A new synthetic route to Mg2Na2NiH6 where a [NiH4] complex is for the first time stabilized by alkali metal counterions

Article abstract of DOI:10.1021/ic0620586

Mg₂Na₂NiH₆ was synthesized by reacting NaH and Mg₂NiH₄ at 310°C under hydrogen pressure. The novel structure type was refined from neutron-diffraction data in the orthorhombic space group Pnma (No. 62), with unit cell dimensions of a = 11.428(2), b = 8.442(2), and c = 5.4165(9) A and a unit cell volume = 523 A³ (Z = 4). The structure can be described by (Mg ₂H₂)²⁺ layers intersected by (Na ₂NiH₄)^2- layers. The [NiH₄] ^4- complex is approximately tetrahedral, indicating formal zerovalent nickel. This is the first example of a solid-state hydride where a [NiH ₄]^4- complex is directly stabilized by alkali metal ions instead of the more polarizing Mg²⁺ ions. A rather long nickel-hydrogen bond distance of 1.65 A indicates a weaker Ni-H bond as a result of the weaker support from the less polarizing alkali metal counterions.

Full text of DOI:10.1021/ic0620586

Inorg. Chem. 2007, 46, 2220−2223
A New Synthetic Route to Mg Na NiH Where a [NiH ] Complex Is for
2
2
6
4
the First Time Stabilized by Alkali Metal Counterions
Karim Kadir* and Dag Nor e´ us
Department of Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity,
S-106 91 Stockholm, Sweden
Received October 27, 2006
2 2 6 2 4
Mg Na NiH was synthesized by reacting NaH and Mg NiH at 310 °C under hydrogen pressure. The novel structure
type was refined from neutron-diffraction data in the orthorhombic space group Pnma (No. 62), with unit cell
3
dimensions of a
structure can be described by (Mg
)
11.428(2), b
)
8.442(2), and c
)
5.4165(9) Å and a unit cell volume
)
523 Å (Z
)
4). The
H
2 2
)2 layers intersected by (Na
+
2
NiH
4
)2 layers. The [NiH
-
4
-
]4 complex is
approximately tetrahedral, indicating formal zerovalent nickel. This is the first example of a solid-state hydride
-
+
where a [NiH
rather long nickel
support from the less polarizing alkali metal counterions.
4
]4 complex is directly stabilized by alkali metal ions instead of the more polarizing Mg2 ions. A
−
hydrogen bond distance of 1.65 Å indicates a weaker Ni H bond as a result of the weaker
−
Introduction
With the use of a new synthetic technique for metal
hydrides, a quaternary Mg Na NiH hydride was discovered.
It has a structure related to but also interestingly different
from Mg NiH . Both hydrides have high hydrogen contents,
.8 and 3.6 wt %, respectively, making them candidates for
hydrogen storage. Both contain 18 electrons, within [NiH
relieve the high electron density on the central atom. This is
conventionally referred to as electron “back-donation”. In
homoleptic complexes with hydrogen as the ligand this is
not directly possible. In solid-state hydrides, however, the
easily deformable electron density of the very polarizable
2
2
6
2
4
-
hydride ion H offers a mechanism for relief of the high
electron density indirectly by transferring it to the counterions
3
4
]
in the solid-state lattice. In the related hydrides of Li
and Na PdH , this leads to metallic conductivity when a small
number of electrons is transferred to the alkali metal matrix,
from the linear 14 electron [PdH ] complex with formal
2 2
PdH
complexes, and with formal zerovalent nickel. This implies
unusual and interesting structural and bonding properties.
2
2
In Mg
2
NiH
4
, the rather weak Ni-H bond in the [NiH
4
]
2
complex is strengthened by the support from strongly
1
0
2,3
2
+
zerovalent d palladium.
polarizing Mg ions. This makes Mg
2 4
NiH too stable with
As the lattice is directly involved in the stabilization
mechanism, it implies that lattice defects such as disorder
and impurities may have profound effects on stability as well
respect to hydrogen release for most practical applications.
In Mg
2
Na
2
NiH
6
, the [NiH
4
] complex is surrounded by less
+
polarizing Na ions, which results in weaker Ni-H bonds
2 4
as the electric properties. Mg NiH is a semiconductor with
as will be described below.
a not too wide band gap of 1.7 eV similar to that of silicon.
Doping from impurities or nonstoichiometry can give it
metal-like electric conductivity, and we have reported that
this conductivity is connected to the presence of a stacking
fault in the lattice and described how it can be switched on
Mg
discovery by Reilly and Wiswall in 1968. Mg
first sight an unlikely compound. The structure is built by
close-to-tetrahedral [NiH ] complexes counterbalanced by
2 4
NiH became a target for many studies since its
1
2 4
NiH is at
4
magnesium ions. The central nickel atom is in a formal
zerovalent d10 oxidation state surrounded by four hydride
ligands. Such low oxidation states usually require good
electron-accepting ligands, for example, carbonyl, cyanide,
or phosphine groups, where suitable ligand orbitals help to
4
and off by manipulating the density of stacking faults. The
4
nickel-hydrogen bond in the [NiH ] complex is rather weak,
and theoretical calculations of the electron structure show
(
2) Kadir, K.; Kritikos, M.; Nor e´ us, D.; Andresen, A. F. J. Less-Common
Met. 1991, 36, 172-174.
*
To whom correspondence should be addressed. E-mail: karim@
struc.su.se. Tel: +46 8 16 23 83. Fax: +46 8 15 21 87.
1) Reilly, J. J.; Wiswall, R. H., Jr. Inorg. Chem. 1968, 7, 2254.
(3) Olofsson-Mårtensson, M.; H a¨ ussermann, U.; Tomkinson, J.; Nor e´ us,
D. J. Am. Chem. Soc. 2000, 122, 6960.
(
(4) Blomqvist, H.; Nor e´ us, D. J. Appl. Phys. 2002, 91, 5141.
2220 Inorganic Chemistry, Vol. 46, No. 6, 2007
10.1021/ic0620586 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/17/2007

2 2 6
New Synthetic Route to Mg Na NiH
that the complex needs support from the small and strongly
2 4
fine powders of Mg NiH and NaH were mixed in a 1:1 molar
2
+
5,6
ratio by grinding them together in a mortar. This mixture (∼2 g)
was compacted in a tablet press at 3 kbar and sintered in a tube
furnace at 300-315 °C in hydrogen of 50 bar for about 20 h. After
cooling, the tablets were crushed and a small amount was removed
for X-ray analysis. To obtain a fairly “single-phased” sample, the
tablet had to be repressed again for two or three subsequent sintering
periods. Additional starting material was added until we reached a
polarizing Mg counterions for stability. Similarly, all of
the transition metal hydrido complexes of the first row
2
+
transition metals have mainly been synthesized with Mg
counterions either alone or in combination with other
counterions. There are now a number of these as shown by
the examples of Mg
3
CrH∼6, Mg
3
MnH∼6, Mg
3 7 2
MnH , Mg -
FeH
LaMg
6
, Mg
2
CoH
5
-12
, CaMgNiH , SrMgNiH
Of all of these complexes, only [FeH ]
6
4
4
, YbMgNiH
4
, and
2 4
molar ratio of 1:2 for Mg NiH to NaH, whereby an olive-green
7
4-
2
NiH
7
.
powder was obtained. Several attempts were made in this iterative
search before the optimum preparative conditions could be found.
A typical successful synthesis started with a 1:2 molar mixture of
2+
has been synthesized without Mg counterions, but then only
with low yield in incomplete reactions (i.e., in Ca FeH and
with a not too electron-dense d configuration).
2
6
6
13
Sr
2
FeH
6
2 4
Mg NiH to NaH, which was compacted and sintered as described
above. Regrinding, recompacting, and resintering were more
essential than the length of the sintering period. During this period,
we were able to add the balancing amounts of NaH and Ni powders.
With each such cycle the amount of impurity phases was reduced.
As this consumed a lot of deuterium, we stopped after three sintering
periods for the larger sample used in the diffraction experiment.
X-ray and Neutron Powder Diffraction Data Collection and
Structure Refinements. X-ray diffraction photographs were ob-
tained from a subtraction-geometry Guinier-H a¨ gg focusing camera,
We also tried to make the corresponding hydrides with alkali
metal counterions by reacting powders of all the first row
transition metals with the alkali metals of Li, Na, and K or
corresponding binary hydrides at temperatures and hydrogen
pressures of up to 600 °C and 150 bar. In no case could we
obtain a reaction. On the other hand, transition metal hydrido
complexes of the second and third row transition metals,
which are held together by stronger bonds, have been
synthesized in large numbers with counterions of most of
the alkali earth as well as alkali metals group. This shows
that magnesium is especially important for stabilizing first
row transition metal hydrido complexes, but in this paper
we report the first quaternary hydride of this type, where an
alkali metal is directly involved in the stabilization of a
1
using strictly monochromatized Cu KR radiation. A single-coated
X-ray film (CEA Reflex 15) was used in all the work. The films
were evaluated by means of a computer-controlled single-beam
microdensitometer designed for the scanning analysis of X-ray
powder photographs.14 The θ scale was calibrated by means of the
internal (silicon) standard technique, using a parabolic correction
curve. To localize the hydrogen atoms in the new hydrides we
[
4
NiH ] complex as its closest neighbor. It is also supported
2 2 6
prepared a Mg Na NiD sample for neutron diffraction. The
2+
by Mg ions but at a significantly longer distance from the
complex. This hydride could not be synthesized directly from
diffraction patterns were recorded at room temperature at the R2
reactor in Studsvik, Sweden, using a cylindrical vanadium sample
holder (diam ) 5 mm) and a neutron wavelength of 1.47 Å.
2 4
the elements but was made by first creating Mg NiH , which
was subsequently reacted with NaH.
15
With the use of the TREOR program and the least-squares
refinements program PIRUM,16 the final X-ray powder pattern of
Experimental Section
2 2 6
Mg Na NiH could be indexed on the basis of an orthorhombic
The starting sodium metal ingot, with 99.95% purity, was
obtained from Aldrich. The Mg Ni alloy ingot was obtained from
Ergenics, USA (as their HYSTOR-301 Alloy). This alloy contains
about 10 wt % extra magnesium to prevent the formation of the
structure with unit cell dimensions of a ) 11.428(2), b ) 8.442(2),
and c ) 5.4165(9) Å and unit cell volume of 523 Å . The deuteride,
3
2
Mg Na NiD , has unit cell dimensions of a ) 11.403(2), b )
2
2
6
3
8.411(3), and c ) 5.405(2) Å and a unit cell volume of 518 Å .
Metal deuterides often have smaller unit cell dimensions than those
of the corresponding hydrides due to unharmonicity in the hydrogen
vibrations. Several syntheses of both the hydride and the deuteride
were made to verify that the difference in unit cell volume was not
due to differences in hydrogen content. All syntheses, how-
ever, gave the same unit cell dimensions as given above, indicating
that the compound had a preference for formation with full
stoiciometry.
non-hydrogen absorbing MgNi
Mg NiH /D were synthesized by reaction with H
forming fine powders. The extra Mg in the Mg Ni alloy formed
MgH /D . The X-ray powder diffraction patterns for the MgH
, NaH/D, and Mg NiH /D produced were consistent with data
2
phase during casting. NaH/D and
2
4
4
2
and D gases
2
2
2
2
2
/
D
2
2
4
4
in the JCPDS tables. To protect the material from air or moisture,
all storing and handling was done in a glovebox under continuously
purified argon gas.
As nickel powder could not be made to react with Na or NaH,
the quaternary hydride was made in a two-step reaction. First, the
Systematic absences were consistent with space group Pnma (No.
2).17 The positions of the heavy atoms were found by direct
6
18
methods (SHELXS ), and subsequent Rietveld refinements RI-
(
5) H a¨ ussermann, U.; Blomqvist, H.; Nor e´ us, D. Inorg. Chem. 2002, 41,
19,20
21
ET94
and FULLPROF based on X-ray and neutron powder
3684.
(
(
6) Miller, G. J.; Deng, H.; Hoffmann, R. Inorg. Chem. 1994, 33, 1330.
7) R o¨ nnebro, E.; Kyoi, D.; Blomqvist, H.; Nor e´ us, D.; Sakai, T. J. Alloys
Compd. 2004, 279 (1-2), 368.
(14) Johansson, K. E.; Palm, T.; Werner, P. E. J. Phys. 1980, E13, 1289.
(15) Werner, P. E.; Eriksson, L.; Westdahl, M. J. Appl. Cryst. 1985, 18,
367.
(16) Werner, P. E.; Eriksson L. World Directory of Powder Diffraction
Programs, Release 2.12; International Union of Crystallography:
Chester, 1993.
(17) International Tables for Crystallography: Space Group Symmetry;
Hahn, T., Ed.; Kluwer Academic Publishers: Dordrecht, Germany,
1989; Vol A.
(
8) Blomqvist, H.; R o¨ nnebro, E.; Kyoi, D.; Sakai, T.; Nor e´ us, D. J. Alloys
Compd. 2003, 358 (1-2), 82-86.
(
9) Bortz, M.; Bertheville, B.; Yvon, K.; Movlaev, E. A.; Verbetsky, V.
N.; Verbetsky, V. N.; Fauth, F. J. Alloys Compd. 1998, 279 (2), L8-
L10.
(
10) Selvam, P.; Yvon, K. Int. J. Hydrogen Energy 1991, 16 (9), 615-
617.
(11) Renandin, G.; Guenee, L.; Yvon, K. J. Alloys Compd. 2003, 350, 145.
(12) Huang, B.; Yvon, K. J. Alloys Compd. 1994, 204, L5.
(13) Huang, B.; Bonhomme, F.; Selvam, P.; Yvon, K. J. Less-Common
Met. 1991, 179, 301.
(18) (a) Sheldrick, G. M. SHELXS-97 Acta Crystallogr. 1990, A46, 467.
(b) Sheldrick, G. M. SHELXL-97, Computer Program for the Refine-
ment of Crystal Structures; University of G o¨ ttingen: G o¨ ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 46, No. 6, 2007 2221

Kadir and Nor e´ us
Figure 1. Rietveld-fitted profile and difference plot for Mg2Na2NiD6. The
neutron-diffraction data is obtained at room temperature with a wavelength
)
1.47 Å. Tick marks and refined weight percentage are (top) for Mg2-
Na2NiD6 and (below) for the impurity phases of NaMgD3, NaD, MgO, and
Mg2NiD4, respectively.
Figure 2. The structure of Mg2Na2NiD6 viewed along the c-axis. The
unit cell axes are marked with green lines. Ni atoms and NiH4 tetrahedra
are red. The hydrogen atoms (small, blue) are connected to the closest Ni
Table 1. Atomic Parameters from Rietveld-Fitted Neutron Powder
Diffraction Data for Mg2Na2NiD6 (Space Group Pnma; Z ) 4)
2
+
and Mg atoms. The structure can be visualized as (Mg2H2) layers
atom
site
x
y
z
Biso
Occ
intersected by (Na NiH )2- layers.
2
4
Ni
4c
8d
4c
4c
4c
8d
4c
8d
-0.001(3)
0.270(3)
0.402(8)
0.165(7)
0.099(4)
0.022(3)
-0.141(4)
-0.135(3)
1/4
0.942(6)
0.863(7)
0.030(2)
0.573(2)
0.160(8)
0.747(6)
0.025(9)
0.933(7)
2.3(8)
0.6(8)
2.3(3)
0.2(2)
4.1(2)
1.9(8)
2.6(1)
4.5(1)
1
1
1
1
1
1
1
1
polarizing Mg2 ions compress the complex. In Mg
NiD
complex is a long >2.7 Å compared with 1.9-2.1 Å in Mg
+
5
-
-
Mg
Na1
Na2
D1
D2
D3
D4
0.013(4)
2
Na
2
1
/4
/4
/4
1
6
the Mg-D distance to the hydrogen atoms in the
1
2
0.102(3)
NiD
4
2
. A short Mg-D comparable to that in MgD is only
found within the (Mg
Na-D distances in the (Na
ing considerable electron overlap and covalent bonding
contribution to the complex. The Na-D distances are
significantly shorter compared with NaD (2.45 Å), whose
bonding we earlier described with a substantial covalent
1
/4
2+
D
2 2
)
layers. On the other hand, the
-0.016(5)
2-
2 4
NiD )
layers are short, indicat-
Table 2. Shortest Interatomic Distances (Å) and Angles (deg) Found in
Mg2Na2NiD6 in the NiD4 Complexa
Ni-D1 ) 1.65(2)
D1-Ni-D2 ) 110.6(2)
D2-Ni-D2 ) 96.0(2)
D1-Ni-D3 ) 118.0(3)
D2-Ni-D3 ) 110.0(2)
Na1-D2 ) 2.40(3)
Na1-D3 ) 2.47(4)
Na2-D1 ) 2.36(3)
Na2-D2 ) 2.23(2)
Na2-D3 ) 2.27(3)
Mg-Na1 ) 2.64(3)
Mg-Na2 ) 2.59(2)
Mg-Mg ) 2.75(1)
2
× Ni-D2 ) 1.66(1)
Ni-D3 ) 1.64(2)
22,3
character as compared to the ionic and isostructural NaCl.
Mg-D4 ) 1.90(2)
Mg-D4 ) 1.94(2)
Mg-D3 ) 2.73(2)
D1-D4 ) 2.06(2)
+
2+
This means that Na has overtaken the role of Mg as a
stabilizer for the NiH complex and could open the possibility
of the synthesis of Na NiH or even Li NiH with possibly
better hydrogen storage properties. Unfortunately, so far we
have failed to do this. It could be due to some kinetic
restriction necessitating the presence of Mg for formation
4
4
4
4
4
Ni-Na1 ) 2.80(4)
Ni-Na2 ) 2.75(3)
Na1-Na2 ) 3.07(4)
2-
a
of the NiH
4
complex or that the (Na
the support from the (Mg
2 4
NiD )
layers still need
layers for stability. In this
Estimated standard deviations in parentheses.
2
+
2 2
D )
respect, it is interesting to note that the unusually short D-D
patterns of Na
2
Mg
2
NiD
6
gave the atomic coordinates listed in Table
) 4.6, R
5.5, and RWP ) 7.05% (cf. Figure 1). The interatomic distances
1. The final Rietveld R values were Bragg R ) 7.3, R
F
P
distance (2.06 Å) between the hydrogen atoms in the
2
-
2+
)
(Na
2 4
NiD )
and (Mg
D
2 2
)
layers is shorter than the
are given in Table 2.
minimum distance of 2.1 Å stipulated by the “Switendick
2
3,24
criterion”.
This indicates a fairly strong interaction
Results and Discussion
between the layers mediated by the hydrogen atoms. We
hope to elucidate this phenomenon by synthesizing and
characterizing additional quaternary hydrides of this kind.
The structure of Mg
2
Na
2
+
NiD
6
, as depicted in Figure 2 can
2
2-
be described by (Mg
2
-
D
2
)
2 4
layers intersected by (Na NiD )
4
layers. The [NiD
indicating a formal zerovalent oxidation state for the central
nickel atom. It is, however, slightly more distorted in Mg
4
]
complex is approximately tetrahedral,
Acknowledgment. We are grateful to Håkan Rundl o¨ f for
collecting the neutron-diffraction pattern at the R2 reactor
in Studsvik and Dr. Lars Eriksson for fruitful discussions
and supplying us with Grusplot and Pfilm programs. Partial
funding by the European Commission DG Research (Con-
tract No. SES6-2006-518271/NESSHY) is gratefully ac-
knowledged by the authors.
2
-
Na
Mg
2
NiD
Na
6
than in Mg
2
NiD
4
. The average Ni-D distance in
2
2
NiD
6
is 1.65 Å. This is significantly longer than the
average distance of 1.54 Å in the corresponding complex in
Mg NiD , indicating a weaker bond. In Mg NiD the small
2
4
2
4
(
19) Wiles, D. B.; Sakthivel, A.; Young R. A. Program DBW 3.2S; School
of Physics, Georgia Institute of Technology: Atlanta, 1988.
(22) The Na-H distance in NaH was estimated as a ) 4.880(1) Å/2. See:
Zintl, E.; Harder, A. Z. Phys. Chem. 1931, B14, 265-284.
(23) Switendick, A. C. Z. Phys. Chem. (Munchen) 1979, 117, 89.
(24) Rao, B. K.; Jena, P. Phys. ReV. B 1985, 31, 6726.
(
20) Wiles, D. B.; Young, R. A. J. Appl. Crystallogr. 1981, 14, 149.
21) Rodriguez-Carvajal, J. FULLPROF.2k, Version 3.00; LLB JRC: Gif-
sur-Yvette, Cedex, France, 2004.
(
2222 Inorganic Chemistry, Vol. 46, No. 6, 2007

2 2 6
New Synthetic Route to Mg Na NiH
Supporting Information Available: Observed, calculated, 2θ,
d-values, and intensities for Na Mg NiH using Guinier-H a¨ gg X-ray
diffraction Cu KR radiation and with Si as the internal standard at
293 K. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0620586
2
2
6
1
Inorganic Chemistry, Vol. 46, No. 6, 2007 2223