Intramolecular apical C-H...M interactions in square-planar nickel(II) complexes with dianionic tridentate ligands and 2-phenylimidazole

Article abstract of DOI:10.1002/ejic.200600594

Two square-planar ternary nickel(II) complexes [Ni(bhac)-(phim)] (1) and [Ni(ahac)(phim)] (2) with O,N,O-donor Schiff bases [acetylacetone benzoylhydrazone (H₂bhac) and acetylacetone acetylhydrazone (H ₂ahac)] and sp² N-donor 2-phenylimidazole (phim) as the ancillary ligand have been synthesised and characterised by analytical, spectroscopic, magnetic and electrochemical methods. The solid-state structures of both complexes have been determined by X-ray crystallography. The asymmetric unit of 1 contains a single complex molecule while that of 2 contains four complex molecules with different conformations. The molecular structures of both complexes reveal that one of the two ortho C-H groups of the pendant phenyl ring of the phim ligand is very close to the metal centre at the apical site indicating the presence of an intramolecular C-H...Ni interaction. In the structures of 1 and 2, the shape of the C-H...Ni interaction is determined by the twisting of the phim ligand and the extent of orthogonality between the planes containing the chelate rings formed by the tridentate ligand and the imidazole ring. A survey of the reported X-ray structures of similar d ⁸ metal ion complexes containing intramolecular C-H...M interactions has been performed for comparison with the structural features of 1 and 2. The optimised molecular structures of 1 and 2 generated by calculations based on the density functional method are compared with the experimental structures. Calculations also reveal that the four molecules present in the asymmetric unit of 2 are very close in energy. Theoretical studies and the down-field shift of the proton resonance on cooling in the NMR spectra suggest three-centre four-electron hydrogen-bond character of the observed apical C-H...Ni interaction in 1 and 2. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600594

FULL PAPER
DOI: 10.1002/ejic.200600594
Intramolecular Apical C–H···M Interactions in Square-Planar Nickel(II)
Complexes with Dianionic Tridentate Ligands and 2-Phenylimidazole
Abhik Mukhopadhyay[a] and Samudranil Pal*[a]
Keywords: Nickel(II) complexes / Square-planar complexes / Apical C–H···Ni interactions / X-ray structures / Theoretical
calculations
Two square-planar ternary nickel(II) complexes [Ni(bhac)-
(phim)] (1) and [Ni(ahac)(phim)] (2) with O,N,O-donor Schiff
bases [acetylacetone benzoylhydrazone (H2bhac) and acetyl- survey of the reported X-ray structures of similar d8 metal
nality between the planes containing the chelate rings
formed by the tridentate ligand and the imidazole ring. A
acetone acetylhydrazone (H2ahac)] and sp2 N-donor 2-phen-
ylimidazole (phim) as the ancillary ligand have been synthe-
sised and characterised by analytical, spectroscopic, mag-
netic and electrochemical methods. The solid-state structures
of both complexes have been determined by X-ray crystal-
lography. The asymmetric unit of 1 contains a single complex
molecule while that of 2 contains four complex molecules
with different conformations. The molecular structures of
both complexes reveal that one of the two ortho C–H groups
of the pendant phenyl ring of the phim ligand is very close
to the metal centre at the apical site indicating the presence
of an intramolecular C–H···Ni interaction. In the structures of
1 and 2, the shape of the C–H···Ni interaction is determined
by the twisting of the phim ligand and the extent of orthogo-
ion complexes containing intramolecular C–H···M interac-
tions has been performed for comparison with the structural
features of 1 and 2. The optimised molecular structures of 1
and 2 generated by calculations based on the density func-
tional method are compared with the experimental struc-
tures. Calculations also reveal that the four molecules pres-
ent in the asymmetric unit of 2 are very close in energy.
Theoretical studies and the down-field shift of the proton res-
onance on cooling in the NMR spectra suggest three-centre
four-electron hydrogen-bond character of the observed api-
cal C–H···Ni interaction in 1 and 2.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
tron) in character because of the availability of the empty
orbital as well as the lone pair of electrons on the metal
ion. Such apical C–H···M interactions in square-planar d8
systems have been described as weakly agostic bonds, non-
classical hydrogen bonds and also as repulsive because of
the filled metal dz2 and C–H σ-orbitals.[2,12–14] Compared
with the agostic interaction the nonclassical hydrogen bond
is not common in metal complexes. For a metal centre to
participate as the proton acceptor in a hydrogen bond, it is
essential for it to be electron rich. Examples of such com-
plexes occur mostly with d8 and d10 metal centres. It has
been demonstrated that such hydrogen bonds can facilitate
the oxidative addition of H–X to a metal centre.[15]
Herein we report two square-planar nickel(II) complexes
with the tridentate Schiff bases H2bhac and H2ahac and the
monodentate 2-phenylimidazole as the ancillary ligand. The
deprotonated Schiff bases (bhac2– and ahac2–) satisfy the
+2 charge on the metal ion and three coordination sites.
The sp2 N atom of the neutral 2-phenylimidazole (phim)
occupies the fourth coordination site and completes the
N2O2 square plane around the metal ion. The ancillary li-
gand 2-phenylimidazole has been specifically chosen so that
one of the two ortho C–H groups of the phenyl ring can be
in close proximity with the metal ion at the apical site. The
complexes, [Ni(bhac)(phim)] (1) and [Ni(ahac)(phim)] (2),
Introduction
The long standing interest in the interaction of transition
metal ions with proximate C–H fragments is primarily due
to the C–H activation processes involved in many catalytic
reactions. Such interactions have been scrutinised inten-
sively for a better understanding of their structural and
bonding features.[1–12] The C–H···M interaction in coordi-
natively unsaturated and electron-deficient (16-electron)
species is called an agostic interaction or agostic bond
where the filled C–H σ orbital interacts with an empty
metal orbital. In these agostic interactions, generally the C–
H fragment is side-on to the metal ion. This T-shape of the
agostic C–H···M interaction facilitates the involvement of
the filled metal dπ orbital and the empty C–H σ* orbital in
back bonding. However, the nature of the apical C–H···M
interactions in square-planar 16-electron complexes of d8
metal ions is not very clear.[9–12] In such species, these inter-
actions can be agostic bond (three-centre two-electron) as
well as non-classical hydrogen bond (three-centre four-elec-
[a] School of Chemistry, University of Hyderabad,
Hyderabad 500 046, India
Fax: +91-40-2301-2460
E-mail: spsc@uohyd.ernet.in
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2006, 4879–4887
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Mukhopadhyay, S. Pal
FULL PAPER
have been characterised by analytical, magnetic, spectro- the lowest unoccupied molecular orbital (LUMO) is pri-
scopic and electrochemical measurements. The X-ray struc- marily localised on the ancillary ligand phim and the high-
tures of 1 and 2 indeed reveal the presence of an apical est occupied molecular orbital (HOMO) is composed of d
C–H···M interaction in both complex molecules. We have and p orbitals of the metal and coordinated O atoms,
examined the reported structures of the square-planar d8 respectively. The spectra of the free Schiff bases and 2-phen-
systems with similar intramolecular C–H···M interactions ylimidazole in methanol solutions display a single absorp-
available in the Cambridge Structural Database for a com- tion in the range 250–215 nm. Thus the absorptions ob-
parison with the structural features of 1 and 2. Calculations served for 1 and 2 are possibly due to the metal-to-ligand
based on density functional methods have been performed and inter- or intra-ligand charge-transfer transitions.
to understand the molecular conformations and the nature
Electron transfer properties of 1 and 2 have been investi-
of the C–H···Ni interaction observed in the X-ray structures gated by cyclic voltammetry using acetonitrile solutions of
of 1 and 2. Variable temperature NMR spectroscopic mea- the complexes. Both 1 and 2 display an irreversible oxi-
surements have been carried out to probe the nature of this dation response at 0.69 and 0.71 V, respectively. The current
interaction in solution.
height of this response is comparable with known one-elec-
tron redox processes under identical conditions.[19,20] No
such response is observed for the deprotonated Schiff bases
and 2-phenylimidazole under the same conditions. The
iron(III) and copper(II) complexes with bhac2– show metal-
centred oxidation responses in the potential range 0.4–
0.8 V.[21–23] Therefore, the oxidation responses observed for
1 and 2 are assigned to a NiII Ǟ NiIII process. The nature
of the HOMO in both complexes also supports this assign-
ment. The irreversible nature of the response suggests that
in each case the corresponding oxidised species is unstable
on the cyclic voltammetry time scale.
Results and Discussion
Synthesis and Some Properties
Description of X-ray Structures and C–H···Ni Interactions
The reactions of 1 equiv. each of Ni(O2CCH3)2·4H2O,
the Schiff bases (H2bhac and H2ahac) and 2-phenylimid-
azole in boiling methanol afford the dark brown complexes
in moderate to good yields. Elemental analysis data are con-
sistent with the molecular formula [Ni(bhac)(phim)] (1) and
[Ni(ahac)(phim)] (2). Both 1 and 2 are electrically noncon-
ducting in methanol solutions. The complexes are diamag-
netic and NMR spectroscopically active. Thus in each com-
plex, the nickel ion is in the +2 oxidation state and the
coordination geometry around the metal centre is square
planar.
The infrared spectra of 1 and 2 do not display any band
assignable to the amide or secondary amine N–H stretch.[16]
Several sharp weak bands observed in the range 2900–
3150 cm–1 are likely to be due to the C–H stretches. The
absence of any band for the N–H group of the ancillary
ligand phim is consistent with its involvement in strong
intermolecular hydrogen bonding (vide infra). Free H2bhac
and H2ahac display the amide C=O stretch[17] near
Ϸ1675 cm–1. The absence of any such band in the spectra
of 1 and 2 indicates complete deprotonation of the Schiff
bases in the complexes. Thus in both complexes, the dian-
ionic tridentate ligands (bhac2– and ahac2–) act as the enol-
ate-O, the imine-N and the deprotonated amide-O donor.
A strong band observed near 1590 cm–1 might involve the
C=N stretches.[18,19]
The molecular structures of 1 and 2 are depicted in Fig-
ure 1. Bond parameters associated with the metal ions are
listed in Table 1. The asymmetric unit of 1 contains one
complex molecule while that of 2 contains four complex
molecules. In each molecule, the enolate-O, the imine-N
and the deprotonated amide-O donor tridentate ligand and
the imine-N donor monodentate phim form a N2O2 square
plane around the metal centre. There is no deviation of the
metal centre from the N2O2 square plane. The Ni–O(enol-
ate), Ni–N(imine), Ni–O(amide) and Ni–N(imidazole)
bond lengths are comparable with those observed in nickel-
The electronic spectra were collected using methanol
solutions of the complexes. Both complexes display several
absorptions. For 1 the absorption bands are in the range
565–234 nm and those for 2 are within 425–235 nm. The
Figure 1. Molecular structures of (a) [Ni(bhac)(phim)] (1) and
(b) [Ni(ahac)(phim)] (2) with the atom labelling scheme. All non-
hydrogen atoms are represented by their 40% probability thermal
density functional calculations show that in both 1 and 2 ellipsoids.
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Apical C–H···M Interactions in Square-Planar Nickel(II) Complexes
FULL PAPER
(II) complexes having the same coordinating atoms.[24–26] along the C6–C16 bond. The O1–Ni1–N3–C9 torsion angle
The average N–C and C–O bond lengths in the –N=C(O–)– (θ) is 48.9° (Table 2). Thus the chelate forming fragment
fragments[21–26] and the C–C and C–O bond lengths in the (O1,O2,N1,N2,C1–C6) of bhac2– and the metal coordi-
–C=C(O–)– fragments[21–24] are consistent with the enolate nated imidazole ring plane are not orthogonal (Figure 1).
form of both amide and acetylacetone moieties of the tri- The phim moiety is not planar because of the twisting of
dentate ligands.
the imidazole ring plane and the phenyl ring plane along
the C9–C10 bond (Figure 1). The extent of twisting (ψ) is
measured by taking the average of the N3–C9–C10–C15
and N4–C9–C10–C11 torsion angles. The value of ψ is cal-
culated as 35.8°. As a result an ortho C–H group of the
phim phenyl ring is close to the metal centre at the apical
site (Figure 1). The Ni···H and Ni···C distances and the
Ni···H–C angle are 2.79(2), 3.321(3) Å and 116.8(2)°,
respectively (Table 2).
Table 1. Selected bond lengths [Å] and bond angles [°] for 1 and 2.
[Ni(bhac)(phim)] (1)
Ni(1)–O(1)
Ni(1)–N(1)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(3)
O(2)–Ni(1)–N(3)
1.8195(17)
1.8269(18)
175.76(7)
89.07(7)
Ni(1)–O(2)
Ni(1)–N(3)
O(1)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
N(1)–Ni(1)–N(3)
1.8443(16)
1.9136(18)
96.15(7)
84.13(7)
172.37(8)
91.08(7)
[Ni(ahac)(phim)] (2)
The bond lengths and bond angles of the four molecules
present in the asymmetric unit of 2 are very similar
(Table 1). However, the orthogonality (θ) between the che-
late forming fragment of ahac2– and the metal coordinated
imidazole ring plane and the twisting of phim (ψ) differ
significantly. The θ and ψ values are in the ranges 72.9–
91.6° and 1.6–21.6°, respectively (Table 2). Most import-
antly in two molecules the orientation of the phim phenyl
ring with respect to the {Ni(ahac)} plane is on one side and
in the other two molecules it is on the other side (Figure 2).
As in 1, one of the two ortho C–H groups of the phim
phenyl ring is close to the metal centre at the apical site in
each of the four molecules. The Ni···H and Ni···C distances
and the C–H···Ni angles are within 2.42–2.69 Å, 3.20–
3.36 Å, and 130.1–142.2°, respectively (Table 2).
Molecule 1
Ni(1)–O(1)
Ni(1)–N(1)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(3)
O(2)–Ni(1)–N(3)
Molecule 2
Ni(2)–O(3)
Ni(2)–N(5)
O(3)–Ni(2)–O(4)
O(3)–Ni(2)–N(7)
O(4)–Ni(2)–N(7)
Molecule 3
Ni(3)–O(5)
Ni(3)–N(9)
O(5)–Ni(3)–O(6)
O(5)–Ni(3)–N(11)
O(6)–Ni(3)–N(11)
Molecule 4
1.829(5)
1.826(6)
178.3(2)
91.4(2)
Ni(1)–O(2)
Ni(1)–N(3)
O(1)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
N(1)–Ni(1)–N(3)
1.851(5)
1.923(6)
95.6(3)
84.0(2)
173.0(3)
89.0(2)
1.818(5)
1.821(7)
177.8(2)
90.3(3)
Ni(2)–O(4)
Ni(2)–N(7)
O(3)–Ni(2)–N(5)
O(4)–Ni(2)–N(5)
N(5)–Ni(2)–N(7)
1.852(5)
1.909(6)
96.0(3)
83.8(3)
173.6(3)
89.8(3)
1.820(5)
1.820(6)
178.3(3)
89.4(3)
Ni(3)–O(6)
Ni(3)–N(11)
O(5)–Ni(3)–N(9)
O(6)–Ni(3)–N(9)
N(9)–Ni(3)–N(11)
1.855(5)
1.918(6)
95.6(3)
84.3(3)
174.2(3)
90.5(3)
Ni(4)–O(7)
Ni(4)–N(13)
O(7)–Ni(4)–O(8)
O(7)–Ni(4)–N(15)
O(8)–Ni(4)–N(15)
1.832(6)
1.838(6)
179.6(3)
89.4(3)
Ni(4)–O(8)
Ni(4)–N(15)
O(7)–Ni(4)–N(13)
O(8)–Ni(4)–N(13)
N(13)–Ni(4)–N(15)
1.853(5)
1.928(7)
96.0(3)
83.7(3)
173.5(3)
91.0(3)
In 1, the tridentate ligand bhac2– is not planar because
of the twisting of the phenyl ring plane along the C6–C16
bond (Figure 1). The dihedral angle between the phenyl
ring plane and the plane constituted by the rest of the
atoms in bhac2– is 25.56(8)°. It is also interesting to note
that one of the two ortho C–H groups of the phenyl ring of
bhac2– is involved in a C–H···π interaction with the phenyl
ring of the ancillary ligand phim (Figure 1). The H···Cg dis-
tance and the C–H···Cg angle are 2.962 Å and 153°, respec-
tively. Possibly this C–H···π interaction is primarily respon-
Figure 2. Overlay diagrams of (a) the four molecules present in the
asymmetric unit of [Ni(ahac)(phim)] (2), and (b) the molecule of
[Ni(bhac)(phim)] (1) and the four molecules of [Ni(ahac)(phim)]
sible for the twisting of the phenyl ring plane of bhac2– (2).
Table 2. Structural parameters related to C–H···Ni interactions and molecular conformations of 1 and 2.
Parameters
[Ni(bhac)(phim)] (1)
Experimental Calculated
[Ni(ahac)(phim)] (2)
Experimental
Calculated
Molecule 1
Molecule 2
Molecule 3 Molecule 4
Ni···H distance [Å]
Ni···C distance [Å]
C–H···Ni angle [°]
ψ[a] [°]
2.79(2)
3.321(3)
116.8(2)
35.8
2.48
3.26
125
28
2.54
2.42
2.47
2.69
2.51
3.23
127
27
3.288(9)
137.7
12.9
3.207(9)
142.2
1.6
3.236(12)
139.3
11.3
3.364(11)
130.1
21.6
θ[b] [°]
48.9
56
73.1
91.6
76.4
72.9
58
[a] Average of N3–C9–C10–C15 and N4–C9–C10–C11 torsion angles. [b] O1–Ni–N3–C9 torsion angle.
Eur. J. Inorg. Chem. 2006, 4879–4887
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4881

A. Mukhopadhyay, S. Pal
FULL PAPER
The following trends are very clear from the above obser- ion complexes are mainly repulsive in nature.[10] But a
vations on the molecular structures of 1 and 2. In 1, only minor attractive contribution to the C–H···M interaction
one complex molecule is present in the asymmetric unit and can not be ruled out in the present complexes. The Ni···H
both the phenyl ring and the imidazole ring planes of phim distances in both complexes and shorter Ni···H distances in
are tilted toward the phenyl ring of the benzoyl fragment 2 compared with that in 1, unaided and unhindered by any
of bhac2– (Figure 1). In spite of being weak in nature we geometrical constraint in the former, substantiate this idea.
believe that the intramolecular C–H···π interaction plays a
major role in restricting the phim moiety from taking ran- Intermolecular Hydrogen Bonds and Self-Assembly of 1
dom orientations and a single conformation of it is stabi- and 2
lized in 1. Because of the replacement of the phenyl group
Both complex molecules contain the imidazole N–H
of bhac2– by a methyl group in ahac2– the C–H···π interac-
group and metal coordinated O atoms, which are conven-
tion of the type observed in 1 is not possible in 2. Complex
tional hydrogen-bond donors and acceptors, respectively. In
2 crystallises with four molecules in the asymmetric unit
the crystal lattice, the molecules of each of the two com-
and the phim moieties are randomly oriented on both sides
plexes are involved in intermolecular N–H···O hydrogen
of the plane containing the {Ni(ahac)} moiety. Thus the lack
bonding interactions involving the imidazole N–H groups
of the C–H···π interaction facilitates free rotation of phim
and the metal coordinated amide O atoms of the tridentate
about the Ni–N(imidazole) bond. In addition, the imid-
ligands. In the case of 1, the N···O distance and the N–
azole plane in 2 is closer to the orthogonal arrangement
H···O angle are 2.856(3) Å and 160(2)°, respectively. There
with the plane containing the two chelate rings (θ = 72.9–
are some variations in the geometrical parameters related
91.6°) compared with that in 1 (θ = 48.9°). This difference
to the N–H···O interactions for the four molecules present
is also likely to be because of the C–H···π interaction in 1.
in the asymmetric unit of 2. The N···O distances are in the
The internal twisting of phim in 1 (ψ = 35.8°) is signifi-
range 2.788(8)–2.846(9) Å and the N–H···O angles are
cantly larger than that in 2 (ψ = 1.6–21.6°). Possibly this
within 148–163°. In each case, self-assembly of the complex
difference is also partly due to the intramolecular C–H···π
molecules through these intermolecular N–H···O hydrogen
interaction in 1. Despite the differences in the θ and ψ val-
bonds leads to a one-dimensional supramolecular structure
ues one of the two ortho C–H groups of the phim phenyl
in the crystal lattice (Figure 3).
ring is near to the apical position of the metal ion in all the
structures. In this context, it may be noted that the crystal
structure of the protonated 2-phenylimidazole (Hphim+) is
known.[27] Here the ψ value is 22.32°. Thus in all prob-
ability significantly different twisting of phim in 1 and 2
compared with that in Hphim+ is due to the intramolecular
C–H···π and C–H···Ni interactions observed in the present
complex molecules.
A close scrutiny of the structural parameters (Table 2)
related to the C–H···Ni interactions and the molecular con-
formations of 1 and 2 reveals the following. The longer the
Ni···H distance the shorter the C–H···Ni angle. There is a
satisfactory linear correlation between the Ni···H distance
and the C–H···Ni angle (Figure S1, Supporting Infor-
mation). The values of the Ni···H distances and the corre-
sponding C–H···Ni angles strongly depend on the molecu-
lar conformations. In general, the Ni···H distance increases
and the C–H···Ni angle decreases with the increase of the Figure 3. One-dimensional ordering of (a) [Ni(bhac)(phim)] (1) and
(b) [Ni(ahac)(phim)] (2) through intermolecular N–H···O hydrogen
bonds.
twisting (ψ) of phim and the decrease of the orthogonality
(θ) between the imidazole ring plane and the plane contain-
ing the two chelate forming fragments of the tridentate li-
gands (Table 2). The Ni···H distance is linearly related to
Survey of the d8 Metal Ion Complexes with Intramolecular
the ψ values as well as θ values with opposite slopes (Fig-
C–H···M Interactions
ure S1). Thus a large twist of the phim ligand forces a T-
shape of the C–H···Ni interaction with an increase in the
The molecular structures of 1 and 2 show that the Ni···H
Ni···H distance, while a small twist of the phim ligand distance, the C–H···Ni angle and hence the shape (T or lin-
makes the C–H···Ni interaction more linear with a decrease ear) of the C–H···Ni interaction largely depend on the twist-
in the Ni···H distance. It is clear from the above facts that ing (ψ) of 2-phenylimidazole (vide supra). The Ni···H dis-
the geometrical arrangement of the C–H···Ni interaction tance is linearly related with ψ and the C–H···Ni angle (Fig-
largely depends on the twist of the phim ligand in 1 and 2. ure S1). To verify whether the trends observed for 1 and 2
A detailed theoretical study reported previously has sup- are general or not we have performed a Cambridge Struc-
ported the idea that the axial M···H interactions in d8 metal tural Database (version 5.26) search for d8 metal ion com-
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Eur. J. Inorg. Chem. 2006, 4879–4887

Apical C–H···M Interactions in Square-Planar Nickel(II) Complexes
FULL PAPER
plexes having intramolecular C–H···M interactions. The se- cluded. The number of hits obtained was 44. Selected struc-
arch was based on the following criteria: (i) the metal ion is tural parameters of these 44 X-ray structures are listed in
always coordinated to a N atom that is bonded to the atom Table 3.
X (elements of groups 14–16), (ii) X is connected to an un-
substituted/substituted phenyl ring by a single bond,
(iii) the phenyl ring ortho C–H at the δ position is involved
in the C–H···M interaction, (iv) the M···H separation is in
the range 2.0–3.0 Å, (v) the Nα–Xβ–Cγ angle is within 110–
140°, (vi) the Nα–Xβ–Cγ–Cδ torsion angle (ψ) is in the range
In the structures collected in Table 3, the mean M···H
0–180° and (vii) structures with R factors Ͼ 10 are ex-
distance increases in the order NiII Ͻ PdII Ͻ PtII. These
values are 2.795, 2.841 and 2.932 Å for NiII, PdII and PtII,
respectively. This order is expected as the van der Waals
Table 3. Structural data for the intramolecular C–H···M interaction
radius increases in the order NiII Ͻ PdII Ͻ PtII.[10] In con-
in d8 metal ion complexes.
trast to the structures of 1 and 2, there is no readily appar-
Refcode
MII M···H dist. [Å] C–H···M angle [°] ψ [°]
ent relationship between the M···H distance and the Nα–
Xβ–Cγ–Cδ torsion angle (ψ). However, the scattergram of ψ
against the M···H distance (Figure 4a) shows the prevailing
trend of long M···H distances for large ψ values. On the
other hand, the M···H distance generally increases with the
decrease of the C–H···M angle (Figure 4b) as observed for
1 and 2. In other words, the C–H···M interaction becomes
more T-shaped with the increase of the M···H distance.
COPGET
DAWGUD
ERURAK
ERUREO
FAKVIW
NiII 2.668
NiII 2.885
NiII 2.885
NiII 2.733
NiII 2.894
125.102
102.297
94.712
109.016
93.133
107.358
136.801
129.004
128.477
101.781
90.816
123.717,
99.198
105.555
140.411
90.055
114.987
90.748
90.750
111.106
108.425
97.906
119.125
117.971
125.092,
115.835
112.824
102.092
99.833
107.519
115.268
100.278,
96.338
102.679
132.141
108.260
106.715
119.920,
128.876
108.391
133.835
122.982
110.515,
114.306,
122.513
107.154
117.149
101.858
90.958
33.573
85.365
87.400
77.854
85.339
88.293
12.763
36.500
55.681
88.010
86.319
31.403,
31.403
77.495
14.618
86.797
75.856
37.947
37.948
77.368
82.605
90.369
73.137
70.944
15.776,
1.934
15.596
43.831
45.921
31.598
16.080
85.396,
84.995
79.360
54.848
79.553
88.674
69.726,
62.872
57.224
54.013
68.115
14.014,
10.811,
3.029
GIGVEX10 NiII 2.859
IFAFIE
NiII 2.476
NiII 2.792
NiII 2.810
NiII 2.919
NiII 2.833
NiII 2.607,
2.972
KIYQUE
KOHHAQ
LOKZOA
LUQJOW
MIYQEQ
NOKVOY
QOZROM
RORHUB
SENHIC
SOSVAX
NiII 2.663
NiII 2.353
NiII 2.869
NiII 2.874
NiII 2.786
SOSVAX10 NiII 2.786
TISPEQ
TISPIU
VETPEP
BABDIS
BECGEV
CAPVOE
NiII 2.844
NiII 2.967
NiII 2.979
PdII 2.703
PdII 2.949
PdII 2.585,
2.732
DAGGUN
DUHYUA
PdII 2.946
PdII 2.783
DUNWAK PdII 2.944
EFODEI
HAPNIV
HOZNUF
PdII 2.884
PdII 2.926
PdII 2.880,
2.836
Figure 4. Scattergrams of (a) ψ (see text for definition) vs. M···H
distance and (b) C–H···M angle vs. M···H distance. The straight
line in (b) represents the least-squares fit.
JEDDIF
KIMXOT
LANBIL
LIJHUH
NINMAY
PdII 2.865
PdII 2.871
PdII 2.968
PdII 2.946
PdII 2.933,
2.744
Computational Results
Calculations based on density functional methods have
been performed for structural optimisations of 1 and 2.
Each of the two complex molecules was computed as a
complete system to consider all steric and electronic factors
of the tridentate ligand (bhac2– or ahac2–) and of the mono-
dentate ancillary ligand 2-phenylimidazole (phim). In both
cases, the atomic coordinates of the molecules obtained in
the crystal structures were used as the starting points and
references for geometry optimisation. The calculated struc-
tural parameters related to the C–H···Ni interaction and the
molecular conformations are listed in Table 2.
NOKGUP
SUGWEW
TURKIA
UGIKOK
PdII 2.823
PdII 2.596
PdII 2.679
PdII 2.983,
2.953,
2.843
XODPEL
YIYHIX
ZODQOY
PdII 2.815
PdII 2.833
PdII 2.868
48.564
72.866
86.739
68.292
12.480,
33.951
HAPZPT10 PtII 2.966
HIXJUT
PtII 2.854,
2.976
125.650,
131.625
Eur. J. Inorg. Chem. 2006, 4879–4887
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4883

A. Mukhopadhyay, S. Pal
FULL PAPER
In the case of 1, the overall conformation of the molecule
in the optimised structure is comparable with that found in
the solid-state X-ray structure. With regard to the molecu-
lar conformation the only major difference between the op-
timised and the experimental structures is in the dihedral
angle between the phenyl ring plane and the plane contain-
ing the rest of the atoms of the bhac2– ligand. In the op-
timised structure the whole tridentate ligand is essentially
planar. It may be noted that the X-ray structure of 1 shows
an intramolecular C–H···π interaction between the phenyl
ring of bhac2– and that of phim (vide supra). It is very likely
that this interaction is responsible for the nonplanarity of
bhac2– in the experimental structure. For the present level
of calculation we could not reproduce the C–H···π interac-
tion in the optimised geometry. The calculated geometrical
parameters indicate a more linear C–H···Ni interaction
compared with that in the experimental structure (Table 2).
The same optimised molecular structure is obtained from
the X-ray structural coordinates of the four molecules pres-
ent in the asymmetric unit of 2. The optimised molecule of
2 is very similar to that of 1 with regard to the Ni···H dis-
tance, C–H···Ni angle and ψ and θ values (Table 2). How-
ever, this optimised structure is rather different when com-
pared with the structures of the four molecules obtained in
the X-ray structure of 2. In general, the twisting of phim
(ψ) is much less, the imidazole ring plane and the plane
containing the two chelate rings are more orthogonal (θ)
and the Ni···H–C interaction is more linear in the experi-
mental structures than those in the optimised structure
(Table 2).
Figure 5. Relative energies of [Ni(bhac)(phim)] (1) (–) and
[Ni(ahac)(phim)] (2) (----) as a function of twist angles (ψ) of 2-
phenylimidazole (phim).
it can also accept electrons from the filled C–H σ orbital.
The delocalisation energies for the C–H σ donation to the
metal calculated through natural bond orbital analysis[28]
are 0.58 and 0.63 kcalmol–1 for 1 and 2, respectively. These
delocalisation energies are insignificant compared with
those obtained for strong agostic interactions and are com-
parable with C–H···M hydrogen-bonding interactions.[29,30]
The "atoms in molecules" theory[31] has also been used for
further probing of the topological properties of the C–
H···Ni interaction in 1 and 2. The values obtained for the
electron densities (ρb = 0.011 and 0.012 a.u. for 1 and 2,
respectively) and the Laplacians (ٌ2ρb = 0.039 and
0.041 a.u. for 1 and 2, respectively) at the bond critical
points are well within the range reported for C–H···M inter-
actions that are hydrogen bonds in character.[30,32–35]
The dependence of the conformational energies of 1 and
2 on the twisting (ψ) of phim has been analysed by per-
forming single point energy calculations by varying ψ from
0 to 90°. The experimental structures of 1 and molecule 1
of 2 are used for these calculations. The relative energies
(∆E) with respect to the lowest energy (at ψ Ϸ 35° for 1
Proton NMR Spectroscopic Studies
NMR spectroscopy is a useful tool for the diagnosis of
and ψ Ϸ 15° for 2) are plotted against the ψ values (Fig- the agostic or hydrogen-bond character of C–H···M interac-
ure 5). For 1 it is a symmetric well-like plot. On the other tions in square-planar d8 metal ion complexes. The proton
hand, for 2 below 20° the change in energy is very low com- resonance shifts to an up-field position for agostic interac-
pared with that above 20°. The energy increases by only tions while it shifts down-field for hydrogen-bond interac-
0.20 kcalmol–1 due to the gradual decrease of ψ from 20 to tions compared with that of the free C–H group.[9,11,36] The
0°. The small increase of energy indicates the absence of any room temperature proton NMR spectra were recorded
significant steric or electronic constraint for the twisting of using (CD3)2CO solutions of 1 and 2. The protons of the
the phim in this range of ψ. For this reason, it is very likely two methyl groups of the acetylacetone fragment in 1 ap-
that four molecules with ψ values in the range 1.6–21.6° pear as two singlets at δ = 1.47 and 2.20 ppm. The singlet
have been found in the asymmetric unit of 2.
observed at δ = 5.07 ppm is assigned to the –CH= group
The nature of the C–H···Ni interaction in 1 and 2 has proton. The imidazole N-H proton resonates as a doublet
been examined by several computational methods. In a at δ = 8.87 ppm (J = 8 Hz). The multiplet observed in the
three-centre four-electron C–H···M interaction, the inter- range δ = 7.2–7.6 ppm is likely to be due to the imidazole
acting hydrogen atom should have a more positive charge ring C-H and aromatic protons. The absence of any cross-
compared with the other hydrogen atoms that are away coupled peak in the 2D NMR spectrum of 1 indicates that
from the metal centre. The natural population analysis[28] the C–H···π interaction is probably absent in the solution
shows that the charge on the ortho-hydrogen atom of the state. In addition to all the above signals a broad singlet is
phim phenyl ring, which is at the apical site of the metal observed at δ = 12.07 ppm. This down-field signal is attrib-
centre, is more positive (by +0.009e and +0.010e for 1 and uted to the ortho-C-H proton of the phim phenyl ring,
2, respectively) compared with the charge on the other or- which is proximal to the metal centre. It may be noted that
tho-hydrogen atom of the same phenyl ring. The nickel(II) the free phim or the Schiff bases do not show any resonance
centre has an empty d orbital as well as a lone pair. Thus in this region. The protons of the three methyl groups of
4884
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4879–4887

Apical C–H···M Interactions in Square-Planar Nickel(II) Complexes
FULL PAPER
ahac2– in 2 are observed as three singlets at δ = 2.01, 2.11 C–H···Ni interaction depends on the twisting (ψ) of the
and 2.27 ppm. The proton of the –CH= group resonates as phim and the extent of the orthogonality (θ) between the
a singlet at δ = 5.51 ppm. A broad singlet at δ = 8.89 ppm plane containing the chelate rings and the imidazole plane.
is assigned to the imidazole N-H proton. As observed for 1 The observed trends are compared with the structures of
the signal from the phim phenyl ring ortho C-H proton, similar species reported in the literature. Theoretically op-
close to the apical site of the metal centre, appears down- timised structures of both 1 and 2 are very similar. Energy
field (δ = 11.63 ppm) as a broad singlet. The multiplet in the calculations were performed by varying ψ from 0 to 90° for
range δ = 7.1–7.8 ppm corresponds to the imidazole ring C- both 1 and 2. The conformer of 1 having a ψ value of Ϸ35°
H and the rest of the aromatic protons in the molecule.
(experimental ψ = 35.8°) is at the lowest energy and the
We have recorded the NMR spectra of both complexes plot of the relative energies (∆E) of various conformers
in the temperature range 20 to –80 °C and monitored the against ψ provides a reasonably symmetric well-type curve.
broad singlet observed at δ = 12.07 and 11.63 ppm for 1 For 2 the lowest energy conformer has a ψ value of Ϸ20°.
and 2, respectively. In each case, the signal becomes sharper However, the energy change is very little below ψ = 20°.
and is shifted further down-field on cooling (Figure 6). The This observation explains the presence of four molecules
shifts are 0.75 and 0.90 ppm for 1 and 2, respectively. The having ψ in the range 1.6–21.6° in the asymmetric unit of
observation of the down-field signal and its behaviour on 2. Theoretical investigations suggest that a hydrogen-bond
lowering the temperature suggest that the C–H···Ni interac- description of the C–H···Ni interaction in both 1 and 2 is
tion present in the solid-state structures of both 1 and 2 is more appropriate. In the NMR spectra of 1 and 2, the ap-
also present in solution and it is essentially hydrogen bond pearance of this interacting proton at down-field and fur-
in character.
ther down-field shifts, because of the lowering of the tem-
perature, substantiates the hydrogen-bond character of this
interaction.
Experimental Section
Materials: The Schiff bases H2bhac and H2ahac were prepared by
condensation reactions of acetylacetone with benzoylhydrazine and
acetylhydrazine, respectively.[21] All other chemicals and solvents
used in this work were of analytical grade, available commercially
and were used without further purification.
Physical Measurements: Microanalytical (C, H, N) data were ob-
tained with a Thermo Finnigon Flash EA1112 series elemental
analyser. Infrared spectra were collected by using KBr pellets with
a Jasco-5300 FT-IR spectrophotometer. A Shimadzu 3101-PC UV/
Vis/NIR spectrophotometer was used to record the electronic spec-
tra. The proton NMR spectra were recorded with a Bruker
400 MHz spectrometer. Solution electrical conductivities were mea-
sured with a Digisun DI-909 conductivity meter. A Sherwood Sci-
entific balance was used for magnetic susceptibility measurements.
A CH-Instruments model 620A electrochemical analyser was used
for cyclic voltammetric experiments with acetonitrile solutions of
the complexes containing tetrabutylammonium perchlorate
(TBAP) as the supporting electrolyte. The three electrode measure-
ments were carried out at 298 K under a dinitrogen atmosphere
with a platinum disc working electrode, a platinum wire auxiliary
electrode and an Ag/AgCl reference electrode.
Figure 6. Temperature dependence of the down-field signal in the
proton NMR spectrum of [Ni(bhac)(phim)] (1).
Conclusions
The tridentate O,N,O-donor benzoyl- and acetylhydra-
zone of acetylacetone (H2bhac and H2ahac) and mono-
dentate N-donor 2-phenylimidazole (phim) yielded square-
Synthesis of the Complexes
planar nickel(II) complexes [Ni(bhac)(phim)] (1) and [Ni- [Ni(bhac)(phim)] (1): A dry methanol solution (15 cm3) of Ni-
(O2CCH3)2·4H2O (125 mg, 0.5 mmol) was added to a dry methanol
solution (10 cm3) of H2bhac (110 mg, 0.5 mmol) and 2-phenylimid-
azole (72 mg, 0.5 mmol). The resulting deep brown mixture was
kept under reflux for 2 h and then evaporated on a steam bath to
approximately half of the original volume. The brown needles that
separated after cooling to room temperature were collected by fil-
tration, washed with a little ice-cold methanol and finally dried in
air. The yield obtained was 160 mg (76%). A single crystal suitable
for X-ray structure determination was selected from this material.
NiC21H20N4O2 (419.12): calcd. C 60.18, H 4.81, N 13.37; found C
(ahac)(phim)] (2). X-ray crystal structures reveal that the
asymmetric unit of 1 contains a single molecule while that
of 2 contains four molecules with different conformations.
The conformation of 1 is significantly different compared
with that of any of the four molecules of 2. Perhaps the
intramolecular C–H···π interaction present in 1 is one of
the factors for this difference. In each of 1 and 2, one of
the two ortho hydrogen atoms of the phim phenyl ring is
very close to the metal centre at the apical site suggesting
an intramolecular C–H···Ni interaction. The shape of the 59.95, H 4.78, N 13.24. Electronic spectroscopic data in CH3OH:
Eur. J. Inorg. Chem. 2006, 4879–4887
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4885

A. Mukhopadhyay, S. Pal
FULL PAPER
λmax (ε) = 565 sh (94), 380 (14200), 366 sh (12700), 267 (19800),
234 sh (21100) nm.
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[Ni(ahac)(phim)] (2): A dry methanol solution (15 cm3) of Ni-
(O2CCH3)2·4H2O (125 mg, 0.5 mmol) was added to a dry methanol
solution (10 cm3) of H2ahac (70 mg, 0.5 mmol) and 2-phenylimid-
azole (72 mg, 0.5 mmol), and the mixture was kept under reflux for
2 h. A brown crystalline material was deposited on the wall of the
round-bottomed flask along the surface of the solvent and formed
a ring. The reaction mixture was cooled to room temperature and
the almost colourless and clear mother liquor was removed care-
fully by using a dropper. The crystalline material was collected after
drying in air. The yield obtained was 110 mg (62%). A single crys-
tal suitable for X-ray structure determination was selected from this
material. NiC16H18N4O2 (357.05): C 53.82, H 5.08, N 15.69; found
C 53.54, H 4.86, N 15.47. Electronic spectroscopic data in CH3OH:
λmax (ε) = 425 sh (370), 358 sh (3500), 344 (4700), 330 sh (4400),
267 (18400), 235 sh (18600) nm.
Computational Methods: DFT calculations for 1 and 2 were per-
formed at the B3LYP/6-311G(d,p) level.[43–45] The starting points
of the geometry optimisations were the X-ray structural coordi-
nates of the single molecule of 1 and of the four independent mole-
cules of 2 found in the corresponding asymmetric units. The
Gaussian 03[46] suite of programmes was used for all calculations.
Supporting Information (see footnote on the first page of this arti-
cle): Plots of C–H···Ni angles, ψ and θ (see text for definitions)
against the Ni···H distances for 1 and 2 (Figure S1).
Acknowledgments
Financial support for this work was provided by the Council of
Scientific and Industrial Research (CSIR) [Grant No. 01(1880)/03/
EMR-II]. A. Mukhopadhyay thanks the CSIR for a research fel-
lowship. We thank B. Pathak and T. S. Thakur for helpful dis-
cussions on the theoretical calculations. All calculations were done
using the computational facility at the Centre for Modelling, Simu-
lation and Design, University of Hyderabad. X-ray crystallographic
studies were performed at the National Single Crystal Dif-
fractometer Facility, School of Chemistry, University of Hyderabad
(funded by the Department of Science and Technology). We thank
the University Grants Commission for the facilities provided under
the UPE and CAS programmes.
X-ray Crystallography: Complexes 1 and 2 crystallise in the space
groups C2/c and P21/c, respectively. Unit cell parameters and the
intensity data were obtained with a Bruker-Nonius SMART APEX
CCD single-crystal diffractometer, equipped with a graphite mono-
chromator and a Mo-Kα fine-focus sealed tube (λ = 0.71073 Å)
operated at 2.0 kW. The detector was placed at a distance of 6.0 cm
from the crystal. Data were collected at 298 K with a scan width
of 0.3° in ω and an exposure time of 30 sec/frame. The SMART
software was used for data acquisition and the SAINT-Plus soft-
ware was used for data extraction.[37] In each case, an absorption
correction was performed with the help of the SADABS pro-
gramme.[38] The structures were solved by direct methods and re-
fined on F2 by full-matrix least-squares procedures. In both struc-
tures, all non-hydrogen atoms were refined with anisotropic ther-
mal parameters. Hydrogen atoms were added at idealised positions
by using a riding model. For 1 the hydrogen atoms were refined
isotropically while for 2 they were not refined. The SHELX-97 pro-
grammes[39] of the WinGX package[40] were used for structure solu-
tion and refinement. The ORTEX6a[41] and Platon[42] packages
were used for molecular graphics. Significant crystallographic data
for 1 and 2 are summarised in Table 4.
[1] M. Brookhart, M. L. H. Green, J. Organomet. Chem. 1983,
250, 395.
[2] J.-Y. Saillard, R. J. Hoffman, J. Am. Chem. Soc. 1984, 106,
2006.
[3] M. Brookhart, M. L. H. Green, L. L. Wong, Prog. Inorg.
Chem. 1988, 36, 1.
[4] R. H. Crabtree, D. G. Hamilton, Adv. Organomet. Chem. 1988,
28, 299.
[5] A. D. Ryabov, Chem. Rev. 1990, 90, 403.
[6] R. H. Crabtree, Angew. Chem. Int. Ed. Engl. 1993, 32, 789.
[7] A. J. Canty, G. van Koten, Acc. Chem. Res. 1995, 28, 406.
[8] D. Braga, F. Grepioni, E. Tedesco, K. Biradha, G. R. Desiraju,
Organometallics 1997, 16, 1846.
Table 4. Selected crystallographic data for 1 and 2.
[9] W. Yao, O. Eisenstein, R. H. Crabtree, Inorg. Chim. Acta 1997,
Complex
[Ni(bhac)(phim)] (1) [Ni(ahac)(phim)] (2)
254, 105.
Empirical formula
Formula mass [gmol–1
Crystal system
Space group
a [Å]
C21H20N4O2Ni
419.12
monoclinic
C2/c
43.532(3)
11.8872(9)
7.6363(6)
99.3510(10)
3899.1(5)
8
C16H18N4O2Ni
357.05
monoclinic
P21/c
24.450(2)
8.5337(7)
33.518(3)
108.985(2)
6613.2(9)
16
[10] T. W. Hambley, Inorg. Chem. 1998, 37, 3767.
[11] M. J. Calhorda, Chem. Commun. 2000, 801.
[12] L. Brammer, Dalton Trans. 2003, 3145.
[13] A. Albinati, P. S. Pregosin, F. Wombacher, Inorg. Chem. 1990,
29, 1812.
[14] M. Bortolin, U. E. Bucher, H. Rüegger, L. M. Venanzi, A. Al-
binati, F. Lianza, S. Trofimenko, Organometallics 1992, 11,
2514.
]
b [Å]
c [Å]
β [°]
V [Å–3
]
[15] T. Hascall, M.-H. Baik, B. M. Bridgewater, J. H. Shin, D. G.
Churchill, R. A. Friesner, G. Parkin, Chem. Commun. 2002,
2644.
Z
µ [mm–1
]
1.019
1.188
Reflections collected
Reflections unique
Reflections [IՆ2σ(I)]
Parameters
19894
3853
3013
333
49271
8643
4844
841
[16] W. Kemp, Organic Spectroscopy, Macmillan, Hampshire, 1987,
pp. 62–66.
[17] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley, New York, 1986, pp. 241–242.
[18] S. Das, S. Pal, J. Organomet. Chem. 2004, 689, 352.
[19] S. G. Sreerama, S. Pal, Inorg. Chem. 2005, 44, 6299.
[20] S. G. Sreerama, S. Pal, Eur. J. Inorg. Chem. 2004, 4718.
[21] N. R. Sangeetha, C. K. Pal, P. Ghosh, S. Pal, J. Chem. Soc.,
Dalton Trans. 1996, 3293.
R1, wR2 [IՆ2σ(I)]
R1, wR2 (all data)
GOF on F2
0.0361, 0.0889
0.0488, 0.0970
0.913
0.0713, 0.1155
0.1408, 0.1375
1.025
Largest peak, hole [eÅ–3
]
0.388, –0.201
0.400, –0.332
[22] S. Das, G. P. Muthukumaragopal, S. N. Pal, S. Pal, New J.
Chem. 2003, 27, 1102.
[23] S. Das, S. Pal, J. Mol. Struct. 2005, 741, 183.
CCDC-294281 and CCDC-294282 contain the supplementary
crystallographic data for 1 and 2, respectively. These data can be
4886
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4879–4887

Apical C–H···M Interactions in Square-Planar Nickel(II) Complexes
FULL PAPER
[24] A. Mukhopadhyay, G. Padmaja, S. N. Pal, S. Pal, Inorg. Chem.
Commun. 2003, 6, 381.
[36]
[37]
A. J. Canty, G. van Koten, Acc. Chem. Res. 1995, 28, 406.
SMART V5.630 and SAINT-plus V6.45, Bruker-Nonius Ana-
lytical X-ray Systems Inc., Madison, WI, USA, 2003.
G. M. Sheldrick, SADABS Program for area detector absorp-
tion correction, University of Göttingen, Göttingen, Germany,
1997.
G. M. Sheldrick, SHELX-97 Structure Determination Software,
University of Göttingen, Göttingen, Germany, 1997.
[25] A. Mukhopadhyay, S. Pal, Polyhedron 2004, 23, 1997.
[26] G. V. Karunakar, N. R. Sangeetha, V. Susila, S. Pal, J. Coord.
Chem. 2000, 50, 51.
[38]
[39]
[27] D. R. Trivedi, A. Ballabh, P. Dastidar, CrystEngComm 2003, 5,
358.
[28] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88,
899.
[29] R. B. Cracknell, A. G. Orpen, J. L. Spencer, J. Chem. Soc.,
Chem. Commun. 1984, 326.
[40]
[41]
L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837.
P. McArdle, J. Appl. Crystallogr. 1995, 28, 65.
[30] T. S. Thakur, G. R. Desiraju, Chem. Commun. 2006, 552.
[31] R. F. W. Bader, Atom in Molecules, A Quantum Theory, The
Clarendon Press, Oxford, 1990.
[32] U. Koch, P. L. A. Popelier, J. Phys. Chem. 1995, 99, 9747.
[33] P. L. A. Popelier, G. Logothetis, J. Organomet. Chem. 1998,
555, 101.
[34] S. Pillet, G. Wu, V. Kulsomphob, B. G. Harvey, R. D. Ernst, P.
Coppens, J. Am. Chem. Soc. 2003, 125, 1937.
[35] C.-S. Liu, P.-Q. Chen, E.-C. Yang, J.-L. Tian, X.-H. Bu, Z.-M.
Li, H.-W. Sun, Z. Lin, Inorg. Chem. 2006, ASAP article.
[42] A. L. Spek, PLATON A Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 2002.
[43] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[44] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
[45] T. H. Dunning Jr, P. J. Hay, Modern Theoretical Chemistry
(Ed.: H. F. Schaefer III), Plenum, New York, 1976, pp. 1–28.
[46] M. J. Frisch et al., Gaussian 03 Revision B05, Gaussian Inc.,
Pittsburgh, PA, 2004.
Received: June 24, 2006
Published Online: October 10, 2006
Eur. J. Inorg. Chem. 2006, 4879–4887
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4887