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446 Metal Hydrides

stable. Enthalpies of formation of ionic hydrides range radii. EuH 2 and YbH 2 crystallize as the dihydrides of
typically from −70 to −180 kJ/mol (Table II) while those Ca, Sr, and Ba in the PbCl 2 type structure. In contrast
of metal fluorides are generally found to be −250 to to the main group hydrides, those with Eu are colored
−600 kJ/mol. The enthalpy of formation of the hydride (red, brown, or violet) and are ferromagnetic semicon-
−
ion H = −72 kJ/mol by the reaction H (g) + e → H − ductors. Simple ionic structure types are adopted by many
(g)
is much less negative than that of the corresponding reac- ternary hydrides of the type A a M m H a+2m (A = Li–Cs,
tion for the fluoride anion (−333 kJ/mol). Binary metal M = Mg–Ba, Eu, Yb), e.g., the perovskite-type struc-
5
hydrides decompose under 10 Pa H 2 atmosphere upon tures. Examples of the latter are RbMgH 3 (hexagonal
heating before they melt because of the low thermody- perovskite (BaTiO 3 ) type), NaMgH 3 (orthorhombic per-
namic stability. As an exception LiH melts at 953 K ovskite (GdFeO 3 ) type), CsCaH 3 , SrLiH 3 , BaLiH 3 , and
−
without decomposition. As H is a very strong base, EuLiH 3 (cubic perovskite (SrTiO 3 ) type). In the last struc-
ture (Fig. 2, left) Eu is surrounded by 12 H cuboctahedrally
ionic hydrides react violently with water, producing H 2
gas. Ionic hydrides are not soluble in common solvents. and Li by 6 H octahedrally. Despite the different stoi-
They are used as reducing agents, e.g., for the reduc- chiometry there is a close relationship to the crystal struc-
tion of oxides to metals, for the convenient production ture of EuMg 2 H 6 (Fig. 2, right), a further ternary metal hy-
of pure hydrogen on a laboratory scale, but are thermo- dride with ionic character. Every other Eu layer is missing
dynamically too stable for reversible hydrogen storage in the latter with respect to the former as required by the
(Table II). electroneutrality on replacing Li by Mg . This surpris-
+
2+
As for the alkaline earth metal hydrides, the cova- ing structural resemblance may be explained by the diag-
lent character increases in the series BaH 2 –SrH 2 –CaH 2 – onal relationship between Li and Mg. Some examples of
MgH 2 –BeH 2 . Unlike the corresponding fluorides, BaH 2 , ternary hydrides with group 3a and 4a metals are NaAlH 4
SrH 2 , and CaH 2 adopt the PbCl 2 type structure. MgH 2 (CaWO 4 type), NaGaH 4 (CaSO 4 type), Na 3 AlH 6 (cryo-
crystallizes in the rutile type and BeH 2 in its own type with lite (Na 3 AlF 6 ) type), and Ca 3 SnH 2 (anti-CsCu 2 Cl 3 type).
a framework of corner-sharing BeH 4 tetrahedra (H–Be–H The alkaline aluminum and gallium hydrides show com-
◦
bond angles between 107 and 113 ). AlH 3 is often de- plex anions [AlH 4 ] , [GaH 4 ] , and [AlH 6 ] 3− in which
−
−
◦
scribed as a covalent hydride, and 3-center-2-electron hydrogen is covalently bonded to the metal. These com-
bonds Al–H–Al (bond angle 141 ) are discussed in anal- pounds are soluble in ethoxyethane. Alanates with transi-
◦
ogy to boranes. However, it is an extended solid with a tion metals have also been reported, such as Ti(AlH 4 ) 4
typical fluoride structure of corner-sharing AlH 6 octahe- and Fe(AlH 4 ) 2 , but not structurally characterized. The
dra (VF 3 type structure), which derives from hexagonal AlH unit also serves as a ligand in organometallic com-
−
4
closest packed Al (Mg type) by filling one-third of the plexes. With BeH 2 , MgH 2 , and CaH 2 , ternary hydrides
octahedral holes with H. GaH 3 is less stable and decom- A(AlH 4 ) 2 (A = Be, Mg, Ca) are formed with a more cova-
poses at room temperature to the elements. Its solid struc- lent character. AlH 3 reacts with diborane to give Al(BH 4 ) 3
ture is unknown; in the gas phase it dimerizes to digallane, and with gallane to give Ga(AlH 4 ) 3 . LiAlH 4 is widely
Ga 2 H 6 . The existence of InH 3 , TlH 3 , and TlH is not yet used in preparative chemistry as a versatile reducing
proven. Distinct molecules as expected for typical cova- agent.
lent compounds are found in GeH 4 and SnH 4 . SnH 4 is a Some ternary main-group metal hydrides are very metal
very volatile hydride (melting point 123 K, boiling point rich and were first reported as being new intermetallic
221 K) that decomposes at room temperature. In the solid compounds with unusual properties, as hydrogen has been
state, weak Sn–H interactions between neighboring tetra- overlooked in the X-ray structure determination. Com-
hedral SnH 4 units are present. In the gas phase, distannane mon sources of hydrogen are the commercially avail-
Sn 2 H 6 has also been reported. The existence of PbH 4 is able divalent metals Ca, Sr, Ba, Eu, Sm, Yb used for
not yet clear. BiH 3 seems to be extremely unstable and has synthesis, which may contain as much as 10–20 at % H.
never been produced in high yields. Unrecognized hydrogen content has led to confusion in
view of valence electron rules in compounds considered
as Zintl phases, e.g., the compounds of the formerly as-
2. Ternary Main Group Metal Hydrides
signed “β-Yb 5 Sb 3 ” type structure. It was shown that the
Ternary hydrides containing alkaline and alkaline earth true composition is A 5 M 3 H(A = Ca, Sr, Ba, Sm, Eu, Yb;
metals only adopt typical ionic structure types and are M = Sb, Bi), and the crystal structure and properties are
very air sensitive. In the following discussion Eu and in agreement with the ionic formula (A ) 5 (M ) 3 H .
2+
−
3−
Yb are included as they are divalent in hydrides only Further examples for metal rich main group hydrides
(except binary YbH 2.4 ) and greatly resemble Sr and Ca are A 3 MH 2 (A = Ca, Yb; M = Sn, Pb) with the ionic
in their hydrides because of their almost identical ionic formula (A ) 3 M (H ) 2 .Ba 5 Ga 6 H 2 contains both a
2+
4−
−