Download Metal Hydrides: Transition Metal Hydride Complexes

Metal Hydrides: Transition Metal Hydride
Complexes
Solid-state metal hydrides provide a safe and efficient
way of storing hydrogen (Schlapbach 2002). They
are commercialized for use in rechargeable batteries
and developed for energy conversion devices such as
fuel cells (see Hydrogen Metal Systems: Electrochemical Reactions). A great majority of metal hydrides
derives from intermetallic compounds and alloys (see
Hydrogen Metal Systems: Hydride Forming Alloys).
They have metallic properties, are nonstoichiometric,
and are usually called ‘‘interstitial’’ because hydrogen occupies interstices in the metal atom network
(Yvon and Fischer 1988). Due to their high mobility
the hydrogen atoms are usually disordered and may
enter and quit the metal matrix near ambient
conditions. ‘‘Complex’’ metal hydrides form a
subclass of metal hydrides. They derive their name
from the presence of discrete metal–hydrogen complexes in the crystal structure that are centered by dor p-elements. The compounds are stoichiometric
and usually nonmetallic, and the hydrogen ligands
are usually ordered at ambient conditions. Consequently, hydrogen in these compounds is less mobile
and leaves less readily the metal atom network.
Among the hydrides based on d-elements, the
historically first member and textbook example is
K2ReH9. It was reported in 1964 and found to
contain tricapped trigonal prismatic [ReH9]2 complexes. Since then the number of complex metal
hydrides has continuously increased and now totals
over 80 compounds containing some 30 different
metal hydride complexes. They have been reviewed
at various time intervals (Bronger 1991, Yvon 1994,
1998, Bau 1997, King 2000) and incorporated into a
public online database on metal hydrides (Sandrock
and Thomas 1997). The present update covers the
currently known solid-state complex transition metal
hydrides and some of their properties of interest for
science and technology. Both mononuclear (terminal
hydrogen ligands) and polynuclear (bridging hydrogen ligands) complexes are covered. Furthermore,
some recently reported examples for hydrogenationinduced complex formation and metal–insulator
transitions in typically metallic (interstitial) hydrides
are also included.
where ½THn d are T-metal hydride complexes that
are stabilized by charge transfer from the surrounding metal cations Md þ (M ¼ alkali, alkaline earth or
divalent rare earth element). The second family
contains hydrogen bonded to T elements and
hydrogen anions H bonded to metal cations Md þ
only, corresponding to the general composition
d dþ Mdþ
m ½THn Mo Hp ðm; n; o; p; ¼ 1; 2; 3; :::Þ
ð2Þ
These ‘‘composite’’ hydrides are of particular interest
because they combine different types of metalhydrogen bonding in the same structure.
Various synthetic routes have been explored of
which the one-step solid-state reaction from the
elements under high (up to 2 kbar) hydrogen pressure
is the most frequently used. Interestingly, the great
majority of hydrides found combine metals that do
not form stable binary compounds between themselves, such as magnesium and iron (2Mg þ Fe þ
3H2 ¼ Mg2FeH6) or magnesium and manganese
(3Mg þ Mn þ 72H2 ¼ Mg3MnH7). Only few hydrides
derive from stable binary metal compounds such as
Mg2NiH4 (Mg2Ni þ 2H2) or from two-phase mixtures
such as Ba7Cu3H17 (Ba–Cu alloy with compositional ratio Ba=CuB73). Alternatively, some hydrides
of this class can be prepared by inexpensive ball milling (Mg2FeH6) and combustion synthesis (Mg2NiH4).
Solution methods are rarely used (BaReH9) because
the complexes are usually insoluble.
Crystal structures are mainly determined by x-ray
and neutron powder diffraction on deuterides.
Complications may arise from multiphase samples, and from temperature-induced phase transitions that lead to micro-twinning and disorder in the
metal–hydrogen ligand spheres. Structural dynamics
are investigated by spectroscopic methods (INS,
Raman, IR, NMR) and magnetic properties on
SQUID magnetometers. Due to the absence of single
crystals of suitable size, reliable measurements of the
electric conductivity are rare. Enthalpies of hydride
formation, DH, are usually determined from pressure-composition isotherms as measured on Sievert’s
apparatus or on a thermobalance, according to the
relation
lnðpeq Þ ¼ DH=RT þ DS=R; Van’t Hoff equation
ð3Þ
1. Composition, Synthesis, and Characterization
There exist two broad families of complex transition
(T) metal hydrides. The first contains hydrogen
bonded to T elements only and has the general
composition
d
Mdþ
m ½THn ðm; n; d ¼ 1; 2; 3; :::Þ
ð1Þ
where peq is the hydrogen equilibrium pressure and T
the absolute temperature. Given that the entropy
term, DS, does not change much from one system
to another (DSB130 J K1 mol1 ), the hydrogen
decomposition temperatures at a given pressure scale
roughly with enthalpy (20 1C : DHB30 kJ/H2 for
peq ¼ 1 bar). Finally, the reaction products ought to
be handled with care because some are air sensitive
1
Metal Hydrides: Transition Metal Hydride Complexes
and pyrophoric. For these reasons quite a few
complex metal hydrides are known to exist but have
not yet been fully characterized.
2. Mononuclear Complexes
The great majority of complexes known are centered
by one T-metal atom only and have terminal
hydrogen ligands. Their compositions and their
distribution over the periodic system are summarized
in Table 1, and a list of currently known and
structurally characterized hydride representatives is
given in Table 2. Over 30 different complexes have
been identified in some 70 compounds that crystallize
with 33 different structure types (for details and
structure drawings see Yvon 1994, 1998). They
include transition elements from group 7 (Mn) to
closed d-shell elements of group 12 (Zn). No
complexes have been reported for group 4 (Ti), 5
(V), and 6 (Cr), and none for Ag, Au, and Hg.
Interestingly, most complexes contain T elements
that do not form stable binary hydrides such as iron
([FeH6]4 in Mg2FeH6) and cobalt ([CoH5]4 in
Mg2CoH5), or form relatively unstable hydrides such
as nickel ([NiH4]4 in Mg2NiH4). As shown in Fig. 1
the ligand geometries range from tricapped trigonal
prismatic (CN ¼ 9: [ReH9]2), pentagonal bipyramidal (CN ¼ 7: [RuH7]3), octahedral (CN ¼ 6:
[FeH6]4), square-pyramidal (CN ¼ 5: [CoH5]4),
planar (CN ¼ 4: [RhH4]3), tetrahedral (CN ¼ 4:
[NiH4]4), saddle-like (CN ¼ 4: [IrH4]5), and triangular (CN ¼ 3: [PdH3]3) to linear (CN ¼ 2:
[PdH2]2). No trigonal bipyramidal (CN ¼ 5) and
no cubic antiprismatic (CN ¼ 8) configuration has
been reported as yet, although indications exist for
the presence of [OsH8]2 complexes in Cs3OsH9 and
Rb3OsH9 (Bronger et al. 2002). The complexes are
usually ordered at room temperature and tend to
become disordered at high temperature such as
tetrahedral [NiH4]4 in Mg2NiH4 (Tl B240 1C). Only
few complexes are disordered at room temperature
and ordered at low temperature such as [PtH4]2 in
K2PtH4 (Tl ¼ 195 K). No dihydrogen complexes have
been reported for this class of hydrides as yet.
Generally speaking, the ligand geometries found
resemble those in coordination compounds (e.g.,
carbonyls and cyano compounds) and are consistent
with ‘‘magic’’ electron counts such as 18 electrons for
octahedral, square-pyramidal and tetrahedral configurations, 16 electrons for planar, and 14 electrons for
linear configurations (see last column of Fig. 1). The
tetrahedral 13-electron [MnH4]2 complex in
K3MnH5 is remarkable as it displays a half-filled
d-shell and leads to a pink-colored hydride that
orders magnetically. Another interesting case is
the saddle-like 16-electron [RuH4]4 complex in
Mg2RuH4, because it provides a possible link to
polynuclear complexes. A given complex usually
occurs in different structure types such as octahedral
[FeH6]4 in cubic Mg2FeH6, cubic Ca4Mg4Fe3H22,
and trigonal SrMg2FeH6, and tetrahedral [NiH4]4 in
monoclinic Mg2NiH4 and cubic CaMgNiH4. Only
few hydrides contain different complexes within the
same structure, such as Mg6Co2H11 (square-pyramidal [CoH4]5 and saddle-like [CoH5]4), whereas
hydrides containing complexes centered by different
T elements have not been reported as yet. An
example of a series of ‘‘composite’’ structures made
up by slabs of covalent-bonded [OsH6]4 complexes
and sheets of ionic-bonded Li þ H is the series
LiHnMg2 OsH4 ðn ¼ 1; 2; NÞ as shown in Fig. 2.
Finally, only few 3d analog exist for the more
numerous 4d- and 5d-hydrides, i.e., only one manganese analog for the rhenium compounds, one iron
analog for the ruthenium compounds (but none for
the osmium series of composite structures), and no
nickel analog for the platinum compounds.
Table 1
Transition metal–hydrogen complexes in solid-state metal hydrides.a
Mn
[MnH4]2
[MnH6]5
Fe
[FeH6]4
Co
[CoH4]5
av
[CoH5]4
Ni
[NiH4]4
Cu
[CuH4]3
Zn
[ZnH4]2
Tc
[TcH9]2
Ru
[RuH4]4
½RuH5 5
av
[RuH6]4
[RuH7]3
Rh
[RhH4]3
½RhH5 4
av
[RhH6]3
Pd
[PdH2]2
[PdH3]3
[PdH4]2
[PdH4]4
Ag
Cd
[CdH4]2
Re
[ReH6]3
[ReH6]5
[ReH9]2
Os
[OsH6]4
[OsH7]3
Ir
[IrH4]5
[IrH5]4
[IrH6]3
Pt
[PtH2]2
[PtH4]2
[PtH6]2
Au
Hg
av ¼ average due to disorder.
a Included are mononuclear complexes only that are structurally characterized, as of beginning of 2003.
2
Metal Hydrides: Transition Metal Hydride Complexes
Table 2
Table 2
Metal hydride complexes in ternary and quaternary
transition metal hydrides.a
Contd.
Manganese, technetium, rhenium
[MnH4]2 tet M3MnH5, M ¼ K, Rb, Cs
[MnH6]5 oct Mg3MnH7
[TcH9]2 ttp
K2TcH9
[ReH6]3 oct
K3ReH6
[ReH6]5 oct
Mg3ReH7
[ReH9]2 ttp
K2ReH9
BaReH9
KNaReH9
M3ReHd10, M ¼ K, Rb
Iron, ruthenium, osmium
[FeH6]4 oct
M2FeH6, M ¼ Mg, Ca, Sr, Eu, Yb
M4Mg4Fe3H22, M ¼ Ca, Yb
MMg2FeH6, M ¼ Sr, Ba, Eu
[RuH4]4 sad Mg2RuH4
[RuH5]5 pyr Mg3RuHd6
[RuH6]4 oct M2RuH6, M ¼ Mg, Ca, Sr, Ba, Eu, Yb
M4RuH6, M ¼ Li, Na
LiMg2RuH7
LiMg4Ru2H13
BaMg2RuH8
[RuH7]3 pbp Na3RuH7
[OsH6]4 oct
M2OsH6, M ¼ Ca, Mg, Sr, Ba
Li4OsH6
LiMg2OsH7
LiMg4Os2H13
BaMg2OsH8
[OsH7]3 pbp Na3OsH7
Cobalt, rhodium, iridium
[CoH4]5 sad Mg6Co2Hd11
[CoH5]4 pyr Mg6Co2H11
Mg2CoHd5
M4Mg4Co3Hd19, M ¼ Ca, Yb
3
[RhH4] pla Li3RhH4
[RhH5]4 pyr Ca2RhHd5.4, Sr2RhHd5
[RhH6]3 oct M3RhH6, M ¼ Li, Na
M8Rh5H23, M ¼ Ca, Sr
Mg6Ir2H11
[IrH4]5 sad
[IrH5]4 pyr
Mg6Ir2H11
M2IrHd5 , M ¼ Ca, Mg, Sr, Eu
Mg6Ir2H11
[IrH6]3 oct
M3RhH6, M ¼ Li, Na
Ba3Ir2H12
Nickel, palladium, platinum
[NiH4]4 tet
Mg2NiH4
MMgNiH4, M ¼ Ca, Sr, Eu, Yb
LaMg2NiH7
[PdH2]2 lin
M2PdH2, M ¼ Li, Na
M3PdH3, M ¼ K, Cs, Rb
MPdH2 þ x,M ¼ Ca, Sr, Eu, Ybd
[PdH3]3 T-sh LiSr2PdHd5
[PdH3]3 tri
NaBaPdH3
[PdH4]2 pla
[PdH4]4 tet
[PtH2]2 lin
[PtH4]2 pla
[PtH6]2 oct
M2PdH4, M ¼ Na, K
M2PdH4, M ¼ Sr, Ba, Eu
Li2PtH2
Na2PtH4
M2PtH4, M ¼ Li, Na, K
M3PtH5, M ¼ K, Rb, Cs
M2PtH6, M ¼ Sr, Ba
M2PtH6, M ¼ Na, K, Rb, Cs
Copper, silver, gold
Ba7Cu3H17
[CuH4]3 tet
Zinc, cadmium, mercury
[ZnH4]2 tet
M2ZnH4, M ¼ K, Rb, Cs
M3ZnH5, M ¼ K, Rb, Cs
Cs3CdH5
[CdH4]2 tet
a Included are mononuclear complexes only that are structurally
characterized; as of beginning of 2003; ttp ¼ tricapped trigonal prismatic,
oct ¼ octahedral, tet ¼ tetrahedral, pla ¼ square planar, tri ¼ trigonal,
sad ¼ saddle like, pbp ¼ pentagonal bipyramid, lin ¼ linear, T-sh ¼
T-shaped; d: hydrogen ligands are partially disordered.
3. Polynuclear Complexes
Hydride complexes containing more than one metal
nucleus are rare but of considerable interest with
respect to their bonding. Dimers can be identified in
Li5Pt2H9 (Bronger and à Brassard 1995) and
Mg3RuH3 (Yuon 1994). While the former displays
planar PtH4 units that are connected via Pt–H–Pt
bonds to [Pt2H9]5 complexes, the latter displays
T-shaped [RuH3]6 units that are possibly connected
via Ru–Ru bonds to [Ru2H6]12 complexes. Tetramers displaying linear T–H–T bonds can be discerned
in MgRhH0.94 (Yuon 1994) and NdMgNi4HB4
(Guénée et al. 2003), and one-dimensional polymers
displaying T–T bonds in Mg2RuH4. However, in view
of their relatively low hydrogen-to-metal ratios
(H/Mo1) some of these hydrides are presumably
metallic and should be classified as ‘‘interstitial’’
rather than as ‘‘complex’’ metal hydrides. A twodimensional network of T-shaped [PdH3]3
av units
occurs in LiSr2PdH5, and three-dimensional networks
of corner-sharing ordered RhH6 octahedrons occurs
in Ca8Rh5H23 (½Rh4 H17 n13n ), brass colored
Ca8Rh6H24 (½Rh3 H12 8n
n , Bronger and Breil 1998)
and in the perovskite structure of metallic EuPdH3
(½PdH3 2n
n , Kohlmann et al. 2001).
4. Complex Formation in Intermetallic
(‘‘Interstitial’’) Hydrides
Until recently the only intermetallic compound
known to yield a nonmetallic hydride was Mg2Ni.
The brownish-colored hydride showed a disordered
3
Metal Hydrides: Transition Metal Hydride Complexes
CN
Complex
Example
Electrons/
complex
9
[ReH9]2[TcH9]2-
K2ReH9
K2TcH9
18
18
7
[OsH7]3[RuH7]3-
Na3OsH7
Na3RuH7
18
18
[MnH6]5[ReH6]5[FeH6]4[RuH6]4[OsH6]4[RhH6]3[IrH6]3[PtH6]2-
Mg3MnH7
Mg3ReH7
Mg2FeH6
Mg2RuH6
Mg2OsH6
Na3RhH6
Na3IrH6
Na2PtH6
18
18
18
18
18
18
18
18
[RuH5]av5[CoH5] 4[IrH5]4-
Mg3RuH6
Mg2CoH5
Mg6Ir2H11
18
18
18
[RhH4]3[PdH4]2[PtH4]2-
Li3RhH4
Na2PdH4
Na2PtH4
16
16
16
4
[MnH4]2[NiH4]4[PdH4]4[CuH4]3[ZnH4]2[CdH4]2-
K3MnH5
Mg2NiH4
Ba2PdH4
Ba7Cu3H17
K2ZnH4
Cs3CdH5
13
18
18
18
18
18
4
[CoH4]av5[RuH4]4[IrH4]5-
Mg6Co2H11
Mg2RuH4
Mg6Ir2H11
18
16
18
3
[PdH3]3-
NaBaPdH3
[PdH3]av3-
LiSr2PdH5
18
[PdH2]2[PtH2]2-
Na2PdH2
Li2PtH2
14
14
6
5
4
Geometry
18
3
2
Figure 1
Coordination numbers (CN), ligand geometries and
electron counts in transition metal–hydride complexes
(av: average due to disorder).
hydrogen distribution in its cubic high-temperature
phase and was originally classified as an ‘‘interstitial’’
hydride. Only after its monoclinic room-temperature
structure had been found to contain an ordered array
4
of tetrahedral [NiH4]4 complexes was it classified as
a ‘‘complex’’ metal hydride. It has the stoichiometric
composition Mg2NiH4 and shows a relatively important but reversible rearrangement of its metal
atom substructure. Systems showing similar behavior
include Li2TH2 (T ¼ Pd, Pt), MPdH2þx (M ¼ Ca, Sr,
Eu,Yb), and YbNiH2.7. Very recently some striking
examples of hydrogenation-induced complex formation and metal–insulator transitions in metallic
compounds have been reported. The compounds
and their properties are summarized in Table 3.
Hydrogenation of the hexagonal compound Mg3Ir,
for example, yields an intensely red-colored non$
y
metallic hydride of composition Mg6Ir2H11 (Cern!
et al. 2002). Its monoclinic structure can be rationalized in terms of square-pyramidal [IrH5]4 and
saddle-like [IrH4]5 18-electron complexes and hydrogen nions H, in agreement with the limiting ionic
formula 4Mg6Ir2H11 ¼ 5MgH2 19Mg2 þ 2[IrH5]4 5
6[IrH4] . This suggests that the compound is a
‘‘complex’’ rather than an ‘‘interstitial’’ metal
hydride. Hydrogenation of metallic LaMg2Ni leads
to a stoichiometric hydride of composition LaMg2
NiH7 that is nonmetallic (Renaudin et al. 2003). It
contains tetrahedral [NiH4]4 complexes and hydrogen anions H corresponding to the limiting ionic
formula La3 þ Mg2þ 2 [NiH4]4 3H. However, hydrogenation of the metallic compounds NdMgNi4
and MgRh leads to nonstoichiometric hydrides for
which electric conductivity measurements have not
yet been performed. While NdMgNi4HB4 contains
[Ni4H4]5 tetramers in which hydrogen bridges three
edges and one face of a nickel tetrahedron (Gu!en!ee
et al. 2003), MgRhHB0.9 contains [Rh4H4]8 tetramers in which hydrogen bridges the edges of a
rhodium square (Yuon 1994). These findings suggest that hydride complex formation could be a
general phenomenon in ‘‘interstitial’’ hydrides, at
least on a local level. The well-known metallic (and
commercial) hydrogen storage material TiFeH2x,
for example, can be described in terms of linear
[FeH2] units, whereas ZrCr2H3.8 can be described as a
network of planar and saddle-like [CrH4] units
(Kohlmann et al 1999). However, the presumably
metallic ternary hydride Mg4IrH5 contains disordered T-shaped [IrH3]6 complexes and H anions,
but does not appear to derive from a stable binary
compound.
5. Bonding
The compositions and ligand geometries of transition
metal hydride complexes can be rationalized in terms
of ‘‘magic’’ electron counts based on full charge
transfer from the surrounding cation matrix, and
s2p2d hybridization schemes involving two-center–
two-electron bonds such as (d10)sp3 for 18-electron
tetrahedral [NiH4]4 and [PdH4]4, (d8)dsp2 for
Metal Hydrides: Transition Metal Hydride Complexes
Figure 2
Composite structures of (LiH) n(Mg2OsH6).
Table 3
Hydrogen-induced transition metal complex formationa and property changes in intermetallic compounds.
Compound
Mg2Ni
Mg3Ir
LaMg2Ni
NdNi4Mg
MgRh
Hydride
Mg2NiH4
Mg6Ir2H11
LaMg2NiH7
NdNi4MgHB4
MgRhHB0.9
T complexes and H anions
4
[NiH4]
3[IrH4]5, [IrH5]4, 5H
[NiH4]4, 3H
[Ni4H4]5
[Rh4HB3.6]4
Hydride properties
Hydride formation
Nonmetallic brownish
Nonmetallic red
Nonmetallic dark gray
Metallic? dark gray
Metallic? dark gray
B280 1C, B1 bar
B400 1C, B50 bar
o200 1C, o8 bar
B50 1C, B8 bar
B20 1C, 1–40 bar
a Reactions are reversible.
planar 16-electron [PdH4]2, (d8)dsp3 for squarepyramidal 18-electron [CoH5]4, (d6)d 2 sp3 for octahedral 18-electron [FeH6]4, and (d10)sp for linear
14-electron [PdH2]2. However, more elaborate electron counting rules based on partial charge transfer
from the cation matrix and s2d hybridization
schemes involving three-center–four-electron bonds
(Firman and Landis 1998) are also possible. The
formal oxidation numbers of the T element range
from I ([CoH4]5] to þ VII ([ReH9]2) and are
consistent with configurational differences as found,
for example, in palladates that change from tetrahedral [Pd(0)H4]4 to planar [Pd(II)H4]2 (OlofssonMa( rtensson et al. 2000). As expected, high oxidation
states occur mainly with heavier members of a group
such as Re(VII) in [ReH9]2, Os(IV) in [OsH7]3, and
Pt(IV) in [PtH6]2 for which no 3d analogs exist.
Complexes having more than 18 electrons have not
been reported as yet. As the H/M ratios decrease,
the systems tend to become metallic and electron
counting becomes less obvious. Two interesting examples in that respect are Mg2RuH4 and Mg3RuH3
that contain saddle-like 16-electron [RuH4]4 and
T-shaped 17-electron [RuH3]6 complexes, respectively. Both systems would be 18-electron if Ru–Ru
bonds occurred that join the complexes to linear
½Run H4n 4n polymers and [Ru2H6]12 dimers, respectively. The possibility of Ru–Ru bonds in these
compounds, however, has been questioned on theoretical grounds (Miller et al. 1994). Clearly, limiting
ionic formulas based on full charge transfer and
‘‘magic’’ electron counts are not suited to describe all
bonding aspects of complex metal hydrides. However,
they are useful for rationalizing their hydrogen
contents, which is generally not possible with their
‘‘interstitial’’ (metallic) counterparts. Finally, an important bonding feature of complex metal hydrides
are the interactions between the hydrogen ligands of
the complexes and the surrounding cation matrix. As
can be seen from the isoelectronic series Mg2FeH6–
Mg2CoH5–Mg2NiH4 shown in Fig. 3, the cations
adopt cubic—or nearly cubic—configurations that
maximize the Md þ –H interactions. Clearly these
interactions not only stabilize the complexes but also
contribute greatly to the thermal stability of the
overall structure. As to the T–H interactions they do
5
Metal Hydrides: Transition Metal Hydride Complexes
Figure 3
Cation environments of 18-electron complexes. Large spheres: Mg.
not appear to contribute much to thermal stability, as
shown by the relatively weak force constants measured for Mg2FeH6 (Fe–H stretching mode: B1.9
( 1, Parker et al. 1997) and the elongation of
mdyne A
the Fe–H bonds in the thermally more stable calcium
analog Ca2FeH6. Theoretical band structure calculations for some of these systems are available (e.g.,
Mg3MnH7, Orgaz and Gupta 2002) but have not yet
addressed the issue of thermal stability.
6. Properties and Possible Applications
In contrast to their ‘‘interstitial’’ counterparts complex
metal hydrides have no apparent homogeneity range
and are usually nonmetallic. They are often colored
and sometimes transparent. Notable exceptions are
Li2PdH2 and Na2PdH2 that are metallic, at least in
certain directions. Hydrides that derive from intermetallic compounds such as Mg2NiH4, Mg6Ir2H11,
and LaMg2NiH7 show hydrogenation-induced transitions from a metallic (Mg2Ni, Mg3Ir, LaMg2Ni) to a
nonmetallic (brownish-red Mg2NiH4, red Mg6Ir2H11,
dark gray LaMg2NiH7) state. Such transitions are of
both fundamental and technological interest as was
shown for binary systems, such as Y–H2 (switchable
mirrors), and ternary systems, such as Mg2Ni–H2
(tuneable optic windows, Griessen 2001, Richardson
et al. 2002). Magnetic properties are known for
hydrides containing magnetic ions that order, such
as rose-colored K3MnH5 (Mn(II), meff ¼ 4.5mB,
TN ¼ 28 K, Bronger and Auffermann 1998) and faint
violet-colored Eu2PdH4 (Eu(II), meff ¼ 8mB, TC ¼ 15 K,
Kohlmann et al. 2001). The vibrational spectra
indicate stretching and bending modes of the complexes in the expected ranges 1600–2000 cm1 and
800–1000 cm1, respectively (Parker et al. 1997).
6
As to properties of interest for hydrogen-storage
applications the hydrogen weight and volume
efficiencies and desorption temperatures of a few
complex T-metal hydrides are summarized in Table 4.
Clearly, some compounds have outstanding properties, such as Mg2FeH6 that shows the highest known
volume efficiency of all materials known (150 g L1,
i.e., more than twice that of liquid hydrogen) and
BaReH9 that has an H/M ratio that surpasses the
hydrogen-to-carbon ratio of methane (H/C ¼ 4). The
weight efficiencies of complex metal hydrides are also
remarkable as shown by Mg2FeH6 and Mg3MnH7
that can store up to 5 wt.% hydrogen, i.e., more than
currently used interstitial hydrides such as AB5HB6
(A ¼ La etc., B ¼ Ni, etc.), AB2HB4 (A ¼ Ti, etc.,
B ¼ Mn, etc.), and ABHB2 (A ¼ Fe, etc., B ¼ Ti, etc.)
that storeB2 wt.% hydrogen. Unfortunately, regarding thermal stability, complex metal hydrides perform less well than their interstitial counterparts.
Only few decompose near room temperature such as
BaReH9 and Ba7Cu3H17 that are, however, relatively
heavy and expensive and not completely reversible.
Mg2NiH4, which is the only commercialized hydride
of this class, decomposes only above 250 1C, corresponding to a desorption enthalpy of DH ¼ 64 kJ/
mol H2. Most other complex metal hydrides are more
stable (decomposition temperatures 4300 1C, DH4
80 kJ/H2) and must be heated to recover hydrogen,
which represents a penalty in energy. Fuel cells, for
example, require metal hydrides having desorption
temperatures B80 1C (DHB30–40 kJ/H2) and—at
least for the transportation sector—hydrogen storage
capacities of B5 wt.%. So far no known metal
hydride fulfills both requirements simultaneously.
Currently commercialized metallic (interstitial) hydrides fulfill the first condition (LaNi5Hx: 20 1C,
DH ¼ 30 kJ/H2) but not the second (ABHx: 2 wt.%),
Metal Hydrides: Transition Metal Hydride Complexes
Table 4
Hydrogen-storage properties of some complex transition metal hydrides.a
Hydrogen density
Formula
Hydrogen anions
Mg3MnH7
BaReH9
NaKReH9
Mg2FeH6
Ca4Mg4Fe3H22
SrMg2FeH8
LiMg2RuH7
LiMg4Os2H13
BaMg2RuH8
Mg2RuH4
Mg2CoH5
Mg6Co2H11
Ca4Mg4Co3H19
Mg6Ir2H11
Mg2NiH4
CaMgNiH4
LiSr2PdH5
Eu2PdH4
Ba7Cu3H17
K2ZnH4
K3ZnH5
[MnH6]5, H
[ReH9]2
[ReH9]2
[FeH6]4
[FeH6]4, H
[FeH6]4, H
[RuH6]4, H
[OsH6]4, H
[RuH6]4, H
[RuH4]4n
n
[CoH5]4
5
[CoH4] , [CoH5]4, H
[CoH5]4, H
[IrH4]5, [IrH5]4, H
[NiH4]4
[NiH4]4
[PdH3]3
[PdH4] 4
[CuH4]3, H
[ZnH4]2
[ZnH4] 2, H
gL1
Desorption
temperature (1 bar)
5.2
2.7
3.5
5.5
5.0
4.0
4.3
2.6
2.7
2.6
4.5
4.0
4.2
2.0
3.6
3.2
1.7
1.0
1.5
2.7
2.7
119
134
117
150
122
115
113
121
98
95
126
97
106
89
98
87
74
68
63
57
56
B280 1C
o100 1C
o100 1C
320 1C
395 1C
440 1C
4400 1C
4400 1C
4400 1C
4400 1C
280 1C
370 1C
4480 1C
o400 1C
280 1C
405 1C
4400 1C
o300 1C
20 1C
310 1C
360 1C
7.7
100
109
71
280 1C
253 1C
wt.%
a-MgH2
H2(liquid)
a Found and/or characterized in Geneva, list completed and updated from Yvon (1998).
whereas complex metal hydrides fulfil either the
first (BaReH9) or the second condition (Mg2FeH6),
but not both together. However, compounds such
as Mg2FeH6 are of interest for high-temperature
applications because of their high thermal stability
and the ease of cycling (Bogdanovic et al. 2002).
Another drawback of complex T-metal hydrides is
their tendency to liberate hydrogen only in steps such
as the quaternary hydride Ca4Mg4Fe3H22 and its
ytterbium congener that decompose according to the
reactions
Ca4 Mg4 Fe3 H22 ) 2Ca2 FeH6 þ4Mg þ Fe þ 5H2
ð4Þ
Yb4 Mg4 Fe3 H22 ) 4YbH2 þ4Mg þ 3Fe þ 7H2
ð5Þ
As can be seen from the pressure–composition
isotherms shown in Fig. 4 they display several
plateaus that do not allow one to take full advantage of the theoretical hydrogen storage capacity.
Clearly, these compounds share this undesirable
feature with other families of potentially useful
hydrogen-storage material hydrides such as alanates
(NaAlH4 ) 13Na3 AlH6 þ 23Al þ H2 ). As to the factors responsible for thermal stability the nature of
the cation matrix clearly plays a major role. As can
be seen from the desorption enthalpies summarized
in Table 5 and the slopes of the Van’t Hoff plots
shown in Fig. 5, the calcium-containing hydrides
Ca4Mg4Fe3H22 and CaMgNiH4 are much more
stable as the calcium-free hydrides Mg2FeH6 and
Mg2NiH4, respectively, in accordance with the
stability of the corresponding binary hydrides. This
suggests that the interactions between the metal
cations and hydrogen of the complexes govern to a
large extent the thermal stability of complex metal
hydrides.
7. Conclusions and Outlook
The currently known complex T-metal hydrides are
capable of storing hydrogen (energy) at volume
densities that exceed by far those of compressed
hydrogen gas and liquid hydrogen. Their weight
efficiencies reach 5% and their hydrogen dissociation
temperatures at 1 bar hydrogen pressure are in the
range 100–400 1C. Some hydrides are relatively
inexpensive to fabricate but thermally too stable for
room-temperature applications, whereas others are
sufficiently unstable but too expensive for large-scale
applications. A few systems show hydrogen-induced
7
Metal Hydrides: Transition Metal Hydride Complexes
Figure 5
Van’t Hoff plots (desorption) of complex T-metal
hydrides. For DH values see Table 5.
Acknowledgments
Figure 4
Desorption isotherms of Yb4Mg4Fe3H22 and
Ca4Mg4Fe3H22 (from J. Alloys Compnds. with
permission).
Table 5
Thermal stability of calcium substituted complex
transition metal hydrides.
Hydride
DHdes (kJ/H2)
Tdes (1 bar H2)
Mg2NiH4
CaMgNiH4
64
129
250 1C
400 1C
Mg2FeH6
Ca4Mg4Fe3H22
98
122
320 1C
400 1C
MgH2
CaH2
74
183
280 1C
550 1C
transitions from delocalized (alloy) to localized
(hydride) electron states, which could be of interest
for optical applications. At present the potential of
complex metal hydrides appears to be limited to
niche markets where materials price plays a minor
role. A challenge for future work is to synthesize
new compounds based on light and inexpensive
3d-elements, and to reach a better understanding of
the metal–hydrogen interactions that govern thermal
stability.
8
This review covers recent work performed by my
collaborators whose names appear in the list of
references. Particular thanks are due to G. Renaudin
and J.-Ph. Soulie! for help with the illustrations. The
work was supported by the Swiss National Science
Foundation and the Swiss Federal Office of Energy.
See also: Hydrogen Metal Systems: Applications of
Gas–Solid Reactions; Hydrogen Metal Systems:
Basic Properties (1) and (2); Hydrogen Metal
Systems: Technological and Engineering Aspects;
Metal Hydrides: Electronic Band Structure; Metal
Hydrides: Sorption Kinetics
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K. Yvon
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