A Simple Route to Calcium and Strontium Hydride Clusters

Article abstract of DOI:10.1002/anie.201706786

The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe₃)₂]₂ with PhSiH₃ in the presence of PMDTA. The Sr complex Sr₆H₉[N(SiMe₃)₂]₃?(PMDTA)₃ crystallizes as an “inverse cryptand”: an interstitial H^? is surrounded by a Sr₆H₈⁴⁺ cage decorated with amide and PMDTA ligands. The analogous Ca complex could also be obtained and both retain their solid-state structures in solution: ¹H NMR spectra in C₆D₆ show two doublets and one nonet (4:4:1). Up to 90 °C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+1.14) and H (?0.59) which signifies an unexpectedly low ionicity. AIM analysis shows hydride???hydride bond paths with considerable electron densities in the bond critical point. The clusters thermally decompose into larger, undefined, metal hydride aggregates.

Full text of DOI:10.1002/anie.201706786

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: A Simple Route to Calcium and Strontium Hydride Clusters
Authors: Sjoerd Harder, Brant Maitland, Michael Wiesinger, Jens
Langer, Gerd Ballmann, Jürgen Pahl, Holger Elsen, and
Christian Färber
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706786
Angew. Chem. 10.1002/ange.201706786
Link to VoR: http://dx.doi.org/10.1002/anie.201706786
http://dx.doi.org/10.1002/ange.201706786

10.1002/anie.201706786
Angewandte Chemie International Edition
Table of Contents
The first strontium hydride complex has been prepared by a surprisingly simple reaction.
The cluster consists of an interstitial hydride, surrounded by an Sr6 octahedron that carries
eight hydrides at its faces and is decorated with ligands. Together with its calcium analogue
1
these clusters can be seen as “inverse cryptands”. H NMR shows H∙∙∙H magnetic coupling.
3+
Theoretical calculations on the Ca cluster suggest unusually high covalency in the Ca6H9
core. AIM analysis shows clear H∙∙∙H bond critical points.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
2
A Simple Route to Calcium and Strontium Hydride Clusters**
Brant Maitland, Michael Wiesinger, Jens Langer, Gerd Ballmann, Jürgen Pahl, Holger Elsen,
Christian Färber, Sjoerd Harder*
[a] Prof. Dr. S. Harder, Dr. B. Maitland, M. Wiesinger, Dr. J. Langer, G. Ballmann, J. Pahl, H.
Elsen, Dr. C. Färber
Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany
Fax: (+49) 9131-8527387
E-mail: sjoerd.harder@fau.de
[**] We thank the Deutsche Forschungsgemeinschaft for financial support of this project
(HA3218/7-1)
Dedicated to Gerard van Koten on the occasion of his 75th birthday
Supporting information for this article is available on the WWW under
http//:www.angewandte.chemie.org or from the author.
Keywords: Strontium – Calcium – Hydrides – Alkaline-earth metals – Self-assembly
Abstract: The first Sr hydride complex has been obtained by simply reacting Sr[N(SiMe3)2]2 with
PhSiH3 in the presence of PMDTA. The Sr complex Sr6H9[N(SiMe3)2]3∙(PMDTA)3 crystallizes as an
4+
“inverse cryptand”: an interstitial Hˉ is surrounded by a Sr6H8 cage decorated with amide and
PMDTA ligands. The analogue Ca complex could also be obtained and both retain their solid state
structures in solution: 1H NMR spectra in C6D6 show two doublets and one nonet (4:4:1). Up till 90 °C,
no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically
low charges on Ca (+1.14) and H (-0.59) which signifies an unexpectedly low ionicity. AIM analysis
shows hydride∙∙∙hydride bond paths with considerable electron densities in the bond critical point.
The clusters thermally decompose in larger, undefined, metal hydride aggregates.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
3
The rich history of the transition metal hydrides[1,2] stands in stark contrast to early main
group metal hydride chemistry. Recent recognition that s-block metal hydride complexes
play a key role in catalysis,[3] however, fuelled interest in their challenging syntheses.[4]
In 2006, we reported on Rational Design of a Well-Defined Soluble Calcium Hydride Complex,
introducing the first calcium hydride complex (1).[5] Ironically, the same issue contains the
paper “Design” in Chemical Synthesis—An Illusion? in which the authors question the ability
of chemists to shape a material to their will.[6] Design presupposes the existence of creative
freedom but atoms always occupy discrete equilibrium positions beyond human influence.
Thus, “design is not possible, and humans are the explorers and not the creators of chemical
worlds.” Although we agree with the fact that there is no complete freedom in building
molecules, we like to think that, at least in “designing” organic ligands, there still is an
enormous freedom to shape and control your ligand within the boundaries of the laws of
molecular bonding.
The key to the succesful isolation of alkaline-earth (Ae) metal hydrides of the type LAeH is
the anionic spectator ligand L (Scheme 1). The standard route generally starts with the
synthesis of a heteroleptic intermediate LAeR in which R is a highly reactive group.
Subsequent reaction with PhSiH3 gives the hydride complex. Subsequent in-situ generation
of LAeH is a prerequisite for its successful isolation. The spectator ligand L should be bulky,
multidentate and strongly bound in order to prevent formation of homoleptic species
through the Schlenk equilibrium: 2 LAeH ⇄ L2Ae + AeH2↓.[7] The high lattice energies of
[AeH2]∞ salts[8] and their associated poor solubilities may redirect the synthesis of LAeH to
isolation of L2Ae. As the homoleptic complex Ca(DIPP-nacnac)2 is surprisingly stable,[9] the
DIPP-nacnac‾ ligand stabilizes complex 1 kinetically. Apart from the spectator ligand also the
neutral co-solvent (THF) plays a crucial role in its stability.[10] Only few other igands are
known to stabilize calcium hydride complexes. Tetradentate macrocyclic ligands, Me3TACD‾
or Me4TACD, gave access to cationic calcium hydride complexes (2, 3), clearly demonstrating
that multiple coordination causes hydride dissociation.[11,12] Using carefully controlled
reaction conditions, gave the first amidinate calcium hydride complex (4).[13]
While lessons learned now allow for access to a growing number of calcium hydride
complexes, the heavier and much more challenging strontium and barium hydride
complexes hitherto remained elusive. Increasing the size of the metal does not only result in
faster ligand exchange[14] but also makes it increasingly difficult to prevent Schlenk equilibria
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
4
by steric hindrance. Additionally, the inherently higher reactivity of heavier group 2 metal
complexes poses problematic decomposition pathways. Our attempts to control ligand
exchange by use of extraordinarily large ligands has so far failed.[15] Herein, we report on a
remarkably simple synthesis of a strontium hydride complex without bulky ligands, thus
rejecting the assumption that large spectator ligands are needed to stabilize heavier
alkaline-earth metal hydride complexes.
Scheme 1. Synthetic routes to heavier
alkaline-earth metal hydride complexes.
[16]
Reaction of the well-known strontium complex SrN’’2 with 1.5 equivalents of PhSiH3 and
PMDTA (N,N,N’,N’’,N’’-pentamethyldiethylenediamine) in cyclohexane/benzene gave
colorless crystals of Sr6H9N’’3∙(PMDTA)3, or alternatively (N’’SrH)3(SrH2∙PMDTA)3, in 54%
3+
yield. The crystal structure shows a hexanuclear cluster with an Sr6H9 core (Figure 1). The
central interstitial H‾ ion[17] (Hcenter) is surrounded by an octahedron of Sr2+ ions that binds
at each of its eight faces an H‾ ion (Houter) lying at the corners of a cube. Combining the Sr6
octahedron with the H8 cube gives an Sr6H8 cage that could be described as an Oh symmetric
rhombic dodecahedron. Anionic N’’ and neutral tridentate PMDTA ligands are bound to the
Sr corners, both in a meridional arrangement (mer,mer). The PMDTA ligands show the
typical twist disorder that is well-known for the similar but bidentate TMEDA ligand and
often simulates higher symmetry.[18] The cluster resides on a crystallographic mirror plane.
While the ligands are disordered to fulfill this symmetry, the core of the cluster conforms to
this symmetry and is highly ordered. All hydride atoms could be clearly identified in the
difference Fourier map and were refined isotropically.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
5
Figure 1. (a) Crystal structure of Sr6H9N’’3∙(PMDTA)3. For clarity, the H atoms and Me groups of the PMDTA and
3+
N’’ ligands, as well as the disorder, have been omitted. (b) The Sr6H9 core of the complex; the Sr atoms span
an octahedron (red). (c) Crystal structure of Ca6H9N’’3∙(PMDTA)3. The H atoms of the ligands and Me groups of
N’’, as well as the disorder, have been omitted for clarity.
Table 1. Selected distances (Å) and 1H NMR data (ppm) for clusters Ca6H9N’’3∙(PMDTA)3 and Sr6H9N’’3∙(PMDTA)3.
Average values shown in bold. Average of calculated values (B3PW91/6-311++G**) shown in bold between
squared brackets.
Complex
Ca6H9N’’3∙(PMDTA)3
Sr6H9N’’3∙(PMDTA)3
M∙∙∙M
3.4322(8)-3.6058(9)
3.7520(7)-3.8777(8)
3.5113 [3.534]
3.8012
M-Hcenter
2.43(3)-2.60(3)
2.63(8)-2.78(8)
2.48 [2.499]
2.69
M-Houter
2.17(3)-2.40(3)
2.38(4)-2.55(5)
2.30 [2.317]
2.43
Hcenter∙∙∙ Houter
2.43(5)-2.62(6)
2.57(9)-2.71(8)
2.53 [2.539]
2.64
Houter∙∙∙ Houter
2.78(4)-3.05(5)
2.89(6)-3.14(6)
2.92 [2.931]
3.05
3.75 (2JHH = 3.0 Hz, 1H)
4.48 (2JHH = 2.0 Hz, 1H)
(1H) Hcenter
(1H) Houter
4.35 (2JHH = 3.0 Hz, 4H)
4.48 (2JHH = 3.0 Hz, 4H)
5.63 (2JHH = 2.0 Hz, 4H)
5.85 (2JHH = 2.0 Hz, 4H)
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
6
A similar synthetic method gave the analogous calcium hydride complex of the same
constitution in high yield (79 %). The hydride-rich complex Ca6H9N’’3∙(PMDTA)3 is to date the
largest calcium hydride cluster that has been structurally characterized. The somewhat
smaller size of Ca2+ (1.00 Å) compared to Sr2+ (1.18 Å)[19] forces one of the PMDTA ligands to
coordinate in a bidentate fashion.
Apart from minor differences in ligand bonding, both metal hydride clusters show a very
similar topology. Despite the two different types of terminal ligands (N’’ and PMDTA), the
Ae6 octahedrons are surprisingly regular with Ae∙∙∙Ae distances in narrow ranges (Table 1).
The average Ca∙∙∙Ca’ distance of 3.5113 Å is similar to that in the dimer 1 (3.524 Å).
Although hydride positions are generally ill-defined, the Ca-H bond distances clearly fall in
two different categories: Ca-Hcenter (2.48 Å) bonds are significantly longer than Ca-Houter
bonds (2.30 Å). Both bond types are considerably longer than the Ca-H bonds in the dimers 1
(2.15 Å) or 4 (2.18 Å). This is likely due to different bridging modes: 3-H and 6-H in
Ca6H9N’’3∙(PMDTA)3 versus 2-H in 1 and 4. Indeed, the Ca-(3-Houter) bonds of 2.30 Å
compare well to the Ca-(3-H) bonds of 2.37 Å in 2. The same trends are observed for the Sr-
H bonds. Given that Sr2+ is 0.18 Å larger than Ca2+, the Sr-Hcenter bond distances are longer
and Sr-Houter shorter than expected but considering the uncertainty in the hydride positions
these differences are marginal.
1H NMR spectra in C6D6 prove that the structures of both clusters are retained in solution.
Although there are two possible isomers, (mer,mer) and (fac,fac), there is no indication for
the latter (Figure 2). The N’’ ligands give two sharp singlets in the expected 2:1 ratio.
Resonances for PMDTA ligands are broadened and overlap while the hydride ligands can
clearly be observed as a nonet and two doublets in the expected integral ratio of 1:4:4
1
(Figure 2, Table 1). As confirmed by H,1H-COSY, the two types of Houter nuclei couple with
Hcenter but only very weakly with each other. If H∙∙∙H distance would be a criterion for
magnetic coupling, the shorter Hcenter∙∙∙Houter versus Houter∙∙∙Houter distances would support
this observation. Larger Hcenter∙∙∙Houter distances in Sr6H9N’’3∙(PMDTA)3 would also explain its
2
somewhat smaller JHH of 2.0 Hz compared to that found in Ca6H9N’’3∙(PMDTA)3 (3.0 Hz).
There are no previous reports on hydride∙∙∙hydride coupling constants in Ca or Sr complexes.
2
Comparison with JHH values in Mg hydride complexes (4.5-28.6 Hz)[20-22] shows that
hydride∙∙∙hydride coupling decreases down the group Mg > Ca > Sr.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
7
Figure 2. The two possible isomers for Ae6H9N’’3∙(PMDTA)3 clusters; P represents Ae(PMDTA) and N’’
represents AeN(SiMe3)2. (a) Meridional arrangement of P and N’’ gives hydride signals in a 1:4:4 ratio. (b) Facial
1
arrangement of P and N’’ gives hydride signals in a 1:3:3:1:1 ratio. (c) Hydride signals in the H and COSY NMR
spectrum of Ca6H9N’’3∙(PMDTA)3 in C6D6 (25 °C); The Sr complex gives a similar coupling pattern.
1
The H NMR chemical shifts for the hydride nuclei in Ca6H9N’’3∙(PMDTA)3 are comparable to
those in 1 (4.45 ppm), 2 (3.99 ppm), 3 (4.72 ppm) and 4 (3.09 ppm). The chemical shifts for
the hydrides in Sr6H9N’’3∙(PMDTA)3 are shifted considerably down-field by 0.73-1.37 ppm.
This fits with the observation that hydrides in Mg[20-24] or Be[25] complexes are generally
found up-field (although those for Mg-H cover a large range: 0.55-6.07 ppm). The chemical
shifts for solid CaH2 (4.5 ppm), SrH2 (6.7 ppm) and BaH2 (8.9 ppm)[26] support the trend of
increasing deshielding down the group: Be → Ba. Since Ae-H bond polarity increases along
the same row, one would expect the contrary: a high negative charge on the hydride nuclei
in heavier Ae metal hydride complexes should give rise to strong shielding and low, or even
negative, chemical shifts. We attribute this counterintuitive downfield shift to deshielding
spin-orbit-induced-heavy-atom effects[27] which are very pronounced for atoms close to the
heavy nuclei (rˉ3 dependence) and increase with the atom number of the heavy atom. As
they depend strongly on involvement of valence s-orbitals, hydride nuclei directly bound to
heavy metals are particularly susceptible to deshielding by spin-orbit coupling. This heavy
atom effect also explains the considerable differences in the hydride chemical shifts for 1
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
8
(4.45 ppm) and its Yb(II) analogue (9.92 ppm).[28] The structures and electron distribution for
Ca/Yb pairs are astoundingly similar.[29] Therefore, the considerable down-field shift for the
Yb-bound hydride can only be explained by stronger spin-orbit coupling of the heavier Yb.
The extremely high chemical shift for the hydride in MeHgH (16.8 ppm) is in agreement with
this explanation.[30]
1H NMR spectra at temperatures up to 90 °C do not lead to coalescence of Hcenter/Houter
signals in either cluster. At this temperature, the decomposition into AeN’’2∙PMDTA and
undefined metal hydride clusters slowly starts. Heating the solutions above 200 °C gives H2
elimination, presumably by PMDTA deprotonation (Me2NCH=CH2 is observed).
Optimization of the lighter of the two hydride clusters, Ca6H9N’’3∙(PMDTA)3, by DFT
calculations at the B3PW91/6-311++G** level closely reproduces the experimentally
observed structure (Table 1). The NPA charge analysis shows a remarkably low positive
charge on the Ca2+ ions. Whereas organocalcium compounds are circa 80% ionic and
generally show charges around +1.60 on Ca,[31] the Ca atoms bound to N’’ have a charge of
+1.196 and those with PMDTA ligands are even less positive: +1.085. As the N’’ ligands carry
negative charges in the expected range (-0.782), the low Ca charges must be due to low
electron densities at the hydrides (Houter: -0.589, Hcenter: -0.606) and PMDTA → Ca electron
transfer (PMDTA: +0.278). Absolute NPA charges on Ca and H in CaH2∙PMDTA are somewhat
higher (Ca: +1.153, H: -0.677, PMDTA: +0.201). The rather low charge difference on Ca and H
3+
suggests an unexpectedly high degree of covalency in the Ca6H9 core. It is, however, less
2-
covalent than bonding in boranes (charges at the same level for B6H6 : B -0.286, H -0.04) and
therefore questionable whether Wade or Hirsch’s rule for 3D aromaticity[32] can be applied.
Bonding in these clusters should stimulate further theoretical work.
AIM analysis does not show bond critical points (BCP’s) between Ca atoms but there are
eight Houter∙∙∙Hcenter BCP’s with considerable electron densities (Figure 3). The BCP of 0.102
e∙Åˉ3 is more than twice the value of that reported for hydride∙∙∙hydride contacts in a Mg
hydride cluster (0.047 e∙Åˉ3, H∙∙∙H = 3.106 Å)[21] and higher than that for an extremely short
B-H∙∙∙HB contact in solid LiNH2BH3 (0.091 e∙Åˉ3, H∙∙∙H = 2.10 Å).[33] The surprisingly large BCP
for Houter∙∙∙Hcenter contacts are even similar to the BCP for Ca-Hcenter bonds (0.119 e∙Åˉ3). The
lack of BCP’s between the outer hydrides can be attributed to the much larger Houter∙∙∙Houter
distances (Table 1) and is in agreement with the absence of strong magnetic coupling
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
9
between these hydrides. The considerable electron density along the Houter∙∙∙Hcenter bond
paths suggests that such counterintuitive hydride∙∙∙hydride interactions may be a first step to
hydrogen formation: Hˉ + Hˉ → H2 + 2eˉ.[34] As solids, the Ca and Sr hydride clusters
decompose at a temperature of 135 °C. There is, however, no evidence for H2 formation
through hydride∙∙∙hydride coupling.
Figure 3. AIM analysis of the electron density in Ca6H9N’’3∙(PMDTA)3. Cross section through two Ca (red) and
Houter / Hcenter atoms (blue); BCP’s shown in green.
In summary, the first Sr hydride cluster and its lighter Ca congener could be obtained.
With metal/hydride ratios of 1/1.5, these are among the most hydride-rich Ae metal clusters
described in literature.[11a,24] Similar to self-organized “inverse crown ethers”,[35] their
structures could be seen as “inverse cryptands”: Hˉ is octahedrally encapsulated by the six
Ae2+ ions of an Ae6H84+ cage. It is especially surprising that after our attempts to synthesize
LSrH by use of extremely bulky ligands, simply reacting SrN’’2 with PhSiH3 in the presence of
PMDTA met with success. We generally believe in rational design in chemical synthesis but
the herein observed self-assembly is unforeseeable. Although the same synthetic protocol
does not work for isolation of a corresponding Ba hydride cluster, we currently vary metal
amide precursors and neutral co-solvents in order to meet this last remaining challenge.
References
[1]
(a) W. Hieber, F. Leutert Ber. Dtsch. Chem. Ges. B. 1931, 64, 2832−2839. (b) W. Hieber, F. Leutert,
Naturwissenschaften, 1931, 19, 360−361. (c) H. D. Kaesz, R. B. Saillant, Chem. Rev. 1972, 72, 231-281
and references cited therein.
This article is protected by copyright. All rights reserved.

10.1002/anie.201706786
Angewandte Chemie International Edition
10
[2]
[3]
[4]
Revived interest in transition metal hydride chemistry is demonstrated by a special issue: J. R. Norton,
J. Sowa, Chem. Rev. 2016, 116, 8315-8317.
(a) S. Harder, Chem. Rev. 110, 3852–3876 (2010). (b) M. S. Hill, D. J. Liptrot, C. Weetman, Chem. Soc.
Rev. 2016, 45, 972–988.
(a) S. Harder, Chem. Commun. 2012, 48, 11165-11177. (b) L. Fohlmeister, A. Stasch, Aust. J. Chem.
2015, 68, 1190-1201.
[5]
[6]
[7]
[8]
S. Harder, J. Brettar, Angew. Chem. Int. Ed. 2006, 45, 3474-3478.
M. Jansen, J. C. Schön, Angew. Chem. Int. Ed. 2006, 45, 3406-3412.
W. Schlenk, W. Schlenk, Jr., Chem. Ber. 1929, B62, 920.
Handbook of Chemistry and Physics, Eds. R. C. Weast and M. J. Astle, 61st ed. 1981, Florida, CRC Press,
p. D-94.
[9]
S. Harder, Organometallics 2002, 21, 3782-3787.
[10]
[11]
A. Causero, G. Ballmann, J. Pahl, C. Färber, J. Intemann, S. Harder, Dalton Trans. 2017, 46, 1822-1831.
(a) P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2012, 51, 4452 –
4455. (b) V. Leich, T. P. Spaniol, J. Okuda, Inorg. Chem. 2015, 54, 4927-4933.
V. Leich, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2016, 55, 4794-4797.
A. Causero, G. Ballmann, J. Pahl, H. Zijlstra, C. Färber, S. Harder, Organometallics. 2016, 35, 3350-3360.
Mean life times for water molecules in the first coordination shell rapidly decrease from Be2+ (10ˉ3 s)
to Ba2+ (10ˉ9 s): L. Helm, A. E. Merbach, Chem. Rev. 2005, 105, 1923-1959.
Manuscript in preparation, B. Freitag, S. Harder.
(a) M. Westerhausen, Inorg. Chem. 1991, 30, 96-101. (b) M. Westerhausen, M. Hartmann, N.
Makropoulos, B. Wieneke, M. Wieneke, W. Schwarz, D. Stalke, Z. Naturforsch. B 1998, 53, 117-125.
Interstitial hydrides have been observed previously in early main group metal chemistry: (a) D. R.
Armstrong, W. Clegg, R. P. Davies, S. T. Liddle, D. J. Linton, P. R. Raithby, R. Snaith, A. E. H. Wheatley,
Angew. Chem., Int. Ed. 1999, 38, 3367-3370. (b) J. Haywood, A. E. H. Wheatley, Dalton Trans. 2008,
3378-3397 and references cited therein.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
(a) H. Köster, E. Weiss, Chem. Ber. 1982, 115, 3422-3426. (b) S. Harder, J. Boersma, L. Brandsma, J. A.
Kanters, W. Bauer, R. Pi, P. Von R. Schleyer, H. Schöllhorn, U. Thewalt, Organometallics 1989, 8, 1688-
1696.
[19]
[20]
Radii for six-coordinate metal cations taken from: R. D. Shannon, Acta Cryst. 1976, A32, 751-767.
S. Harder, J. Spielmann, J. Intemann, H. Bandmann, Angew. Chem. Int. Ed. 2011, 50,
4156–4160.
[21]
[22]
[23]
J. Intemann, J. Spielmann, P. Sirsch, S. Harder, Chem. Eur. J. 2013, 19, 8478–8489.
L. Fohlmeister, A. Stasch, Chem. Eur. J. 2016, 22, 10235-10246.
D. Martin, K. Beckerle, S. Schnitzler, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed. 2015, 54,
4115–4118.
[24]
[25]
M. Arrowsmith, M. S. Hill, D. J. MacDougall, M. F. Mahon, Angew. Chem. Int. Ed. 2009, 48, 4013–4016.
M. Arrowsmith, M. S. Hill, G. Kociok-Köhn, D. J. MacDougall, M. F. Mahon, Angew. Chem. Int. Ed. 2012,
51, 2098 –2100.
[26]
[27]
A. T. Nicol, R. W. Vaughan, J. Chem. Phys. 1978, 69, 5211-5213.
(a) M. Kaupp, O. L. Malkina, V. G. Malkin, P. Pyykkö, Chem. Eur. J. 1998, 4, 118-126. (b) ) M. Kaupp, O.
L. Malkina, J. Chem. Phys. 1998, 108, 3648-3658.
[28]
[29]
[30]
[31]
C. Ruspic, J. Spielmann, S. Harder, Inorg. Chem. 2007, 46, 5320–5326.
S. Harder, Angew. Chem. Int. Ed. Engl. 2004, 43, 2714-2718.
K. Kwetkat, W. Kitching, J. Chem. Soc., Chem. Commun. 1994, 345-347.
Charges between +1.387 and +1.822 are found on Ca: (a) B. Freitag, H. Elsen, J. Pahl, G. Ballmann, A.
Herrera, R. Dorta, S. Harder, Organometallics 2017, 36, 1860-1866. (b) A. Causero, H. Elsen, J. Pahl, S.
Harder, Angew. Chem. Int. Ed. 2017, 56, 6906 –6910. (c) L. Orzechowski, G. Jansen, S. Harder, J. Am.
Chem. Soc. 2006, 128, 14676-14684.
[32]
[33]
(a) K. Wade, Adv. Inorg. Chem. Radiochem. 1976, 18, 1–66. (b) A. Hirsch, Z. Chen, H. Jiao, Angew.
Chem. Int. Ed. 2000, 39, 3915–3917.
D. J. Wolstenholme, J. T. Titah, F. N. Che, K. T. Traboulsee, J. Flogeras, G. S. McGrady, J. Am. Chem. Soc.
2011, 133, 16598–16604.
[34]
[35]
D. J. Wolstenholme, J. L. Dobson, G. S. McGrady, Dalton Trans. 2015, 44, 9718–9731.
R. E. Mulvey, Chem. Commun. 2001, 1049-1056.
This article is protected by copyright. All rights reserved.