EPR spectra from EPR-silent species: high-frequency and high-field EPR spectroscopy of pseudotetrahedral complexes of nickel(II).

Article abstract of DOI:10.1021/ic020198j

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 90-550 GHz and fields up to approximately 15 T) has been used to probe the non-Kramers, S = 1, Ni(2+) ion in a series of pseudotetrahedral complexes of general formula NiL(2)X(2), where L = PPh(3) (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), /E/ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh(3))(2)Cl(2). These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh(3))(2)Br(2), HFEPR suggested /D/ = 4.5(5) cm(-1), /E/ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh(3))(2)I(2) was unsuccessful; however, magnetic susceptibility measurements indicated /D/ = 27.9(1) cm(-1), /E/ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting ( approximately 840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.

Full text of DOI:10.1021/ic020198j

Inorg. Chem. 2002, 41, 4478−4487
EPR Spectra from “EPR-Silent” Species: High-Frequency and
High-Field EPR Spectroscopy of Pseudotetrahedral Complexes of
Nickel(II)
J. Krzystek,§ Ju-Hyun Park,# Mark W. Meisel,# Michael A. Hitchman,‡ Horst Stratemeier,‡
,†
Louis-Claude Brunel,§ and Joshua Telser*
Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory,
Florida State UniVersity, Tallahassee, Florida 32310, Department of Physics and Center for
Condensed Matter Sciences, UniVersity of Florida, GainesVille, Florida 32611-8440,
Department of Chemistry, UniVersity of Tasmania, Hobart, Tasmania, Australia, and
Chemistry Program, RooseVelt UniVersity, Chicago, Illinois 60605-1394
Received March 12, 2002
High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of ∼90−
550 GHz and fields up to ∼15 T) has been used to probe the non-Kramers, S ) 1, Ni2+ ion in a series of
pseudotetrahedral complexes of general formula NiL2X , where L ) PPh3 (Ph ) phenyl) and X ) Cl, Br, and I.
2
Analysis based on full-matrix solutions to the spin Hamiltonian for an S ) 1 system gave zero-field splitting
parameters: D ) +13.20(5) cm-1, |E| ) 1.85(5) cm-1, g ) g ) g ) 2.20(5) for Ni(PPh3)2Cl2. These values
x
y
z
are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization
measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh3)2Br2, HFEPR
suggested |D| ) 4.5(5) cm-1, |E| ) 1.5(5) cm-1, g ) g ) 2.2(1), and g ) 2.0(1), which are in agreement with
x
y
z
concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were
performed on a minor crystal form, while the present study was performed on the major form, and apparently the
electronic parameters differ greatly between the two. HFEPR of Ni(PPh3)2I2 was unsuccessful; however, magnetic
susceptibility measurements indicated |D| ) 27.9(1) cm-1, |E| ) 4.7(1), g ) 1.95(5), g ) 2.00(5), and g )
x
y
z
2.11(5). This magnitude of the zero-field splitting (∼840 GHz) is too large for successful detection of resonances,
even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their
molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting
of experimental data to ligand-field parameters, shows that the halo ligands act as strong π-donors, while the
triphenylphosphane ligands are π-acceptors.
Introduction
the electronic structure transition metal ions with half-integer-
spin ground states (Kramers systems).2 This method has been
much less successful when applied to transition metal ions
with integer-spin ground states (non-Kramers systems). In
such cases, unless the symmetry about the metal ion is high
(nearly cubic), the magnitude of the axial zfs parameter, |D|,
is often larger than the available microwave quantum and
the system is “EPR-silent”. In high-spin integer systems with
S g 2 and rhombic symmetry, some nominally spin-
forbidden transitions can be detected in certain cases at
frequencies much lower than |D| (and usually at very low
Standard EPR1 spectroscopy at conventional microwave
frequencies (X-band ∼9 GHz (microwave quantum energy:
0.3 cm-1); Q-band ∼35 GHz (1.2 cm-1)) and magnetic fields
(up to ∼1.5 T) has been fruitfully employed to determine
* Author to whom correspondence should be addressed. E-mail: jtelser@
roosevelt.edu.
§ Florida State University.
# University of Florida.
‡ University of Tasmania.
† Roosevelt University.
(1) Abbreviations used are as follows: AOM, angular overlap model; EPR,
electron paramagnetic resonance; FC, field-cooled; HFEPR, high-
frequency and -field EPR; HS, high-spin; LS, low-spin; S/N, signal-
to-noise ratio; ZFC, zero-field cooled; zfs, zero-field splitting.
(2) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Dover Publications: New York, 1986.
4478 Inorganic Chemistry, Vol. 41, No. 17, 2002
10.1021/ic020198j CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/31/2002

EPR Spectra from “EPR-Silent” Species
magnetic fields),2 especially when using a parallel mode
detection of the EPR signal.3,4 However, in transition metal
ions with S ) 1 ground states, such “non-Kramers” signals
are not observable as they would appear at fields and/or
frequencies beyond the range of conventional EPR spec-
trometers, even in the case of rhombic symmetry.
forms pseudotetrahedral complexes that are “HS d8”. The
molecular and electronic structure of these complexes has
been of interest for many years. Crystal structures for the
series Ni(PPh3)2X2 (X ) Cl, Br, and I) were originally
reported by Garton et al.17 Much more accurate structures,
which in some cases resolved crystallographic ambiguities,
were subsequently reported for each of the following series:
Cl,20 Br,21 and I.22 Each complex clearly has pseudotetrahe-
dral geometry, with approximately C2V symmetry about Ni2+.
The electronic absorption spectra of the chloro and bromo
complexes have also been carefully examined,23,24 as have
their magnetic properties.24 The study by Gerloch and co-
workers is particularly noteworthy in that single-crystal
magnetic susceptibility measurements were made and were
combined with an analysis of the optical spectra, thereby
providing a complete description of the electronic structure
of Ni(PPh3)2Cl2 and Ni(PPh3)2Br2.24
Consequently, with the aim of extending the applicability
of HFEPR and of providing information that complements
and extends the range available by other techniques, the
Ni(PPh3)2X2 series present excellent candidates for study by
HFEPR. Magnetic susceptibility measurements were also
made on these complexes to provide important corroborating
evidence concerning the electronic parameters determined
by HFEPR. The combination of HFEPR with magnetic
measurements has been useful in other studies of transition
metal complexes.25
High-frequency and high-field EPR (HFEPR; ν > 90 GHz;
B0 up to ∼25 T)5-7 has the ability to overcome this difficulty
by the combination of sufficiently high-mm/sub-mm fre-
quencies and magnetic fields, so that EPR signals are
observable in S ) 1 systems characterized by large zfs,
whether of axial or rhombic symmetry. Such an observation
has been recently made for two S ) 1 ions: Ni2+ (3d8) and
V
3+ (3d2), both with pseudooctahedral symmetry. In several
of these HFEPR studies, the paramagnetic ions were dopants
into diamagnetic hosts: V3+ was in a Ga3+ host with an O6
donor set8 and Ni2+ was in a Zn2+ host with either a N6 9 or
O4N2 10 donor set. Another study on powder Ni2+ complexes
with N6 and N4O2 donor sets employed 9- and 35-GHz as
well as 180-GHz HFEPR.11
As part of our efforts to expand the use of HFEPR in the
study of “EPR-silent” molecules,12-15 we describe here the
use of this technique to study a series of solid molecular
complexes of Ni2+ in a highly distorted pseudotetrahedral
environment, which produces an S ) 1 ground state of
significant zfs, and thus no observable conventional EPR
spectrum.
The specific systems investigated are a series of dihalo-
bistriphenylphosphane complexes of Ni2+, thus having the
general formula NiL2X2, where L ) PPh3 and X ) Cl, Br,
and I. The discovery of these complexes by Venzani over
forty years ago was itself a significant development in
inorganic chemistry.16,17 Four-coordinate transition metal
complexes of nd8 electronic configuration are typically found
in square-planar geometry and are thus diamagnetic (“LS
d8”).18 With certain, sufficiently bulky ligands, such as
triphenylphosphane16,17 and triphenylphosphane oxide,19 Ni2+
Experimental Section
Materials. Ni(PPh3)2Cl2 was obtained from Aldrich and used
without further purification. Ni(PPh3)2Br2 was obtained from
Aldrich as well, but was also synthesized from NiBr2 and PPh3 in
1-butanol, as described by Venzani,16 to give dark green needle
crystals. The bulk crystalline product presumably corresponds to
that crystallographically characterized,21 synthesized using the
same procedure. HFEPR results on both commercial and synthetic
Ni(PPh3)2Br2 were essentially identical. Ni(PPh3)2I2 was synthesized
following the same procedure to give dark red block crystals.
EPR Spectroscopy. HFEPR spectra were recorded on a spec-
trometer that has been previously described in detail.26 Briefly,
spectra can be recorded over the field range of 0-15 T at
fundamental frequencies of 95 and 110 GHz, and at harmonic
multiples of these frequencies (e.g., 190 or 220 GHz) up to the
fifth harmonic (475 or 550 GHz). Mechanical tuning of the Gunn
oscillator is possible within the limit of about 3 GHz above, and
(3) Hendrich, M.; Debrunner, P. Biophys. J. 1989, 56, 489-506.
(4) Campbell, K. A.; Yikilmaz, E.; Grant, C. V.; Gregor, W.; Miller, A.-
F.; Britt, R. D. J. Am. Chem. Soc. 1999, 121, 4714-4715.
(5) Barra, A.-L.; Brunel, L.-C.; Gatteschi, D.; Pardi, L.; Sessoli, R. Acc.
Chem. Res. 1998, 31, 460-466.
(6) Brunel, L.-C. Physica B 1995, 360-362.
(7) Hagen, W. Coord. Chem. ReV. 1999, 190, 209-229.
(8) Tregenna-Piggott, P. L. W.; Weihe, H.; Bendix, J.; Barra, A.-L.; Gu¨del,
H.-U. Inorg. Chem. 1999, 38, 5928-5929.
(9) van Dam, P. J.; Klaassen, A. A. K.; Reijerse, E. J.; Hagen, W. R. J.
Magn. Reson. 1998, 130, 140-144.
(18) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. In
AdVanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; pp
840-842.
(10) Pardi, L. A.; Hassan, A. K.; Hulsbergen, F. B.; Reedijk, J.; Spek, A.
L.; Brunel, L.-C. Inorg. Chem. 2000, 39, 159-164.
(11) Collison, D.; Helliwell, M.; Jones, V. M.; Mabbs, F. E.; McInnes, A.
J. L.; Riedi, P. C.; Smith, G. M.; Pritchard, R. G.; Cross, W. I. J.
Chem. Soc., Faraday Trans. 1998, 94, 3019-3025.
(12) Goldberg, D. P.; Telser, J.; Krzystek, J.; Montalban, A. G.; Brunel,
L.-C.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1997,
119, 8722-8723.
(19) Cotton, F. A.; Goodgame, D. M. L. J. Am. Chem. Soc. 1960, 82, 5771-
5774.
(20) Brammer, L.; Stevens, E. D. Acta Crystallogr. 1989, C45, 400-403.
(21) Jarvis, J. A. J.; Mais, R. H. B.; Owston, P. G. J. Chem. Soc. (A) 1968,
1473-1486.
(22) Humphry, R. W.; Welch, A. J.; Welch, D. A. Acta Crystallogr. 1988,
C44, 1717-1719.
(13) Telser, J.; Pardi, L. A.; Krzystek, J.; Brunel, L.-C. Inorg. Chem. 1998,
37, 5769-5775.
(23) Fereday, R. J.; Hathaway, B. J.; Dudley, R. J. J. Chem. Soc. (A) 1970,
571-574.
(14) Krzystek, J.; Telser, J.; Pardi, L. A.; Goldberg, D. P.; Hoffman, B.
M.; Brunel, L.-C. Inorg. Chem. 1999, 38, 6121-6129.
(15) Krzystek, J.; Telser, J.; Hoffman, B. M.; Brunel, L.-C.; Licoccia, S.
J. Am. Chem. Soc. 2001, 123, 7890-7897.
(24) Davies, J. E.; Gerloch, M.; Phillips, D. J. J. Chem. Soc., Dalton Trans.
1979, 1836-1842.
(25) Fanucci, G. E.; Krzystek, J.; Meisel, M. W.; Brunel, L.-C.; Talham,
D. R. J. Am. Chem. Soc. 1998, 120, 5469-5479.
(26) Hassan, A. K.; Pardi, L. A.; Krzystek, J.; Sienkiewicz, A.; Goy, P.;
Rohrer, M.; Brunel, L.-C. J. Magn. Reson. 2000, 142, 300-312.
(16) Venzani, L. M. J. Chem. Soc. 1958, 719-724.
(17) Garton, G.; Henn, D. E.; Powell, H. M.; Venzani, L. M. J. Chem.
Soc. 1963, 3625-3629.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4479

Krzystek et al.
below, the fundamental frequency. This tuning produces a range
that increases with the harmonic number, so for example at the 5th
harmonic of the 95 GHz source, the available frequency range is
quite significant, amounting to ca. 460-490 GHz. Thus using the
combination of available harmonics and source tuning ability
provides the possibility of collecting spectra spanning a broad range
of frequencies.
Typically, 30 mg of sample was used for HFEPR samples of
solids. In previous HFEPR studies of pure solid samples character-
ized by S ) 2, and negative D, magnetic field-induced torquing of
microcrystallites occurred, so that quasi-single-crystal spectra were
obtained at low temperatures.12,14,15 In the NiL2X2 series, this effect
was much weaker, with only partial orientation occasionally
observed. These torquing effects were further prevented by the
immobilization procedures used previously (embedding the sample
in a KBr pellet or n-eicosane mull). A more frequent phenomenon
in loose samples was the appearance of “pseudonoise” originating
from a superposition of narrow “single-crystal-like” spectra, such
as can be obtained in a simulated EPR spectrum when insufficient
single-crystal orientation averaging is used.27 We thus ground the
samples before the experiment to ensure a random distribution of
the crystallites in space. Even so, the high-field dispersion provided
by HFEPR sometimes precluded the recording of ideal powder-
pattern spectra.
surements were run from 2 to 300 K. The sample was zero-field
cooled (ZFC) to 2 K before a measuring field of 0.01 or 0.1 T was
applied. Data were then recorded while warming the sample from
the lowest temperature. The sample was then cooled again to 2 K,
but in the presence of a 0.1 T (1000 G) field, and additional field-
cooled (FC) data were acquired. Magnetization versus field
measurements were performed at 2 K over the field range 0-5 T.
The background signal was estimated by independently measur-
ing a separate can and straw, and these values were subtracted from
the results. The experiment uncertainty arising from the sample
holder and SQUID itself contribute a maximum of 5.3 × 10-4 emu/
mol in the high-temperature region (T J 40 K). The SQUID
magnetometer automatically calculates an intrinsic uncertainty by
using an averaging process for a measurement at a given temper-
ature. In addition, the initial room temperature magnetization value
is compared with the value recorded after the entire cool down,
measurement, and warm up process is completed. This procedure
provides a check on the reproducibility of the measurement and an
estimate of the overall uncertainty.
The diamagnetic contribution of each sample, estimated from
Pascal’s constants,29 øD ) -407.4 × 10-6 emu/mol for Ni(PPh3)2-
Cl2, -429.8 × 10-6 emu/mol for Ni(PPh3)2Br2, and -461.8 × 10-6
emu/mol for Ni(PPh3)2I2, was then subtracted from each background
corrected value to yield the paramagnetic molar susceptibility, øP.
The resulting values of the susceptibility were compared to the
theoretical expectations in the high-temperature limit, i.e., near 300
K. For Ni(PPh3)2Cl2 and Ni(PPh3)2I2, these comparisons were in
exact agreement. For Ni(PPh3)2Br2, a direct measurement of
background signal and estimated Pascal’s constants corrections
indicated that within the overall experimental uncertainty (see
above), the resulting susceptibility agreed with the theoretical
expectation in the high-temperature limit.
Magnetic susceptibility data were fit by using the same spin
Hamiltonian (eq 1) and nonlinear least-squares routine as for the
EPR data. Programs employed either analytical solutions to the spin
Hamiltonian for an axial system29 or exact solutions (matrix
diagonalization, using the EISPACK routines) for either axial or
rhombic systems. These methods have been used by some of us
previously.30,31 Analytical and exact methods gave essentially the
same results for fitting to the axial case. For fits to rhombic systems,
all three g values were allowed to vary independently. The rhombic
splitting, E, was either allowed to vary independently of D or
constrained to a magnitude eD/3. Because magnetic susceptibility
data are relatively insensitive to the sign of D, fits were performed
with D constrained either to <0 or to >0, with E given the same
sign, where applicable. Analogous procedures were employed to
fit the field-dependent magnetization data. All EPR and magnetic
data fitting software are available as FORTRAN source code from
the corresponding author.
EPR Analysis. The magnetic properties of an ion with S ) 1
can be described by the standard spin Hamiltonian comprised of
Zeeman and zfs terms:2
2
H ) âB‚g‚S + D(Sz2 - S(S + 1)/3) + E(Sx2 - Sy )
(1)
Two different procedures were used to extract numerical values of
the spin Hamiltonian parameters from the experimental spectra. In
the first procedure, numerical methods were used to calculate the
EPR transition energies and probabilities from the eigenvalues and
eigenvectors, respectively, obtained by diagonalization of the spin
Hamiltonian matrix resulting from eq 1. These results were used
to create characteristic canonical resonance field versus EPR
operating frequency dependencies. The program has been modified
to allow nonlinear least-squares fitting (using the program DSTEPIT
from QCPE, Bloomington, IN) of the spin Hamiltonian parameters
to the experimental dependencies of the same kind. This procedure
was used in conjunction with human judgment, which was used to
eliminate unphysical, but mathematically possible, results to obtain
best fit parameters for the entire field vs frequency array of EPR
transitions for a given complex. As an alternative procedure, as in
our previous studies, a program written by Weihe28 was used to
generate powder pattern EPR spectra for individual frequencies,
ensuring the correctness of assigning the observed EPR transitions
through the first procedure. Spectral simulation also aided in
determining the precise resonance fields, since the experimental
line shapes often did not have an ideal first derivative appearance.
Magnetic Measurements. Bulk magnetization measurements
were obtained from a standard Quantum Design MPMS SQUID
magnetometer. The samples consisted of randomly oriented mi-
crocrystals with a total mass of 170 mg for Ni(PPh3)2Cl2, 140 mg
for Ni(PPh3)2Br2, and 100 mg for Ni(PPh3)2I2. A small plastic can
(0.273-mL polyethylene vial, Scienceware from Bel-Art Products,
Pequannock, NJ) and plastic straw were used as the sample holder
during the measurements. Magnetization versus temperature mea-
Ligand-Field Analysis. Ligand-field parameters from the AOM
model32 were estimated for the Ni(PPh3)2X2 complexes by using
two programs in tandem: CAMMAG, written by Gerloch and co-
workers,33 and AOMX, written by Adamsky.34 Both programs use
(29) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203-283.
(30) Telser, J.; Drago, R. S. Inorg. Chem. 1984, 23, 3114-3120.
(31) Eichhorn, D. M.; Telser, J.; Stern, C. L.; Hoffman, B. M. Inorg. Chem.
1994, 33, 3533-3537.
(32) Scha¨ffer, C. E. Struct. Bonding 1968, 5, 68-95.
(33) Gerloch, M. CAMMAG, a FORTRAN program for AOM calculations;
University of Cambridge: Cambridge, U.K.
(34) Adamsky, H. AOMX, an Angular Overlap Model Computer Program;
Department of Theoretical Chemistry, Heinrich-Heine-Universita¨t:
Du¨sseldorf, Germany, 1994. For more information see the WWW
site: http://www.theochem.uni-duesseldorf.de/∼heribert/aomx/aomxhtml/
aomxhtml.html.
(27) Mombourquette, M. J.; Weil, J. A. J. Magn. Reson. 1992, 99, 37-44.
(28) Simulation software is available from Dr. H. Weihe; for more
information see the WWW page: http://sophus.kiku.dk/software/epr/
epr.html.
4480 Inorganic Chemistry, Vol. 41, No. 17, 2002

EPR Spectra from “EPR-Silent” Species
Figure 2. Experimental (top) and simulated (bottom) HFEPR spectra of
Ni(PPh3)2Cl2 immobilized in n-eicosane mull at 434.67 GHz. Other
experimental conditions and identification of particular transitions are as
in Figure 1. The parameters used in the simulation were the following: D
) +13.20 cm-1; |E| ) 1.85 cm-1; gx ) 2.25, gy ) 2.17, and gz ) 2.20;
single-crystal line width, ∆Bx ) ∆Bz ) 50 mT, ∆By ) 5 mT.
Figure 1. HFEPR spectra of loose Ni(PPh3)2Cl2 at several frequencies
(as indicated). Other experimental conditions: temperature, 4.5 K; field
sweep rate, 0.5 T/min; field modulation frequency, 8 kHz; amplitude, 1.5
mT; available sub-mm power, ca. 10 µW. In the bottom spectrum, the
specific transitions corresponding to canonical orientations in the powder
pattern are identified by using the nomenclature common for triplet
systems.36
the entire dn basis set together with geometrical information
provided by X-ray structures to provide energy levels and spin and
symmetry identification of d orbital states. The program AOMX
allows least-squares fitting of energy levels to ligand-field param-
eters. Because of the local C2V symmetry of the Ni(PPh3)2X2
complexes, the AOM parameters of the two triphenylphosphane
ligands and of the two halo ligands were held equivalent. As in
the original study by Davies et al.,24 bonding to PPh3 was described
by the AOM parameters: ꢀσ(P) and ꢀπ(P) respectively for σ- and
cylindrical π-bonding, and bonding to X ) Cl, Br, and I cor-
respondingly by ꢀσ(X) and ꢀπ(X).
Figure 3. Experimental (top) and simulated (middle and bottom) HFEPR
spectra of loose Ni(PPh3)2Cl2 at 490.46 GHz and 5 K. Other experimental
conditions and identification of particular transitions are as in Figure 1.
The parameters used in the simulation were the following: D ) +13.20
cm-1 (solid line) or D ) -13.20 cm-1 (dashed line), |E| ) 1.85 cm-1; g
) 2.20 (isotropic); and single-crystal line width, 50 mT (isotropic).
Results
HFEPR Spectroscopy of Ni(PPh3)2Cl2. Typical HFEPR
spectra at 4.5 K of microcrystalline Ni(PPh3)2Cl2 at several,
closely spaced, sufficiently high sub-mm wave frequencies
are presented in Figure 1. The spectrum at 451.4 GHz shows
a broad, but distinct zero-field signal that becomes visible
in its entirety at 464.9 GHz and develops with increasing
frequency into two distinct transitions visible at ∼2.5 and
∼4 T, respectively, at 490.5 GHz. Consideration of a rhombic
spin triplet system,35 which is assuredly the case for the C2V
symmetry NiL2X2 complexes, allows identification of the
lower field signal as Bmin and that at the higher field as B2y.
Here, and in the following, we are using the notation of
Wasserman36 for identifying the particular canonical orienta-
tions of the zfs tensor relative to the magnetic field. Two
additional features, which are seen in the 450-490 GHz
frequency range, move smoothly to higher field with
increasing frequency and can be identified as the B1y and
B2x transitions, respectively. The B1y transition has a peculiar
shape with a repeatable double “spike” of very narrow signals
(see Figure S1 (Supporting Information), where the spectrum
was taken in a narrower field range). This structure is an
artifact due to partial torquing, and has a beneficial side
effect, namely that this transition is identifiable over a much
wider frequency range than would otherwise be possible. An
experiment on a sample embedded in n-eicosane mull at a
somewhat lower frequency of 435 GHz is shown in Figure
2 and proves that the double-spiked shape of the B1y
transition is an artifact. At this frequency, the two observed
signals correspond to the B1y and B2x transitions. It is possible
to match closely the resonant fields of the observed signals
by using the powder pattern simulation program28 with the
following parameters: |D| ) 13.20 cm-1; |E| ) 1.85 cm-1;
gx ) 2.20; gy ) 2.17; and gz ) 2.20. The zero-field signal
appearing at 451.4 GHz in Figure 1 can be directly identified
as the (D + |E|) transition in the triplet manifold, thus
yielding in a direct way D + |E| ) 15.05 cm-1.
Experimental and simulated spectra at 4.5 K of Ni(PPh3)2-
Cl2 at 490.46 GHz are presented in Figure 3 and correspond-
ing spectra at 535.68 GHz are shown in Figure S2 (Sup-
porting Information). At these highest available frequencies,
corresponding to the fifth harmonic of the 95 and 110 GHz
source, respectively, one finally enters the high-frequency
regime (hν > |D|), resulting in powder pattern EPR spectra
for Ni(PPh3)2Cl2 more typical of triplet states.35 At low field,
Bmin is observed, and B2y and B2x are the main higher field
features. Likely due to their low intensities, no z transitions
are observed. The intensity of the Bmin transition is about 1
(35) Weltner, W., Jr. Magnetic Atoms and Molecules; Dover: New York,
1983.
(36) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763-1772.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4481

Krzystek et al.
triplet spectrum,35,36 with a large Bmin signal at low field and
a powder pattern bounded by two features, which we assign
to B2y,z and B3y,z. This assignment suggests a lower magnitude
of zfs in the bromo complex. The apparent noise in the
spectra is not due to a low signal/noise ratio (S/N) (“actual”
instrument noise), as can be seen by the excellent S/N in
the Bmin region, but to signals from individual microcrys-
tallites giving an incomplete powder pattern orientation that
can be generated by computer simulation with an insuf-
ficiently fine grid.27 This effect is even more recognizable
in a spectrum taken at a somewhat lower frequency, 291
GHz, where the apparent S/N ratio again becomes very good
at high field, after passing through the B3y,z resonance (Figure
S3). Figure S3 also contains a spectrum taken at a higher
frequency of 497 GHz.
The analysis of the complete set of spectra at multiple
frequencies is not straightforward due to the absence of
several expected transitions. Note that for the case of
maximum zfs rhombicity (|E| ) |D|/3), the y and z transitions
(B2y and B2z) overlap, and so do the two x transitions (B2x
and B3x), which could explain the number of observed lines.
The x transitions come rather close to g ) 2.20 at high
frequencies, thereby making attractive its attribution to a
double-quantum transition appearing in other Ni(II) sys-
tems.9-11 At lower frequencies, however, this feature does
not remain at fields corresponding to g ) 2.20, rather it shifts
to higher effective g values, which is not as expected for a
double-quantum transition.9,11 An alternative attribution to
some high-symmetry Ni(II) impurity with isotropic g ) 2.20
fails for the same reason. While it is possible to simulate
the spectrum in Figure 5 (326 GHz) satisfactorily (and
likewise for the spectra in Figure S3), we have not found a
unique set of values that would fully satisfactorily recreate
the spectrum at any single frequency. Thus, no simulations
are included in Figures 5 and S3.
Consequently, we have not identified a unique set of spin
Hamiltonian parameters that fully model the HFEPR behav-
ior for Ni(PPh3)2Br2. Nevertheless, there are two viable
scenarios as shown in Figure 6, which presents the com-
plete resonance field versus frequency dependence for
Ni(PPh3)2Br2 at 4.5 K. There is a single set of experi-
mental points (represented by squares), but two alternative
sets of simulations, represented by the lines, and labeled
accordingly. In Figure 6A, the following spin Hamiltonian
parameters were used: |D| ) 4.5 cm-1; |E| ) 1.5 cm-1; gx,y
) 2.2; and gz ) 2.0. The lower simulation set, Figure 6B,
used the following: |D| ) 4.2 cm-1; |E| ) 1.0 cm-1; and
giso ) 2.2.
The magnitude of D is essentially the same in both cases;
however, case (A) corresponds to one of maximum rhom-
bicity, |E| ) |D|/3, while case (B) has a lower magnitude of
E. Only in case (A) is it possible to simulate the observed
number of branches, without neglecting one or the other
transition branch. Although each version shows some dis-
crepancies between the experimental and simulated resonance
field vs frequency dependencies, we believe that case (A)
better reflects the reality. The discrepancies are the cause of
the relatively large error margins as shown in Table 1.
Figure 4. Complete resonance field vs frequency dependence of HFEPR
spectra of Ni(PPh3)2Cl2 at 5 K. Experimental resonance positions at specific
frequencies are given by the squares and calculated resonances are shown
by lines. Spin Hamiltonian parameters used in the calculated lines were
the following: |D| ) 13.20 cm-1; |E| ) 1.85 cm-1; and g ) 2.20 (isotropic).
The calculated lines are identified by using standard nomenclature36 for
triplet states with rhombic symmetry.
order of magnitude weaker than calculated (compare simu-
lated and experimental spectra in Figures 3 and S2), which
suggests some nonrandom distribution of microcrystallites
in the field due to torquing since Bmin is an off-axis turning
point whose intensity strongly depends on the random
distribution of crystallites in space.36
The entire resonance field vs frequency spectral data set
of Ni(PPh3)2Cl2, of which Figures 1-3, S1, and S2 represent
cross-sections along the field axis, is shown in Figure 4. The
resonances plotted in this representation form characteristic
branches that are labeled according to the terminology of
Wasserman.36 Simulation of individual spectra facilitates
optimal selection of resonant field values. The calculated
lines through these points are based on a combination of
automated nonlinear least-squares fitting with the use of
human judgment to eliminate physically unreasonable results.
The automated fitting procedure with isotropic g led to |D|
) 13.20 cm-1 and |E| ) 1.85 cm-1, and these values
provided the best fits of the individual spectra as well as
of the entire 2-dimensional field-frequency data set. The
fitting parameter values are summarized in Table 1. When
variation in g values was allowed, this procedure did lead
to a slight deviation from giso ) 2.20 (see Table 1). Although
we believe that the g tensor is indeed likely rhombic, the
level of precision warranted by the data does not allow for
g rhombicity to be specified and, therefore, we report the
values obtained with giso ) 2.20. This value above ge is
expected for Ni2+, a greater than half-filled d electron
system.2
HFEPR Spectroscopy of Ni(PPh3)2Br2. The HFEPR
spectra for microcrystalline Ni(PPh3)2Br2 were recorded at
low temperatures, and Figure 5 presents a spectrum at 4.5
K and 325.9 GHz. The transitions in the powder spectrum
are identified tentatively, as discussed below. The weak
transitions marked with an asterisk (*) originate from the
fourth harmonic of the fundamental frequency, which is equal
to 434.5 GHz. In contrast to Ni(PPh3)2Cl2, the Ni(PPh3)2Br2
spectrum at this frequency strongly resembles a “typical”
4482 Inorganic Chemistry, Vol. 41, No. 17, 2002

EPR Spectra from “EPR-Silent” Species
Table 1. Electronic Parameters for NiL2X2 Complexes As Determined by Powder HFEPR and Magnetic Susceptibility Measurements
complex
method
D (cm-1
)
|E| (cm-1
1.85(5)
1.78(5)
)
gx
gy
gz
Ni(PPh3)2Cl2
HFEPRa
+13.20(5)
+12.03(5)
+14
+13.1(2)
+4.5(5)
+5.38(5)
+13.3
+3.8(2)
+27.92(5)
+25.6(2)
2.20(5)
1.99(2)
2.03
2.20 (fix)
2.2(1)
2.06(5)
1.85
2.00 (fix)
1.95(5)
2.00 (fix)
2.20(5)
2.00(2)
2.20(5)
2.40(2)
2.51
2.20 (fix)
2.0(1)
2.22(5)
2.77
2.00 (fix)
2.11(5)
2.00 (fix)
magnetic susceptibility, powdera,b
magnetic susceptibility, crystalc
field-dependent magnetization powdera,d
HFEPRa
0.0(2)
1.5(5)
1.76(5)
2.20 (fix)
2.2(1)
2.00(5)
Ni(PPh3)2Br2
magnetic susceptibility, powdera,b,e,f
magnetic susceptibility, crystalc,e
field-dependent magnetization powdera,d,e
magnetic susceptibility, powdera,b
field-dependent magnetization powdera,d
0.0(2)
4.71(5)
0.0(2)
2.00 (fix)
2.00(5)
2.00 (fix)
Ni(PPh3)2I2
a This work. Values in parentheses give uncertainties. b Powder data over the temperature range 2-300 K at a field of 0.1 T. c Single-crystal data over
the temperature range 20-295 K, taken from Davies et al.,24 with powder average calculated here: øpowder ) [øx + øy + øz]/3 ) [øa* + øb + øc]/3. Since
only the magnitude of |D| is important here, axial fitting was used; the single-crystal data indicated relatively small rhombic effects for both complexes.
d Powder data over the field range 0-5 T at a temperature of 2 K. To obtain a meaningful fit, it was necessary to fix the g values to those obtained from
HFEPR or simply to g ) 2.00. e The powder data for Ni(PPh3)2Br2 are for the bulk crystalline form, which predominantly corresponds to that of the reported
structure.21 The single-crystal data are for a minor form, which was specifically chosen, and is described to be isomorphous with Ni(PPh3)2Cl2, although no
structure was reported.24 f It was possible to fit the powder data for Ni(PPh3)2Br2 by using a negative value for D (and E): D ) -8.10 cm-1, E ) -1.60
cm-1, gx ) 2.38, gy ) 1.89, and gz ) 2.03. These magnitudes of D and E/D for this fit do not correspond to those from HFEPR. In particular, an absence
of detectable low-field EPR transitions in the |D| + |E| ∼ 300 GHz frequency region makes these values improbable.
Figure 5. EPR spectrum of loose Ni(PPh3)2Br2 at 325.9 GHz and 4.5 K.
Other experimental conditions are as in Figure 1 with the exception of
available sub-mm power, which at the third harmonic of the fundamental
frequency is on the order of 1 mW. The transitions in the powder spectrum
are identified and labeled tentatively, following the analysis presented in
Figure 6A. The weak transitions marked with an asterisk (*) originate from
the fourth harmonic, which occurs at 434.5 GHz.
HFEPR Spectroscopy of Ni(PPh3)2I2. No signals what-
Figure 6. Complete resonance field vs frequency dependence of HFEPR
soever were observed for microcrystalline Ni(PPh3)2I2 over
spectra of Ni(PPh3)2Br2 at 4.5 K. There is one set of experimental points
the entire field/frequency range available, and in the tem-
perature range of 4.5-20 K.
(represented by squares) in both plots, but two alternative sets of simulations
(A and B), represented by lines, and labeled accordingly. Simulation A
assumed perfectly rhombic symmetry with the following spin Hamiltonian
parameters: |D| ) 4.5 cm-1; |E| ) 1.5 cm-1; gx,y ) 2.20; and gz ) 2.00.
Simulation set B used the following parameters: |D| ) 4.2 cm-1; |E| )
1.0 cm-1; and g ) 2.20 (isotropic). In spectrum B, the open squares represent
a tentative assignment to a double-quantum transition with g ∼ 2.2 rather
than to the transition B2,3x (see text).
Magnetic Susceptibility Measurements of Ni(PPh3)2Cl2,
Ni(PPh3)2Br2, and Ni(PPh3)2I2. Magnetic susceptibility
measurements were made over the temperature range 2-300
K at fields of 0.01 and 0.1 T on microcrystalline samples of
each of the three compounds of interest. Figure 7 presents
these experimental data in the form of effective magnetic
moment (µeff) versus temperature together with a best fit line
in each case. At high temperatures, for each of the three
complexes µeff ) g[S(S + 1)]1/2 ) 2.8-3.0, exactly as
expected for S ) 1 with g in the range ∼2.0-2.2, consistent
with the g values from EPR. The high-temperature behavior
of these systems is thus that of simple, mononuclear
paramagnets. Qualitatively, the observed decrease in µeff at
low temperature from these high-temperature, spin-only
values is indicative of zfs effects.29
) 1 system with D ) +12.03 cm-1, |E| ) 1.78 cm-1, gx )
1.99, gy ) 2.00, and gz ) 2.40. Fits to an axial system (E )
0, gx ≡ gy) were acceptable and gave essentially the same
values for D and g, but allowance of rhombic symmetry
greatly improved the fit and is more physically meaningful
in this system. These fit parameters are in good agreement
with those obtained by HFEPR. The variation in g values is
not significant, as the average g value (2.13) is close to that
obtained from HFEPR and, more importantly, the value for
D agrees to within 10%. The parameters obtained by HFEPR
are likely more accurate, as they derive from simulation of
resonant transitions, but there is clearly a consistent picture
Quantitatively, magnetic susceptibility data for Ni(PPh3)2-
Cl2 were well fit over the entire temperature range to an S
Inorganic Chemistry, Vol. 41, No. 17, 2002 4483

Krzystek et al.
appearance of the µeff versus temperature curve for this
complex relative to the other two shows a much more rapid
decrease in µeff as the temperature decreases (Figure 7). This
behavior is the consequence of a much larger magnitude of
D, as given by the following best fit parameters: D )
+27.92 cm-1, |E| ) 4.71 cm-1, gx ) 1.95, gy ) 2.00, and gz
) 2.11. This magnitude of D, ∼840 GHz, with its positive
sign as in Ni(PPh3)2Cl2, would preclude the observation of
EPR signals, even at the highest frequency available to us.
This situation arises because all the potentially observable
transitions in the available frequency/field range in the case
of positive D originate from the excited spin sublevels (MS
) (1), which are only negligibly populated at low temper-
atures given the magnitude of |D|.
Last, an interesting feature of the temperature-dependent
magnetization of Ni(PPh3)2I2 was observed below 4 K, which
behavior was not present in the other systems (see Figure
S4). Briefly stated, we observed the zero-field cooled (ZFC)
magnetization signals to be time dependent below 4 K,
possessing a relaxation toward the field-cooled (FC) values
with a time constant of about 28 min. This behavior is
believed to be intrinsic to the material and not an experi-
mental artifact associated with thermal equilibrium. The
observed behavior may indicate the presence of a spin
glasslike transition,37 but detailed studies were not performed
at this time as the behavior is likely unrelated to the
properties of the individual Ni(PPh3)2I2 molecules.
Figure 7. Plots of effective magnetic moment (µeff) versus temperature
for the three Ni(PPh3)2X2 complexes discussed in this work. The applied
field was 0.1 T. Squares represent experimental points while lines were
drawn with the following spin-Hamiltonian parameters: (a) Ni(PPh3)2Cl2,
D ) +12.03 cm-1, |E| ) 1.78 cm-1, gx ) 1.99, gy ) 2.00, and gz ) 2.40;
2.00, and gz ) 2.22; and (c) Ni(PPh3)2I2, D ) +27.92 cm-1, |E| ) 4.71
cm-1, gx ) 1.95, gy ) 2.00, and gz ) 2.11. Error bars, determined as
described in the Experimental Section, are included for all three complexes
but are smaller than the symbol size for Ni(PPh3)2Cl2 and Ni(PPh3)2I2.
(b) Ni(PPh3)2Br2, D ) +5.38 cm-1, |E| ) 1.76 cm-1, gx ) 2.06, gy
)
Field-Dependent Magnetization Measurements of
Ni(PPh3)2Cl2, Ni(PPh3)2Br2, and Ni(PPh3)2I2. The magnetic
susceptibility measurements were performed at low fields
(0.1 T) over a broad temperature range (2-300 K), while
the HFEPR resonances generally occurred at fields of several
T, but at low temperature (∼5 K). Field-dependent magne-
tization measurements provide a link between these two
techniques in that a bulk magnetization measurement is made,
but at a single, low temperature (2 K) and over a relatively
broad field range (0-5 T).
Figure 8 presents the experimental magnetization versus
applied field data for microcrystalline samples of the three
compounds of interest together with a best fit line in each
case. Qualitatively, it can be seen that in none of the samples
is saturation magnetization reached at the maximum field
of 5 T. However, for Ni(PPh3)2Br2 (Figure 8B) for which
the magnitude of D is smallest, the magnetization curve is
the closest to achieving saturation, while for Ni(PPh3)2I2
(Figure 8C), with the largest D, the magnetization curve
shows no hint of saturation. Quantitatively, it is possible to
fit each of these curves; however, the fitting procedure is
complicated by the lack of a saturation magnetization value.
In particular, to obtain meaningful fits it was necessary to
fix the g values as follows: for Ni(PPh3)2Cl2, an isotropic g
) 2.20, as found by HFEPR; for Ni(PPh3)2Br2, the g values
from HFEPR are less reliable so an isotropic g ) 2.00 was
employed; for Ni(PPh3)2I2, no HFEPR data are available so
an isotropic g ) 2.00 was also employed. Given these
of the spin Hamiltonian parameters for Ni(PPh3)2Cl2 based
on the two techniques.
As was the case with HFEPR, the situation for Ni(PPh3)2-
Br2 regarding magnetic susceptibility data is not so straight-
forward. It was possible to fit the observed data quite well
by using a negative D (ca. -8 cm-1; see Table 1), but the
magnitudes of both D and E/D are too large and too small,
respectively, to be consistent with HFEPR data. Fits con-
strained to a positive D were somewhat more problematic
in that the magnitude of E was increased above D/3, which
simply corresponds to a reorienting of the coordinate system,
and thus is merely a fitting artifact. When the constraint E/D
e 1/3, with E, D > 0 was used, it was possible to fit the
observed data, yielding the following parameters: D ) +5.38
cm-1, E ) +1.76 cm-1, gx ) 2.06, gy ) 2.00, and gz )
2.22. These values are at least in rough agreement with those
from HFEPR. The magnetic data favor a system with high,
essentially maximum, rhombicity and a magnitude of D that
is comparable to that from HFEPR. The average g value
(2.09) is reasonably close to that from HFEPR (2.13) as well.
Clearly, neither the magnetic nor HFEPR data provide a
description of Ni(PPh3)2Br2 that is as satisfying as for
Ni(PPh3)2Cl2. It is nevertheless reasonable to conclude that
Ni(PPh3)2Br2 is highly rhombic with D positive and about
4.5 ( 1.0 cm-1, much smaller than in Ni(PPh3)2Cl2.
The magnetic susceptibility studies for Ni(PPh3)2I2 were
the most useful, since no HFEPR spectra were obtainable
for this complex. The reason for this negative result appears
to come from the magnetic data, where even the qualitative
(37) Mydosh, J. A. Spin Glasses: An Experimental Introduction; Taylor
& Francis: London, UK, 1993.
4484 Inorganic Chemistry, Vol. 41, No. 17, 2002

EPR Spectra from “EPR-Silent” Species
Cl2, our best fit of the single-crystal data to an axial powder
model gave |D| ) 14.0 cm-1, gx,y ) 2.03, and gz ) 2.51 (fit
parameters are also given in Table 1). Nevertheless, the fit
was adequate and the resulting spin Hamiltonian parameters
are in reasonable agreement with those obtained both by our
powder susceptibility measurements and by HFEPR (see
Table 1). For Ni(PPh3)2Br2, our powder fit to their single-
crystal data gave |D| ) 13.3 cm-1, gx,y ) 1.85, and gz )
2.77. These values are in substantial disagreement with those
obtained here from powder susceptibility, magnetization, and
HFEPR. Possible reasons for this discrepancy will be
discussed in the next section.
Discussion
The use of high fields and frequencies has been shown to
succeed in producing EPR spectra from an integer spin
system that is “EPR-silent” with use of conventional
methods. In Ni(PPh3)2Cl2, a significant number of powder
pattern EPR transitions were observed over a wide field range
at numerous resonant frequencies. These include not only
the allowed transitions, but also nominally forbidden ones,
such as B1y in Figures 1 and 2. The axial zfs, |D| ≈ 13 cm-1,
resulting from analysis of the two-dimensional field-
frequency data set represents what we believe is a new high
value in terms of what is now accessible by HFEPR. Because
of the large magnitude of the zfs in this complex, it was
possible to determine unequivocally the sign of D. Specif-
ically, consideration of which transitions are detectable, and
which are not, supports a positive sign of D. In particular,
the B3x,y transitions, which are spin-allowed, are not de-
tected in the experiment at all, even though much greater
mm power is available in the 92-300 GHz range of
frequencies in which they are expected to occur than in the
sub-mm 330-550 GHz range where other transitions actually
appear. Since these two transitions originate from a MS )
+1 or -1 spin level, these levels must be the excited levels,
and the ground level is MS ) 0. This situation corresponds
to a positive D in the standard representation. Additional
proof is offered by spectral simulations for individual
frequencies as in Figure 3.
Figure 8. Plots of magnetization versus applied field for the three
Ni(PPh3)2X2 complexes discussed in this work. The temperature was 2 K.
Squares represent experimental points while lines were drawn by using the
following spin-Hamiltonian parameters: (a) Ni(PPh3)2Cl2, D ) +13.1 cm-1
,
E ) 0.0 cm-1, giso ) 2.20; (b) Ni(PPh3)2Br2, D ) +3.82 cm-1, E ) 0.0
cm-1, and giso ) 2.00; and (c) Ni(PPh3)2I2, D ) +25.6 cm-1, E ) 0.0
cm-1, and giso ) 2.00. The experimental uncertainties are less than the size
of the data points.
constraints, the fits to the experimental data are quite good,
as seen in Figure 8. For Ni(PPh3)2Cl2 and Ni(PPh3)2Br2 the
resulting fit parameters are quite close to those obtained by
HFEPR (see Table 1); indeed curves employing the exact
parameters from HFEPR visually match the experimental
magnetization data quite well (not shown). An important
distinction between the HFEPR data and the field-dependent
magnetization data is that the latter are almost totally
insensitive to the rhombic zfs parameter, E. Field-dependent
magnetization thus provides an important counterpart to
HFEPR, but cannot provide as detailed information on
electronic structure.
Analysis of Previously Reported Magnetic Susceptibil-
ity Measurements of Ni(PPh3)2Cl2 and Ni(PPh3)2Br2.
Single-crystal magnetic susceptibility measurements over the
temperature range 20-295 K for Ni(PPh3)2Cl2 and Ni(PPh3)2-
Br2 were performed by Davies et al.24 They reported the
molar susceptibility data for the two complexes at each of
the three orthogonal principal orientations with respect to
the external magnetic field.38 For direct comparison with our
powder magnetic susceptibility results, their øl (l ) a*, b,
c) values at each temperature were averaged and the resulting
“powder” values fit by using the same procedure as with
our data. It is unfortunate that their data did not extend to
lower temperature as the magnetic moment changes very
little over the temperature range 20-295 K. For simplicity,
therefore, only axial zfs fitting was performed. For Ni(PPh3)2-
The analysis of the powder patterns in the Ni(PPh3)2Br2
HFEPR spectra was more complicated and hence the
accuracy of the extracted spin Hamiltonian parameters was
not as high as in the case of Ni(PPh3)2Cl2. Also, the sign of
D could not be determined. Interestingly, magnetic studies
of Ni(PPh3)2Br2 encountered similar problems. While the
exact cause of these difficulties is not yet clear, one
possibility is the coexistence of two or more distinct crystal
structures of this complex.24 Nevertheless, the HFEPR and
magnetic data agree that Ni(PPh3)2Br2 exhibits a strongly
rhombic zfs tensor, possibly of the maximum allowed
rhombicity (|E| ) |D|/3), with zfs parameters reduced in
comparison with Ni(PPh3)2Cl2: |D| ) 4.5 ( 0.5 cm-1 and
|E| ) 1.5 ( 0.5 cm-1. We note that in the case of Ni(PPh3)2I2,
the axial zfs determined by magnetic measurements, |D| ≈
27 cm-1, is still outside the range accessible to our current
HFEPR instrumentation. Planned extension of this spectro-
(38) In these complexes, the principal crystal susceptibilities (crystal
axes: a*, b, c) are identical with the principal molecular susceptibilities
(x, y, z).
Inorganic Chemistry, Vol. 41, No. 17, 2002 4485

Krzystek et al.
scopic technique into the THz range will eventually allow
even such compounds to be amenable to study by EPR.
We next turn to an analysis of the spin Hamiltonian
parameters obtained for each of the three complexes. As has
been known for many years, the bulky triphenylphosphane
ligands enforce a pseudotetrahedral geometry about Ni2+
leading to an S ) 1 spin ground state.16,17 As would be
expected from the steric size and differing donor abilities of
the halogen versus P donor ligands, the geometry of the
complexes is quite far from tetrahedral. For example, in
Ni(PPh3)2Cl2, the X-Ni-X bond angle is 128.0° (X ) Cl)
and the P-Ni-P bond angle is 111.4°;20 for Ni(PPh3)2Br2,
these bond angles are 126.3° (X ) Br) and 110.4°,
respectively;21 and for Ni(PPh3)2I2, the bond angles are 118.1°
(X ) I) and 105.3°.22 Using the coordinate system defined
by Gerloch and co-workers,24 the P-Ni bonds define the xz
plane and the X-Ni bonds define the yz plane. The structural
and bonding differences between these two sets of ligands
gives rise to C2V rather than D2d symmetry, and a significant
rhombic distortion about Ni, as manifest by E * 0.
The molecular structure of the Ni(PPh3)2X2 complexes
qualitatively supports the spin Hamiltonian parameters
determined by HEPR. It is possible to make a more
quantitative correlation by making use of other data, par-
ticularly from electronic absorption spectroscopy. This
method was used in our previous study of aqueous Cr2+,13
which parametrized the metal ion’s ligand field with use of
the parameters defined by Ballhausen.39 A similar crystal-
field method was used by Gerloch and Slade in their early
studies of tetrahedral Ni2+.40 Subsequently, Gerloch and co-
workers employed the angular overlap model (AOM)32,41 in
their magnetic and spectroscopic studies of these com-
plexes.24,42 The AOM method has also been used by
McGarvey in developing the theory of paramagnetic NMR
of pseudotetrahedral complexes of Ni2+ and Co2+ with D2d
point-group symmetry.43 Because of the previous applications
of AOM to pseudotetrahedral complexes of Ni2+, we employ
it here as well. Recent HFEPR studies on other integer-spin
transition metal complexes have also employed AOM to
analyze spin Hamiltonian parameters.8,44
Figure 9. Energy levels of triplet states of Ni2+ 3d8 as a free-ion, in a
tetrahedral (Td) ligand-field (hypothetical [NiL4]2+ or [NiX4]2- complex),
and in a rhombic (C2V) ligand-field (actual NiL2X2 complex). The energy
level spacings of the free-ion and Td cases are approximate, but those of
the C2V case are as observed by single-crystal electronic absorption
spectroscopy of Ni(PPh3)3Cl2.23 These electronic transitions are also
indicated, together with their polarization. The state labeled 3A1 also includes
singlet states of similar energy. There are no symmetry-allowed transitions
from the 3B1 ground state to excited states of 3B2 symmetry; the energy
levels of these states are determined by calculation (see text). Inclusion of
spin-orbit coupling with singlet states (not shown) would split each of the
triplets into three nondegenerate levels, affording the rhombic zfs seen by
HFEPR.
The analysis of the electronic structure of Ni(PPh3)2Cl2
by Davies et al.24 was based upon their single-crystal
magnetic susceptibility measurements, the electronic absorp-
tion spectra reported by Fereday et al.,23 and structural
information from the original X-ray crystallographic study.17
The crystal structure of Ni(PPh3)2Cl2 has since been rede-
termined with much greater precision20 and we have used
these recent structural data in our analysis. The combination
of better structural data with our experimental results neces-
sitated changes in the ligand-field parameters of Ni(PPh3)2-
Cl2 originally proposed by Davies et al.24 Our ligand-field
results for Ni(PPh3)2Cl2 nevertheless clearly confirm their
proposal24 that the PPh3 ligands are σ-donors and π-acceptors
and the halo ligands are σ-donors and π-donors.
Table 2 presents the optimal values of ligand-field
parameters for the Ni(PPh3)2X2 series resulting from our
fitting procedure employing the programs CAMMAG33 and
AOMX.34 The procedure by which these parameters were
obtained is described in detail in the Supporting Information.
We note here only that the optimum estimate of ꢀπ(Cl) is
very large (2421(29) cm-1). However, the electronic transi-
tion energies for Ni(PPh3)2Cl2 can be fitted almost as well
(see Table S1, Supporting Information) if this parameter is
constrained to a value similar to that in other complexes,
although this causes the rhombic component of the zfs tensor
to be considerably larger than is observed experimentally
(Table 2).
For illustrative purposes, we show in Figure 9 the 3d8
triplet state energy levels of a Ni2+ free ion with successive
application of a tetrahedral and of a rhombically distorted
ligand field, as found for Ni(PPh3)2Cl2. The electronic
transitions for this complex, as determined by single-crystal
optical spectroscopy,23 are specifically indicated in the figure
and are roughly to scale.
(39) Ballhausen, C. J. In Introduction to Ligand Field Theory; McGraw-
Hill: New York, 1962; pp 99-103.
(40) Gerloch, M.; Slade, R. C. J. Chem. Soc. (A) 1969, 1012-1022.
(41) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and its
Applications; Wiley-VCH: New York, 2000. See Chapter 3 and
references therein for a discussion of the chemical significance of AOM
parameters. See p 110 for free-ion Racah and spin-orbit coupling
parameters.
(42) Gerloch, M.; McMeeking, R. F. J. Chem. Soc., Dalton Trans. 1975,
2443-2451.
(43) McGarvey, B. R. Inorg. Chem. 1995, 34, 6000-6007.
(44) Barra, A.-L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.;
Fabretti, A. C.; Uytterhoeven, M. G. Angew. Chem., Int. Ed. Engl.
1997, 36, 2329-2331.
4486 Inorganic Chemistry, Vol. 41, No. 17, 2002

EPR Spectra from “EPR-Silent” Species
Table 2. Ligand-Field Parameters (in cm-1) for Ni(PPh3)2X2 (X ) Cl, Br, I) and Related Complexes
complex
ꢀσ(P)
ꢀπ(P)
ꢀσ(X)
ꢀπ(X)
B
ú
a,b
a,c
a,d
Ni(PPh3)2Cl2
Ni(PPh3)2Cl2
Ni(PPh3)2Br2
Ni(PPh3)2Br2
5509(43)
4192(34)
4292(1351)
4000
5509
5000
-1235(33)
-1674(35)
-501(286)
-1500
5227(51)
5689(43)
3184(3293)
4000
2000
3000
2421(29)
1138
517(3003)
1500
600
700
481(1)
461(1)
590(40)
550
481
620
435(2)
347(2)
264(0.5)
300
e
a,f
Ni(PPh3)2I2
-1235
550
[Ni(PPh3)Br3]- g
[Ni(PPh3)I3]- g
-1500
-1500
6000
2000
600
490
a This work; standard deviations from fits are given in parentheses. All fits used the following relationship between the Racah parameters: C ) 4.7B.41
b Parameters determined by using recent crystallographic data20 and by analysis of HFEPR and magnetic measurements and previous electronic absorption
data. These parameters yield the following zfs: D ) +13.2, |E| ) 1.8 cm-1
.
c In this case, the constraint ꢀσ(Cl)/ꢀπ(Cl) ) 5 was imposed (see Supporting
Information). These parameters yield the following zfs: D ) +12.6, |E| ) 3.6 cm-1
.
d Results for major, crystallographically characterized21 form. Parameters
determined by analysis of HFEPR and magnetic measurements and previous electronic absorption data. These parameters yield the following zfs: D )
+7.1, |E| ) 1.4 cm-1
.
e Parameters reported by Davies et al. for the minor form reported to be isomorphous with Ni(PPh3)2Cl2.24 Use of the crystallographic
data for the major form with these parameters yields the following zfs: D ) +7.6, |E| ) 0.5 cm-1
.
f Parameters determined by using crystallographic data22
and by analysis of HFEPR and magnetic measurements and correspondence with values determined for other entries in the table (see text). These parameters
yield the following zfs: D ) +23.9, |E| ) 5.8 cm-1. Standard deviations are not meaningful as there are no electronic absorption data to fit. g Parameters
reported by Gerloch and Hanton.45
For Ni(PPh3)2Br2, we employed structural data for the
more abundant form,21 rather than the form isomorphous with
Ni(PPh3)2Cl2.24 The bromo complex also lacks high-quality
electronic absorption data. Use of these structural data21 and
our magnetic and HFEPR data, as described in the Support-
ing Information, made it possible to obtain very rough
estimates for the ligand-field parameters for this complex
(see Table 2). This analysis reproduces the rhombic zfs
observed by HFEPR and suggests a reduction in spin-orbit
coupling relative to the chloro complex, which may result
from increased covalency in the bromo versus the chloro
complex. In the case of Ni(PPh3)2I2, only structural22 and
our magnetic data are available. As described in the
Supporting Information, use of data for a related iodo
complex45 combined with our results for Ni(PPh3)2Cl2 allow
estimates as to ligand-field parameters that provide zfs in
good agreement with experiment (see Table 2). The relatively
large spin-orbit coupling in this complex may be the effect
of spin delocalization onto the iodo ligands.
concurrent powder magnetic susceptibility and field-depend-
ent magnetization measurements indicate a magnitude of zfs
(∼27 cm-1) that is too large too allow observation of
resonances at the fields, frequencies, and temperatures
employed here. The spin Hamiltonian parameters observed
experimentally for Ni(PPh3)2Cl2, in combination with previ-
ous electronic absorption data and a recent crystal structure
of the complex, allowed determination of a set of electronic
and bonding parameters that describe well the electronic
structure of this complex. The AOM parameters for
Ni(PPh3)2Cl2 so obtained confirm previous conclusions that
the phosphane ligand acts as a π-acceptor and the chloro
ligand as a strong π-donor. The data for Ni(PPh3)2Br2 do
not allow unambiguous derivation of ligand field parameters,
but the zfs and optical transition energies may be reproduced
satisfactorily by assuming similar bonding parameters to the
chloro complex, but with a significant lowering of the spin-
orbit coupling due to greater covalency. In contrast, the large
zfs observed for Ni(PPh3)2I2 implies a strong spin-orbit
interaction, and this may be ligand based, due to the high
spin-orbit coupling constant of iodine.
Conclusions
High-frequency and high-field EPR spectroscopy has been
used to probe the non-Kramers, S ) 1, Ni2+ ion in a series
of complexes of pseudotetrahedral symmetry and general
formula Ni(PPh3)2X2 (X ) Cl, Br, I). For Ni(PPh3)2Cl2,
numerous EPR transitions were observed at multiple mm/
sub-mm frequencies and analysis of the full, 2-dimensional
field-frequency data set yielded spin Hamiltonian parameters
in agreement with, but of much higher accuracy than,
concurrent powder magnetic susceptibility and magnetization
measurements. For Ni(PPh3)2Br2, HFEPR spectra were again
successfully recorded; however, the analysis was less con-
clusive. The combination of HFEPR and powder magnetic
measurements (both susceptibility and field-dependent mag-
netization) suggests highly rhombic spin Hamiltonian pa-
rameters, with the magnitude of zfs much less than in the
chloro complex (∼5 vs ∼13 cm-1, respectively). For
Ni(PPh3)2I2, no HFEPR spectra were recorded; however,
Acknowledgment. This work was supported in part by
the NHMFL (J.K. and L.-C.B.) and the Petroleum Research
Fund, administered by the American Chemical Society, ACS-
PRF 36163-AC5 (J.-H.P. and M.W.M.). J.T. is supported
by Roosevelt University and the NHMFL User Program. We
thank Prof. B. R. McGarvey, University of Windsor, Canada,
for initial assistance with the ligand field analysis and Dr.
H. Weihe from the Chemistry Department, University of
Copenhagen, Denmark, for the simulation software and great
help in implementing it. We also thank Z. X. Zhou and Dr.
G. Cao, NHMFL, for initial magnetic measurements.
Supporting Information Available: Details of the deter-
mination of ligand-field parameters; four figures showing HFEPR
spectra at different conditions and magnetization data; and a table
of experimental and calculated electronic absorption data for
Ni(PPh3)2Cl2. This material is available free of charge via the
Internet at http://pubs.acs.org.
(45) Gerloch, M.; Hanton, L. R. Inorg. Chem. 1981, 20, 1046-1050.
IC020198J
Inorganic Chemistry, Vol. 41, No. 17, 2002 4487