Probing the 'Venus fly-trap' parameters of cyclo-octadiene in selected β-diketonato complexes of platinum(II) and the nickel-triad from a spectroscopic, X-ray crystallographic and DFT study

Article abstract of DOI:10.1016/j.poly.2012.10.043

Synthetic, structural and theoretical studies of Pt(II) complexes of a series of β-diketonato ligands with a variation of sterics, namely acetylacetonato (acac) and thenyoltrifluoroactonato (thtfac), are reported. The crystal structures of [Pt(cod)(acac)]PF₆(I), [Pt(cod)(acac)]BF ₄(II) and [Pt(cod)(thtfac)]BF₄(III) are presented while the complexes were characterized spectroscopically by IR, ¹H and ¹³C NMR spectroscopy. DFT calculations of the group 10 transition metals in the nickel triad, utilising the model complex, [M(cod)(acac)] ⁺, to gain further insights in the variation of the transition metal within the complex and the interactions with both the β-diketonato and cis,cis-1,5-cyclo-octadiene (cod) co-ordinating ligands are reported. In particular, the variation in the 'Venus fly-trap' bite and jaw dihedral angles, χ and ψ, is ca. 2° and 4°, respectively, and therefore quite small, but significant. The HOMO and LUMO energy gaps between the Ni(II) and Pd(II) complexes were insignificant, while that of Pt(II) is >1 eV higher than the corresponding 3rd and 4th row congener complexes.

Full text of DOI:10.1016/j.poly.2012.10.043

Polyhedron 50 (2013) 82–89
Contents lists available at SciVerse ScienceDirect
Polyhedron
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Probing the ‘Venus fly-trap’ parameters of cyclo-octadiene in selected
b-diketonato complexes of platinum(II) and the nickel-triad from a spectroscopic,
X-ray crystallographic and DFT study
⇑
T.N. Hill, A. Roodt , G. Steyl
Department of Chemistry and Strategic Academic Cluster: Materials and Nanosciences, University of the Free State, Bloemfontein 9300, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthetic, structural and theoretical studies of Pt(II) complexes of a series of b-diketonato ligands with a
variation of sterics, namely acetylacetonato (acac) and thenyoltrifluoroactonato (thtfac), are reported.
The crystal structures of [Pt(cod)(acac)]PF6(I), [Pt(cod)(acac)]BF4(II) and [Pt(cod)(thtfac)]BF4(III) are pre-
sented while the complexes were characterized spectroscopically by IR, 1H and 13C NMR spectroscopy.
DFT calculations of the group 10 transition metals in the nickel triad, utilising the model complex,
[M(cod)(acac)]+, to gain further insights in the variation of the transition metal within the complex
and the interactions with both the b-diketonato and cis,cis-1,5-cyclo-octadiene (cod) co-ordinating
Received 1 September 2012
Accepted 29 October 2012
Available online 5 November 2012
Keywords:
b-Diketonato
Platinum(II)
Crystallography
DFT
ligands are reported. In particular, the variation in the ‘Venus fly-trap’ bite and jaw dihedral angles,
v
and , is ca. 2° and 4°, respectively, and therefore quite small, but significant. The HOMO and LUMO
w
energy gaps between the Ni(II) and Pd(II) complexes were insignificant, while that of Pt(II) is >1 eV
higher than the corresponding 3rd and 4th row congener complexes.
Ó 2012 Elsevier Ltd. All rights reserved.
Spectroscopy
Cyclooctadiene
Crystal structures
Venus fly-trap angles
1. Introduction
For these reasons we have focused our investigation to well
known b-diketonate ligands which were commercially available
Diolefins act as ligands for a large number of metal centres and
forms an important range of complexes for the study of -bonding
(acetylacetone (Hacac) and thenoyltrifluoroacetone (Hthtfac)). In
this paper we present the crystal structure of several b-diketonato
cod platinum(II) complexes of the type [Pt(cod)(LL0-Bid)]A (where
LL0-Bid = acac, thtfac; A = BF4ꢀ, PF6ꢀ) along with theoretical calcu-
lations on the [M(cod)(acac)]+ complex for the nickel triad. The
aim is the further understanding of the co-ordination of a model
diolefin ligand such as 1,5-cylo-octadiene and the influence that
the b-diketonate systems, while varying different counter ions,
have on the respective structures and comparing the data for the
cod ligand with the previously defined ‘Venus fly-trap’ bite and
p
interactions. Bidentate olefins can also act as inhibitors in certain
homogeneous catalytic processes and a more detailed knowledge
on the bonding modes therein maybe exploited by more detailed
and systematic variation of effects, which influences transition
metal interactions therewith [1]. Similarly, classic b-diketone and
related ligands have been studied for more than a century and their
ability to give rise to rich and interesting co-ordination chemistry
is well known. b-Diketonato ligand systems act as monovalent
O,O0-chelating donors capable of stabilizing mononuclear transi-
tion metal complexes. These O,O0-bidentate ligands may be under-
utilised in middle and late transition metal complexes [2], to
evaluate dynamics and reaction mechanisms [3] as well as poten-
tial application in radiopharmaceuticals [4,5] and homogeneous
catalysis [6–12]. The keto-enol tautomerism of this class of ligand
has been widely studied [13,14]. The stepwise and subtle system-
atic ligand manipulation ability of b-diketonato ligand systems
therefore allow investigation into the bonding modes and co-
ordination of cis,cis-1,5-cyclo-octadiene (cod).
jaw dihedral angles,
v and w, [15] and a simultaneous evaluation
of spectroscopic data.
2. Experimental
2.1. Materials and general procedures
All reagents used for synthesis and characterization were of
analytical grade, purchased from Sigma–Aldrich, unless otherwise
stated. Reagents were used as received, without purification.
NMR spectra were recorded on a Bruker Advance II 600
(1H: 600.28 MHz; 13C: 150.96 MHz), 1H NMR spectra were refer-
enced internally using residual protons in the deuterated solvent
⇑
Corresponding author.
E-mail address: roodta@ufs.ac.za (A. Roodt).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.10.043

T.N. Hill et al. / Polyhedron 50 (2013) 82–89
83
(CDCl3: d 7.28; CD2Cl2: s 5.32). 13C NMR spectra were similarly ref-
erenced internally to the solvent resonance (CDCl3: t 77.36; CD2-
Cl2: m 53.8) with values reported relative to tetramethylsilane (d
0.0). Chemical shifts are reported in ppm.
atoms by a full-matrix least-squares method. The positions of the
hydrogen atoms were calculated using a riding model to the adja-
cent carbon, unless otherwise stipulated. Hydrogen interactions
were calculated using the PLATON [23–25] and PARST [26] programs.
Molecular graphics were obtained using DIAMOND [27], while overlay
illustrations were generated using HyperChem™ 7.5 [28].
Details of the crystal data, intensity measurements and data
processing are summarized in Table 1 for [Pt(cod)(acac)]PF6(I),
[Pt(cod)(acac)]BF4(II) and [Pt(cod)(thtfac)]BF4(III) while selected
bond distances and angles are given in Table 2. The molecular
structure with thermal ellipsoids at 50% probability and number-
ing scheme for complex I are shown in Fig. 1, complex II is likewise
numbered, the counter-ions and hydrogen atoms have been omit-
ted for clarity. The numbering scheme of complex III is represented
in Fig. 2, crystallizing with two independent complexes in the
asymmetric unit, labelled as IIIa and IIIb, respectively. The atoms
are prefixed with 1 or 2, respectively, e.g. O11 and O21 for the first
oxygen atom of the b-diketonato ligand, and again the counter-
ions and hydrogen atoms have been omitted for clarity.
Infrared spectra were recorded on a Bruker Tensor 27 Standard
System spectrophotometer with a laser range of 4000–370 cmꢀ1
Solid samples were prepared as potassium bromide disks.
.
Preparation
of
dichlorido-1,5-cyclo-octadieneplatinum(II)
starting material was achieved by the previously published meth-
od [16] from the reaction of potassium tetrachloroplatinate in
water and propanol with cis,cis-1,5-cyclo-octadiene (cod).
2.2. Synthesis
2.2.1. Preparation of [Pt(cod)(acac)]PF6(I) and [Pt(cod)(acac)]BF4(II)
[Pt(cod)Cl2] (100 mg, 0.27 mmol) was dissolved in dichloro-
methane (5 ml). To this AgA (A = PF6, BF4, 135.1 mg or 104.1 mg
0.53 mmol) was added, the resulting solution was stirred for ca.
15 min. An equivalent of acetylacetone (Hacac, 27 mg, 0.27 mmol)
was added and the mixture stirred for a few minutes to allow the
reaction to complete. The solution was filtered and diethyl ether
was added to the filtrate to precipitate the product. The resulting
solution was again filtered and the precipitate washed with diethyl
ether. Crystals suitable for X-ray determination were obtained by
DCM/Et2O vapour diffusion.
2.4. Computational methodology
The DFT (density functional theory) molecular orbital calcula-
tions were carried out using the GAUSSIAN 03 [29] software suite.
Becke’s three parameter hybrid (B3LYP) [30,31] exchange correla-
tion functional was used. The basis set employed in this study
was 6-311++G(d,p) [32–34] for the main group elements and
LanL2DZ [35] for the middle to late transition metals. Vibrational
frequencies were calculated at the 6-311++G(d,p) level for the
main group elements and at the LanL2DZ level for the middle to
late transition metals with minimum energies confirmed to have
zero imaginary frequencies. The frequencies were unscaled and
used to compute the zero-point vibrational energies, while the cal-
culated harmonic wavenumbers were used in the analysis of the
experimental IR spectra. The values were scaled down by the factor
0.9982, to account for anharmonicity effects and limitations of the
basis set [36].
Yield (I) 42 mg (53%); (II) 100 mg (74%).
I: IRKBr
:
vC@C 1526, vC@O 1556, vPt–O 647 and 473, vPt–[C@C]
558 cmꢀ1
.
1H NMR (600.28 MHz, CD2Cl2) d 2.008 (s, 6H, CH3),
2.492 (q, 4H, CH2), 2.836 (d, 4H, CH2), 5.744 (t, 4H, CH), 5.861 (s,
1H, CH). 13C NMR (150.96 MHz, CD2Cl2) d 26.70 (CH3), 29.95
(CH2), 99.51 (CH), 103.02 (CH), 187.84 (CO).
II: IRKBr
:
vC@C 1529, vC@O 1555, vPt–O 645 and 474, vPt–[C@C]
521 cmꢀ1
.
1H NMR (600.28 MHz, CD2Cl2) d 2.209 (s, 6H, CH3),
2.499 (q, 4H, CH2, IIa), 2.254 (q, 4H, CH2, IIb), 2.683 (dd, 4H, IIb),
2.841 (d, 4H, CH2, IIa), 5.556 (t, 4H, CH, IIb), 5.75 (t, 4H, CH, IIa),
5.862 (s, 1H, CH). 13C NMR (150.96 MHz, CD2Cl2) d 26.81 (CH3),
29.97 (CH2, IIa), 31.23 (CH2, IIb), 99.53 (t, CH, IIa), 100.94 (CH,
IIb), 103.02 (CH), 187.81 (CO).
2.2.2. Preparation of [Pt(cod)(thtfac)]BF4(III)
3. Results and discussion
Using the preparation method presented in Section 2.2.1,
changing the b-diketonato ligand to thenoyltrifluoroacetone (Htht-
fac) complex III was synthesized. Crystals suitable for X-ray deter-
mination were obtained by DCM/Et2O vapour diffusion.
Yield (III) 90 mg (54%).
3.1. Synthesis and characterization
The platinum(II) complexes (I–III) were synthesized utilising
potassium tetrachloroplatinate in a mixture of water and propanol
followed by the addition of a catalytic amount of tin chloride and
an excess of cis,cis-1,5-cyclo-octadiene (cod). The mixture was
stirred for several days and allowed to evaporate to dryness where
upon it was extracted with dichloromethane. The resulting di-
chloro(1,5-cyclo-octadiene)platinum(II) complex was then further
reacted with an equivalent of the b-diketonato ligand in the pres-
ence of a silver salt (Scheme 1). The compounds described in this
paper were characterized by IR, 1H and 13C NMR spectroscopy
(data in Section 2.2).
III: IRKBr
: mC@C 1540, mC@O 1579, mPt–O 613 and 473, mPt–[C@C]
522 cmꢀ1 1H NMR(600.28 MHz, CDCl3) d 2.604 (m, 4H, CH2),
.
2.727 (m, 4H, CH2), 5.626 (t, 4H, CH), 6.717 (s, 1H, CH), 7.969 (d,
1H, CH), 8.085 (d, 2H, CH). 13C NMR (150.96 MHz, CDCl3) d 30.22
(CH2), 31.28 (CH2), 100.42 (t, CH), 120.94 (CH), 130.59 (CH),
136.72 (CH), 139.73 (CF3), 198.87 (CO).
2.3. X-ray crystallographic data collection and refinement
Data collections for [Pt(cod)(acac)]PF6(I) was obtained on a
Bruker APEX II 4K CCD diffractometer. The data for [Pt(cod)
(acac)]BF4(II) and [Pt(cod)(thtfac)]BF4(III) were collected on an
Oxford Diffraction Xcalibur 3 Crysalis CCD system [17]. Both sys-
The IR spectra of the compounds showed characteristic bands at
613–647 cmꢀ1 and 474 cmꢀ1 assigned to mPt–O
+ Dring (in-plane
ring distortion) and mPt–O while mPt–[C@C] bands were observed
around 558–521 cmꢀ1. These bands were assigned based on the
previous assignments of similar compounds by Mikami et al. [37]
and Nakamoto et al. [38] who studied a range of bis-acetylacetona-
to transition metal complexes. The O–Pt–O bite angle decreases
when varying the b-diketonato ligands from acac to thtfac leading
to an increase in strain on the metallocycle, this is seen in the IR by
ca. 34 cmꢀ1 shift down-field.
tems were equipped with a graphite-monochromatted Mo Ka radi-
ation, for the Bruker system all the reflections were merged and
integrated with SAINT-PLUS [18] and corrected for Lorentz, polariza-
tion and absorption effects with SADABS [19], while Crysalis RED
[20] was used for the Oxford Diffraction system.
Structures were solved by direct and conventional Patterson
methods using SHELX-97 [21] as part of the WINGX [22] package,
and anisotropic refinement was performed on all non-hydrogen
The 1H NMR data is given in Section 2.2 and the complexes have
been described considering the atomic numbering scheme given in

84
T.N. Hill et al. / Polyhedron 50 (2013) 82–89
Table 1
ꢀ
ꢀ
Crystallographic data and refinement parameters for [Pt(cod)(LL0-Bid)]A, where A = PF6 and BF4 , LL0-Bid = acac, thtfac.
Crystal data
[Pt(cod)(acac)]A
[Pt(cod)(thtfac)]A
PF6ꢀ(I)
BF4ꢀ(II)
BF4(III)
Empirical formula
Formula weight
T (K)
C
13H19F6O2PPt
C13H19BF4O2Pt
489.18
100(2)
C16H16BF7O2PtS
611.25
100(2)
547.34
100(2)
k (Å)
0.7103
monoclinic
P21/c
0.71073
orthorhombic
Pca21
0.7103
monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
9.3082(3)
26.3980(4)
7.19670(10)
7.71820(10)
90
14.4456(2)
15.1804(1)
19.5643(3)
90
15.4486(5)
12.6335(3)
90
a
(°)
b (°)
119.164(2)
90
1586.38(8)
4
2.292
9.013
90
90
117.551(1)
90
3803.74(8)
8
2.135
7.563
c
(°)
V (Å3)
1466.29(4)
4
2.216
9.613
928
Z
Dcalc (Mg mꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
1040
2320
Crystal size (mm)
Theta range for data collection (°)
Index ranges
0.39 ꢁ 0.22 ꢁ 0.12
3.22 to 28.38
ꢀ12h 6 10,
ꢀ20 6 k 6 18,
ꢀ16 6 l 6 16
15026
0.26 ꢁ 0.18 ꢁ 0.03
2.83 to 27.98
ꢀ34 6 h 6 34,
ꢀ9 6 k 6 7,
ꢀ10 6 l 6 8
11111
0.51 ꢁ 0.26 ꢁ 0.13
2.53 to 28.00
ꢀ19 6 h 6 19,
ꢀ20 6 k 6 11,
ꢀ25 6 l 6 25
29977
Reflections collected
Independent reflections
3963
2884
9095
[Rint = 0.0479]
28.38, 99.4
0.411 and 0.127
Full-matrix least-squares on F2
3963/0/210
1.037
[Rint = 0.0233]
27.98, 99.4
0.761 and 0.189
Full-matrix least-squares on F2
2884/1/186
1.054
[Rint = 0.0333]
28.00, 99.1
0.440 and 0.113
Full-matrix least-squares on F2
9095/0/505
0.947
Completeness to h (°; %)
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0286,
wR2 = 0.0558
R1 = 0.0406,
wR2 = 0.0597
1.431 and ꢀ1.369
R1 = 0.0175,
wR2 = 0.0439
R1 = 0.0195,
wR2 = 0.0445
1.468 and ꢀ0.721
R1 = 0.0257,
wR2 = 0.0554
R1 = 0.0387,
wR2 = 0.0591
1.964 and ꢀ1.270
Largest difference in peak and hole (e Åꢀ3
)
Table 2
Selected experimental and optimised bond distances (Å) and angles (°) for complexes
I–III.
I
II
Calca
IIIa
IIIb
Pt–O1
Pt–O2
Pt–C1
Pt–C2
Pt–C5
Pt–C6
C1–C2
C5–C6
2.011(3)
2.009(3)
2.161(4)
2.146(4)
2.148(5)
2.154(4)
1.401(6)
1.393(6)
2.015(3)
2.013(3)
2.160(3)
2.153(4)
2.164(3)
2.147(4)
1.388(8)
1.411(8)
2.041
2.041
2.221
2.205
2.221
2.205
1.399
1.399
2.011(3)
2.015(3)
2.152(4)
2.166(4)
2.152(4)
2.157(4)
1.395(5)
1.392(5)
2.007(3)
2.025(3)
2.151(4)
2.165(4)
2.152(4)
2.161(4)
1.398(5)
1.399(5)
O1–Pt–O2
C1–Pt–C2
C5–Pt–C6
C1–Pt–C6
C2–Pt–C5
vb
93.5(1)
38.0(2)
37.8(2)
82.3(1)
82.3(1)
86.8(4)
78.9(5)
93.5(1)
37.6(2)
38.2(2)
82.2(2)
82.0(2)
86.9(2)
79.1(2)
91.95
36.84
36.84
81.41
81.41
85.9
92.4(1)
37.7(1)
37.7(1)
82.9(1)
88.7(1)
87.4(1)
78.9(2)
92.8(1)
37.8(1)
37.9(1)
82.1(1)
82.9(1)
87.4(2)
80.5(2)
Fig. 1. The molecular structure of I. Displacement ellipsoids are drawn at 50%
probability. Hydrogen atoms and counter ion have been omitted for clarity.
wc
82.1
a
Calculated [Pt(cod)(acac)]+ moiety.
for complex II but was not present in complexes I and III; this was
ascribed to the folding of the cod in solution. The 13C{1H} NMR res-
onances (data in Section 2.2), including the Pt satellites, were in
accordance with the proposed structures.
b
See below for definition of
v and w.
Fig. 1. The resonances of methine hydrogen atoms of the cod dem-
onstrated the classic triplet of 195Pt coupling [3JPt–H = 33 Hz]. With
a singlet observed for the b-diketonato hydrogen (H10), for com-
plex III a shift higher field (6.71 ppm) was found as a result of
the additional electron withdrawing of the substituents of the tht-
3.2. Structural description
The platinum metal centres of complexes I–III are bonded to the
two oxygen atoms of the b-diketonato ligands and p-bonded with
fac ligand. Both complexes
(5.86 ppm) leading to the conclusion that the counter ions do not
play a significant role in the cationic species. A rotamer was found
I
and II showed similar shifts
the alkene bonds of the cis,cis-1,5-cyclo-octadiene (cod) ligand giv-
ing rise to a square planar co-ordination geometry around the me-
tal centre. A low temperature re-determination of II at 100 K was

T.N. Hill et al. / Polyhedron 50 (2013) 82–89
85
Fig. 2. The molecular structure of IIIa. Displacement ellipsoids are drawn at 50% probability. Hydrogen atoms and counter ion have been omitted for clarity.
metal chelate ring interactions of the platinum co-ordinated acet-
ylacetonato metallocycle between the metal chelate ring and the
methine (H10) hydrogen of the acac moiety are illustrated in
Fig. 4 and exhibit distances of 3.5804 Å with symmetry operator
3/2 ꢀ x, y, ½ + z, while the methyl (H12C)ꢂmetal chelate ring has a
distance of 2.762 Å.
3.3. Computational data
Scheme 1. Synthesis of the platinum(II) cyclo-octadiene b-diketonato complexes.
No accurate relevant solid state data for isostructural com-
plexes of all members of the nickel triad are available to enable
comparison for the different metal centres since isolation of suit-
able and stable crystals proved to be a significant challenge. Thus,
DFT calculations were performed to compare the geometry with
that obtained from the solid state studies, gain insight into the
electronic structures and explain the bonding nature and spectral
properties of the complexes. An excellent correlation between
the computed structures and that of the experimental structures
was obtained as can be seen in Fig. 5, which presents the Hyper-
Chem™ superimposed image of complexes I and II with the calcu-
lated [Pt(cod)(acac)]+ moiety with r.m.s. values of 0.067 and
0.076 Å, respectively. In general, the optimised geometric parame-
ters are in very good agreement with the values based upon the
X-ray crystal structure data as can be seen in Table 2, and the com-
mon trends observed in the experimental data are excellently
reproduced in the calculations. It should be noted that the theoret-
ical calculations do not consider the effects of the chemical
environment for example intra- and intermolecular packing inter-
actions. As a result the distances for the theoretical calculations are
fractionally larger (1–3%) than those observed for the
X-ray crystal structures.
done (original by Venkatasubramanian [39]), to ensure accurate
data and thus allowing for the direct comparison with complex I.
Due to the cationic nature of the complexes the resulting charge
is balanced with either the hexafluorophosphate (PF6ꢀ) or tetraflu-
rorborate (BF4ꢀ) counter-ions. The ions display classic distorted
tetragonal (BF4ꢀ) and octahedral (PF6ꢀ) co-ordination geometries
with no disorder present. The varying of the counter-ion does
not significantly change the bond distances observed for the plat-
inum centre, nor the general coordination mode, see Table 2. More-
over, the variation of the solid state packing mode and
intermolecular packing effects proved to be negligible. The alkene
(C@C) bond lengths of cod were not affected by the increased elec-
tron withdrawing groups on the trans acac-type ligand and gener-
ally remained constant with a mean value of 1.397(6) Å.
The distortion from square planar geometry as measured by the
intersection of the planes defined by the mid-point of the alkene
carbons of the cod ring with the oxygen atoms of the b-diketones
and the platinum metal centre with the b-diketonato back-bone
is as follows: complex IIIa > IIIb > II > I being 5.48(5)°, 4.77(5)°,
4.19(6)° and 2.57(6)°, respectively.
The hydrogen interactions for complex I and II originate mostly
between the cod moieties and the counter-ions. However, hydro-
gen bond interactions with the b-diketonato moieties are present
as listed in Table 3 and illustrated in Fig. 3. For both complex I
In the calculated IR spectrum of [Pt(cod)(acac)]+ moiety with
the experimental spectra of complexes I and II are graphically dis-
played, while selected vibrations are presented in the Supplemen-
tary material (Fig. S5, Table S2); with no deconvolution of
experimental bands performed. Therefore, the number of IR signals
shown by the theoretical spectrum of a given complex may be
larger with respect to that shown by the experimental spectrum
of the same complex.
-
and II the different fluorido ligands on the BF4 counter ion (F3) is
split between the methine hydrogen (H10) and the methyl hydro-
gen (I: H12B and II: H13B) atoms. An additional ‘splitted’ fluorido
ligand interaction (F4) is observed for II. For complex I a further
interaction with the cod moiety with the fluorido ligand (F5) was
found.
Due to variations in the hydrogen interactions, crystal system
and the presence of metal chelate ring interactions (Fig. 4) the
packing of I was found to be a clustered head-to-tail whist a classic
head-to-tail packing for II was seen (Supplementary data). The
ꢀ
The PF6 anion stretching frequencies are found in the region
between 920 and 740 cmꢀ1 while that of the BF4 is located at
ꢀ
980–1150 cmꢀ1. The strongest bands of the experimental spectra
ꢀ
were assigned as the anions, for complex II the BF4 anion bands
were found at 1097, 1057, 1038 and 1011 cmꢀ1 representing the
-
four mB–F frequencies, while for complex I the PF6 anion was seen

86
T.N. Hill et al. / Polyhedron 50 (2013) 82–89
Table 3
additional expected decrease compared to an uncoordinated b-
Hydrogen bond distances and angles for complexes I–II [Å and °].
diketone ligand. For complex I and II nearly identical values of
1556 cmꢀ1 were obtained with the computed value in good agree-
ment with the assignment. A similar lowering of the frequencies
due to the conjugation for mC@C was also found with values of
1526 cmꢀ1 for I and 1529 cmꢀ1 for II, respectively. Of further inter-
est in this study are the mPt–O and mPt–[C@C] frequencies describing
the co-ordination environment of the platinum metal centre.
Moreover, it is noted that the counter-ion neither plays a signifi-
cant role in the co-ordination polyhedron’s geometry, nor does
the hydrogen interactions found between the cod and b-diketonato
back-bone and the counter-ions significantly alter the harmonics.
An additional weak band at 1350 cmꢀ1 observed in the exper-
imental data (Supplementary data Fig. S5) may be attributed to mas
of the conjugated ketones of the acetylacetonato ligands. For the
computed spectra this stretching harmonic is combined with the
D–Hꢂ ꢂ ꢂA
I
d(D–H)
d(Hꢂ ꢂ ꢂA)
d(Dꢂ ꢂ ꢂA) \(DHA)
C5–H5ꢂ ꢂ ꢂF5
0.98
0.97
0.98
0.98
0.96
0.93
2.37
2.52
2.53
2.48
2.55
2.39
3.140(6) 135.5
3.332(6) 140.8
3.360(5) 142.8
3.242(5) 134.7
3.432(6) 152.6
3.271(5) 158.7
#1
C7–H7Aꢂ ꢂ ꢂF4
#2
C2–H2ꢂ ꢂ ꢂF3
C6–H6ꢂ ꢂ ꢂF6
#3
#4
C
C
12–H12Bꢂ ꢂ ꢂF3
#4
10–H10ꢂ ꢂ ꢂF3
II
C2–H2ꢂ ꢂ ꢂF1
C2–H2ꢂ ꢂ ꢂF3
C3–H3Bꢂ ꢂ ꢂF4
C7–H7Aꢂ ꢂ ꢂF2
1.00
1.00
0.99
0.99
1.00
0.98
0.99
0.98
0.95
0.98
2.54
2.47
2.39
2.40
2.31
2.65
2.55
2.48
2.44
2.48
3.255(6) 128.1
3.145(6) 123.9
3.320(7) 156.5
3.266(7) 146.2
3.192(6) 146.7
3.609(6) 166.0
3.360(7) 138.7
3.445(5) 166.9
3.328(5) 156.5
3.368(6) 151.2
#5
#6
#7
C6–H6ꢂ ꢂ ꢂF4
#7
C13–H13Cꢂ ꢂ ꢂF1
#8
C7–H7Bꢂ ꢂ ꢂF1
#9
C
C
C
12–H12Bꢂ ꢂ ꢂF4
wagging (x) of the methylene (CH2) groups of the cod moiety as
illustrated in Fig. 6, resulting in a broader band.
#9
10–H10ꢂ ꢂ ꢂF3
13–H13Bꢂ ꢂ ꢂF3
#9
In the comparison of the different calculated IR spectra the un-
scaled values were used (Table 4). It is clear from the discussion
above that the scaled values are in good agreement with the exper-
imentally observed bands (Supplementary data Fig. S5 and
Table S2). Small changes with maximum differences of 8 cmꢀ1, to
the symmetrical stretching frequency of the co-ordinated conju-
Symmetry transformations used to generate equivalent atoms
#1 ꢀ x, y ꢀ 1/2,
#2x, ꢀy + 1/
#3x ꢀ 1, ꢀy + 1/2,
z ꢀ 1/2
ꢀz + 3/2
2, z + 1/2
#4 ꢀ x + 1, y ꢀ 1/2,
ꢀz + 3/2
#5x, y ꢀ 1, z
#6 ꢀ x + 1, ꢀy + 1,
z ꢀ 1/2
#7x, y ꢀ 1, z ꢀ 1
#8x, y, z ꢀ 1
#9 ꢀ x + 3/2,
y ꢀ 1, z ꢀ 1/2
gated ketone functionality of the acetylacetonato moiety (mC@O
)
was observed, while the largest variation was found for the mC@C
acac ligand with a variation of 14 cmꢀ1 when varying the group
10 metal centre as presented in Fig. 6.
Two geometric angles are specifically highlighted in this inves-
tigation, namely the chi (v) or ‘bite’ angle characterized by the
at 842 cmꢀ1 a single band was observed due to the octahedral sym-
metry (Oh) of the anion.
Due to the conjugation present in the b-diketonato back-bone
intersection of the planes, defined by adjacent alkene carbons of
the
mC@O bands were expected at a lower wavenumber region than
the cod ligand and the platinum metal centre. Secondly, the psi
(w) or ‘jaw’ angle characterized by the intersection between planes
for the standard ketone frequencies (1725–1705 cmꢀ1), with an
Fig. 3. Molecular diagram of (a) I and (b) II illustrating hydrogen interactions by a dashed bond. Only relevant hydrogen atoms have been included.

T.N. Hill et al. / Polyhedron 50 (2013) 82–89
87
Table 4
Selected calculated IR (cmꢀ1),
v
(°) and
w
(°) values for the Group 10 metal
complexes. Unscaled.
[Ni(cod)(acac)]+
[Pd(cod)(acac)]+
[Pt(cod)(acac)]+
mC@O
1577
1569
1572
mC@Cacac
mC@Ccod
mas
1553
1543
1557
1605 w
1593 w
1377
86.6
85.9
1567 w
1559 w
1378
84.8
84.9
1541 w
1531 w
1384
85.9
82.1
C@Ccod
mas
va
wa
C@O
a
See Fig. 7 for definition of
v and w.
defined by the alkane carbons of the cod moiety, as depicted
graphically in Fig. 7.
By comparing the crystallographic and computed values of the
‘jaw’ and ‘bite’ angles, for I and II with [Pt(cod)(acac)]+ (Table 2)
it was observed that the values remain similar, with a few degrees
difference detected.
Upon descending the Ni-triad the theoretical values (Table 4)
for the ‘bite’ angle follow a similar trend to the covalent radii of
the triad, namely Ni < Pt < Pd, whilst the ‘jaw’ angle closes as one
proceeds down the triad with a difference of ca. 3.8° from top to
bottom, and clearly represents quite a small change. This is quite
interesting since, upon variation of the electronic properties of
the trans bidentate acac type ligand for the Pd(II) only, a significant
effect in the jaw angle of up to 8 degrees was observed [15].
Expansion of the use of the jaw and bite angles in follow-up
Fig. 4. Molecular diagram of II illustrating metal chelate ring interactions by
dashed bonds. Only selected hydrogen atoms are included; tetrafluoroborate
counter ions omitted for clarity. Symmetry operators [3/2 ꢀ x, y, 1/2 + z] and
[1/2 ꢀ x, y, ꢀ1/2 + z].
Fig. 5. HyperChem™ superimposed image of (a) [Pt(cod)(acac)]BF4(II) (black) vs. [Pt(cod)(acac)]+ (calculated) (red) with an RMS error of 0.067 Å (b) [Pt(cod)(acac)]PF6(I)
(black) vs. [Pt(cod)(acac)]+ (calculated) (red) with an RMS error of 0.076 Å. The counter-ions and hydrogen atoms of the X-ray crystal structures have been omitted for clarity.
(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Calculated IR spectrum of the finger print region of [Ni(cod)(acac)]+ (blue), [Pd(cod)(acac)]+ (red) and [Pt(cod)(acac)]+(green).
wagging; unscaled. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
m – stretching, c – rocking, scissoring, x –

88
T.N. Hill et al. / Polyhedron 50 (2013) 82–89
Fig. 7. Left, the interplanar bond angle
v
, the ‘bite’ of the Venus fly-trap, formed by the olefinic moieties though the platinum metal centre. Right, the extent of the opening of
the Venus fly-trap ‘jaw’ angle,
w, as defined by the planes though the alkane carbons of the cyclo-octadiene moiety.
Table 5
Molecular orbitals of calculated structures [M(cod)(acac)]+.
HOMOꢀ1
HOMO
LUMO
LUMO+1
Ni(II)
Pd(II)
Pt(II)
Table 6
investigations are therefore paramount in defining the diolefinic
coordination in these cod systems more accurately.
Molecular orbital energies (Hartree) and energy gaps (eV) of the [M(cod)(acac)]+
(M = Ni, Pd, Pt) calculated structures.
The visual representation of the molecular orbitals (MOs) is an-
other method whereby interactions of the cyclo-octadiene with the
various metal centres and the trans b-diketonato ligand may be ob-
served. The second-highest (HOMOꢀ1) and highest (HOMO) occu-
pied MOs and the lowest (LUMO) and second-lowest (LUMO+1)
unoccupied MOs for the calculated complexes of [M(cod)(acac)]+
where M = Ni, Pd, Pt (group 10 metals) are presented in Table 5
and Table 6, along with the differences in the respective
M(II) Molecular orbital energies (Hartree)
HOMOꢀ1 HOMO LUMO LUMO+1
Ni(II) ꢀ0.41512 ꢀ0.37704 ꢀ0.2485 ꢀ0.19038 1.036 3.498 1.582
Pd(II) ꢀ0.41333 ꢀ0.37277 ꢀ0.24407 ꢀ0.18711 1.104 3.502 1.550
Pt(II) ꢀ0.41536 ꢀ0.37796 ꢀ0.2089 ꢀ0.19198 1.018 4.600 0.460
Energy Gaps (eV)
Egꢀ1 Eg Eg+1
orbitals (Egꢀ1
Eg+1
ꢃ
D
|HOMOꢀ1| ꢀ |HOMO|; Eg ꢃ
D
|HOMO| ꢀ |LUMO|;
ꢃ
D
|LUMO| ꢀ |LUMO+1|). The contour plots shown with the
The HOMOꢀ1 MOs of nickel and palladium are comparable with
MOs in Table 5 are viewed along the z-axis. The gap energies reflect
the chemical activity of the molecules ability of the MOs to donate
(HOMO) or accept (LUMO) an electron.
p
orbital delocalization occurring between both the cyclo-
octadiene moiety and the acetylacetonato moiety. Platinum
however only shows delocalization between the metal and the

T.N. Hill et al. / Polyhedron 50 (2013) 82–89
89
acetylacetonato moiety. For the HOMO MOs delocalization on the
back-bone of the acetylacetonato moiety is seen for all three com-
plexes, with the platinum complex showing some electron density
around the cyclo-octadiene alkene carbons, with a dyz orbital cen-
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.poly.2012.10.043.
tred on the metals. A dx2
orbital contribution is observed on the
ꢀy2
metal centres for the LUMO MOs.
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found to be small whereas the palladium complex showed almost
none. This capability of back bonding is assumed to be the reason
why the platinum complexes are the most stable of the triad.
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Acknowledgements
Financial assistance from the University of the Free State under
the Strategic Academic Cluster initiative (Materials and Nano-
sciences) is gratefully acknowledged as well as the use of the High
Performance Cluster for the GAUSSIAN03 calculations. We thank Prof.
Ola Wendt from the University of Lund, Sweden for the use of their
Oxford X-ray diffractometer. We also express our gratitude to
SASOL, the South African National Research Foundation (SA-NRF/
THRIP) and the Swedish International Development Co-operation
Agency (SIDA) for financial support of this project. Part of this
material is based on work supported by the SA-NRF/THRIP under
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mendations expressed in this material are those of the authors and
do not necessarily reflect the views of the SA-NRF.
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CCDC 883760 (I), 883761 (II) and 883762 (III) contain the sup-
plementary crystallographic data for this article. These data can be