Syntheses and Structures of [(THF)nM{(NSiMe3)2PPh2}2] Complexes (M = Be, Mg, Ca, Sr, Ha; = 0-2): Deviation of Alkaline Earth Metal Cations from the Plane of an Anionic Ligand

Article abstract of DOI:10.1021/ic9613117

The syntheses and solid-state structures of [(THF)_nM{(NSiMe₂)₂PPh₂}₂] (M = Be, n = 0, 1; M = Mg, n = 0, 2; M = Ca, n = 1, 3; M = Sr, n = 2, 4; M = Ba, n = 2, 5) are presented. Comparison of the geomet

Full text of DOI:10.1021/ic9613117

Inorg. Chem. 1997, 36, 2413-2419
2413
Syntheses and Structures of [(THF)nM{(NSiMe3)2PPh2}2] Complexes (M ) Be, Mg, Ca, Sr,
Ba; n ) 0-2): Deviation of Alkaline Earth Metal Cations from the Plane of an Anionic
Ligand†
Roland Fleischer and Dietmar Stalke*,‡
Institut fu¨r Anorganische Chemie der Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
ReceiVed October 31, 1996X
The syntheses and solid-state structures of [(THF)nM{(NSiMe3)2PPh2}2] (M ) Be, n ) 0, 1; M ) Mg, n ) 0, 2;
M ) Ca, n ) 1, 3; M ) Sr, n ) 2, 4; M ) Ba, n ) 2, 5) are presented. Comparison of the geometric parameters
within the homologous series and to related systems uncovers the dication-induced alterations of coordination to,
as well as bonding within, the anionic fragment. The coordination number increases from 4 (Be, Mg) Via 5 (Ca)
to 6 (Sr, Ba). Two of the Ph2P(Me3SiN)2 anions cover the coordination sphere of beryllium and magnesium,
while with calcium one single THF molecule and with strontium and barium two additional THF molecules are
required to complete the metal coordination sphere. Against steric considerations the two THF molecules in 4
and 5 are coordinated to the same hemisphere of the metal leaving the two anions cisoid. Even against sterical
strain the alkaline earth metals leave the plane of one of the Ph2P(Me3SiN)2 anions with increasing mass
demonstrating the preference of Sr and Ba to interact with π electron density. This effect can also be found in
related systems. It might be small and counterbalanced by steric requirements, but it is significant. The metal
π interaction rises continually with the mass of the metal and decreasing bulk of the anion but is independent
from cisoid or transoid arrangement of the anions. From the homologous series of complexes presented three
structure determining factors can be deduced: (i) dicationic size; (ii) bent X-M-X (M ) Sr, Ba) arrangement;
(iii) increasing π interaction with increasing mass of the alkaline earth metal cation.
Introduction
Furthermore, the structures of MX2 molecules (M ) alkaline
earth metal; X ) halides, hydride, organic substituents) are
vigorously discussed: Is the deviation from linearity an intrinsic
structural feature or is the bending induced by additionally
present donor bases in the metal coordination sphere?6,7
Because bonding to alkaline earth metals is regarded predomi-
nantly ionic, simple electrostatic models as well as the first
VSEPR approach8 predicted a linear structure for MX2 species.
However, the bent geometry of, for instance, CaF2 and many
complexes of the heavier group 2 dihalides represent the most
prominent exceptions from the VSEPR theory and, among
others, lead to its present extended form.9-11
This paper is concerned with the synthesis and structural
investigation of the aminoiminophosphoranate complexes
[(THF)nM{(NSiMe3)2PPh2}2] (M ) Be, n ) 0, 1; M ) Mg, n
) 0, 2; M ) Ca, n ) 1, 3; M ) Sr, n ) 2, 4; M ) Ba, n ) 2,
5). The aminoiminophosphoranate ligand is a versatile building
block in transition and main group metal chemistry.12,13 The
chelating [Ph2P(Me3SiN)2] bidentate anion provides sufficient
bulk to prevent the resulting alkaline earth metal complexes
from aggregation. Two of this substituents cover the coordina-
tion sphere of beryllium and magnesium, while with calcium
Up to now most effort in the chemistry of alkaline earth metal
organometallic chemistry still is focused on organomagnesium
compounds,1 because since the 1900s they became a well-
established synthetic tool in organic and organometallic chem-
istry. It is only very recently that not only Grignard reagents
but also the compounds containing the heavier alkaline earth
metals have attracted the interest of both synthetically2 and
theoretically3 oriented chemists. Alkaline earth metal com-
pounds are promising precursors to metal oxides for the
chemical vapor deposition (CVD) process. Volatile and soluble
complexes of these metals are required in the CVD or sol-gel
process to facilitate materials synthesis like high-temperature
super conductors.4,5
† Dedicated to Prof. Dr. W. Adam, Wu¨rzburg, on the occasion of his
60th birthday.
‡E-mail: dstalke@chemie.uni-wuerzburg.de.
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
(1) For review: Markies, P. R.; Akkerman, O. S.; Bickelhaupt, F.; Smeets,
W. J. J.; Spek, A. L. AdV. Organomet. Chem. 1991, 32, 147. Recent
magnesium imides: (a) Hascall, T.; Ruhlandt-Senge, K.; Power, P.
P. Angew. Chem. 1994, 106, 350; Angew. Chem., Int. Ed. Engl. 1994,
33, 356. (b) Hascall, T.; Olmstead, M. M.; Power, P. P. Angew. Chem.
1994, 106, 1056; Angew. Chem., Int. Ed. Engl. 1994, 33, 1000.
(2) For review: (a) Hannusa, T. P. Polyhedron 1990, 9, 1345; Chem.
ReV. 1993, 93, 1023.
(5) Liu, F.-Q.; Stalke, D.; Roesky, H. W. Angew. Chem. 1995, 107, 2004;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1872.
(6) Kaupp, M.; Schleyer, P. v. R.; Dolg, M.; Stoll, H. J. Am. Chem. Soc.
1992, 114, 8202.
(7) Mo¨sges, G.; Hampel, F.; Schleyer, P. v. R. Organometallics 1992,
11, 1769.
(8) Gillespie, R. J.; Nyholm, R. S. Q. ReV. Chem. Soc. 1957, 11, 339.
(9) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry;
Allyn and Bacon: Boston, MA, 1991; p 99.
(10) Bytheway, I.; Gillespie, R. J.; Tang, T.-H.; Bader, R. F. W. Inorg.
Chem. 1995, 34, 2407.
(11) Gillespie, R. J.; Robinson, E. A. Angew. Chem. 1996, 108, 539; Angew.
Chem., Int. Ed. Engl. 1996, 35, 477.
(12) Roesky, H. W. In The Chemistry of Inorganic Ring Systems; Studies
in Inorganic Chemistry; Steudel, R., Ed.; Elsevier: Amsterdam, 1989.
(13) Recent review: Witt, M.; Roesky, H. W. Chem. ReV. 1994, 94, 1163.
(3) (a) Hayes, E. F. J. Phys. Chem. 1966, 70, 3740. (b) Yarkonyi, D. R.;
Hunt, W. J.; Schaeffer, H. F. Mol. Phys. 1973, 26, 941. (c) Gole, J.
L.; Siu, A. K. Q.; Hayes, E. F. J. Chem. Phys. 1973, 58, 857. (d)
Hasset, D. M.; Marsden, C. J. J. Chem. Soc., Chem. Commun. 1990,
667. (e) Dyke, J. M.; Wright, T. G. Chem. Phys. Lett. 1990, 169, 138.
(f) Szentpaly, L. v.; Schwerdtfeger, P. Chem. Phys. Lett. 1990, 70,
555. (g) Salzner, U.; Schleyer, P. v. R. Chem. Phys. Lett. 1990, 172,
461. (h) DeKock, R. L.; Peterson, M. A.; Timmer, L. K.; Baerends,
E. J.; Vernooijs, P. Polyhedron 1990, 9, 1919. (i) Seijo, L.; Baran-
diara´n, Z.; Huzinaga, S. J. Chem. Phys. 1991, 94, 3762. (j) Kapp, J.;
Schleyer, P. v. R. Inorg. Chem. 1996, 35, 2247.
(4) Pandey, S. K.; Steiner, A.; Roesky, H. W.; Stalke, D. Angew. Chem.
1993, 105, 625; Angew. Chem., Int. Ed. Engl. 1993, 32, 596.
S0020-1669(96)01311-0 CCC: $14.00 © 1997 American Chemical Society

2414 Inorganic Chemistry, Vol. 36, No. 11, 1997
Fleischer and Stalke
Scheme 1
Scheme 2
Figure 1. Structure of [Be{(NSiMe3)2PPh2}2] (1) in the solid state.
Anisotropic displacement parameters are depicted at the 50% probability
level.
Table 1. Selected Bond Lengths (pm) and Angles (deg) for 1-5
compound
1
2
3
4
5
M-N1
M-N2
M-N3
M-N4
M-O1
M-O2
M‚‚‚P1
M‚‚‚P2
P1-N1
P1-N2
P2-N3
P2-N4
177.1(4) 208.3(2) 255.1(2) 268.9(3) 279.6(2)
175.3(4) 206.8(2) 239.7(2) 261.0(2) 273.7(2)
177.7(3) 207.2(2) 254.6(2) 263.9(3) 284.9(2)
176.0(3) 207.8(2) 239.3(2) 260.5(2) 274.8(2)
235.3(1) 264.7(2) 281.1(2)
one single THF molecule and with strontium and barium two
additional THF molecules are required to complete the metal
coordination sphere. All complexes 1-5 are quite soluble in
apolar hydrocarbons; hence, synthesis in homogeneous phase
is facilitated. Gels containing M(OH)2‚xH2O (M ) Be, Mg,
Ca, Sr, Ba) can readily be made by hydrolyzation of the
homogeneous solution of the related complex in toluene with
water in methanol or ethanol.
259.8(2) 277.7(2)
231.2(4) 262.8(1) 302.02(7) 321.73(9) 335.20(8)
232.1(4) 262.2(1) 303.92(7) 319.60(9) 337.27(8)
159.0(2) 159.6(2) 158.9(2) 158.2(3) 157.9(2)
160.1(2) 159.6(2) 166.4(2) 158.8(3) 158.9(2)
158.9(2) 159.2(2) 159.0(2) 158.4(3) 157.7(3)
159.5(2) 159.3(2) 164.6(2) 158.8(3) 158.3(2)
N1-P1-N2
N3-P2-N4
99.1(1) 104.3(1) 109.38(8) 109.1(1) 110.00(1)
99.1(1) 104.5(1) 108.29(8) 109.5(1) 109.86(1)
C7-P1-C13 103.0(1) 104.2(1) 104.4(1) 102.4(1) 103.14(1)
C25-P2-C31 105.6(1) 103.4(1) 103.05(9) 102.4(1) 102.98(1)
P1-N1-Si1 136.1(1) 133.5(1) 134.8(2) 134.3(2) 135.0(2)
P1-N2-Si2 133.8(1) 137.2(1) 134.5(1) 131.2(2) 136.6(2)
P2-N3-Si3 135.7(1) 136.8(1) 135.1(1) 132.7(2) 136.9(2)
P2-N4-Si4 132.7(1) 135.1(1) 131.4(1) 135.1(2) 134.0(2)
Results and Discussion
Preparation of 1-5. The three chemically active sites in
the N,N′-bis(trimethylsilyl)aminoiminodiphenylphosphorane are
(i) the acidic NH function (ii) the donating imino nitrogen atom,
and (iii) the potential cleavage of the Si-N or P-N bond,
leading to anionic chelating ligands.14 These properties of the
aminoimino-phosphoranes hastened them to be widely used
ligands in the synthesis of metallacycles.13 The starting
materials are obtained in the reaction of trimethylsilyl azide with
diphenylphosphane in a Staudinger-type reaction.15 The amino
function of the N,N′-bis(trimethylsilyl)aminoiminodiphenyl-
phosphorane can be deprotonated by strong bases. In the
syntheses presented here the alkaline earth metal bis[bis-
(trimethylsilyl)amides] have been used as bases. They can
readily be obtained from the tin bis[bis(trimethylsilyl)amide].16
The beryllium bis[bis(trimethylsilyl)amide] was obtained by a
transmetalation reaction of the lithium derivative with beryllium
dichloride.17 The barium bis[bis(trimethylsilyl)amide] was
obtained by the reaction of bis(trimethylsilyl)amine with
elemental barium.18 After 1-2 h of stirring of the starting
materials in hexane at room temperature the crude product was
obtained as a white precipitate. The solution was decanted and
the product washed twice with hexane. Crystallization from
hexane/THF (1:1) yielded pure alkaline earth metal complexes
(Scheme 2).
P1‚‚‚M‚‚‚P2
175.8(2) 176.11(4) 144.29(2) 132.54(2) 131.52(2)
suitable for cryogenic structural determination. At lower
temperatures they suffer from solid-solid phase transition, and
at higher temperatures they decompose in the X-ray beam.
However, recrystallization of the high-temperature phase from
a THF/hexane mixture at -22 °C yielded crystals of the low-
temperature phase which could be studied at -120 °C. 1 is a
monomer in the solid state (Figure 1).
The central beryllium dication is coordinated to two chelating
Ph2P(Me3SiN)2 monoanions resulting in an almost undistorted
tetrahedral BeN4 arrangement in the core of the complex. The
orthogonals of the two PN2 planes intersect at an angle of 89°.
The average Be-N distance in 1 of 176.5 pm (Table 1) is 6.5
pm longer than the related distance in the recent structure of
the diazadien beryllium complex [Be(NPhdCPhCPhdNPh)2],
which adopts the same tetrahedral BeN4 arrangement.19 The
elongation of the Be-N distance in 1 by about 20 pm compared
to the gas-phase structure of Be{N(SiMe3)2}220 (Be-N 157 pm)
testifies to either the considerable steric requirements of the two
Ph2P(Me3SiN)2 ligands or the increase of the coordination
number. The four independent P-N bond distances are of equal
length within their estimated standard deviation, indicating ideal
charge delocalization in the PN2 fragment of the anion as
observed earlier.21
Crystal Structure of [Be{(NSiMe3)2PPh2}2] (1). Although
1 crystallizes from pure hexane at 0 °C, those crystals are not
(14) Wolfsberger, W.; Hager, W. Z. Anorg. Allg. Chem. 1977, 433, 247.
(15) Paciorek, K. L.; Kratzer, R. H. J. Org. Chem. 1966, 31, 2426.
(16) Westerhausen, M. Inorg. Chem. 1991, 30, 96.
(17) Bu¨rger, H.; Forker, C.; Goubeau, J. Monatsh. Chem. 1965, 96, 597.
(18) Vaartstra, B. A.; Huffman, J. C.; Streib, W. E.; Caulton, K. G. Inorg.
Chem. 1991, 30, 121
(19) Thiele, K.-H.; Lorenz, V.; Thiele, G.; Zo¨nnchen, P.; Scholz, J. Angew.
Chem. 1994, 106, 1461; Angew. Chem., Int. Ed. Engl. 1994, 33, 1372.
(20) Clark, A. H.; Haaland, A. J. Chem. Soc., Chem. Commun. 1969, 912.

[(THF)nM{(NSiMe3)2PPh2}2] Complexes
Inorganic Chemistry, Vol. 36, No. 11, 1997 2415
Figure 2. Structure of [Mg{(NSiMe3)2PPh2}2] (2) in the solid state.
Anisotropic displacement parameters are depicted at the 50% probability
level.
Crystal Structure of [Mg{(NSiMe3)2PPh2}2] (2). Unlike
the crystals of the isotype structure of 1, the crystals of 2 grown
from THF/hexane at room temperature show no phase transition
at low temperature. In the monomeric structure the single
magnesium cation is tetrahedrally coordinated by the four
nitrogen atoms of the two anions (Figure 2).
Figure 3. (a) Left: Structure of [(THF)Ca{(NSiMe3)2PPh2}2] (3) in
the solid state. Anisotropic displacement parameters are depicted at
the 50% probability level. (b) Right: Related mesomeric resonance
form visualizing the bonding in the trigonal bipyramid.
The almost linear P1‚‚‚Mg‚‚‚P2 arrangement (176.1°, Table
1) minimizes steric repulsion. Like in 1 the orthogonals of the
two PN2 planes intersect at an angle of 89°. The average Mg-N
distance is 207.5 pm. The increase of 44 pm in the cationic
radius of Mg2+ (78 pm) versus Be2+ (34 pm)22 is not quite
reflected in the increase of the metal-nitrogen distances, but
the distances of 202 pm in monomeric [Mg{N(SiMe3)2}2‚
2THF]23 and of 198/215 pm in Mg{N(SiMe3)2}2,24 respectively,
are in the same order of magnitude.
Crystal Structure of [(THF)Ca{(NSiMe3)2PPh2}2] (3).
Crystals were grown from a THF/hexane mixture at room
temperature. Exposure to air cause the colorless blocks to turn
opaque. Different from those of 1 and 2, the structure of 3
shows an additional THF molecule coordinated to the calcium
dication. Apparently the radius of 106 pm 22 for Ca2+ is
sufficient to accommodate two Ph2P(Me3SiN)2 ligands and a
solvent molecule (Figure 3a).
Figure 4. Structure of [(THF)2Sr{(NSiMe3)2PPh2}2] (4) in the solid
state. Anisotropic displacement parameters are depicted at the 50%
probability level.
{N(SiMe3)2}2‚dme]25 (dme ) dimethoxyethane) are consider-
ably shorter, but the average Ca-N distance in the Ca2N2 four-
membered ring of donor-free [Ca{N(SiMe3)2}2]225 (µ2-N, 249.5;
terminal N, 228 pm) is almost identical to the average Ca-N
distance in 3.
Crystal Structure of [(THF)2Sr{(NSiMe3)2PPh2}2] (4). The
strontium complex crystallizes from a THF/hexane mixture at
room temperature within a few hours. The solid decomposes
instantaneously when exposed to air. Again, the increased ionic
radius of Sr2+ to 127 pm 22 causes an extension of the
coordination number. In addition to the four nitrogen atoms
of the two aminoiminophosphorane ligands two THF molecules
are coordinated to the metal.
The P1‚‚‚Sr‚‚‚P2 vector is even more acute than in 3 (132.5°,
Table 1) while the orthogonals of the two PN2 planes like in 1
and 2 intersect at an angle of 87°. The coordination sphere of
the strontium metal can be rationalized as a distorted octahedron
with two nitrogen atoms (N2 and N4, Figure 4) and both oxygen
atoms in the equatorial positions and N1 and N3 at the axial
positions. However, the differences in the Sr-N distances in
relation to their positions are not as striking as in the trigonal
bipyramidal coordination of 3 (Sr-N1,3axial, 268.9(3) and
263.9(3) pm; Sr-N2,4eq, 261.0(2) and 260.5(2) pm). Compared
to the Sr-N distance of 244 pm in [Sr{N(SiMe3)2}2‚2dme]26
they are remarkably long. In agreement with that, the four
The coordination polyhedron of the central metal can be
described as a distorted trigonal bipyramid (Figure 3b). The
equatorial positions are occupied by two nitrogen atoms (N2
and N4) and the oxygen atom of the THF molecule, while N1
and N3 are located at the axial positions. The P1‚‚‚Ca‚‚‚P2
vector is clearly bent (144.3°, Table 1), and the orthogonals of
the two PN2 planes intersect at an angle of 68°. Neither the
Ca-N nor the P-N distances are equally long (a and b in
Scheme 4). They can be classified into two different groups.
The shorter Ca-N distances of the equatorial positions (239.3(2)
and 239.7(2) pm) are correlated to the longer P-N distances
(164.6(2) and 166.4(2) pm), while the shorter P-N distances
(158.9(2) and 159.0(2) pm) give rise to the longer axial Ca-N
distances (255.1(2) and 254.6(2) pm). The long ≡P-Nh-
bonds indicate that the electron density is mostly located at the
nitrogen atom; hence, they are attractive to strong electrostatic
interactions with the metal. Compared with that in the shorter
≡PdNh- bonds the electron density is more located in a partial
double bond and the nitrogen atom is less attractive to the
calcium cation. The Ca-N distances of 227 pm in [Ca-
(21) (a) Schmidbaur, H.; Schwirten, K.; Pickel; H.-H. Chem. Ber. 1969,
102, 564. (b) Witt, M.; Roesky, H. W.; Stalke, D.; Pauer, F.; Henkel,
T.; Sheldrick, G. M. J. Chem. Soc., Dalton Trans. 1989, 2173.
(22) Emsley, J. The Elements; Clarendon Press: Oxford, U.K., 1991.
(23) Bradley, D. C.; Hursthouse, M. B.; Ibrahim, A. A.; Abdul Malik, K.
M.; Motevalli, M.; Mo¨seler, M.; Powell, A.; Runnacles, J. D.; Sullivan,
A. C. Polyhedron 1990, 9, 2959.
(25) Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1991, 604,
127.
(24) Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1992, 609,
39.
(26) Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1991, 606,
177.

2416 Inorganic Chemistry, Vol. 36, No. 11, 1997
Fleischer and Stalke
Table 2. Comparison of Geometrical Features of 1-5
compound
1
2
3
4
5
metal
coord no.
Be
4
Mg
4
Ca
5
Sr
6
Ba
6
dicationic radius (pm) 34
78
106
127
143
av M-N (pm)
av P-N (pm)
deviation from
176.6 207.5 254.9 (239.6)a 263.8 278.3
159.4 159.4 159.0 (165.5)a 158.6 158.2
0.2
4.3
16.8
2.7
3.8
best anionic planes 9.8
14.1 36.4
51.1 55.7
(pm)a
a Two values are quoted due to symmetry-independent anions.
Scheme 3. Arrangement of the PN2M Unitsa
Figure 5. Structure of [(THF)2Ba{(NSiMe3)2PPh2}2] (5) in the solid
state. Anisotropic displacement parameters are depicted at the 50%
probability level.
independent P-N bonds are identical within their estimated
standard deviations (Table 1), indicating ideal charge delocal-
ization in the PN2 fragment. Unlike the calcium derivative 3
the different metal-nitrogen distances in 4 are attributed to steric
crowding. The longer Sr-N bonds result from sterical interac-
tion of the two THF molecules and the N-bonded trimethylsilyl
groups.
Crystal Structure of [(THF)2Ba{(NSiMe3)2PPh2}2] (5).
The barium complex is very similar to the strontium derivative.
It crystallizes from THF/hexane within hours as colorless cubes.
Like in 4, the metal in 5 is coordinated by two THF molecules,
apart from the two Ph2P(Me3SiN)2 anions. The increase in the
Ba2+ radius to 143 pm 22 apparently is not sufficient to
accommodate a third THF molecule.
The bending of the P1‚‚‚Ba‚‚‚P2 vector and the intersection
of the orthogonals of the two PN2 planes in 5 (131.5 and 88°,
respectively; Table 1) is almost identical to the values of 4.
There is no obvious difference in the Ba-N distances for the
equatorial and axial positions in the distorted octahedral Ba
coordination polyhedron. The comparison of the mean Ba-N
and Ba-O bond lengths to the related distances in 4 reflect the
expected increase of about 16 pm due to the bigger cationic
radius. However, the mean Ba-N distance of 278.3 pm in 5
is notably longer than the related distance in [Ba{N(SiMe3)2}2‚
2THF]18 (259 pm) but shorter than in Ba[(pz*)3Ge]‚1/2dioxane27
(pz* ) 3,5-dimethylpyrazol-1-yl; Ba-N(σ), 280; Ba-N(η), 293
and 297 pm). The four equally long P-N bonds in 5 indicate
ideal charge delocalization in the PN2 moiety.
a While the alkaline earth metals are located in the plane of one
Ph2P(Me3SiN)2 anion (right; see also Figure 6A), the central phosphorus
atom of the second ligand is bent toward the Ba and Sr metal center
(left; see also Figure 6B).
in the Ca compound with one additional THF molecule the
orientation of the two anions clearly needs to be cisoid. In the
series of the alkaline earth metal aminoiminophosphinates 1-5,
this compound occupies a special position. In the trigonal
bipyramidal coordination polyhedron the presence of partial
PdN double bonds imply long NfM donor bonds in the apical
positions. An ideal trans configuration of the anions could be
realized by coordination of a second THF molecule, e.g. with
Sr (4) and Ba (5), but the cisoid configuration is retained and
even more pronounced along the homologous metals
(P1‚‚‚M‚‚‚P2; M ) Ca 144.3, Sr 132.5, Ba 131.5°; Table 1).
Kaupp, Schleyer, and co-workers found by high-level ab initio
MO calculations the MCp2 metallocenes (M ) Ca, Sr, Ba; Cp
) C5H5) to be bent (Cp‚‚‚Ba‚‚‚Cp 147°) but described them as
“quasilinear” due to the low linearization energy (less than 1.5
kcal/mol).6 For the barium amides, however, they expected a
bent arrangement because of small but significant covalent
σ-bonding contributions involving metal d-orbitals28 and po-
larization of the metal cation by the anions.29 For the Ba(NH2)2-
(HF)4 system they calculated the linearization energy to be ca.
3 kcal/mol.30 From these calculations the bending of 4 and 5
is an intrinsic feature and not induced by solvent coordination.
The cisoid arrangement of the anions leaves a vacant coordina-
tion site which is filled by two THF molecules.
Structural Comparison
Structural Comparison of 1-5 Among Themselves. The
most obvious common structural feature of 1-5 is their
monomeric molecular state. This fact seems not surprising in
view of the considerable steric demand of the two anions. They
shield the dication quite well. There is only room for one or
two THF molecules but not for oligomerization. Hence they
are soluble even in apolar organic solvents. As a consequence
of the increasing cationic radius the coordination number
increases from 4 (Be, Mg) Via 5 (Ca) to 6 (Sr, Ba). All M-N
distances are within the range of known alkaline earth metal-
nitrogen distances.
However, there are some remarkable structural trends to be
discussed along this homologous series of alkaline earth metal
compounds summarized in Table 2. While in the Be and Mg
derivatives 1 and 2 the two anions are located strictly trans and
orthogonal with respect to each other to minimize sterical strain,
The most surprising structural feature of the alkaline earth
metal complexes presented here is the different coordination
of the two anions in the strontium and barium derivatives 4
and 5 compared to the lighter alkaline earth metal compounds
1-3. Figure 6 shows the least-squares fit of the best planes of
both moieties (see also Table 2). While the metals remain nearly
in the plane of one Ph2P(Me3SiN)2 anion (Figure 6A), the
deviation from the plane of the second anion increases gradually
from 9.8 for Be to 55.7 pm for Ba (Figure 6B).
(28) Kaupp, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 491.
(29) (a) Kaupp, M.; Schleyer, P. v. R.; Stoll, H.; Preuss, H. J. Chem. Phys.
1991, 94, 1360. (b) Kaupp, M.; Schleyer, P. v. R.; Stoll, H.; Preuss,
H. J. Am. Chem. Soc. 1991, 113, 6012.
(30) Mo¨sges, G.; Hampel, F.; Kaupp, M.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1992, 114, 10880.
(27) Steiner, A.; Stalke, D. Inorg. Chem. 1995, 34, 4846.

[(THF)nM{(NSiMe3)2PPh2}2] Complexes
Inorganic Chemistry, Vol. 36, No. 11, 1997 2417
Table 3. Deviation from Anionic Best Plane (pm)
- a
RC(NR)2
carbazolate
metal
Mg
Ca
Sr
5.4
20.6
32.0
81.6
73.8
136.9
135.5
19.4
136.9
98.2
Ba
24.0
a If only one number is quoted, the two anionic planes are symmetry
related.
Table 4. NMR Spectroscopic Data for 1-5 from Solution
compound
Figure 6. Superposition of the two best PN2 anionic planes in 1-5.
While the metals remain nearly in the plane of one Ph2P(Me3SiN)2
anion (A), the deviation from the plane of the second anion increases
gradually from 9.8 for Be to 55.7 pm for Ba (B).
1
2
3
4
5
31P-NMR (H3PO4) (δ) 33.46 22.48
14.13
11.10
8.02
29Si-NMR (SiMe4) (δ) -3.23 -7.71 -11.40 -13.14 -14.44
2J(Si,P) (Hz)
3.9
7.5
11.5
13.7
15.7
Scheme 4. Resonance Structures of the Ph2P(Me3SiN)2
Aniona
Scheme 5. Coordination of One Ge(pz*)3 Anion to the
Central Metal in Ba[(pz*)3Ge]2
a While the more localized P-N double bond of a and b is detected
in the Ca complex 3, the delocalized structure of c was found in the
Be (1), Mg (2), Sr (4), and Ba (5) complexes.
π-electron-density interaction. Like the heavier alkaline metals
the higher alkaline earth metals were found to prefer the
multihapto mode of bonding to delocalized anions rather than
monohapto bonding.30 This makes the out-of-plane arrangement
of the metals energetically attractive even against steric require-
ments. The heavier (i.e. the easier to polarize) the metals get
the more they want to interact with the (nondirected) electron
density of the π system. In Ba[(pz*)3Ge]2 multihapto bonding
of an heteroaromatic ring to barium was observed for the first
time. Four of the six present 3,5-dimethylpyrazol-1-yl groups
were exclusively N σ-bonded to the barium while two were
η2-coordinated (Scheme 5).
Structural Comparison of 1-5 to Related Homologous
Systems. To test this theory we looked for related complexes
and compared them to the complexes discussed here. The
benzamidinate PhC(Me3SiN)2 anion is very similar to the
Ph2P(Me3SiN)2 anion. Only the central Ph2P unit is substituted
by a PhC group. The solvated magnesium to barium benz-
amidinates have been synthesized and structurally characterized
by Westerhausen et al.32 The out-of-plane arrangement of the
metals should even be more pronounced because of the smaller
anion compared to the Ph2P(Me3SiN)2 anion. Even though the
plane of the phenyl ring is more or less perpendicular to the
CN2 π-system, this anion is quite flat compared to the latter.
Hence the weak π-interaction to the CN2 moiety is not as much
counterbalanced by steric requirements. And in fact the out-
of-plane deviation increases from 5.4 pm in [(C6H5CtN)Mg-
{(NSiMe3)2CPh}2]32a to 81.6 pm in [(dme)(THF)Ba{(NSiMe3)2-
CPh}2].32d Even the deviation of the Ba atom from the plane
of the second benzamidinate anion (24.0 pm) is much higher
than in 5 (Table 3).
At first sight this out-of-plane bending along the N‚‚‚N vector
is surprising, because it increases anion repulsion. The phenyl
group of the one anion interacts with the trimethylsilyl group
of the second anion. To minimize steric strain one would expect
a planar arrangement for both PN2M units similar to the situation
found in 1 and 2. Therefore one would expect the angle φ in
Scheme 3 to be close to 180° for both anions, in particular,
since there are no obvious packing effects which cause the
bending of the central phosphorus atom of one ligand toward
the Ba and Sr metal center. Therefore the angle φ of 167.2° in
the Sr complex and of 167.0° in the Ba complex (this
corresponds to a deviation of the position of the metal from the
PN2 plane by 51.1 (Sr) and 55.7 pm (Ba), respectively; Figure
6b) may be taken as an indication of increasing metal π-interac-
tion of the heavier alkaline earth metals to the delocalized
electron density of the PN2 moiety (c in Scheme 4). A similar
trend was observed in the homologous series of the related alkali
metal derivatives [(THF)nM{(NSiMe3)2PPh2}2]m (M ) Li, Na,
K, Rb, Cs; n ) 1, 2, 4, m ) 1, 2, ∞),31 although this appeared
not as so systematically as in the structures discussed here. The
change of the coordination environment and the degree of
oligomerization does not progress as gradually as in 1-5
(deviation from the PN2 best plane (pm): Li, 6.4; Na, 4.9 and
19.2; K, 10.5; Rb, 71.7; Cs, 15.5).
Apart from the structure of 3, which is a special case, the
electron density is delocalized within the PN2 moiety (c in
Scheme 4). In tune with the increasing ordering number of the
metal the average P-N bond length decreases (Table 2),
indicating electron accumulation on the anion along the line
Be to Ba. The barium dication obviously is more easy to
polarize than the beryllium dication. This is supported by the
NMR chemical shifts and coupling constants (Table 4). Both
the signal for the central phosphorus atom as well as the signal
for the silicon atoms in the ligand are detected more and more
Figure 7 shows the least-squares fit of the best planes of the
CN2 fragments. Clearly the same structural variations found
in 1-5 can be seen in the alkaline earth metal benzamidinates.
The lithium and sodium benzamidinate exhibit dimeric struc-
2
upfield as the metal gets heavier. The JSi,P coupling constant
increases constantly. Since the Ph2P(Me3SiN)2 anion in 5 (Ba)
is more electron rich than in 1 (Be) and the barium dication is
easy to polarize, 5 is the best candidate to observe metal
(32) (a) Mg: Westerhausen, M.; Hausen, H.-D. Z. Anorg. Allg. Chem. 1992,
615, 27. (b) Ca: Westerhausen, M.; Schwarz, W. Z. Naturforsch.,
Sect. b 1992, 47, 453. (c) Sr: Westerhausen, M.; Hausen, H.-D.;
Schwarz, W. Z. Anorg. Allg. Chem. 1992, 618, 121. (d) Ba:
Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1993, 619,
1455.
(31) Steiner, A.; Stalke, D. Inorg. Chem. 1993, 32, 1977.

2418 Inorganic Chemistry, Vol. 36, No. 11, 1997
Fleischer and Stalke
(ii) Like in many other heavier alkaline earth metal com-
plexes, the anions in 3-5 were found to be cisoid with respect
to each other. Against sterical considerations the two THF
molecules in 4 and 5 are coordinated to the same hemisphere
of the metal. Apart from the ab initio MO computational
prediction (significant covalent σ-bonding contributions involv-
ing metal d-orbitals and polarization of the metal cation by the
anions) there is now experimental evidence of a bent arrange-
ment presented here (structural strain, shortening of the average
P-N distance and NMR upfield shift).
Figure 7. Superposition of the best CN2 anionic plane in the solvated
alkaline earth metal benzamidinates [(donor)nM{(NSiMe3)2CPh}2] (M
) Mg, Ca, Sr, Ba) illustrating the increasing out-of-plane coordination
with increasing ordering number of the metals.
(iii) Although multihapto π interaction with an anion has been
calculated to be favorable for the heavier alkaline earth cations,
this effect has not yet been reported experimentally. While the
π interaction of the central Ba dication and one pyrazolyl ring
in Ba[(pz*)3Ge]2 might be sterically induced, this is clearly not
the case in the systems presented here. Even against steric strain
the heavier alkaline earth metals Sr and Ba leave the plane of
one anion, demonstrating their preference to interact with π
electron density. This effect can also be found in related
systems. Certainly the π-interaction is smaller than in the
alkaline earth metallocenes and counterbalanced by steric
requirements, but it is significant. The metal π-interaction rises
with the mass of the metal and decreasing bulk of the anion
but is independent from cisoid or transoid arrangement of the
anions.
tures.33 The structures of the solvated alkaline earth carbazolate
complexes of Ca, Sr, and Ba with two unsubstituted C12H8N
anions are known.30 The coordinating nitrogen atom in the
anions is integrated in a NC5 aromatic ring. Unlike in the
structures of the heavier alkali metal carbazolates,34 in the
structures of the alkaline earth derivatives there are no short
metal-carbon distances observed. They are regarded as
exclusively monohapto σ-bonded without any multihapto π-bond-
ing to the metal,30 but the deviation of the position of the metals
from the plane of the anions is even more striking than in the
benzamidinate complexes. Table 3 summarizes the deviations
of the metal from the best planes of the central NC5 perimeters
in the structures of the alkaline earth metal carbazolate
structures. Obviously the flat carbazole anion allows even more
significant deviation from the anionic plane than the benz-
amidinate ions. In the strontium and barium derivative the metal
is more than 135 pm out of the plane of the anion (Table 3).
Like in the other structures, a careful inspection of possible short
inter- and intramolecular distances, which may account for the
out-of-plane deviation, did not support the hypothesis that
packing forces might cause this effect. In the alkaline earth
metal mercaptobenzoxazolate35 and aminothiatriazolate36 com-
plexes reported by Snaith et al. the deviation from the plane is
prevented by additional M-S side arm donation. However, the
following statement can be made: The higher deviation of the
alkaline earth metals from the plane of the anion with increasing
mass demonstrates their rising preference to interact with
π-electron density, even against steric strain.
Experimental Section
All manipulations were performed under inert gas atmosphere of
dry N2 with Schlenk techniques or in an argon glovebox. All solvents
were dried over Na/K alloy and distilled prior to use. NMR spectra
were obtained in benzene-d6 as solvent with SiMe4 or H3PO4 as external
reference on a Bruker AM 250. Mass spectra were recorded on a
Finnigan Mat 8230 or Varian Mat CH5 spectrometer. Elemental
analyses were performed by the Analytische Laboratorium des Instituts
fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
Hazards. Beryllium compounds are extremely toxic and supposed
to cause cancer.37 Handle with care!
N,N′-bis(trimethylsilyl)aminodiphenylphosphinimine15 and alkaline
earth metal bis[bis(trimethylsilyl)amides]16,17 were prepared according
to known literature procedures. The alkaline earth metal N,N′-bis[bis-
(trimethylsilyl)aminodiphenylphosphinimides)] were obtained using the
following general procedure: A solution of alkaline earth metal bis-
[bis(trimethylsilyl)amide] (2.5 mmol) in 10 mL of hexane is added to
a solution of N,N′-bis(trimethylsilyl)aminodiphenylphosphinimine (5
mmol, 1.8 g) in 10 mL of hexane and stirred overnight. After the
precipitate has settled, the reaction solution is decanted. Recrystalli-
zation of the precipitate from THF/hexane give colorless crystals,
suitable for X-ray structure analysis.
Conclusion
The question of what factors govern the coordination
geometry of the alkaline earth metal has to be answered in three
parts:
(i) Like in most compounds, as well the coordination
geometry of the alkaline earth metals is governed by anion cation
size (miss)match. Not surprisingly, the coordination number
increases from 4 (Be, Mg) Via 5 (Ca) to 6 (Sr, Ba). Two of the
Ph2P(Me3SiN)2 anions cover the coordination sphere of beryl-
lium and magnesium, while with calcium one single THF
molecule and with strontium and barium two additional THF
molecules are required to complete the metal coordination
sphere.
Be[(NSiMe3)2PPh2]2 (1): M ) 28.13 g/mol; yield 31%; mp 53 °C;
1H (C6D6) δ 0.06 (s, 18H), 7.05 (m, 12H), 7.75 (m, 8H); 29Si (C6D6) δ
2
-3.23 (d, JSi,P ) 3.9 Hz); 31P (C6D6) δ 33.46; MS (70 eV) m/z 727
(100%, M+), 368 (46%, M+ - C18H28N2PSi2). No elemental analysis
was attempted because of the extreme toxicity of beryllium compounds.
Mg[(NSiMe3)2PPh2]2 (2): M ) 743.43 g/mol; yield 93%; mp 268
1
°C; H (C6D6) δ 0.130 (s, 18H), 7.15 (m, 12H), 7.92 (m, 8H); 29Si
(C6D6) δ -7.71 (d, 2JSi,P ) 7.5 Hz); 31P (C6D6) δ 22.48. MS (70 eV)
m/z 742 (42% M+), 383 (12% M+ - C18H28N2PSi2), 356 (100%
C15H34N2PSi3+). Anal. Calcd (found): C, 58.2 (55.78); H, 7.60 (7.80);
N, 7.5 (7.01).
(33) Stalke, D.; Wedler, M.; Edelmann, F. T. J. Organomet. Chem. 1992,
431, C1.
(34) (a) Hacker, R. Kaufmann, E.; Schleyer, P. v. R.; Mahdi, W.; Dietrich,
H. Chem. Ber. 1987, 120, 1533. (b) Gregory, K. Diplomarbeit, Univ.
Erlangen-Nu¨rnberg, 1988. (c) Gregory, K.; Bremer, M.; Schleyer, P.
v. R.; Klusener, P. A. A.; Brandsma, L. Angew. Chem. 1989, 101,
1261; Angew. Chem., Int. Ed. Engl. 1989, 28, 1224. (d) Gregory, K.
Dissertation, Univ. Erlangen-Nu¨rnberg, 1991.
(35) Mikulcik, P.; Raithby, P. R.; Snaith, R.; Wright, D. S. Angew. Chem.
1991, 103, 452; Angew. Chem., Int. Ed. Engl. 1991, 31, 428.
(36) Banbury, F. A.; Davidson, M. G.; Martin, A.; Raithby, P. R.; Snaith,
R.; Verhorevoort, K. L.; Wright, D. S. J. Chem. Soc., Chem. Commun.
1992, 1152.
Ca[(NSiMe3)2PPh2]2‚THF (3): M ) 831.30 g/mol; yield 88%; mp
1
240 °C; H (C6D6) δ 0.17 (s, 18H), 1.29 (m, 4H), 3.67 (m, 4H), 7.17
2
(m, 12H), 7.94 (m, 8H); 29Si (C6D6) δ -11.40 (d, JSi,P ) 11.5 Hz);
31P (C6D6) δ 14.13; MS (70 eV) m/z 758 (38% M+ - THF), 399 (69%
M+ - C22H36N2OPSi2), 364 (100% M+ - [Ph2P(NHPh)(NSiMe3)+].
Anal. Calcd (found): C, 57.8 (57.54); H, 7.8 (7.77); N, 6.7 (6.52).
(37) Kumberger, O.; Schmidbaur, H. Chem. Unserer Zeit 1993, 27, 310.

[(THF)nM{(NSiMe3)2PPh2}2] Complexes
Table 5. Crystal Data of 1-5 at T ) 153(2) K
Inorganic Chemistry, Vol. 36, No. 11, 1997 2419
compound
1
2
3
4
5
formula
fw
cryst size (mm)
space group
a (pm)
b (pm)
c (pm)
C36H56BeN4P2Si4
C36H56N4MgP2Si4
743.46
0.7 × 0.6 × 0.4
P21/c
1275.9(3)
1803.0(4)
1939.2(4)
90
106.21(2)
90
4.284(2)
4
C40H64CaN4OP2Si4
831.33
C44H72N4O2P2Si4Sr
950.98
0.3 × 0.2 × 0.2
P21/n
1119.50(10)
2429.9(3)
1838.8(2)
90
92.829(8)
90
5.1255(9)
4
C44H72BaN4O2P2Si4
1000.70
0.8 × 0.7 × 0.2
P21/n
1131.80(10)
2519.8(2)
1831.40(10)
90
93.106(6)
90
5.2153(7)
4
728.16
0.6 × 0.6 × 0.5
Cc
1884.5(8)
1147.4(3)
1983.6(8)
90
100.92(3)
90
4.211(3)
4
0.8 × 0.8 × 0.8
P1h
1133.00(10)
1156.40(10)
1994.8(2)
104.162(7)
97.393(6)
97.258(6)
2.4790(4)
2
R (deg)
â (deg)
γ (deg)
V (nm3)
Z
Fc (Mg m-3
)
1.148
0.246
1560
1.153
0.257
1592
1.114
0.320
892
1.232
1.245
2016
1.274
0.951
2088
µ (mm-1
F(000)
)
2θ range (deg)
8-45
8-45
8-45
8-45
8-45
no. of reflns measd
no. of unique reflns
no. of restraints
refined params
7910
5670
6353
7174
6842
5515
2
436
0.026
0.067
0.039; 1.40
0.16
5608
0
436
0.037
0.094
0.042; 3.08
0.38
6217
437
595
0.032
0.087
0.048; 1.29
0.27
6701
279
564
0.035
0.100
0.031; 5.47
0.29
6838
0
526
0.028
0.068
0.033; 4.10
0.35
R1a [I > 2σ(I)]
wR2b (all data)
g1; g2c
highest diff peak (10-6 e pm-3
)
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fo )2]}1/2
.
c w ) 1/[σ2(Fo ) + (g1P)2 + g2P]; P ) (Fo + 2Fc2)/3.
In 3 the disordered trimethylsilyl moiety (Si1-C3) was refined to a
split occupancy of 0.7/0.3, while the disordered phenyl ring (C7-C12)
was refined to a split occupancy of 0.5/0.5 and the disordered donating
THF molecule (O1-C40) was refined to a split occupancy of 0.9/0.1,
using bond length and similarity restraints.
Sr[(NSiMe3)2PPh2]2‚2THF (4): M ) 970.94 g/mol; yield 80%; mp
1
155 °C; H (C6D6) δ 0.16 (s, 36H), 1.33 (m, 8H), 3.59 (m, 8H), 7.16
2
(m, 12H), 7.93 (m, 8H); 29Si (C6D6) δ -13.14 (d, JSi,P ) 13.7 Hz);
31P (C6D6) δ 11.10; MS (70 eV) m/z 806 (40% M+ - 2THF], 447
(84% M+ - C26H44N2O2PSi2], 387 (100% M+ - C26H48N2OPSi2]. Anal.
Calcd (found): C, 54.4 (54.11); H, 7.5 (7.59); N, 5.8 (5.95).
Ba[(NSiMe3)2PPh2]2‚2THF (5): M ) 1000.65 g/mol; yield 78%;
The two disordered THF molecules in 4 were refined to split
occupancy of 0.7/0.3 (O1-C40) and 0.5/0.5 (O2-C44). Selected bond
lengths and angles are presented in Table 1. Further details on the
structure investigation can be obtained from the Director of the
Cambridge Crystallographic Data Center, 12 Union Road, GB-
Cambridge CB2 1EZ, U.K., by quoting the full journal citation.
1
mp 148 °C; H (C6D6) δ 0.17 (s, 18H), 1.35 (m, 8H), 3.581 (m, 8H),
2
7.16 (m, 12H), 7.93 (m, 8H); 29Si (C6D6) δ -14.44 (d, JSi,P ) 15.7
Hz); 31P (C6D6) δ 8.02; MS (70 eV) m/z 856 (2% M+ - 2THF), 497
(11% M+ - C26H44N2O2PSi2). Anal. Calcd (found): C, 52.8 (51.99);
H, 7.3 (7.29); N, 5.6 (5.61).
X-ray Measurements of 1-5. Crystal data for the five structures
are presented in Table 5. All data were collected at low temperatures
using oil-coated shock-cooled crystals38 on an Stoe-Siemens AED using
monochromated Mo KR radiation (λ ) 0.710 73 Å). A semiempirical
absorption correction was applied.39 The structures were solved by
Patterson or direct methods with SHELXS-86.40 All structures were
refined by full-matrix least-squares procedures on F2, using SHELXL-
93.41 All non-hydrogen atoms were refined anisotropically, and a riding
model was employed in the refinement of the hydrogen atom positions.
Acknowledgment. Both authors wish to thank the Deutsche
Forschungsgemeinschaft, the Volkswagen-Stiftung, and the
Fonds der Chemischen Industrie for financial support. D.S.
wishes to thank axs-Analytical X-Ray Systems, Karlsruhe,
Germany.
Supporting Information Available: X-ray crystallographic files
in CIF format for compounds 1-5 are available on the Internet only.
Access information is given on any current masthead page.
(38) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615. (b)
Kottke, T.; Lagow, R.; Stalke, D. J. Appl. Crystallogr. 1996, 29, 465.
(39) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351.
IC9613117
(41) Sheldrick, G. M. Program for crystal structure refinement, University
of Go¨ttingen, 1993.
(40) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.