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454 Metal Hydrides

of a material. Inhomogeneities during hydrogenation of- hydrides. Also, no hydrido complexes of rare earth or
ten introduce internal strain and stress, and conversely, actinide elements are known. Some 80 complex transi-
mechanical stress may introduce hydrogen concentration tion metal hydrides are known and well characterized at
inhomogeneities. The latter is utilized in Gorsky effect present.
measurements, in which the hydrogen redistribution after
mechanical deformation of a sample is studied, in order to 1. Thermodynamic and Chemical Properties
investigate long-range hydrogen diffusion. The lattice ex-
Complex hydrides of the late transition metals (M = 8b
pansion on formation of a metal hydride cracks the metal
group element) are formed at relatively moderate hydro-
or intermetallic compound into a fine powder. The vol-
gen pressures with generally low oxidation states for M,
ume effect upon hydrogenation can be as large as 25% for II 2−
e.g., [Pt H 4 ] in K 2 PtH 4 . Synthesizing compounds with
the fully hydrided LaNi 5 H 7 as compared to LaNi 5 . Even IV 2−
higher oxidation states such as [Pt H 6 ] in K 2 PtH 6 or
higher values are found for the system CeRu 2 (37% vol-
complex hydrides of transition metals other than from
ume increase for CeRu 2 H 5 ) due to a valence change for group 8b requires much higher hydrogen pressures. A re-
6
Ce. As a rule of thumb a volume increase of 2–3 × 10 pm 3
markable exception is the solution synthesis of the first
per absorbed hydrogen atoms occurs. On repeated hydro- VII
complex metal hydride ever reported, K 2 Re H 9 . No
gen absorption–desorption cycles phase segregation may
compounds for M = 3b, 4b, or 5b elements are known
occur. The formation of very small Ni clusters in LaNi 5
thus far. A possible explanation for this fact might be the
powder cycled 1500 times was observed by the occur- higher thermodynamic stability of the binary hydrides of
63
rence of a magnetic hyperfine splitting in Ni M¨ossbauer
the early as compared to the late transition metals. Hydrido
spectroscopy. The high surface area of cycled materials
complexes of Ag, Au, and Hg are also unknown. The
engenders a higher sensitivity toward contaminants such
scarce thermodynamic data on complex hydrides sug-
as oxygen or water.
gest a thermal stability between those of ionic hydrides
and metallic hydrides (Table II) with hydride enthalpy
C. Complex Transition Metal Hydrides of formation H ranging between −64 (Mg 2 NiH 4 ) and
−137 kJ/mol (Yb 4 Mg 4 Fe 3 H 22 ). As, in contrast to ternary
The characteristic part of a complex transition metal hy-
hydrides containing transition metals only, the complex
dride is the anionic homoleptic transition metal (M) hy-
transition metal hydrides are usually not based on sta-
drido complex [M m H h ] , which is balanced by the cation ble intermetallic compounds, the models for the predic-
x −
+
A or A 2+ (A = Li–Cs, Mg–Ba, Eu, Yb). The formation of
tion of the enthalpy of hydride formation as discussed in
anextendedsolidisaconsequenceoftheattractiveelectro-
Section III.B.2 are of limited use here. Considering the
static Coulomb interaction between cations and complex high weight and volume efficiencies for hydrogen stor-
anions, whereas within the [M m H h ] x − complex, hydrogen
age, e.g., that of Mg 2 FeH 6 , which is more than twice
is bound covalently to the metal M. The hydride fluoride
that of liquid hydrogen, the less stable complex transi-
analogy known from ionic hydrides is less pronounced for
tion metal hydrides are interesting candidates for hydro-
complex transition metal hydrides; however, some repre-
gen storage applications. Most complex transition metal
sentatives show structural resemblance to the correspond-
hydrides are air sensitive and insoluble in commonly used
ing halides. The hydrido complexes follow the 18-electron
solvents. Compounds with organometallic cations such as
rule known from coordination chemistry, e.g., [ReH 9 ] 2−
4− [MgBr(THF) 2 ] 4 [FeH 6 ] (THF = tetrahydrofuran) show a
in K 2 ReH 9 or [NiH 4 ] in CaMgNiH 4 . They may also
− moderate THF solubility.
contain free hydride anions H that are not part of the
−
complex anions, e.g., in K 3 PtH 5 (=(K ) 3 [PtH 4 ] H ).
+
2−
2. Geometry and Properties of Transition Metal
According to their bonding properties, complex transition Hydrido Complexes
metal hydrides are stoichiometric, electron-precise com-
pounds that often are colored, are nonmetallic, and have The geometries found for transition metal hydrido com-
an ordered hydrogen distribution. However, because of the plexes resemble those in inorganic transition metal coor-
high mobility of hydrogen as a ligand, some of them un- dination compounds following the well-known rule that
dergo a transition to a disordered high-temperature phase. a total of 18 electrons are required for a stable complex.
In metal-rich compounds with less electropositive ele- Examples for 18-electron complexes are the tricapped
I
ments A (=Li, Mg), metal–metal interactions may occur, trigonal prismatic [Re VII H 9 ] , octahedral [Mn H 6 ] ,
2−
5−
I
II
II
II
and they are border cases toward metallic metal hydrides, [Re H 6 ] , [Fe H 6 ] 4− (Fig. 6), [Ru H 6 ] , [Os H 6 ] ,
5−
4−
4−
III
IV
III
2−
whereas for heavier, electropositive metals A (= Rb, [Rh H 6 ] , [Ir H 6 ] , [Pt H 6 ] , square pyramidal
3−
3−
II
0
I
2−
Cs, Ba) a more saltlike character is found. No dihydro- [Co H 5 ] 4− (Fig. 6), tetrahedral [Mn H 4 ] , [Ni H 4 ] 4−
II
I
0
gen complexes as found in metal organic coordination (Fig. 6), [Pd H 4 ] , [Cu H 4 ] , Zn H 4 ] 2− and
3−
4−
II
chemistry are known for inorganic homoleptic complex [Cd H 4 ] . Electron-deficient complexes show different
2−