A Mercury → antimony interaction

Article abstract of DOI:10.1002/anie.201002995

Fatal attraction: As a result of a unique iodide push-stibonium pull effect, the dinuclear complex (see picture) displays an unprecedented Hg→Sb interaction. The existence of this interaction was examined experimentally and theoretically. Copyright

Full text of DOI:10.1002/anie.201002995

Angewandte
Chemie
DOI: 10.1002/anie.201002995
Metal–Metal Interactions
A Mercury!Antimony Interaction**
Tzu-Pin Lin, Casey R. Wade, Lisa M. Pꢀrez, and Franꢁois P. Gabbaꢂ*
In the periodic table, mercury is classified as a post-transition
metal element because it possesses a filled d shell in most of
its compounds. In the past few decades, efforts to challenge
this classification have been recurrent,[1] leading, in 2007, to
the observation of the d8 mercury complex HgF4.[2] This
complex, the thermodynamic stability of which has been
computationally established,[3] was identified on the basis of a
single IR band in the reaction of Hg with F2 under photolytic
conditions in a neon matrix at 4 K.[4]
interaction.[8] The choice of the Lewis acid was dictated by a
series of earlier studies that have shown that antimony(V)
species are among the most potent Lewis acids.[9] Reaction of
1,8-dilithionaphthalene[10] with Ph3SbBr2 in THF followed by
addition of HgBr2 afforded 1 as a yellow solid in 54% yield
(Scheme 1). This compound could be converted into the
Unlike mercury, late transition metals, such as gold and
platinum, form numerous complexes in which the metal
involves its d electrons in bonding. Prototypical examples of
such complexes include the d8 complexes AuCl4ꢀ and PtCl4
.
2ꢀ
More recently, it has been shown that these late transition
metals can engage two of their d electrons in the formation of
a dative bond with a Lewis acid to afford complexes of type
A
[5] and B,[6] among others.[7] Although the electrons involved
Scheme 1. Synthesis of the mercury–antimony complexes. (TBAI=
tetra-n-butylammonium iodide, tmeda=tetramethylethylenediamine.)
in the dative bond remain polarized toward the metal,[6a,b] the
ability of gold and platinum to form such interactions by
donation of their d electrons is another signature of their
transition metal identity. Stimulated by the notion that
mercury might behave as a transition metal,[2] we questioned
whether complexes of type C could be observed.
To address this question, we sought a compound featuring
a divalent mercury center held in close proximity to a highly
Lewis acidic center. These considerations led us to select the
1,8-naphthalenediyl dinucleating ligand with a narrow bite
angle that would promote a putative Hg!Lewis acid
corresponding hexafluorophosphate salt 2-PF6 by abstraction
of the bromide anion with TlPF6. Alternatively, reaction of 1
with TlPF6 and subsequently TBAI (tetra-n-butylammonium
iodide) afforded the iodide complex 3. These compounds
have been fully characterized. The 199Hg NMR chemical shifts
of complexes 1 (ꢀ68.2 ppm, [D6]DMSO), 2-PF6 (ꢀ271.9 ppm,
[D6]DMSO), and 3 (ꢀ71.8 ppm, [D6]DMSO) exhibit a con-
siderable downfield shift compared to that of Ph2Hg
(ꢀ808.5 ppm, [D6]DMSO).[11] This observed downfield shift
is noteworthy and may reflect an increase in the coordination
number of the mercury atom caused by coordination of
solvent molecules, or a halide ligand in the case of 1 and 3.[12]
An examination of the crystal structure of [2]+ (Figure 1a)
indicates the presence of a dinuclear core with a short
transannular mercury–antimony separation of 3.0601(7) ꢀ.[13]
This separation, which lies between the sum of the metallic
(3.17 ꢀ) and covalent radii (2.71 ꢀ) of the two elements,
indicates that these two atoms are within bonding distance.[14]
Further inspection of the coordination environment of the
mercury atom suggests a T-shape geometry, as indicated by
the C1-Hg-Sb, C11-Hg-Sb, and C1-Hg-C11 angles of 87.1(2),
86.3(2), and 170.6(3)8, respectively. The secondary coordina-
tion sphere of the mercury atom also involves two hexafluor-
ophosphate fluorine atoms that form weak contacts of
3.114(5) and 2.823(5) ꢀ. The Hg-Sb-C21 angle of 174.2(2)8
[*] T.-P. Lin, C. R. Wade, Dr. F. P. Gabbaꢀ
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
E-mail: francois@tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/gabbai
Dr. L. M. Pꢁrez
Laboratory for Molecular Simulation, Texas A&M University
College Station, TX 77843 (USA)
[**] Support by the National Science Foundation (CHE-0646916 and
CHE-0952912), the Welch Foundation (A-1423), Texas A&M
University (Davidson Professorship), and the Laboratory for
Molecular Simulation at Texas A&M University (software and
computation resources) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002995.
Angew. Chem. Int. Ed. 2010, 49, 6357 –6360
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6357

Communications
trigonal bipyramidal, as indicated by the Hg-Sb-C21 angle of
179.2(2)8 and a sum of angles in the equatorial plane of 349.38.
The structure of 1 resembles that of 3 with a bromide instead
of an iodide coordinated to the mercury center (see the
Supporting Information, Figure S1).[13] Unlike in 3, however,
the bromide ligand of 1 is also involved in an interaction with
the antimony atom of a neighboring molecule, leading to the
formation of a centrosymmetrical dimeric structure.
ꢀ
The short Hg Sb separation observed in these dinuclear
complexes and also the distorted trigonal bipyramidal geom-
etry displayed by the antimony atom suggests the possibility
of a Hg!Sb interaction. To gain more insight into the
presence of such an interaction, we carried out DFT
calculations on [2]+ and 3 using the ADF program. For
comparative purposes, analogous calculations were carried
out on dimeric naphthalenediyl mercury (4), a compound that
[17]
ꢀ
features a Hg Hg distance of only 2.8 ꢀ,
and on the
mercury–indium complex (C10H6)2Hg(InBr-THF2) (5), the
structure of which has been previously reported.[8] All
calculations, which were carried out at the BP86/TZP level
of theory using the zero-order regular approximation
(ZORA), produced optimized structures that are in excellent
agreement with those observed experimentally (see the
Supporting Information). The molecular orbitals obtained
by these calculations for [2]+ and 3 are extensively delocalized
on the ligands and do not provide a clear depiction of the
antimony–mercury transannular interaction. For this reason,
we decided to use the electron localization function (ELF),
which can be used to map the electron-pair localization in a
molecule.[18] The ELF values near the mercury–mercury
centroid for 4 (Figure 1b) and mercury–indium centroid for
5 (Supporting Information, Figure S2) drop to almost zero,
thus indicating the absence of covalent bonding between the
two heavy atoms (Figure 1b). In the case of 4, the ELF
analysis is in agreement with the work of Pyykkꢁ who showed
that the short mercury–mercury contact present in 4 is
nonbonding and repulsive.[19] In the case of [2]+, the region
of low ELF values separating the two heavy atoms shows a
distinct thinning, thus signaling the onset of covalent bonding.
This phenomenon becomes much more acute in 3, as shown
by a continuum of elevated ELF values along the Hg!Sb
vector. This feature indicates that the coordination of the
iodide to the mercury center increases the donor capability of
the latter toward the antomony atom. Further evidence for
the electron sharing in the Hg!Sb linkage was derived from
a Boys localization analysis (Figure 1c). In all cases, this
analysis identifies a lone pair at mercury of d character that is
clearly extended along the transannular vector. This lone pair
shows essentially no polarization in the case of 4 (Figure 1c)
and 5 (Supporting Information, Figure S2). On going from
[2]+ to 3, this orbital becomes more polarized toward the
antimony atom, thus supporting the crucial role played by the
iodide in increasing the donor capability of the mercury atom.
Altogether, these computational results show that the Hg!
Sb present in 3 involves a polar covalent component that
complements the strong mercurate–stibonium electrostatic
interaction already present in this compound.[20]
Figure 1. a) Crystal structure of 2-PF6 (left) and 3 (right). Thermal
ellipsoids are set at 50% probability; hydrogen atoms and the
hexafluorophosphate anion of 2-PF6 are omitted for clarity. Pertinent
metrical parameters can be found in the text and in the Supporting
Information. b) ELF maps for [2]+, 3, and 4. The maps are drawn in the
plane containing the two central heavy atoms and one of mercury-
bound carbon atoms. c) Mercury-centered Boys orbitals for [2]+, 3, and
4 (at 0.02 isosurface value).
indicates that the antimony atom approaches a distorted
trigonal bipyramidal coordination geometry with the mercury
atom as one of the axial ligands. This view is confirmed by the
sum of the C27-Sb-C8, C27-Sb-C18, and C8-Sb-C18 angles of
344.08, which approaches the ideal value of 3608. The crystal
structure of
3
(Figure 1a), which displays a virtually
ꢀ
unchanged Hg Sb separation of 3.073(1) ꢀ, shows that the
iodide anion is coordinated to the mercury center.[13] As non-
fluorinated diarylmercury complexes do not form isolable
adducts with any halides,[12,15] the coordination of the iodide
ligand to the mercury atom of 3 constitutes a distinctive and
noteworthy feature. The resulting mercury–iodine distance of
2.991(1) ꢀ in 3 is also shorter than that observed in complexes
involving highly Lewis acidic fluorinated organomercurials,
such as [(C6F5)2HgI]ꢀ (3.118(2) ꢀ).[16] Owing to the presence
of this iodide ligand, the mercury atom has a distorted square
planar geometry, as indicated by the C1-Hg-C11 and I-Hg-Sb
angles of 163.1(3)8 and 162.24(2)8, respectively. The coordi-
nation geometry at the antimony center in 3 remains distorted
Although these results suggest the participation of
d orbitals in the mercury–antimony interaction present in 3,
6358
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6357 –6360

Angewandte
Chemie
resulting in the immediate precipitation of TlBr, which was removed
it is also possible that this linkage benefits from a I-Hg-Sb
three-center–two-electron (3c,2e) interaction that would
involve an iodide lone pair orbital, a mercury 6p orbital,
and a Sb-CPh s* orbital (Figure 2a). In support of this view, we
note that the LUMO of [2]+ bears a large contribution from
the latter two orbitals (Figure 2b).
by filtration through celite. Pentane (10 mL) was added to the filtrate,
resulting in the precipitation of 2-PF6 (29.8 mg, 68% yield) as a white
solid. Further recrystallization of the solid from MeCN (3 mL)
afforded colorless crystals of 2-PF6·MeCN. 1H NMR (400 mhz;
3
[D6]DMSO) d = 8.28 (d, 2H, Naph-CH, JH-H = 7.7 Hz), 8.20 (d, 2H,
Naph-CH, 3JH-H = 6.7 Hz), 8.09 (d, 2H, Naph-CH, 3JH-H = 8.0 Hz),
7.80–7.77 (m, 4H, p-Ph-CH and Naph-CH), 7.58–7.49 (m, 8H, m-Ph-
CH and Naph-CH), 7.19 ppm (bs, 4H, o-Ph-CH); 13C NMR
(100 mhz; [D6]DMSO) d = 171.8, 142.5, 140.4, 138.0, 136.5, 135.0,
133.3, 132.6, 131.9, 130.6, 128.7, 127.0, 125.4 ppm (one of the
quaternary carbon nuclei was not detected); 199Hg NMR (71mhz;
[D6]DMSO) d = ꢀ271.9 ppm (s). HRMS: m/z calcd for C32H22HgSb+
729.0461; found: 729.0463.
3: To a yellow suspension of 1 (20 mg, 0.025 mmol) in THF
(2 mL), TlPF6 (1.2 equiv, 10.4 mg, 0.030 mmol) was added at room
temperature, resulting in the formation of a white precipitate of TlBr.
2.0 equiv of solid tetrabutylammonium iodide (18.5 mg, 0.050 mmol)
was added to this mixture in one portion. After stirring for 10 min, the
precipitate was removed by filtration through celite, and the resulting
solution was allowed to stand for 24 h at ꢀ208C, leading to the
Figure 2. a) I-Hg-Sb 3c,2e interaction in 3. b) Lowest unoccupied
molecular orbital of [2]+ (at 0.02 isosurface value) showing the large
ꢀ
contribution from the mercury 6p and Sb CPh s* orbitals.
formation
3
as yellow crystals (18 mg, 85% yield). 1H NMR
(400 mhz; [D6]DMSO) d = 8.84 (d, 2H, Naph-CH, 3JH-H = 6.8 Hz),
8.27 (d, 2H, Naph-CH, 3JH-H = 8.0 Hz), 8.09 (d, 2H, Naph-CH, 3JH-H
=
In conclusion, we have demonstrated that the mercury
center of 3 can act as a Lewis base, a phenomenon that had
thus far only been observed for transition metals. In the case
of 3, this phenomenon is the result of a unique iodide push–
stibonium pull effect, which polarizes the diffuse closed shell
of the mercury atom, thus promoting its engagement in a
polar bonding interaction.
8.0 Hz), 7.78 (d, 2H, Naph-CH, 3JH-H = 7.4 Hz), 7.73 (t, 2H, p-Ph-CH,
3JH-H = 7.5 Hz), 7.59–7.46 (m, 8H, m-Ph-CH and Naph-CH), 7.20 ppm
3
(d, 4H, o-Ph-CH, JH-H = 7.6 Hz); 13C NMR (100 mhz; [D6]DMSO)
d = 174.0, 144.4, 142.4, 140.6, 137.3, 135.7, 134.3, 133.1, 132.7, 131.6,
131.4, 129.6, 128.0, 126.0 ppm; 199Hg NMR (71mhz; [D6]DMSO) d =
ꢀ71.8 ppm (s). Elemental analysis calcd (%) for C32H22HgISb:
C 44.91, H 2.59; found: C 44.97, H 2.52.
DFT structural optimizations were carried out using the ADF
program (2008.01).[22] All calculations were carried out using the
BP86 functional[23] with the all-electron TZP basis sets for all
atoms.[24] These calculations were performed using the zero-order
regular approximation (ZORA).[25] Electron localization function
(ELF)[18] and Boys[26] localization analyses were carried out in the
ADF program. ELF plots and Boys localized orbitals were visualized
in the ADF program.
Experimental Section
Caution! Mercury, antimony, and thallium compounds are highly
toxic and should be handled cautiously. Mercuric bromide and
triphenylantimony were purchased from Aldrich. Thallium hexa-
fluorophosphate was purchased from Alfa Aesar. 1,8-Dilithionaph-
thalene-tmeda[10] and triphenyldibromoantimony[21] were prepared
according to published procedures. All preparations were carried out
under an atmosphere of dry N2 employing either a glove box or
standard Schlenk techniques. Solvents were dried by passing through
an alumina column (n-pentane and MeCN) or refluxing under N2
over Na/K (Et2O, n-hexane, and THF). NMR spectra were recorded
on a Varian Unity Inova 400 FT NMR spectrometer (1H: 399.59 mhz,
13C: 100.45 mhz, 199Hg: 71.20 mhz) at ambient temperature. Chemical
Received: May 17, 2010
Published online: July 26, 2010
Keywords: antimony · hypervalence · Lewis acids · mercury ·
.
transition metals
shifts d are given in ppm and are referenced to residual H and 13C
solvent signals and external neat HgMe2.
1
[1] a) R. L. Deming, A. L. Allred, A. R. Dahl, A. W. Herlinger,
M. O. Kestner, J. Am. Chem. Soc. 1976, 98, 4132 – 4137; b) P.
Hrobꢃrik, M. Kaupp, S. Riedel, Angew. Chem. 2008, 120, 8759 –
8761; Angew. Chem. Int. Ed. 2008, 47, 8631 – 8633; c) S. Riedel,
M. Straka, M. Kaupp, Chem. Eur. J. 2005, 11, 2743 – 2755.
[2] X. Wang, L. Andrews, S. Riedel, M. Kaupp, Angew. Chem. 2007,
119, 8523 – 8527; Angew. Chem. Int. Ed. 2007, 46, 8371 – 8375.
[3] a) M. Kaupp, H. G. von Schnering, Angew. Chem. 1993, 105,
952 – 954; Angew. Chem. Int. Ed. Engl. 1993, 32, 861 – 863; b) M.
Kaupp, M. Dolg, H. Stoll, H. G. von Schnering, Inorg. Chem.
1994, 33, 2122 – 2131.
1: A solution of Ph3SbBr2 (500 mg, 0.976 mmol) in THF (10 mL)
was added dropwise to a solution of 1,8-dilithionaphthalene-tmeda
(250 mg, 0.976 mmol) in THF (5 mL) at ambient temperature. The
mixture was allowed to stir for 10 min and was then transferred into a
THF solution (3 mL) of HgBr2 (352 mg, 0.976 mmol). The resulting
clear yellow solution was allowed to stand overnight at room
temperature, yielding yellow crystals of complex 1 (213 mg, 54%
yield), which were filtered, washed with THF (3 ꢂ 5 mL), and dried
under vacuum. 1H NMR (400 mhz; [D6]DMSO) d = 8.98 (d, 2H,
Naph-CH, 3JH-H = 6.7 Hz), 8.24 (d, 2H, Naph-CH, 3JH-H = 8.0 Hz),
8.08 (d, 2H, Naph-CH, 3JH-H = 8.0 Hz), 7.76 (d, 2H, Naph-CH, 3JH-H
7.2 Hz), 7.71 (t, 2H, p-Ph-CH, JH-H = 7.4 Hz), 7.57–7.44 (m, 8H, m-
Ph-CH and Naph-CH), 7.19 ppm (d, 4H, o-Ph-CH, JH-H = 7.1 Hz);
=
[4] W. B. Jensen, J. Chem. Educ. 2008, 85, 1182 – 1183.
[5] a) H. Braunschweig, K. Gruss, K. Radacki, Angew. Chem. 2007,
119, 7929 – 7931; Angew. Chem. Int. Ed. 2007, 46, 7782 – 7784;
b) H. Braunschweig, K. Radacki, K. Schwab, Chem. Commun.
2010, 46, 913 – 915; c) H. Braunschweig, K. Gruss, K. Radacki,
Angew. Chem. 2009, 121, 4303 – 4305; Angew. Chem. Int. Ed.
2009, 48, 4239 – 4241.
3
3
13C NMR (100 mhz; [D6]DMSO) d = 173.9, 145.3, 142.4, 140.6, 137.3,
135.8, 135.6, 134.3, 132.7, 131.7, 131.4, 129.7, 128.1, 126.0 ppm;
199Hg NMR (71mhz; [D6]DMSO) d = ꢀ68.2 (s). Elemental analysis
calcd (%) for C32H22BrHgSb: C 47.52, H 2.74; found: C 48.10, H 2.74.
2-PF6: Solid TlPF6 (17.5 mg, 0.05 mmol) was added in one portion
to a MeCN solution (2 mL) of complex 1 (40.6 mg, 0.05 mmol),
[6] a) S. Bontemps, G. Bouhadir, K. Miqueu, D. Bourissou, J. Am.
Chem. Soc. 2006, 128, 12056 – 12057; b) M. Sircoglou, S. Bon-
Angew. Chem. Int. Ed. 2010, 49, 6357 –6360
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6359

Communications
temps, M. Mercy, N. Saffon, M. Takahashi, G. Bouhadir, L.
[15] a) T. J. Taylor, C. N. Burress, F. P. Gabbaꢄ, Organometallics 2007,
26, 5252 – 5263; b) T. J. Wedge, M. F. Hawthorne, Coord. Chem.
Rev. 2003, 240, 111 – 128.
[16] F. Schulz, I. Pantenburg, D. Naumann, Z. Anorg. Allg. Chem.
2003, 629, 2312 – 2316.
[17] H. Schmidbaur, H. J. Oeller, D. L. Wilkinson, B. Huber, G.
Mueller, Chem. Ber. 1989, 122, 31 – 36.
[18] A. D. Becke, K. E. Edgecombe, J. Chem. Phys. 1990, 92, 5397 –
5403.
Maron, D. Bourissou, Angew. Chem. 2007, 119, 8737 – 8740;
Angew. Chem. Int. Ed. 2007, 46, 8583 – 8586; c) M. Sircoglou, M.
Mercy, N. Saffon, Y. Coppel, G. Bouhadir, L. Maron, D.
Bourissou, Angew. Chem. 2009, 121, 3506 – 3509; Angew.
Chem. Int. Ed. 2009, 48, 3454 – 3457; d) P. Gualco, T.-P. Lin, M.
Sircoglou, M. Mercy, S. Ladeira, G. Bouhadir, L. M. Perez, A.
Amgoune, L. Maron, F. P. Gabbai, D. Bourissou, Angew. Chem.
2009, 121, 10076 – 10079; Angew. Chem. Int. Ed. 2009, 48, 9892 –
9895.
[19] P. Pyykkꢁ, M. Straka, Phys. Chem. Chem. Phys. 2000, 2, 2489 –
2493.
[7] a) H. Braunschweig, R. D. Dewhurst, A. Schneider, Chem. Rev.
2010, 110, 3924-3957; b) J. S. Figueroa, J. G. Melnick, G. Parkin,
Inorg. Chem. 2006, 45, 7056 – 7058; c) K. Pang, S. M. Quan, G.
Parkin, Chem. Commun. 2006, 5015 – 5017; d) A. F. Hill, G. R.
Owen, A. J. P. White, D. J. Williams, Angew. Chem. 1999, 111,
2920 – 2923; Angew. Chem. Int. Ed. 1999, 38, 2759 – 2761; e) A. F.
Hill, Organometallics 2006, 25, 4741 – 4743; f) V. K. Landry, J. G.
Melnick, D. Buccella, K. Pang, J. C. Ulichny, G. Parkin, Inorg.
Chem. 2006, 45, 2588 – 2597; g) J. Wagler, E. Brendler, Angew.
Chem. 2010, 122, 634 – 637; Angew. Chem. Int. Ed. 2010, 49, 624 –
627.
[8] F. P. Gabbaꢄ, A. Schier, J. Riede, A. Sladek, H. W. Goerlitzer,
Inorg. Chem. 1997, 36, 5694 – 5698.
[9] a) V. Gutmann, H. Hubacek, A. Steininger, Monatsh. Chem.
1964, 95, 678 – 686; b) G. A. Olah, W. S. Tolgyesi, S. J. Kuhn,
M. E. Moffatt, I. J. Bastien, E. B. Baker, J. Am. Chem. Soc. 1963,
85, 1328 – 1334.
[20] A Morokuma-type energy decomposition analysis carried out
with the ADF program on the model complex IH2Hg!SbH2Ph2
indicates that the IH2Hg and SbH2Ph2 fragments are held by an
electrostatic interaction of ꢀ354.390 kJmolꢀ1 and an orbital
interaction of ꢀ171.351 kJmolꢀ1. Further details can be found in
the Supporting Information.
[21] M. Kimura, A. Iwata, M. Itoh, K. Yamada, T. Kimura, N.
Sugiura, M. Ishida, S. Kato, Helv. Chim. Acta 2006, 89, 747 – 783.
[22] a) ADF2008.01. SCM, Theoretical Chemistry, Vrije Universiteit,
Amsterdam, The Netherlands, http://www.scm.com; b) C. Fon-
seca Guerra, J. G. Snijders, G. te Velde, E. J. Baerends, Theor.
Chem. Acc. 1998, 99, 391 – 403; c) G. te Velde, F. M. Bickelhaupt,
E. J. Baerends, C. F. Guerra, S. J. A. van Gisbergen, J. G. Snijd-
ers, T. Ziegler, J. Comput. Chem. 2001, 22, 931 – 967.
[23] a) A. D. Becke, Phys. Rev. A 1988, 38, 3098 – 3100; b) J. P.
Perdew, Phys. Rev. B 1986, 33, 8822 – 8824.
[10] W. Neugebauer, T. Clark, P. v. R. Schleyer, Chem. Ber. 1983, 116,
3283 – 3292.
[24] E. van Lenthe, E. J. Baerends, J. Comput. Chem. 2003, 24, 1142 –
1156.
[11] M. A. Sens, N. K. Wilson, P. D. Ellis, J. D. Odom, J. Magn. Reson.
1975, 19, 323 – 336.
[12] V. B. Shur, I. A. Tikhonova, Russ. Chem. Bull. 2003, 52, 2539 –
2554.
[13] CCDC 771798, CCDC 771799, and CCDC 771800 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J.
Echeverria, E. Cremades, F. Barragan, S. Alvarez, Dalton
Trans. 2008, 2832 – 2838.
[25] a) E. van Lenthe, E. J. Baerends, J. G. Snijders, J. Chem. Phys.
1993, 99, 4597 – 4610; b) E. van Lenthe, E. J. Baerends, J. G.
Snijders, J. Chem. Phys. 1994, 101, 9783 – 9792; c) E. van Lenthe,
R. van Leeuwen, E. J. Baerends, J. G. Snijders, Int. J. Quantum
Chem. 1996, 57, 281 – 293; d) E. van Lenthe, J. G. Snijders, E. J.
Baerends, J. Chem. Phys. 1996, 105, 6505 – 6516; e) E. van
Lenthe, A. Ehlers, E.-J. Baerends, J. Chem. Phys. 1999, 110,
8943 – 8953.
[26] a) J. M. Foster, S. F. Boys, Rev. Mod. Phys. 1960, 32, 300 – 302;
b) C. Edmiston, K. Ruedenberg, Rev. Mod. Phys. 1963, 35, 457 –
464; c) W. Von Niessen, J. Chem. Phys. 1972, 56, 4290 – 4297.
6360 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6357 –6360