Binary compounds of hydrogen
Binary compounds of hydrogen are
These hydrogen compounds can be grouped into several types.Overview
Binary hydrogen compounds in
Hydrides in
Hydrides in the
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
Elements in group 13 to 17 (
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
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 | 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 |
Molecular hydrides
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
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
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 123
2[8] |
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 162
2[19] |
GaH 3[20] 151 |
GeH 4 92 |
AsH 3 67 |
H 2Se 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 222
3[24] |
SnH 4 163 |
SbH 3 146 |
H 2Te 100 |
HI 27 | |
CsH 119 (−50) |
BaH 2 213 (−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 293
3[28] |
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.
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 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 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][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 |
Notes
- ^ Upper limit imposed by phase diagram, taken at 454 K.
- ^ Upper limit imposed by phase diagram, taken at 383 K.
- ^ Upper limit imposed by phase diagram, taken at 650 K and 25 MPa.
- ^ Upper limit imposed by phase diagram, taken at 556 K.
- ^ Upper limit imposed by phase diagram.
- ^ Upper limit imposed by phase diagram, taken at 500 K.
- ^ Lower limit imposed by phase diagram.
- ^ Limit imposed by phase diagram.
- ^ Limit imposed by phase diagram.
- ^ Upper limit imposed by phase diagram, taken at 500 K.
- ^ Upper limit imposed by phase diagram, taken at 1000 K.
- ^ Upper limit at 500 K.
- ^ Upper limit imposed by phase diagram.
- ^ Lower limit imposed by phase diagram.
- ^ Lower limit imposed by phase diagram.
- ^ Limit imposed by phase diagram.
- ^ Upper limit imposed by phase diagram.
- ^ Limit imposed by phase diagram.
- ^ Lower limit imposed by phase diagram.
- ^ Lower limit imposed by phase diagram.
- ^ Lower limit imposed by phase diagram.
References
- ^ Concise Inorganic Chemistry J.D. Lee
- ^ Main Group Chemistry, 2nd Edition, A. G. Massey
- ^ Advanced Inorganic Chemistry F. Albert Cotton, Geoffrey Wilkinson
- ^ Inorganic chemistry, Catherine E. Housecroft, A. G. Sharpe
- ^ Inorganic Chemistry Gary Wulfsberg 2000
- ^ Data in KJ/mole gas-phase source: Modern Inorganic Chemistry W.L. Jolly
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