Halide ligands - More than just σ-donors? A structural and spectroscopic study of homologous organonickel complexes

Article abstract of DOI:10.1021/ic8007365

The isoleptic organonickel complexes [(bpy)Ni(Mes)X) (bpy = 2,2′-bipyridine; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I, and for comparison X = OMe and SCN) have been investigated by multiple spectroscopic means. Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer) obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X) + Solv ? [(bpy)Ni(Mes)(Solv)] ⁺ + X^-, which occurs very fast for X = I (k = 0.176(4) M^-1 s^-1) or Br but very slow for X = Cl (k = 5.18(5) × 10^-5 M^-1 cm^-1) or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all halide coligands but marginal π-donation.

Full text of DOI:10.1021/ic8007365

Inorg. Chem. 2008, 47, 11324-11333
Halide LigandssMore Than Just σ-Donors? A Structural and
Spectroscopic Study of Homologous Organonickel Complexes
Axel Klein,†,* Andre´ Kaiser,† Wolfram Wielandt,‡,§ Ferdinand Belaj,‡ Eric Wendel,| Helmut Bertagnolli,|
and Stanislav Za´lisˇ⊥
UniVersität zu Köln, Department fu¨r Chemie, Bereich Anorganische Chemie, Greinstraꢀe 6,
D-50939 Ko¨ln, Germany, Karl-Franzens-UniVersita¨t Graz, Institut fu¨r Chemie, Schubertstraꢀe 1,
A-8010 Graz, Austria, UniVersita¨t Stuttgart, Institut fu¨r Physikalische Chemie, Pfaffenwaldring 55,
D-70569, Stuttgart, Germany, and J. HeyroVsky´ Institute of Physical Chemistry, Academy of
Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18 000 Prague 8, Czech Republic
Received April 24, 2008
The isoleptic organonickel complexes [(bpy)Ni(Mes)X] (bpy ) 2,2′-bipyridine; Mes ) 2,4,6-trimethylphenyl; X ) F,
Cl, Br, or I, and for comparison X ) OMe and SCN) have been investigated by multiple spectroscopic means.
Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray
absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer)
obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy
allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X] + Solv a [(bpy)Ni(Mes)(Solv)]+ + X-, which
occurs very fast for X ) I (k ) 0.176(4) M-1 s-1) or Br but very slow for X ) Cl (k ) 5.18(5) × 10-5 M-1 cm-1)
or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic
transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands
play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all
halide coligands but marginal π-donation.
Introduction
various (electro)catalytic C-C coupling reactions.2-9 Paralleling
their use in catalysis, fundamental studies on structures and
Organometallic nickel complexes with R-diimine ligands like
2,2′-bipyridine (bpy), 1,10-phenanthroline, or diazabutadienes
have gained enormous interest in the past decade. This is mainly
due to a number of important catalytic processes like olefin
oligo- or polymerization, olefin/CO copolymerization,1 and
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2007, 692, 3156–3167.
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* Author to whom correspondence should be addressed. E-mail:
axel.klein@uni-koeln.de.
†
Universität zu Köln.
Karl-Franzens-Universita¨t Graz.
Present address: EPCOS OHG, Deutschlandsberg, Austria.
Universita¨t Stuttgart.
‡
§
|
⊥
Academy of Sciences of the Czech Republic.
(1) (a) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis with
Organometallic Compounds, 2nd ed.; Wiley/VCH: Weinheim, Ger-
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2000, 100, 1169–1203. (c) Michalak, A.; Ziegler, T. Organometallics
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2001, 40, 534–540. (e) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
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11324 Inorganic Chemistry, Vol. 47, No. 23, 2008
10.1021/ic8007365 CCC: $40.75  2008 American Chemical Society
Published on Web 10/29/2008

Halide LigandssMore Than Just σ-Donors?
electronic properties of organonickel complexes with R-diimines
have been carried out.9-11 We have contributed to this with
the investigation of a number of organonickel complexes
[(N∧N)Ni(Mes)Br] (N∧N ) R-diimine ligands, Mes ) mesityl
) 2,4,6-trimethylphenyl) with various diimine ligands. Their
structures and reactivity toward ligand exchange reactions have
been studied using X-ray diffraction and absorption spectros-
copy;12,13 their photophysics and photochemistry were explored
by a combination of multiple spectroscopy and quantum
chemical calculations,12,14 and finally, their redox chemistry was
investigated,15 with respect to the application of such systems
in electrocatalytical C-C coupling reactions.5
Summarizing all of these studies, a considerable contribu-
tion of the bromide coligand can be stated. (i) In the presence
of strongly donating solvents like nitriles, N,N-dimethylfor-
mamide (DMF), or dimethylsulfoxide (DMSO), ligand
exchange reactions occur, releasing the bromide ligand (eq
1), whereas in acetone, ethers, or chlorinated solvents, the
complexes were stable.13
the question of whether there are further interactions between
the metal and ligand beyond the M-L σ bond, such as π
bonds (π-accepting, π-donating). Both aspects will be
addressed in this study, to which we have added, for the
sake of comparison, also the complexes with the pseudoha-
lides X ) OMe and SCN.
Experimental Section
Materials. Commercially available PPh3 (Aldrich), NiCl2, NiBr2
(Chempur), and MesBr (Acros) were used without further purifica-
tion.
Instrumentation. 1H NMR spectra were recorded using Bruker
Avance 300 (1H, 300.13 MHz; 13C, 75.47 MHz; 19F, 282.35 MHz)
or Bruker Avance 400 spectrometers (1H, 400.13 MHz; 13C, 100.61
MHz; 19F, 376.50 MHz), the latter using a triple resonance
1H,19F,BB inverse probe head. The unambiguous assignment of the
1
1H and 13C resonances was obtained from H nuclear Overhauser
effect spectrometry; 1H correlation spectroscopy; and gradient-
1
selected H and 13C heteronuclear single quantum coherence and
heteronuclear multiple-bond correlation experiments. All 2D NMR
experiments were performed using standard pulse sequences from
the Bruker pulse program library. Chemical shifts were relative to
tetramethylsilane and CCl3F. UV-vis absorption spectra were
recorded with Varian Cary 05E or Cary 50 scan spectrophotometers.
Kinetic measurements were performed in an acetone solution (2
mL acetone) using a Cary 50 scan spectrophotometer. After the
admixture of 0.5 mL of acetonitrile, the decay of the long-
wavelength absorption band at about 460 nm was recorded and
fitted assuming a first-order mechanism (monoexponential decay).
Elemental analyses were obtained using a HEKAtech CHNS
EuroEA 3000 analyzer. IR and Raman spectra of the complexes
dispersed in KNO3 pellets were recorded with a Bruker IFS/66v/S
spectrometer equipped with a FRA106/S module for Raman
measurements, using a Nd:YAG laser (λ ) 1064 nm) as the
excitation source. Electrochemical measurements were carried out
at a 100 mV/s scan rate in 0.1 M Bu4NPF6 solutions using a three-
electrode configuration (glassy carbon electrode, Pt counter elec-
trode, Ag/AgCl reference) and a Metrohm Autolab PGSTAT30
potentiostat and function generator. The ferrocene/ferrocenium
couple served as the internal reference.
Crystal Structure Analysis for [(bpy)Ni(Mes)Cl] and
[(bpy)Ni(Mes)I]. Crystals of both compounds were obtained from
concentrated solutions in toluene. The measurements for [(bpy)Ni-
(Mes)Cl] were performed at 293(2) K using IPDS II (STOE and
Cie.), and the data for [(bpy)Ni(Mes)I] were obtained using a STOE
four-circle diffractometer at 100(2) K, both using graphite-
monochromatized Mo KR radiation. The structures were solved
by direct methods (SHELXS 97)16 and refined by full-matrix least-
squares techniques against F2 (SHELXL 97).17 The non-hydrogen
atoms were refined with anisotropic displacement parameters
without any constraints. The hydrogen atoms were included by using
appropriate riding models. Details of the crystal structure solutions
and refinements were summarized in Table 1. Full structural
information for [(bpy)Ni(Mes)Cl] and [(bpy)Ni(Mes)I] has been
deposited with the Cambridge Crystallographic Data Centre, No.
CCDC 659485 (X ) Cl) and No. CCDC 659486 (X ) I). Copies
of the data can be obtained, free of charge, upon application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: +44-1223
336033 or e-mail: deposit@ccdc.cam.ac.uk).
[(L)2Ni(Mes)Br] + L′ h [(L)2Ni(Mes)L]+ + Br-
L ) PPh3 or bpy/2; L′ ) strong donation solvents (1)
(ii) A thorough study of the photophysical properties of the
complex [(bpy)Ni(Mes)Br] revealed an appreciable contribu-
tion of the Br coligand to the electronic transitions, as inferred
from quantum chemical calculations of the ground state,
excited states, and electronic transitions by time-dependent
density functional theory (TD DFT).14 (iii) After 1e-
reduction, the complexes [(N∧N)Ni(Mes)Br] (N∧N ) various
R-diimine ligands) undergo a very fast cleavage of the
bromide coligand (eq 2).15
-
[(N∧N)Ni(Mes)Br] y+e z [(N∧N)Ni(Mes)Br]•-h
[(N∧N)Ni(Mes)]• + Br- (2)
As a consequence of these studies, we have prepared the
four isoleptic complexes [(bpy)Ni(Mes)X] (bpy ) 2,2′-
bipyridine; Mes ) 2,4,6-trimethylphenyl; X ) F, Cl, Br, or
I) and investigated them by multiple spectroscopy and
electrochemistry to probe for the role of the coligand X to
both reactivity and electronic states.
There are two main issues in the metal-ligand interaction
in coordination chemistry. One is regarding the character of
the M-L σ bond (covalent vs ionic); the other is addressing
(9) (a) Yamamoto, T.; Abla, M.; Murakami, Y. Bull. Chem. Soc. Jpn.
2002, 75, 1997–2009. (b) Yamamoto, T.; Abla, M. J. Organomet.
Chem. 1997, 535, 209–211. (c) Uchino, M.; Asagi, K.; Ymamoto,
A.; Ikeda, S. J. Organomet. Chem. 1975, 84, 93–103. (d) Yamamoto,
T.; Yamamoto, A.; Ikeda, S. J. Am. Chem. Soc. 1971, 93, 3350–3359.
(10) (a) Tucci, G. C.; Holm, R. H. J. Am. Chem. Soc. 1995, 117, 6489–
6496. (b) Arcas, A.; Royo, P. Inorg. Chim. Acta 1978, 31, 97–99. (c)
Arcas, A.; Royo, P. Inorg. Chim. Acta 1978, 30, 205–207.
(11) De Souza, R. F.; Simon, L. C.; Alves, M. C. J. Catal. 2003, 214 (1),
165–168.
(12) Klein, A. Z. Anorg. Allg. Chem. 2001, 627, 645–650.
(13) Feth, M. P.; Klein, A.; Bertagnolli, H. Eur. J. Inorg. Chem. 2003, 83,
9–852.
(14) Feth, M. P.; Klein, A.; Bertagnolli, H.; Za´lisˇ, S. Eur. J. Inorg. Chem.
2004, 2784–2796.
(15) Klein, A.; Kaiser, A.; Sarkar, B.; Wanner, M.; Fiedler, J. Eur. J. Inorg.
Chem. 2007, 96, 5–976.
(16) Sheldrick, G. M. SHELXS 97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(17) Sheldrick, G. M. SHELXL 97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11325

Klein et al.
Table 1. Selected Crystallographic Data for [(bpy)Ni(Mes)X] (X ) Cl or I)
formula/fw
cryst syst/space group
cell a
b
c
C19H19ClN2Ni/369.52
monoclinic/P21/n
11.323(2) Å
9.7350(19) Å
16.524(3) Å
102.83(3)°
1775.8(6)/4
1.382/768
1.243/293(2)
2.44 to 28.18°
-15 e h e 14, -12 e k e 12, -21 e l e 21
4041/2566
0.075/93.2%
4041/211/0
C19H19IN2Ni/460.95
monoclinic/P21/c
16.744(6) Å
12.624(4) Å
17.824(6) Å
108.98(3)°
ꢀ
vol (Å3)/Z
3563(2)/8
calcd density (Mg/m3)/F(000)
µ (mm-1)/temp (K)
Θ range for data collection
index ranges
1.719/1824
2.822/100(2)
2.57 to 26.00°
-20 e h e 19, -1 e k e 15, -1 e l e 21
8508/6978
0.0289/99.8%
6978/427/0
1.088
R1 ) 0.0456, wR2 ) 0.1040
R1 ) 0.0603, wR2 ) 0.1122
1.108 and -1.255
reflns collected/unique
R(int)/completeness to Θmax
data/params/restraints
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest difference peak and hole (e/Å3)
0.904
R1 ) 0.0401, wR2 ) 0.0904
R1 ) 0.0726, wR2 ) 0.1019
0.508 and -0.382
Table 2. 1H NMR Data of [(bpy)Ni(Mes)X] in Acetone-d6 (Numbering as in Scheme 1)
δ (in ppm)
5, 5′
compound
6, 6′
8.66
3, 3′
8.47
4, 4′
7.91
m-H
o-CH3
p-CH3
bpy (free ligand)
X ) F
7.40
9.46, 7.22
9.43, 7.22
9.43, 7.20
9.70, 7.41
8.92, 7.56
8.60, 7.26
8.36, 8.33
8.37, 8.34
8.36, 8.33
8.41, 8.40
8.34, 8.22
8.44, 8.37
7.73, 7.34
7.73, 7.36
7.71, 7.35
7.69, 7.03
7.72, 7.23
7.88, 7.39
6.46
6.47
6.45
6.48
6.58
6.52
3.06
3.06
3.05
3.00
3.10
3.04
2.19
2.20
2.19
2.20
2.20
2.20
X ) Cl
8.22, 8.17
8.21, 8.15
8.20, 8.19
8.20, 8.06
8.32, 8.17
X ) Br
X ) I
X ) OMe
X ) SCN
Scheme 1. Numbering of the Protons in the Complexes
[(bpy)Ni(Mes)X]
amplitude reduction factor (AFAC) was fixed at 0.8, and an overall
energy shift (Ef) was introduced to give a best fit to the data.
Computational Details. Ground-state electronic structure cal-
culations on complexes [(bpy)Ni(Mes)X] (X ) F, Cl, Br, I) have
been done on the basis of DFT methods using the Gaussian 0321
and ADF2006.0122 program packages. The lowest excited states
of the closed shell complexes were calculated by the TD DFT
method.
Within the ADF program, Slater-type orbital basis sets of triple-ꢂ
quality with two polarization functions for the Ni atom and with
one polarization function for the remaining atoms were employed;
methyl groups on the Mes coligands were described by basis sets
of double-ꢂ quality with polarization functions. The inner shells
were represented by a frozen core approximation (1s for C, N, and
F; 1s-2p for Cl; 1s-3p for Br; and 1s-3p for Ni and 1s-4p for I
were kept frozen). Within ADF, the functional including Becke’s
gradient correction to the local exchange expression in conjunction
X-Ray Spectroscopy. The X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS)
measurements were performed at the XAS beamline, ANKA,
Karlsruhe. For the measurements at the Ni K edge (8333 eV), a
Si(111) double-crystal monochromator was used. The tilt of the
second monochromator crystal was set to 40% harmonic rejection.
The spectra were collected under ambient conditions at 21 °C in
transmission mode with ion chambers. All ion chambers were filled
with nitrogen. Energy calibration was performed with nickel foil.
The solid samples were embedded in a cellulose matrix and pressed
to pellets. Data evaluation started with the removal of background
absorption from the experimental spectrum by subtraction of a
Victoreen-type polynomial, which was fitted to the pre-edge region
of the spectrum. This step and also the following normalization of
the spectrum were performed with the program WinXAS.18 After
the normalization, the program AUTOBK19 was used to remove
the postedge background and to isolate the EXAFS function ꢁ(k).
A curve-fitting analysis of the EXAFS function, weighted with k3,
was performed according to the curved wave formalism of the
program EXCURV98 with XALPHA phase and amplitude func-
tions.20 The mean free path of the scattered electrons was calculated
from the imaginary part of the potential (VPI set to -4.00); the
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(22) ADF 2006.1: (a) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen,
S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler,
T. J. Comput. Chem. 2001, 22, 931–967. ADF 2005.01: (b) SCM;
Vrije Universiteit: Amsterdam, The Netherlands. http://www.scm.com
(accessed Oct 10, 2008).
(18) Ressler, T. J. Synch. Radiat. 1998, 5, 118–122.
(19) Newville, M.; Livins, P.; Yakoby, Y.; Rehr, J. J.; Stern, E. A. Phys.
ReV. B 1993, 47, 14126–14131.
(20) Gurman, S. J.; Binsted, N.; Ross, I. J. Phys. C 1986, 19, 1845–1861.
11326 Inorganic Chemistry, Vol. 47, No. 23, 2008

Halide LigandssMore Than Just σ-Donors?
with Perdew’s gradient correction to local density approximation
with VWN parametrization of electron gas data was used (ADF/
BP).23,24 The scalar relativistic zero-order regular approximation
was used in ADF. Within Gaussian 03, polarized valence double-ꢂ
basis sets25 were used for C, N, H and X atoms and for Ni.26 The
hybrid Becke’s three-parameter functional with the Lee, Yang, and
Parr correlation functional (B3LYP)23 was used in Gaussian 03
calculations (G03/B3LYP). The conductor-like polarizable con-
tinuum model (CPCM)27 was used for modeling of the solvent
influence in TD DFT calculations. The calculations on complexes
were performed without any symmetry constraints. Spectral calcula-
tions were done at optimized geometries using an identical method.
Yield: 0.43 g, 0.926 mmol, 60%. Anal. calcd for C19H19IN2Ni
(460.95): C, 49.50; H, 4.15; N, 6.08. Found: C, 49.81; H, 4.25; N,
1
6.11%. H NMR (acetone-d6): See Table 2.
Preparation of [(bpy)Ni(Mes)(OMe)]. A total 100 mg of solid
sodium (4.35 mmol)] was dissolved in 50 mL of terahydrofuran
(THF) and 10 mL of methanol. To this solution was added 400
mg (0.966 mmol) of [(bpy)Ni(Mes)Br], the resulting solution was
stirred for 12 h. The reaction mixture was evaporated to dryness,
and the dark residue was extracted with 5 × 20 mL of dichlo-
romethane. After evaporation of the solvent, the resulting material
was twice recrystallized from dichloromethane/n-heptane (1:1) to
yield 290 mg (0.79 mmol, 82%) of violet microcrystalline material.
Anal. calcd for C20H22O1N2Ni (365.12): C, 65.79; H, 6.07; N, 7.67.
Preparation of the Precursor Complexes trans-[(PPh3)2-
Ni(Mes)X]. The precursor complexes trans-[(PPh3)2Ni(Mes)X] (X
) Cl or Br) were prepared from anhydrous NiX2, PPh3, and
corresponding Grignard reagents MesMgX, as recently described
for trans-[(PPh3)2Ni(Mes)Br].12 trans-[(PPh3)2Ni(Mes)Cl] was ob-
tained as a yellow powder, yield 17%. Anal. calcd for C45H41ClNiP2
(737.92): C, 73.25; H, 5.60. Found: C, 73.31; H, 5.65%. 1H NMR
(CD2Cl2): δ 7.47 (m, 12H, o-Ph); 7.33 (m, 6H, p-Ph); 7.22 (m,
12H, m-Ph); 5.79 (s, 2H, HMes); 2.34 (s, 6H, o-CH3); 1.89 (s, 3H,
p-CH3). 31P NMR (CD2Cl2): δ 18.67. 13C NMR (CD2Cl2): δ 142.1
(Mes1), 142.3 (Mes2), 135.2 (Ph2), 133.2 (Mes4), 132.5 (Ph1),
130.0 (Ph4), 128.0 (Ph3), 127.5 (Mes3), 26.1 (o-CH3), 19.9 (p-
CH3).
1
Found: C, 65.72; H, 5.99; N, 7.71%. H NMR (acetone-d6): See
Table 2; for the OCH3 protons, a signal was found at 3.29 ppm (s,
3H). 13C NMR (acetone-d6): δ 156.5 (bpy2), 153.5 (bpy2′), 153.1
(bpy6), 151.7 (bpy6′), 148.6 (Mes1), 143.5 (Mes2), 139.8 (bpy4),
137.9 (bpy4′), 133 (Mes4), 127.4 (bpy3), 126.7 (bpy3′), 126.2
(Mes3), 122.6 (bpy5), 121.3 (bpy5′), 49.6 (OCH3), 25.8 (o-CH3),
20.9 (p-CH3). UV-vis (acetone): 468, 540sh nm.
Preparation of [(bpy)Ni(Mes)(SCN)]. To a total of 158 mg
(0.38 mmol) of [(bpy)Ni(Mes)Br] dissolved in 50 mL of THF was
added 131 mg (0.38 mmol) of AgSbF6, and the resulting solution
was stirred in the dark for 90 min. The colorless solution obtained
from careful filtration was added to 37 mg (0.38 mmol) of KSCN
suspended in 10 mL of THF. The mixture was heated under reflux
for 3 h. The resulting orange solution was separated from some
colorless precipitate and evaporated to dryness. Recrystallization
from dichloromethane gave 121 mg (0.308 mmol, 81%) of orange-
red material. Anal. calcd for C20H19S1N3Ni (392.16): C, 61.26; H,
Preparation of the Complexes [(bpy)Ni(Mes)X] (X ) F,
Cl, Br, I). The complexes with X ) Cl or Br were synthesized by
stirring the precursor complexes trans-[(PPh3)2Ni(Mes)X] (typically
1.38 g, 1.87 mmol for X ) Cl) with bpy (310 mg, 1.964 mmol) in
diethyl ether (100 mL) at ambient temperature for 24 h. The
resulting dark red suspensions were filtered; the collected precipitate
was rinsed with n-pentane (50 mL) and dried under a vacuum. The
yield for X ) Cl was 0.57 g, 1.543 mmol, 83%. Anal. calcd for
C19H19ClN2Ni (369.52): C, 61.76; H, 5.18; N, 7.58. Found: C,
1
4.88; N, 10.72. Found: C, 61.22; H, 4.89; N, 10.81%. H NMR
(acetone-d6): See Table 2. 13C NMR (acetone-d6): δ 157.9 (bpy2),
155.4 (bpy2′), 151.8 (bpy6), 149.6 (bpy6′), 147.2 (Mes1), 141.7
(Mes2), 141.2 (bpy4), 140.4 (bpy4′), 131.2 (Mes4), 128.2 (bpy3),
128.0 (bpy3′), 126.9 (Mes3), 123.0 (bpy5), 119.3 (bpy5′), 112.1
(SCN), 24.9 (o-CH3), 20.7 (p-CH3). UV-vis (acetone): 445 nm.
IR (film): 2921s, 2098vs (νN≡C f SCN is S-bonded), 1730m,
1604m, 1442s, 1143m, 1013 m, 852m, 765s, 736s cm-1. The
compound was first mentioned in a report by Seidel in 1985,28 but
characterization remained undone.
1
61.81; H, 5.25; N, 7.61%. H NMR (acetone-d6): See Table 2.
[(bpy)Ni(Mes)Br] was obtained under the same conditions at 95%
yield. Anal. calcd for C19H19BrN2Ni (431.98): C, 55.12; H, 4.63;
1
N, 6.77. Found: C, 55.33; H, 4.65; N, 6.72%. H NMR (acetone-
d6): See Table 2.
Preparation of [(bpy)Ni(Mes)F]. To a solution of [(bpy)Ni-
(Mes)Br] (0.99 g, 2.391 mmol) in 1,2-dichloroethane (30 mL) was
added TlF (0.70 g, 3.133 mmol), and the resulting solution was
stirred at 60 °C for 18 h. The mixture was concentrated under a
vacuum to 10 mL, filtered, and treated with n-pentane (60 mL) to
precipitate the product. Yield: 310 mg, 0.878 mmol, 37%. Anal.
calcd for C19H19FN2Ni (353.08): C, 64.63; H, 5.42; N, 7.93. Found:
Results and Discussion
Preparation and Analyses. The complexes [(bpy)Ni-
(Mes)X] (X ) Cl, Br) were prepared from trans-
[(PPh3)2Ni(Mes)X] and bpy as described for X ) Br.12,14
The complexes [(bpy)Ni(Mes)X] (X ) F or I) were obtained
from [(bpy)Ni(Mes)Br] by a salt metathesis reaction using
TlF or NaI, respectively. The complexes with X ) OMe or
SCN were prepared by abstraction of Br from the bromide
complex using AgSbF6 and the addition of the corresponding
anion (KSCN or NaOMe). All complexes were obtained as
air-stable bright red microcrystalline material in good yields.
For long-time storage, the complexes have to be saved from
1
C, 64.70; H, 5.45; N, 7.94%. H NMR (acetone-d6): See Table 2.
19F NMR (acetone-d6): δ -407.1 (s).
Preparation of [(bpy)Ni(Mes)I]. A solution of [(bpy)Ni(Mes)Cl]
(0.57 g, 1.543 mmol) and dry NaI (1.16 g, 7.713 mmol) in dry
acetone (20 mL) was refluxed for 20 h. All volatile parts were
removed under a vacuum, and the solid residue was extracted with
1,2-dichloroethane. Upon the addition of n-pentane (70 mL), a dark
red precipitate formed that was filtered and dried under a vacuum.
1
humidity. From elemental analyses and H NMR spectros-
(23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Becke,
A. D. Phys. ReV. A 1988, 38, 3098–3100.
(24) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865–3868. (b) Perdew, J. P. Phys. ReV. A 1986, 33, 8822–8824.
(25) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213–222.
(26) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L. J. Chem.
Phys. 1998, 109, 1223–1229.
copy, the entity and high-purity of the obtained materials is
evident (details in the Experimental Section). The fluoride
coligand reveals a 19F NMR signal at -407 ppm, which is
typical for F ligands in square-planar organonickel(II)
(27) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669–681.
(28) Seidel, W. Z. Chem. 1985, 25, 411–412.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11327

Klein et al.
We assume that the system is quite flexible in many
parameters to compensate for various constraints from crystal
packing. At the same time, we can conclude from our study
that the effects imposed by the X coligand on the complex
geometry are not dramatic.
X-Ray Spectroscopy of [(bpy)Ni(Mes)X]. In the Ni K
edge XANES spectra for all complexes (see Figure 2), the
typical two pre-edge peaks of square-planar Ni(II) complexes
can be observed.13,31 The prepeak at about 8333 eV can be
assigned to a 1s f 3d electron transition, while the second
peak, which occurs at about 8338 eV, is due to a 1s f 4p
transition with shakedown contributions.31a-c Both observa-
tions confirm the square-planar geometry around the nick-
el(II) atom. For comparison, the “edge energies” of the
XANES spectra were determined as the energy correspond-
ing to a normalized absorption of 0.5. These edge energies
decrease slightly from F (8341.95 eV) to I (8340.77 eV),
indicating the highest electron-donating power for the I
coligand (see Figure 2, right).
Figure 3 shows the k3-weighted experimental and fitted
EXAFS spectra of the complexes [(bpy)Ni(Mes)X] along
with the magnitudes of their Fourier transforms in r space.
The obtained structural parameters of the investigated
complexes determined by a curve-fitting analysis of the
EXAFS spectra are summarized in Table 4.
The fitting of the Ni K edge EXAFS spectra was
performed using a three-shell model, in which the first
coordination shell at about 1.93 Å comprises the two
coordinating nitrogen atoms of the diimine ligand and the
carbon atom of the mesityl coligand. The second shell
consists of the halogen backscatterer, and the third coordina-
tion shell at ∼2.8 Å contains six backbone-carbon atoms of
the mesityl and diimine ligand. In the first shell, the nitrogen
and carbon backscatterers cannot be distinguished, because
of their similar distances and backscattering amplitudes.
Therefore, they were fitted as one shell with nitrogen
amplitude and phase functions.
The data obtained from EXAFS are in good agreement
with the parameters from single-crystal XRD for X ) Cl,
Br, and I (see Table 4). Only the interatomic distances of
the third shells are on average 0.05 Å smaller than the XRD
values. There are two main reasons for this larger error: (i)
carbon atoms are weak backscatterers with a small signal at
a distance of 2.8 Å; (ii) the third shells consist of six different
carbon atoms with interatomic distances ranging from 2.77
to 3.01 Å. This distance distribution also gives rise to the
relatively high Debye-Waller factors of the third shells. For
the F derivative, the same type of fit was used as for the
other three complexes. Here, the fluorine and N(bpy)/C(Mes)
Figure 1. Molecular structure of [(bpy)Ni(Mes)Cl] showing the atomic
numbering scheme. The probability ellipsoids are drawn at the 50%
probability level. H atoms are omitted for clarity.
complexes.29,30 The OMe and SCN coligands show charac-
1
teristic signals for their H or 13C atoms. A closer look at
the 1H NMR data (Table 2) reveals that the signals of the 6
and 6′ protons are extremely different. The H(6) proton is
strongly deshielded by the close-by halide coligand; the H(6′)
proton in contrast is strongly shielded by the π system of
the aromatic Mes coligand. It is interesting to note that these
effects show almost the same magnitude for F, Cl, and Br.
The deshielding effect of the I coligand is largest in that
series; the pseudohalide ligands OMe and SCN do not exhibit
a deshielding effect of the same magnitude.
Crystal Structure of [(bpy)Ni(Mes)X] (X ) Cl or I).
From concentrated solutions of the complexes [(bpy)Ni-
(Mes)X] (X ) Cl or I) in toluene, we obtained suitable single
crystals for X-ray diffraction (XRD). The compounds were
found to crystallize in the monoclinic space groups P21/c
for X ) I and P21/n for X ) Cl (for details, see Table 1). In
both crystal structures, no intermolecular contacts were
found; for X ) I, the unit cell contains two independent
molecules.
The molecules show an almost planar coordination sphere
around the nickel atom; the mesityl coligand is oriented
almost perpendicular to the coordination plane. As an
example, Figure 1 displays the molecular structure of
[(bpy)Ni(Mes)Cl]; selected structural data is summarized
together with data from the X ) Br derivative13 in Table 3.
As expected from the stronger trans influence of the formal
carbanionic mesityl coligand compared to the weaker X
coligands, the Ni-N(11) bond is markedly longer than
Ni-N(21). The observed Ni-X distances are in excellent
agreement with the sum of the covalent radii (2.15 Å for
Cl, 2.30 Å for Br, and 2.49 Å for I, see Table 4), pointing
to polar covalent character of the Ni-X bond. Other
variations in bond parameters like, for example, the
C(1)-Ni-N(11) angle, which ranges from 170 to 176°, or
the interring dihedral angle of the two pyridine units, which
shows variations from 5 to 12°, do not indicate a clear trend
within the series. The chelate bite angle N-Ni-N seems to
be fixed for all systems to about 82°, a value which has also
been observed for related systems.12-14
(31) (a) Colpas, G. J.; Maroney, M. J.; Bagyinka, C.; Kumar, M.; Willis,
W. S.; Suib, S. L.; Baidya, N.; Mascharak, P. K. Inorg. Chem. 1991,
30, 920–928. (b) Ottenwaelder, X.; Aukauloo, A.; Journaux, Y.;
Carrasco, R.; Cano, J.; Cervera, B.; Castro, I.; Curreli, S.; Munoz,
M. C.; Rosello, A. L.; Soto, B.; Ruiz-Garcia, R. Dalton Trans. 2005,
2516–2526. (c) DuBois, J. L.; Mukherjee, P.; Stack, T. D. P.; Hedman,
B.; Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 2000, 122,
5775–5787. (d) Renner, M. W.; Furenlid, L. R.; Barkigia, K. M.; Fajer,
J. J. Phys. IV France 1997, 7 (C2), 661–663. (e) Kosugi, N.;
Yokoyama, T.; Asakura, K.; Kuroda, H. Chem. Phys. 1984, 91, 249–
256. (f) Kosugi, N.; Yokoyama, T.; Kuroda, H. Chem. Phys. 1986,
104, 449–453.
(29) (a) Steffen, A.; Sladek, M. I.; Braun, T.; Neumann, B.; Stammler,
H.-G. Organometallics 2005, 24, 4062–4064. (b) Braun, T.; Perutz,
R. N. Chem. Commun. 2002, 2749–2757. (c) Braun, T.; Foxon, S. P.;
Perutz, R. N.; Walton, P. H. Angew. Chem., Int. Ed. 1999, 38, 3326–
3329.
(30) Burling, S.; Elliott, P. I. P.; Jasim, N. A.; Lindup, R. J.; McKenna, J.;
Perutz, R. N.; Archibald, S. J.; Whitwood, A. C. Dalton Trans. 2005,
368, 6–3695.
11328 Inorganic Chemistry, Vol. 47, No. 23, 2008

Halide LigandssMore Than Just σ-Donors?
Table 3. Selected Bond Lengths (Å) and Angles (deg) of [(bpy)Ni(Mes)X]
X ) Cl
X ) Br13
X ) I
molecule 1
molecule 2
molecule 1
molecule 2
Bond Lengths (Å)
2.2977(12)
Ni(1)-X(1)
Ni(1)-C(1)
2.1577(11)
1.901(3)
1.985(2)
1.911(2)
1.339(4)
1.345(4)
1.341(4)
1.365(3)
1.469(4)
2.3024(14)
1.887(8)
1.987(6)
2.4678(10)
1.900(5)
1.984(4)
1.926(4)
1.329(6)
1.355(6)
1.349(6)
1.357(6)
1.477(7)
2.4661(10)
1.914(5)
1.984(4)
1.912(4)
1.348(7)
1.355(7)
1.354(6)
1.363(6)
1.475(7)
1.904(8)
1.980(6)
1.895(6)
1.347(10)
1.326(9)
1.361(10)
1.352(10)
1.460(11)
Ni(1)-N(11)
Ni(1)-N(21)
N(11)-C(16)
N(11)-C(12)
N(21)-C(26)
N(21)-C(22)
C(12)-C(22)
1.897(6)
1.324(10)
1.359(10)
1.333(10)
1.376(10)
1.456(11)
Angles (deg)
94.3(3)
176.2(3)
82.2(3)
87.5(2)
177.5(2)
96.0(2)
C(1)-Ni(1)-N(21)
C(1)-Ni(1)-N(11)
N(21)-Ni(1)-N(11)
C(1)-Ni(1)-X(1)
N(21)-Ni(1)-X(1)
N(11)-Ni(1)-X(1)
sum of angles around Ni(1)
92.92(11)
175.40(11)
82.80(10)
89.16(9)
174.91(7)
95.24(8)
360.12
93.2(3)
170.4(3)
83.0(3)
88.3(2)
172.1(2)
96.8(2)
361.30
92.73(19)
173.00(18)
82.81(17)
86.59(14)
171.56(13)
98.62(12)
360.75
92.96(19)
173.9(2)
82.80(18)
87.56(15)
174.11(13)
97.10(13)
360.42
360.00
Dihedral Angles (deg)
N-M-N/X-Ni-CMes
Py(1)/Py(2)a
5
2.2
2.2, 2.5
90.9
8.6
1.9
1.4, 1.8
92.9
12.1
10.5
3.8, 7.1
93.4
10.2
5.0
2.9, 5.0
90.2
c
7.5
5.4
2.1, 4.1
84.7
Py/N,M,N,C(12),C(22)b
Mes/CMes,N,M,N,Xc
a Py represents the pyridine units of the chelate ligand bpy. b N,M,N,C(12),C(22) defines the chelate five ring. CMes,N,M,N,X defines the coordination
plane.
Table 4. Structural Parameters of the Solid Complexes [(bpy)Ni(Mes)X] Determined from the Ni K-Edge EXAFS Spectra in Comparison to XRD Data
and ADF/BP Calculations
X
r (Å)a
Na
σ (Å)a
Ef (eV)a
fit-indexa
28.47
XRD data (Å)
calcd datab (Å)
I
Ni-C/Nc
Ni-I
1.93(2)
2.47(2)
2.81e
1.92(2)
2.29(2)
2.82e
3
1
6
3
1
6
3
1
6
3
1
6
0.074(7)
0.081(8)
0.15(2)
0.074(7)
0.059(6)
0.10(2)
0.089(9)
0.074(7)
0.10(2)
0.055e
8.11
1.94
2.47
2.88
1.93
2.30
2.87
1.93
2.16
2.88
1.940
2.505
2.892
1.934
2.299
2.885
1.933
2.158
2.889
1.917
1.792
2.891
Ni· · ·Cd
Ni-C/Nc
Ni-Br
Br
Cl
F
7.86
7.41
2.89
21.50
33.29
34.56
Ni· · ·Cd
Ni-C/Nc
Ni-Cl
1.93(2)
2.16(2)
2. 84e
2.05e
Ni· · ·Cd
Ni-C/Nc
Ni-F
1.88e
0.045e
0.12(2)
Ni· · ·Cd
2.93e
a Absorber-backscatterer distance, r; coordination number, N; Debye-Waller factor, σ; edge position, Ef (Fermi energy) relative to calculated vacuum
zero; fit-index, R, for a fitted k range from 3.0 to 15.5 Å-1. The numbers in parentheses are uncertainties in the last digit. The errors are estimated to be (1%
for distances, (10% for Debye-Waller factors of shells with r < 2.5 Å, and (15% for Debye-Waller factors of shells with r > 2.5 Å. b From ADF/BP
calculations (see below). c N(bpy) and C(Mes) atoms. d 4C(bpy) + 2C(Mes) atoms. e Errors higher than the estimates given above due to strong correlation
between different shells (see text).
backscatterers could be fitted at 1.88 Å and 2.05 Å,
respectively. The latter value is significantly different from
the first-shell distances of the other complexes (about 1.93
Å). The reason for this discrepancy is that the backscattering
amplitudes and interatomic distances of fluorine, carbon, and
nitrogen are very similar. Therefore, in the fitting procedure,
the N(bpy)/C(Mes) contribution cannot be separated unam-
biguously from the fluorine signal, which leads to this
inaccurate Ni-C/N distance. Accordingly, also the real Ni-F
distance might deviate distinctly from the fitted value of 1.88
Å. The third shell consisting of six backbone-carbon atoms
could be fitted at 2.93 Å, which is in good agreement with
the corresponding XRD values (from single crystals) of the
Cl, Br, and I complexes.
ized organometallic complexes like cis-[(dippe)Ni(CH3)F]
(dippe ) di(isopropylphosphino)ethane), with a Ni-F dis-
tance of 1.860(2),32 or cis-[(dchpe)Ni(C5NF4)F] (dchpe )
di(cyclohexylphosphino)ethane),30 in which the Ni-F dis-
tance lies around 1.85 Å (averaged for two molecules). An
examination of Ni-F distances in the Cambridge Structural
Database reveals a bimodal distribution of Ni-F distances,
with one group showing a mean Ni-F distance of 1.86 Å
and a second group revealing a mean distance of 2.03 Å.
The latter group is represented by complexes like trans-
[(py)4NiF2],33 with an octahedrally configured nickel atom
and two electrons in the eg antibonding orbital. The first
(32) Ca´mpora, J.; Matas, I.; Palma, P.; Graiff, C.; Tiripicchio, A. Orga-
nometallics 2005, 24, 2827–2830.
(33) (a) Halasyamani, P.; Willis, M. J.; Stem, C. L.; Poeppelmeier, K. R.
Inorg. Chim. Acta 1995, 240, 109–115. (b) Holzbock, J.; Sawodny,
W.; Walz, L. Z. Kristallogr. 1997, 212, 115–120.
The observed Ni-F bond length of 1.88 Å is slightly
longer than the sum of the covalent radii of 1.80 Å (see Table
5) and in good agreement with related structurally character-
Inorganic Chemistry, Vol. 47, No. 23, 2008 11329

Klein et al.
Figure 2. Comparison of the normalized XANES spectra of the solid samples [(bpy)Ni(Mes)X] (with X ) F, Cl, Br, I) measured at the Ni K edge. On the
right: Magnified edge region of the four complexes.
Figure 3. Experimental (solid line) and calculated (dotted line) k3 ꢁ(k) EXAFS functions (left) and their Fourier transforms (right) of the solid samples
[(bpy)Ni(Mes)X] measured at the Ni K edge. Spectra for X ) Cl, Br, and I are shifted on the y axes for clarity.
Table 5. Covalent vs Ionic Radii of Ni and the X Atoms and Observed Ni-X Distances in Complexes [(bpy)Ni(Mes)X]
Ni
F
Cl
Br
I
rcov/rionic (Å)
∑(rNi + X) (Å)
rNi-X exp. XRD (Å)
rNi-X exp. EXAFS (Å)
1.16a/0.63
0.64/1.36
1.80/1.99
0.99/1.67
2.15/2.30
2.1577(11)
2.16
1.14/1.82
2.30/2.45
2.3000(13)b
2.29
1.33/2.06
2.49/2.69
2.4669(10)b
2.47
1.88
a The covalent radius for Ni is not unequivocal since it is strongly dependent on the oxidation state and ligand surroundings. The value of 1.16 Å has been
widely accepted for square-planar Ni(II) complexes;36 other values are from ref. b Averaged values.
group comprises exclusively organometallic nickel com-
plexes with a square-planar configuration. The different
electronic situation explains the markedly shortened Ni-F
distance in comparison to the nonorganometallic complexes.
An intermediate case is met in the binuclear cationic complex
[(µ-F){Ni(bpy)2F}2]+ in which the terminal Ni-F bond is
longer (2.005(4) Å) than the bridging one (1.985(3) Å).34
Interestingly, within the group of organometallic complexes,
the orientation (cis or trans) of the F ligand toward the
strongly donating organoligand has no marked impact on
the Ni-F bond length (in terms of a trans influence).
We can thus conclude that, in organometallic derivatives
(including our example), the Ni-F bonds are relatively strong
and the degree of covalency seems to be rather high. Both
(34) Emsley, J.; Arif, M.; Bates, P. A.; Hursthouse, M. B. J. Chem. Soc.,
Dalton Trans. 1989, 1273–1276.
(35) Libri, S.; Jasim, N. A.; Perutz, R. N.; Brammer, L. J. Am. Chem. Soc.
2008, 130, 7842–7844.
11330 Inorganic Chemistry, Vol. 47, No. 23, 2008

Halide LigandssMore Than Just σ-Donors?
assigned to single isolated vibrations; Table 6 indicates the
prevailing contribution to individual vibrations.
Absorption Spectroscopy and Quantum Chemical
Calculations. The absorption spectrum of [(bpy)Ni(Mes)Br]
has been reported and analyzed by quantum chemical
calculations recently, together with the spectra of [(bpy)Ni-
(Mes)2] and [(bpy)Ni(Me)2].14 The long-wavelength absorp-
tion band system for the bromide-containing derivative was
found to be blue-shifted compared to the dimesityl and
dimethyl complexes; the found negative solvatochromism
for the former is largely enhanced, but the overall character
does not differ markedly. The quantum chemical calculations
revealed contributions of bromine p orbitals to the highest
occupied molecular orbitals (HOMOs) up to 42%; resonance
Raman investigations however could not establish unequivo-
cally the bromide coligand as a part of the molecules’
chromophore. The variation of the X coligand was thus
sought in order to support the assignment. However, Figure
5 (data collected in Table 7) reveals that the long-wavelength
absorption bands of the complexes do not vary markedly.
There is only a slight red shift upon going from X ) F to I.
The extinction coefficients which have been determined with
an accuracy of (200 M-1cm-1 show an increase in the same
direction.
Thus, new DFT calculations on the present systems
[(bpy)Ni(Mes)X] were carried out using two different
program packages: Gaussian 03 and ADF. Both approaches
describe the structures of all systems quite well; however,
the ADF/BP method yields structural parameters slightly
closer to the experimental values. ADF/BP calculated
selected bond lengths and angles are compared with experi-
mental ones in Table S11 (Supporting Information).
The composition of the frontier molecular orbitals is almost
the same for all four complexes. The closely lying set of
HOMOs is formed either by Ni 3d orbitals or π orbitals of
the mesityl coligand. The three lowest-lying unoccupied
orbitals formed predominantly by π* bpy orbitals are
followed by the antibonding Ni 3d orbital. A full data
summary can be found in Tables S12-S15 in the Supporting
Information. In contrast to the previous calculations on the
[(bpy)Ni(Mes)Br] system,14 the contribution of the X coli-
gand to the HOMO was found to be marginal for X ) F
and Cl and is only slightly increasing within the series going
from F to I. The reason lies in the different type of calculation
applied. The previous work consisted of ADF in vacuo
calculations,14 whereas the present calculations include the
solvent effect (G03/B3LYP/CPCM).
Figure 4. Raman spectra of solid samples of [(bpy)Ni(Mes)X] in the range
from 100 to 700 cm-1
.
can be explained with the high electron density of the nickel
atom in organometallic derivatives fitting well to the high
electronegativity of the F atom.
Furthermore, if we compare the Ni-X distances for all
four homologous nickel complexes with the calculated sum
of the covalent radii (Table 5), only the Ni-F distance
slightly exceeds the latter, while for X ) Cl, Br, or I, the
two fit extremely well. Thus, it seems that all Ni-X bonds
in our series are highly covalent, and the (bond prolonging)
ionic contribution to the Ni-X bond is highest for fluoride
and lowest for X ) I. This is in good agreement with a recent
report on very strong hydrogen bridging in the system trans-
[(C5F4N)Ni(PEt3)2F · · · H(indole)], indicating a highly polar
Ni-F bond in the organonickel complex.35 In the section
on the ligand exchange reaction (see below), we will try to
further substantiate the question on the polarity of the Ni-F
bond.
Vibrational Spectroscopy (IR and Raman). Assuming
the same geometry for the series of isoleptic complexes
[(bpy)Ni(Mes)X], we have measured IR and Raman spectra
to probe for the influence of the X coligand. A comparison
of the spectra reveals only very subtle changes in the spectra
going through the series of X coligands, as exemplified in
Figure 4 for the Raman spectra in the range 100-700 cm-1
(a detailed lists of Raman frequencies in the range from 3200
to 200 cm-1 and further spectra can be found in the
Supporting Information). Table 6 reveals that DFT calculated
Raman active vibrations only slightly depend on ligand X
and reasonably describe the experimental spectrum. No
resonances from clearly defined vibrations like ν(Ni-X),
which were expected in this region, were discernible. The
calculations show that M-X stretching modes are strongly
coupled to other modes; thus, these bands (at around 400
and 330 cm-1) are almost invariant to the change of X, and
they contribute only to rather weak transitions. Also, other
Raman active features in this part of the spectrum cannot be
Additionally, the corresponding TD DFT (G03/B3LYP/
CPCM) calculated transitions reveal that, although the I atom
and in part also Br contribute to the HOMOs, their involve-
ment in the electronic transitions is negligible, and the
absorption spectra should thus be invariant to the variation
of X. Table 7 lists experimental and TD DFT (G03/B3LYP/
CPCM) calculated wavelengths. Selected lowest calculated
TD DFT excitation energies, together with the composition
of individual transitions, are presented in Tables S16-S19
in the Supporting Information. The simulated electronic
spectrum of [(bpy)Ni(Mes)Br] depicted in Figure 6 quali-
(36) Kilbourne, B. T.; Powell, H. M. J. Chem. Soc. (A) 1970, 1688–1693.
(37) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry,
2nd ed.; Oxford University Press: Oxford, U.K., 1994.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11331

Klein et al.
Table 6. Experimental and DFT/B3LYP Calculated Raman Active Bands for [(bpy)Ni(Mes)X] in the Range from 200 to 700 cm-1
X ) F X ) Cl X ) Br X ) I
exptl
calcda
exptl
calcda
exptl
calcda
exptl
calcda
Prevailing vibrations
660
639
562
658
634
548
543
520
417
374
367
351
253
226
659
638
562
651
622
555
551
552
400
370
339
339
246
233
658
638
562
651
622
554
552
552
389
369
336
317
246
229
658
638
562
652
622
553
548
518
376
368
335
299
244
223
νNi-N (sym) + δ (CH)
νNi-N (antisym) + δ (CH)
δ (N-Ni-C) + δ (Mes)
δ (N-Ni-C) + δ (Mes)
δ (Mes)
529
399
372
340
332
258
216
527
405
373
345
330
257
223
527
387
372
338
310
256
236
527
375
371
338
301
253
231
δ (Mes) + νNi-X
δ (N-Ni-N) (sym)
δ (Mes) + νNi-C
δ (Mes) + νNi-X
δ (bpy)
δ (bpy)
a Calculated values were scaled by the factor 0.963.
Figure 6. The simulated electronic spectrum of [(bpy)Ni(Mes)Br] based
on G03/B3LYP/CPCM (dichloromethane) calculated excitation energies and
oscillator strengths (represented by vertical lines).
(λ3-5). The calculation also reveals, in full agreement with
the experimental data, that the dominant charge transfer band
system in the visible spectrum (λ3 and λ4), which is
responsible for the color of the compounds, is invariant for
the coligand X. They thus confirm the assumptions from the
calculations of the composition of the states involved in the
lowest electronic transitions, which can overall be described
as metal(Ni)-to-ligand(bpy) charge transfer with marked
contributions from the mesityl coligand to the HOMO.
On the other hand, the calculated energies fail to match
the experimentally observed absorption maxima, although a
solvent model was included in the procedure. For example,
the calculated very long wavelength transitions around 620
nm were not observed in the experimental spectra and might
instead be part of the extended band system around 500 nm.
Nevertheless, we were quite confident that qualitatively the
calculations show the correct electronic situation of the
frontier orbitals and the electronic transitions.
A comparison to the complexes containing the pseudoha-
lides OMe or SCN reveals that the latter also show the
characteristic charge transfer band system in the visible
region. Compared to the halide complexes, their band
systems are markedly blue-shifted (Table 7). This is in
agreement with a supposed destabilization of the mainly
nickel-centered HOMO by the stronger ligands OMe and
SCN. The latter is believed to be S-bonded.
Figure 5. Absorption spectra of [(bpy)Ni(Mes)X] in dichloromethane
solution.
Table 7. Experimental Long-Wavelength Absorption Maxima of
Complexes [(bpy)Ni(Mes)X] in Dichloromethane Solution Compared
with TD DFT (B3LYP/CPCM) Calculated Wavelengths and Oscillator
Strengths
λ (nm), ε (M-1 cm-1), or calcd oscillator strength
(X)
λ1
λ2
λ3
λ4
λ5
F
exptl λ 242
295
460sh 482
526sh
1980
610
ε
22500 22640 3210
284 425
3560
476, 493
calcd λ 226
osc. str. 0.318 0.497 0.086 0.009, 0.004 0.008
Cl
Br
I
exptl λ 244
24000 23860 3520
calcd λ 237 285 425
298
463sh 483
530sh
2010
600
ε
3820
476, 493
osc. str. 0.316 0.528 0.086 0.004, 0.009 0.006
exptl λ 245
24900 23820 3810
calcd λ 230 285 429
297
463sh 489
532sh
2320
621
0.003
532sh
2540
521, 653
0.004, 0.003
518sh
720
ε
4290
484
osc. str. 0.460 0.528 0.089 0.012
exptl λ 243 298 467sh 491
26800 25100 3910
calcd λ 237 287 434
ε
4210
493
osc. str. 0.342 0.448 0.091 0.019
OMe exptl λ 243 284 445sh 464
21000 13980 1570 1680
SCN exptl λ 238 289 432sh 452
27700 27100 2900 3000
ε
495
1700
ε
tatively represents the experimental spectrum of this com-
plex.
The calculated and the experimental absorption data agree
qualitatively quite well. The calculation allows good repro-
duction of the intensity ratio between the UV (π-π*) bands
(λ1 and λ2) and the charge transfer bands in the visible region
Ligand Exchange Reactions. In a recent paper, we
reported the instability of the bromide complex [(bpy)Ni-
(Mes)Br] in strongly donating solvents like pyridine, nitriles,
DMF, or DMSO, with a very fast first reaction (eq 3),
cleaving the bromide coligand. This is followed by the
11332 Inorganic Chemistry, Vol. 47, No. 23, 2008

Halide LigandssMore Than Just σ-Donors?
-
[(bpy)Ni(Mes)(Solv)]• yE2 + e z [(bpy)Ni(Mes)(Solv)]•-
splitting of the chelate ligand (eq 4) and in the case of protic
solvents the splitting of the mesityl group.13
k1
\
(6)
[(bpy)Ni(Mes)X] + Solv y
z [(bpy)Ni(Mes)(Solv)]+ + X-
In order to suppress further chemical reactions which
follow the EC process,15 we performed the measurements
at -30 °C (in THF). The cathodic peak potentials determined
for E1 are slightly more negative for X ) Cl and F (-1.98
V) than for Br (-1.92 V) or I (-1.95 V); the E2 process
occurs for all complexes at -2.08 V, as expected. The
differences in the E1 values are definitely too small to be
discussed; the equal E2 values confirm our previous
assignment.
The oxidation of all halide complexes occurs irreversibly
around +0.17 V (in CH2Cl2), in full agreement with the
calculated HOMO of the complexes, which is composed of
nickel or Mes(π) orbitals, but obtains no contribution from
X. The OMe complex (+0.71 V) and the SCN analogue
(+0.55 V) lie markedly higher, which is in agreement with
the results from absorption spectroscopy.
(3)
k2
[(bpy)Ni(Mes)(Solv)]+ + 2Solv y
\z
[(Solv)3Ni(Mes)]+ + bpy (4)
The rate constant for the system [(bpy)Ni(Mes)Br] +
acetonitrile has been determined to be k1 ) 0.324(6) M-1
s-1 for eq 3 and k2 ) 0.0041(3) M-1s-1 for eq 4.13 Since the
splitting of the X coligand is the initial reaction, it was
obligatory to test the same reaction with the complete series
of halide coligands.
For all systems the reactions of the complexes dissolved
in acetone were monitored by UV-vis spectroscopy. Upon
the addition of acetonitrile, a rapid bleaching of the long-
wavelength band system at about 460 nm was observed and
was found to occur qualitatively in the same way for each
system. A monoexponential fit (assuming first-order kinetics)
thus allows the determination of k1. For the iodine derivative,
we found a very rapid reaction with k1 ) 0.176(4) M-1 s-1;
for the F and Cl analogues, the rates are much smaller. For
X ) F, we found k1 ) 1.54(6) × 10-4 M-1 s-1; for X ) Cl,
we determined k1 ) 5.18(5) × 10-5 M-1 s-1. We can
conclude that the substitution reaction (3) occurs very
probably in an associative manner; the rates go along the
series F < Cl < Br < I, which is parallel to their leaving
group character in organic substitution reactions.38 This, in
turn, means that there is no indication that the Ni-X bond
obtains more ionic character for the electronegative elements
Cl or F. This is in excellent agreement with other findings
in this work but contrasts with the findings for the system
trans-[(C5F4N)Ni(PEt3)2F · · · H(indole)] in which a very
strong hydrogen bonding indicates a highly polar Ni-F bond
in the organonickel complex.35 However, the F ligand in this
complex is located trans to the organo coligand, which might
enhance the Ni-F bond polarization, whereas in our system,
the more electronegative N donor atom of bpy is located
trans to the Ni-F bond.
Conclusions
Our studies on the isoleptic organonickel complexes
[(bpy)Ni(Mes)X] with X ) F, Cl, Br, I, OMe, and SCN have
revealed that the halide ligands in these systems exhibit some
particular properties. (i) They show a strong deshielding
NMR effect in their vicinity; for the pseudohalides, this effect
is much weaker. (ii) The Ni-X bond is largely covalent,
even for fluoride. The Ni-F bonds in organonickel com-
plexes are rather short and seem to be governed by the high
electronegativity of the fluorine atom rather than by the trans
influence. (iii) The contribution of the halide coligands to
the HOMOs in these complexes (π donation) is generally
quite weak and decreases along the series I > Br > Cl > F,
which parallels the decreasing overlap integrals assumed
from the covalent radii.
Acknowledgment. We thank Dr. Ingo Pantenburg for
collecting the crystal data for [(bpy)Ni(Mes)Cl], as well as
Dr. Wieland Tyrra and Dr. Harald Scherer (University of
Cologne) for NMR measurements. The ANKA Angstroem-
quelle Karlsruhe is acknowledged for the provision of
beamtime, and we would like to thank Dr. Stefan Mangold
for assistance using beamline XAS. Financial support is
acknowledged from the Academy of Sciences of the Czech
Republic (S.Z., Grant KAN100400702), from the Ministry
of Education of the Czech Republic (S.Z., Grant OC 139),
and from the Deutsche Forschungsgemeinschaft (A.K., Grant
KL 1194/5-1).
Electrochemistry. The electrochemistry of the bromide
complex [(bpy)Ni(Mes)Br] has been studied in detail re-
cently.15 From this, we knew that, after the first one-electron
reduction (E1), the complex rapidly cleaves the bromide
ligand (eq 5), which renders the first reduction wave
irreversible (EC process).
-
C
\z
[(bpy)Ni(Mes)Br] yE1 + e z [(bpy)Ni(Mes)Br]•- y
Supporting Information Available: Further tables with full
structural data for the structures of [(bpy)Ni(Mes)Cl] and [(bpy)Ni-
(Mes)I], tables with the complete results of the quantum chemical
calculations, and further figures showing the crystal structures (unit
cell) of the two complexes, together with further EXAFS, Raman,
and IR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
[(bpy)Ni(Mes)(Solv)]• + Br- (5)
The second reduction (E2) wave is reversible, and is
attributed to the reduction of the solvent complex (Eq. 6).
(38) Jencks, W. P. Acc. Chem. Res. 1980, 13, 161–169.
IC8007365
Inorganic Chemistry, Vol. 47, No. 23, 2008 11333