Effect of hydrogen bonding on the coordination: Part 2. Semi-coordination in trans-di(salicylato)bis(1,3-diaminopropane-N,N')copper(II)

Article abstract of DOI:10.1016/S0020-1693(01)00605-3

The crystal and molecular structure of trans-di(ortho-hydroxobenzoato-O)bis(1,3-diaminopropane-N,N′)copper(II) [ortho-hydroxobenzoate = salicylate hereafter] determined by single crystal X-ray diffraction methods at T = 193(2) K. The structure is composed of monomeric complex units with the central copper(II) ion occupying a centre of symmetry. The coordination polyhedron can be described as axially elongated distorted octahedron with the axial Cu-O bond length of 2.559(3) ? indicating semi-coordination. There is notable asymmetry in the coordinated 1,3-diaminopropane molecule, which forms a six-membered chelate ring displaying Cu-N1-C1 and Cu-N2-C3 angles of 121.5(2) and 117.4(2)°, respectively. This deviation may be correlated to differences in hydrogen bonding between the amino groups and the non-coordinated oxygen atoms of the salicylate anions. The coordinated salicylate anion shows considerable declination of 28.9°, which may be correlated to intramolecular hydrogen bonding. The electronic properties of benzoate anions were studied by Natural Bond Order (NBO) analyses for optimised model compounds at the MP2/6-311 + G*//MP2/6-311 + G* level of theory. The results suggest that semi-coordination is mainly characterised by concomitant electrostatic attraction of the charges at the central copper(II) cation and repulsion between an electron lone pair of a ligand atom and the electron lone pair at the copper(II) 3d_z2 atomic orbital.

Full text of DOI:10.1016/S0020-1693(01)00605-3

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Inorganica Chimica Acta 324 (2001) 212–217
Effect of hydrogen bonding on the coordination
Part 2. Semi-coordination in
trans-di(salicylato)bis(1,3-diaminopropane-N,N’)copper(II)
Markku R. Sundberg *, Raikko Kiveka¨s, Pertti Huovilainen, Rolf Uggla
Department of Chemistry, Laboratory of Inorganic Chemistry, Uni6ersity of Helsinki, PO Box 55 (A. I. Virtasen aukio 1),
FIN-00014 Helsinki, Finland
Received 22 March 2001; accepted 6 August 2001
Abstract
The crystal and molecular structure of trans-di(ortho-hydroxobenzoato-O)bis(1,3-diaminopropane-N,N%)copper(II) [ortho-hy-
droxobenzoate=salicylate hereafter] determined by single crystal X-ray diffraction methods at T=193(2) K. The structure is
composed of monomeric complex units with the central copper(II) ion occupying a centre of symmetry. The coordination
,
polyhedron can be described as axially elongated distorted octahedron with the axial Cu–O bond length of 2.559(3) A indicating
semi-coordination. There is notable asymmetry in the coordinated 1,3-diaminopropane molecule, which forms a six-membered
chelate ring displaying Cu–N1–C1 and Cu–N2–C3 angles of 121.5(2) and 117.4(2)°, respectively. This deviation may be
correlated to differences in hydrogen bonding between the amino groups and the non-coordinated oxygen atoms of the salicylate
anions. The coordinated salicylate anion shows considerable declination of 28.9°, which may be correlated to intramolecular
hydrogen bonding. The electronic properties of benzoate anions were studied by Natural Bond Order (NBO) analyses for
optimised model compounds at the MP2/6-311+G*//MP2/6-311+G* level of theory. The results suggest that semi-coordination
is mainly characterised by concomitant electrostatic attraction of the charges at the central copper(II) cation and repulsion
between an electron lone pair of a ligand atom and the electron lone pair at the copper(II) 3dz atomic orbital. © 2001 Elsevier
2
Science B.V. All rights reserved.
Keywords: Semi-coordination; Natural bond order; Crystal structures; Copper Complexes; Carboxylate Complexes; Hydrogen bonding
hydrogen bonding has been found to stabilise te-
trafluoroborate coordination to copper(II) ion [8]. Am-
1. Introduction
moniumcopper(II) sulfate exhibits a switch in the
direction of the long axis of the Jahn–Teller distortion
upon deuteration, which has been explained by differ-
ences in hydrogen bonding interactions [9].
The concept of semi-coordination was introduced in
1967 to describe a situation, where ‘…a polyatomic
anion occupies the long tetragonal position in an other-
wise square-planar complex of copper(II) with an atom
Hydrogen bonding is very important, e.g. in many
biochemical and bioinorganic processes involving tran-
sition metal complexes. It is well established how hy-
drogen bonding may assist molecular recognition in
complexation [1]. However, there are only few studies
focused into the effect of hydrogen bonding on a
coordinate bond [2–5]. It has been suggested, that
hydrogen bonding might even regulate bonding iso-
merism in copper(I) thiocyanates [6]. Difference in hy-
drogen bonding was found to dictate conformational
isomerism in a cobalt(III) complex [7]. Intramolecular
,
at a distance of 2.5–2.6 A’ [10]. Later on, the elonga-
tion was explained by d–s mixing [11]. However, this
idea has been criticised owing to too large numerical
values obtained for the mixing and differences between
the observed and predicted coordination modes [12].
So far, the substituents in the benzoate anions have
not been capable of marked hydrogen bonding in the
* Corresponding author. Tel.: +358-9-191-50232; fax: +358-9-
191-50198.
E-mail address: markku.sundberg@helsinki.fi (M.R. Sundberg).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00605-3

M.R. Sundberg et al. / Inorganica Chimica Acta 324 (2001) 212–217
213
Cutn2(XbzO)2 (where tn refers to 1,3-diaminopropane
2.3. Crystal data
and XbzO to a substituted benzoate anion) series stud-
ied in our laboratory [13]. Since hydrogen bonding is a
gateway to molecular organisation [14] and even to
crystal engineering, it was considered appropriate to use
a hydroxyl group in an ortho position as a suitable
probe in the present study. Moreover, it was suggested
in an earlier review, that hydrogen bonding to a coordi-
nated carboxylate oxygen atom should result in elonga-
tion in the axial Cu–O bond in trans-bis Werner-type
copper(II) complexes [3]. In Part 1, ortho-, meta- and
para-substituted aminobenzoate anions showed marked
tendency to deform ideal symmetry of the chelate rings
and increase declination of a coordinated benzoate
anion with respect to an equatorial coordination plane
[4]. Here we have synthesised a new analogue with
salicylate anion as counterion [15] and determined the
structure by single-crystal X-ray diffraction methods.
Furthermore, the electronic properties of the salicylate
anion and the related intramolecular hydrogen bonding
were analysed by means of quantum chemical methods
for the optimised species at the MP2/6-311+G* level
of theory. In this work we show, how hydrogen bond-
ing affects the coordination in general and semi-coordi-
nation in particular.
CUSALIA [Cutn2(2OHbzO)2], C20H30CuN4O6, Mr=
486.02, orthorhombic, space group Pbca (no. 61), a=
,
23.672(3), b=12.078(2), c=7.807(3) A, V=2232.1(10)
3
,
A (by least-squares refinement of diffractometer angles
for 25 independent well-centred high-angle reflections)
at T=173 K; Z=4. Dark blue plates. Crystal dimen-
sions 0.50×0.35×0.12 mm3, v=10.22 cm−1
.
2.4. X-ray structure determination of CUSALIA
Single-crystal data collection was performed at 173 K
on a Rigaku AFC7S diffractometer using graphite
monochromatised Mo Ka radiation. 2019 unique
reflections were collected by ꢀ–2q scan mode (2qmax
=
50.5°). The data were corrected for absorption („ scan).
The structure was solved by direct methods and
refined on F2 by the SHELXL-97 program package [17].
Non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Approximate position of the
hydrogen atom H11 bonded to O3 was picked from DF
map, and positional parameters of H11 were then
refined with fixed isotropic thermal displacement
parameter. Rest of the hydrogen atoms were treated as
riding atoms, using the SHELX-97 default parameters.
Final R values: R1=0.0489, wR2=0.1181 [I\2|(I)].
2. Experimental
2.5. Theoretical calculations
2.1. Syntheses
Optimisations for the structures were carried out at
the MP2/6-311+G* level of theory by using GAUS-
SIAN-98 A7 code [18]. The symmetry for every moiety
was Cs. The electronic properties were then analysed by
NBO methods [19] at the MP2/6-311+G* level of
theory.
The complex (hereafter CUSALIA) was synthesised
by dispersing 20 mmol of the salicylic acid (Fluka AG,
pur.) and 10 mmol of CuCO3 (Baker’s Analysed) in
aqueous EtOH (100 ml H2O, 50 ml EtOH). The suspen-
sion was heated and stirred until a complete removal of
CO2. When 20 mmol of tn (Fluka, purum) was added
into the suspension, a deep blue solution was formed.
Blue crystals were separated by filtration and recrys-
tallised from EtOH. The molecular structure was then
determined by single-crystal X-ray diffraction methods.
Anal. Calc. for C20H30CuN4O6: C, 49.40; H, 6.27; N,
11.52. Found C, 50.39; H, 6.26; N, 11.53%.
3. Results and discussion
3.1. General description of the compound
Selected bonding parameters are listed in Table 1.
The numbering scheme for the atoms in the complex
unit is shown in Fig. 1.
2.2. Powder diffraction study
The complex unit has the central copper(II) ion
located at a centre of symmetry. A similar coordination
mode was also found in trans-di(salicylato)-bis(iso-
propyl-2-methylpropane-1,2-diamine-N,N%)copper(II)
[20] (hereafter CUSALIB). Both of the nitrogen atoms
of the tn ligand in the present complex are coordinated
and form six-membered chelate rings in the equatorial
direction. The respective equatorial Cu–N bond lengths
are equal within experimental errors, 2.027(3) and
The dimensions of the unit cell of CUSALIA were
calculated from a photograph produced by monochro-
matised Cu Ka radiation in a focussing camera of
Guinier type with the additional data obtained from
Weissenberg photographs [16]. Orthorhombic, space
group Pbca, a=23.772(3), b=11.977(2), c=7.9672(8)
3
A, V=2268.3(5) A , Dc=1.42 Mg m−3, Dm=1.42 Mg
m−3 at ambient temperature. The listing of observed
and calculated sin2 q values with their hkl indices is
given in Section 4
,
,
,
2.020(3) A. The bite angle N1–Cu–N2 [88.44(12)°]
deviates significantly from 90°. Accordingly, the coordi-

214
M.R. Sundberg et al. / Inorganica Chimica Acta 324 (2001) 212–217
nation polyhedron can be described as axially elongated
distorted octahedron.
The axial salicylate anion is coordinated via one
compound it displays the syn-configuration. Another
marked difference is, that there are no other hydrogen
bonds reported in CUSALIB, where the non-coordi-
nated oxygen O2 would participate in other hydrogen
bonding, either intra or intermolecular ones. The differ-
ences are illustrated schematically in Chart 1 below. In
our compound the non-coordinated oxygen O2 forms
several hydrogen bonds, as discussed later.
Because the conformation of the salicylate anion in
CUSALIA is preferred to that in CUSALIB only by
approximatley 0.9 kcal mol−1, we carried out X-ray
powder diffraction study of the our sample. The peaks
could be interpreted only in terms of unit cell data of
CUSALIA.
oxygen only. The respective axial bond length of
,
2.559(3) A shows elongation, which is originally called
as semi-coordination [10]. In CUSALIB the axial Cu–
,
O distance is significantly longer, 2.606(4) A [19]. How-
ever, there is an interesting difference in the salicylate
anions reported here and in CUSALIA: in our com-
pound a hydrogen is connected to the phenolic oxygen
O3 forming an intramolecular hydrogen bond with the
coordinated oxygen O1. In CUSALIA the hydrogen
atom is bonded to non-coordinated oxygen O2 forming
a hydrogen bond with the phenolic oxygen O3. More-
over, in our compound the phenolic group is anti to the
non-coordinated oxygen O2, whereas in CUSALIB
3.2. The six-membered chelate ring
Table 1
The chelate rings formed by tn display chair confor-
mation. The puckering parameters z1, z2 and z3 are
,
Selected bond lengths (A) and bond angles (°) for trans-di(salicy-
lato)bis(1,3-diaminopropane-N,N%)copper(II)
,
0.857(6), 0.616(6) and 0.954(6) A, respectively. The zi
values describe a distance of a carbon atom from a
CuN1N2-plane. Thus it seems that the chair is slightly
skewed, because the z1 and z3 values are significantly
different. The asymmetry of the ring is seen also in the
angles Cu–N1–C1 and Cu–N2–C3, which have the
values 121.5(2)° and 117.4(2)°, respectively. The former
angle seems quite wide, if we compare it with the one
calculated for the optimised (DFT methods) structure
of trans-diaquabis(1,3-diaminopropane-N,N%)copper(II)
cation, where the calculated values for the Cu–N–C
angles are 117.5 and 117.8° [21]. A plausible explana-
tion for the deviations described above may be an
asymmetric hydrogen bonding network with respect to
the chelate ring.
Bond lengths
Cu–N1
Cu–N2
Cu–O1
O1–C10
2.027(3)
2.020(3)
2.559(3)
1.276(4)
O2–C10
O3–C5
C4–C10
1.248(4)
1.347(4)
1.499(5)
Bond angles
N1–Cu–N2
C10–O1–Cu
C1–N1–Cu
C3–N2–Cu
88.44(12)
127.0(2)
121.5(2)
117.4(2)
O2–C10–O1
O2–C10–C4
O1–C10–C4
123.3(3)
119.5(3)
117.2(3)
3.3. The salicylate anion
The bond lengths and angles in the salicylate anion
are in the normal range found in the Cutn2(XbzO)2
,
series [22]. The C4–C10 bond length of 1.499(5) A
shows elongation, if it is compared with the value
,
1.466(2) A obtained for 2,5-piperazinedione-salicylic
acid complex [23]. However, our value is near to
Fig. 1. ORTEP drawing of trans-di(salicylato)bis(1,3-diaminopropane-
N,N%)copper(II) showing the labelling of the non-hydrogen atoms and
intramolecular hydrogen bonding.
,
1.497(6) A in CUSALIB [20].
In Part 1 we defined inclination and declination as
bowing and lurching of a coordinated benzoate anion
[4]. In the title compound the values for inclination and
declination are −133.4 and 28.9°, respectively. The
inclination value indicates that the carboxylate group
displays clearly syn-coordination (the threshold value is
180°), which is the common coordination mode in the
Cutn2(XbzO)2 series. In Part 1 the absolute declination
values
for
trans-di(3-aminobenzoato-O)bis(1,3-di-
aminopropane-N,N%)copper(II) complex and the corre-
sponding para-aminobenzoate analogue were 28.0 and
38.5°, respectively. In these compounds both of the
Chart 1.

M.R. Sundberg et al. / Inorganica Chimica Acta 324 (2001) 212–217
215
Table 2
2 seem to indicate stronger interactions with the amino
group containing N1. Indeed, this inequality may well
be correlated with the asymmetry of the ligand dis-
cussed above. Moreover, the parameters for the in-
Hydrogen bonds for trans-di(salicylato)bis(1,3-diaminopropane-
N,N%)copper(II)
D–H···A
d(D–H)
d(H···A)
d(D···A)
Ú(DHA)
(°)
,
,
,
(A)
(A)
(A)
tramolecular
hydrogen
bonding
between
the
coordinated oxygen atom and the amino hydrogen
seem reasonable.
O3–H11···O1
N1–H1···O2
0.98(4)
0.92
0.92
1.60(5)
2.02
2.03
2.499(4)
2.917(4)
2.915(4)
3.025(4)
151(4)
162.8
159.7
163.3
The geometrical parameters between the ortho-hy-
droxy group in the salicylate anion and the coordinated
oxygen atom of the carboxylate group are all typical for
the intramolecular hydrogen bonding. Especially the
value of 151(4)° for the angle O3–H11···O1 is suitable
for an interaction between an electron pair of a sp3
hybridised oxygen and hydrogen to form a six-mem-
bered ring O1···H11–O3–C5–C4–C10. A sp3 hybridis-
ation would in turn bring in inclination of the salicylate
anion. Obviously the corresponding angle of 83(2)° in
the analogous ortho-aminobenzoate derivative is too
narrow to require a sp3 hybridisation. This would in-
deed explain the small inclination value of 4.8° in the
latter compound [4].
N1–H2···O2c1
N2–H4···O2c2
0.92
2.13
Symmetry transformations used to generate equivalent atoms: c1 x,
−y+1/2, z+1/2; c2 −x+1, y+1/2, −z+3/2.
Chart 2.
oxygen atoms of the carboxylate groups are hydrogen
bonded. It was then suggested that hydrogen bonding
to the coordinated oxygen would change the electronic
configuration from approximate sp2 towards sp3; the
declination would increase upon this change [4]. Usu-
ally the declination values for the complexes in the
Cutn2(XbzO)2 series are in the range of 0.5–12.0° (e.g.
in the for trans-di(benzoato-O)bis(1,3-diaminopropane-
N,N%)copper(II) the value is 5.5°) [24]. Although there
was a relatively short intramolecular distance in trans-
di(ortho - aminobenzoato - O)bis(1,3 - diaminopropane-
N,N%)copper(II) between the coordinated oxygen and
the amino group in the benzene ring, the absolute
declination value was 4.8°. To explain the small decli-
nation value, the existence of hydrogen bonding was
considered doubtful. Despite of the suitable short dis-
tance, the value of 83(2)° for the N3–H21···O1 angle
was evaluated to be too narrow to indicate hydrogen
bonding [4]. Therefore, the declination value of 28.9°
should imply hydrogen bonding to O1 in the present
compound.
3.5. Quantum chemical studies
In principle the hydrogen atom positions in a single-
crystal X-ray diffraction study always contain errors in
their atomic positions, and therefore we carried out
quantum chemical calculations for the moieties shown
in Chart 2. The structures were optimised at the MP2/
6-311+G* level of theory. Selected bond lengths for
the optimised species are given in Table 3.
In general, the optimised and experimental values
agree quite well. The most noticeable difference is seen
in the C4–C10 bond lengths between SALIA and the
corresponding copper complex CUSALIA. A plausible
explanation is, that the optimised structures refer to gas
phase without any intermolecular interactions. As dis-
cussed above, there are three hydrogen bonds involving
O2 in CUSALIA. In contrast to this, there are no
intramolecular hydrogen bonds in CUSALIB, where
the calculated and experimental values for C4–C10 are
in good accordance. In terms of energy, SALIA is
preferred to SALIB by ca. 0.9 kcal mol−1
.
3.4. Hydrogen bonding
Table 3
,
Selected bond lengths (A) for the optimised anions at the MP2/6–
311+G* level theory and experimental ones (in italics) for CUSALIA
and CUSALIB
Listing of the hydrogen bonds is given in Table 2.
The following parameters were taken as boundary val-
ues with three e.s.d.s as threshold: H···O 1.96(12), N···O
,
2.85(9) A and N–H···O 162(11)° [25]. There are totally
C4–C10
C10–O1
C10–O2
C5–O3
four hydrogen atoms in a 1,3-diaminopropane molecule
capable of hydrogen bond formation. Obviously there
are three of them involved in interactions with a non-
coordinated carboxylate oxygen. This means that the
two amino groups are different with respect to hydro-
gen bonding. Moreover, the parameters shown in Table
SALIA
CUSALIA
SALIB
CUSALIB
BENZOA
1.540
1.499(5)
1.496
1.497(6)
1.552
1.280
1.276(4)
1.225
1.225(6)
1.258
1.244
1.248(4)
1.339
1.272(6)
1.258
1.351
1.348(4)
1.294
1.325(6)

216
M.R. Sundberg et al. / Inorganica Chimica Acta 324 (2001) 212–217
To the best of our knowledge, the conformation
found in CUSALIB compound is unique. No similar
structures were found from CSD.
It is clearly seen, that coordination to copper has
very small influence in C10–O1 bond length. Instead,
when a proton is bonded to O2, the difference between
the C10–O1 and C10–O2 bonds is evident.
In a benzoate anion both of the carboxylate oxygen
atoms are chemically equivalent. However, in a salicy-
late anion (either SALIA or SALIB) the oxygen atoms
must be different. The interesting question is, which one
of the two available oxygen atoms in a carboxylate
group will be coordinated. When we look at the natural
charges shown in Table 4, the answer is clear: the
oxygen carrying the more negative charge will be re-
ferred. It is also evident, that when we compare the
optimised BENZOA and SALIA, the effect of hydro-
gen bonding is clear: intramolecular hydrogen bonding
to O1 makes it more negative than O2.
The values in Table 4 suggest also, that the more a
coordinated oxygen carries a negative charge, the fur-
ther away it will be from a central copper(II) cation.
This can be deduced from the structures CUSALIA
and the corresponding benzoate analogue, where also
the equatorial ligands are the same (namely, tn). In
CUSALIB, the equatorial ligand is different and the
corresponding Cu–N distances are shorter. This results
in elongation in the axial direction, as predicted by the
plasticity effect [27].
The electronic configurations of the oxygen atoms
were analysed also by NBO methods. The results are
given in Table 5. In BENZOA and SALIA all of the
oxygen atoms display sp3 hybridization, whereas in
SALIB the non-coordinated oxygen atom has sp2 hy-
bridization (in SALIB the coordinated oxygen can have
only two electron lone pairs, since it is protonated). It is
also pertinent to note, that HOMO is located at the
non-coordinated oxygen in SALIA and SALIB. As
noted already, CUSALIB does not contain any hydro-
gen bonds involving the non-coordinated oxygen. The
corresponding optimized anion SALIB has clearly the
least negative charge. It would be tempting to speculate
that hydrogen bonding formation should require a neg-
atively charged carboxylate oxygen displaying sp3 (in-
stead of sp2) hybridization. Clearly more data are
needed to evaluate this assumption.
Table 4
Natural charges (e) for the optimised anions and the X-ray experi-
mental Cu–O1 bond lengths for the corresponding copper(II) com-
plexes
,
O3
O2
O1
Cu–O1 (A)
SALIA
SALIB
BENZOA
−0.795
−0.895
−0.842
−0.776
−0.878
−0.906
−0.804
−0.878
2.559(3) a
2.606(4) b
2.468(5) c
O1 refers to the coordinated and O2 to the non-coordinated oxygen
atom; O3 is the oxygen of hydroxyl group.
a CUSALIA.
b CUSALIB.
There seems to be also a correlation between the
amount of negative charge and the energy of an elec-
tron lone pair (in HOMO): the more there is negative
charge, the lower is the energy.
The results obtained here seem to be chemically
unreasonable, if we consider the axial bonding. How-
ever, if we accept the idea of repulsion between the
charges at the electron lone pairs at the copper(II) ion’s
c trans-di(Benzoato-O)bis(1,3-diaminopropane-N,N%)copper(II)
[26].
Table 5
The electron lone pairs (LP) at the oxygen atoms of the carboxylate
groups
Lone pair
Energy (a.u.)
3dz orbital and an electron pair of a ligand atom
presented in the earlier review article [3], the results
make sense.
Further studies are in progress to study the electronic
repulsion as a reason for axial elongation in Werner-
type copper(II) complexes.
2
SALIA
LP1 O1
LP2 O1
LP3 O1
LP1 O2
LP2 O2
LP3 O2
−0.708
−0.330
−0.196
−0.764
−0.237
−0.158
SALIB
LP1 O1
LP2 O1
LP1 O2
LP2 O2
−0.662
−0.318
−0.792
−0.273
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 166920. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44-1223-336033; e-mail: deposit@
aadc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk].
The data obtained from powder diffraction measure-
ments are available from the authors upon request.
BENZOA
LP1 O1
LP2 O1
LP3 O1
−0.750
−0.218
−0.144
The energies are given in atomic units. O1 and O2 refer to the
coordinated and non-coordinated oxygen atoms, respectively.
HOMO is printed in italics. In BENZOA the oxygen atoms have the
same values; the values are given only for one.

M.R. Sundberg et al. / Inorganica Chimica Acta 324 (2001) 212–217
217
or nitro groups. The latest paper in the series: M.R. Sundberg,
J.K. Koskimies, J. Matikainen, H. Tylli, Inorg. Chim. Acta 268
(1998) 21.
Acknowledgements
Generous computing time was provided by Centre of
Scientific Computing, Espoo, Finland.
[14] D.H. Busch, N.A. Stephenson, Coord. Chem. Rev. 100 (1990)
119.
[15] Preliminary structural report: M.R. Sundberg, R. Uggla, The 8th
National Symposium of Inorganic and Analytical Chemistry,
Turku, Finland, Abstracts and Programme 120, 1994.
[16] Experimental details are the same than in: R. Na¨sa¨nen, R.
Uggla, P. Merila¨inen, Suomen Kemistil. Ser. B 34 (1961) 79.
[17] G.M. Sheldrick, SHELX-97, University of Go¨ttingen, German,
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Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S.
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