pm SmH
3.00

[65]
Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa UH
≥3.00

[nb 21][66]
Np Pu Am Cm Bk Cf Es FM Md No Lr
Legend
Miscible Undetermined

Notes

  1. ^ Upper limit imposed by phase diagram, taken at 454 K.
  2. ^ Upper limit imposed by phase diagram, taken at 383 K.
  3. ^ Upper limit imposed by phase diagram, taken at 650 K and 25 MPa.
  4. ^ Upper limit imposed by phase diagram, taken at 556 K.
  5. ^ Upper limit imposed by phase diagram.
  6. ^ Upper limit imposed by phase diagram, taken at 500 K.
  7. ^ Lower limit imposed by phase diagram.
  8. ^ Limit imposed by phase diagram.
  9. ^ Limit imposed by phase diagram.
  10. ^ Upper limit imposed by phase diagram, taken at 500 K.
  11. ^ Upper limit imposed by phase diagram, taken at 1000 K.
  12. ^ Upper limit at 500 K.
  13. ^ Upper limit imposed by phase diagram.
  14. ^ Lower limit imposed by phase diagram.
  15. ^ Lower limit imposed by phase diagram.
  16. ^ Limit imposed by phase diagram.
  17. ^ Upper limit imposed by phase diagram.
  18. ^ Limit imposed by phase diagram.
  19. ^ Lower limit imposed by phase diagram.
  20. ^ Lower limit imposed by phase diagram.
  21. ^ Lower limit imposed by phase diagram.

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  56. ^ Zuzek, E.; Abriata, J. P.; San-Martin, A.; Manchester, F. D. (1 August 1990). "The H-Zr (Hydrogen-Zirconium) System". Bulletin of Alloy Phase Diagrams. 11 (4). Springer-Verlag: 385–395. doi:10.1007/BF02843318.
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  58. ^ a b Materials Science International Team (2006). "Au-H-Pd (Gold - Hydrogen - Palladium)". In Effenberg, G.; Ilyenko, S. (eds.). Noble Metal Systems. Selected Systems from Ag-Al-Zn to Rh-Ru-Sc. Berlin: Springer Berlin Heidelberg. pp. 1–8. ISBN 978-3-540-46994-0.
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  62. ^ Guminski, C. (1 October 2002). "The H-Hg (Hydrogen-Mercury) System". Journal of Phase Equilibria. 23 (5). Springer-Verlag: 448–450. doi:10.1361/105497102770331460.
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  64. ^ Manchester, F. D.; Pitre, J. M. (1 February 1997). "The Ce-H (Cerium-Hydrogen) system". Journal of Phase Equilibria. 18 (1). Springer-Verlag: 63–77. doi:10.1007/BF02646759.
  65. ^ Zinkevich, M.; Mattern, N.; Handstein, A.; Gutfleisch, O. (13 June 2002). "Thermodynamics of Fe–Sm, Fe–H, and H–Sm Systems and its Application to the Hydrogen–Disproportionation–Desorption–Recombination (HDDR) Process for the System Fe
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  66. ^ Manchester, F. D.; San-Martin, A. (1 June 1995). "The H-U (Hydrogen-Uranium) System". Journal of Phase Equilibria. 16 (3). Springer-Verlag: 263–275. doi:10.1007/BF02667312.