Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Sol–gel process wikipedia , lookup
Jahn–Teller effect wikipedia , lookup
Metalloprotein wikipedia , lookup
Metal carbonyl wikipedia , lookup
Hydroformylation wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Spin crossover wikipedia , lookup
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 Bibliography Bau R, Drabnis M H 1997 Structures of transition metal hydrides determined by neutron diffraction. Inorg. Chim. Acta 259, 27–50 Bogdanovic B, Reiser A, Schlichte K, Spliethoff B, Tesche B 2002 Thermodynamics and dynamics of the Mg–Fe–H system and its potential for thermochemical thermal energy storage. J. Alloys Compnds. 345, 77–89 Bronger W 1991 Complex transition metal hydrides. Angew. Chem. Intl. Ed. Engl. 30, 759–68 Bronger W, Auffermann G 1998 New ternary Alkali-metal– transition-metal hydrides synthesized at high pressures: characterization and properties. Chem. Mater. 10, 2723–32 Bronger W, à Brassard L 1995 Li5Pt2H9, a complex hydride containing isolated [Pt2H9]5 ions. Angew. Chem. Int. Ed. Engl. 34, 898–900 Bronger W, Breil L 1998 Calcium–rhodium–hydride—synthese und struktur. Z. anorg. allg. Chem. 624, 1819–22 Metal Hydrides: Transition Metal Hydride Complexes Bronger W, Sommer T, Auffermann G, Muller . P 2002 New alkali metal osmium and ruthenium hydrides. J. Alloys Compnds. 330–332, 536–42 $ Cern! y R, Joubert J-M, Kohlmann H, Yvon K 2002 Mg6Ir2H11, a new metal hydride containing saddle-like [IrH4]5 and square-pyramidal [IrH5]4 hydrido complexes. J. Alloys Compnds. 340, 180–8 Firman T K, Landis C R 1998 Structure and electron counting in ternary transition metal hydrides. J. Am. Chem. Soc. 120, 12650–6 Griessen R 2001 Switchable mirrors. Europhys. News 32, 41–4 Gu!en!ee L, Favre-Nicolin V, Yvon K 2003 Synthesis, crystal structure and hydrogenation properties of the ternary compounds LaNi4Mg and NdNi4Mg. J. Alloys Compnds. 348, 129–37 King R B 2000 Structure and bonding in homoleptic transition metal hydride anions. Coord. Chem. Rev. 200–202, 813–29 Kohlmann H, Fauth F, Yvon K 1999 Hydrogen Order in Monoclinic ZrCr2H3.8. J. Alloys Compnds. 285, 204–11 Kohlmann H, Fischer H E, Yvon K 2001 Europium–palladium hydrides. Inorg. Chem. 40, 2608–13 Miller G J, Deng H, Hoffmann R 1994 Ternary group VIII hydrides: ligand field and cation orbital effects in their electron structures. Inorg. Chem. 33, 1330–9 Olofsson-M(artensson M, H.aussermann U, Tomkinson J, Nor!eus D 2000 Stabilization of electron-dense palladium– hydrido complexes in solid-state hydrides. J. Am. Chem. Soc. 122, 6960–70 Orgaz E, Gupta M 2002 Electronic structure of the new manganese ternary hydride Mg3MnH7. J. Alloys Compnds. 330–332, 323–7 Parker S F, Williams K P J, Bortz M, Yvon K 1997 Inelastic neutron scattering, infrared, and Raman spectroscopic studies of Mg2FeH6 and Mg2FeD6. Inorg. Chem. 36, 5218–21 Renaudin G, Gu!en!ee L, Yvon K 2003 LaMg2NiH7, a novel quaternary metal hydride containing tetrahedral [NiH4]4 complexes and hydride anions. J. Alloys Compnds. 350, 145–50 Richardson T J, Slack J L, Farangis B, Rubin M D 2002 Mixed metal films with switchable optical properties. Appl. Phys. Lett. 80, 1349–51 Sandrock G, Thomas G 1997 Hydrogen information center, http://hydpark.ca.sandia.gov/, IEA/DOE/SNL hydride databases Schlapbach L (guest editor) 2002 Hydrogen as a fuel and its storage for mobility and transport. MRS Bull. 27/9, 675–716 Yvon K 1994 Hydrides: solid state transition metal complexes. In: King R B (ed.) Encycl. Inorg. Chem. Wiley, New York, Vol. 3, pp. 1401–20 Yvon K 1998 Complex transition-metal hydrides. Chimia 52, 613–19 Yvon K, Fischer P 1988 Crystal and magnetic structures of ternary metal hydrides: a comprehensive review. In: Schlapbach L (ed.) Hydrogen in Intermetallic Compounds, I: Electronic, Thermodynamic, and Crystallographic Properties, Preparation. Topics in Appl. Phys. 63, 87–138, Springer, Berlin K. Yvon Copyright r 2004 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials: Science and Technology ISBN: 0-08-043152-6 pp. 1–9 9