Impact of tetrahedral and square planar geometry of Ni(II) complexes with (pseudo)halide ligands to magnetic properties

Article abstract of DOI:10.1016/j.ica.2018.08.029

Four tetracoordinate Ni(II) complexes have been prepared, structurally characterized, and subjected to magnetometric studies. The complexes [Ni(PPh₃)₂(NCS)₂], [Ni(dppp)(NCS)₂], and [Ni(dppm)Br₂] are planar and thus diamagnetic. The complex [Ni(biqu)Br₂] is quasi-tetrahedral, with the geometry close to C_2v symmetry, and paramagnetic. While on one side it resembles a prolate bisphenoid (the angle N-Ni-N = 83 deg), on the second side it mimics an oblate bisphenoid (Br-Ni-Br = 126 deg). It exhibits a zero-field splitting of the ground term ³A₂ into three crystal-field multiplets that can be described by D and E parameters within the spin Hamiltonian formalism. The ab initio calculations confirm this interpretation; however, the evaluation of the spin-Hamiltonian parameters meets difficulties owing to the quasi-degeneracy of the electronic terms.

Full text of DOI:10.1016/j.ica.2018.08.029

Accepted Manuscript
Impact of tetrahedral and square planar geometry of Ni(II) complexes with
(pseudo)halide ligands to magnetic properties
Dominik Lomjanský, Cyril Rajnák, Ján Titiš, Ján Moncoľ, Luká š Smolko,
Roman Boča
PII:
S0020-1693(18)30875-2
https://doi.org/10.1016/j.ica.2018.08.029
ICA 18429
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
6 June 2018
20 July 2018
20 August 2018
Please cite this article as: D. Lomjanský, C. Rajnák, J. Titiš, J. Moncoľ, L. Smolko, R. Boča, Impact of tetrahedral
and square planar geometry of Ni(II) complexes with (pseudo)halide ligands to magnetic properties, Inorganica
Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.08.029
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Impact of tetrahedral and square planar geometry of Ni(II) complexes with
(pseudo)halide ligands to magnetic properties
Dominik Lomjanskýa, Cyril Rajnáka*, Ján Titiša*, Ján Moncoľb, Lukáš Smolkoc, and Roman Bočaa
a Department of Chemistry, Faculty of Natural Sciences, University of SS Cyril and Methodius, 917
01 Trnava, Slovakia, cyril.rajnak@ucm.sk, jan.titis@ucm.sk
b Institute of Inorganic Chemistry, Slovak University of Technology, 812 37 Bratislava, Slovakia
c Faculty of Medicine, J.P. Šafárik University in Košice, Košice, Slovakia
Abstract
Four tetracoordinate Ni(II) complexes have been prepared, structurally characterized, and subjected
to magnetometric studies. The complexes [Ni(PPh3)2(NCS)2], [Ni(dppp)(NCS)2], and
[Ni(dppm)Br2] are planar and thus diamagnetic. The complex [Ni(biqu)Br2] is quasi-tetrahedral,
with the geometry close to C2v symmetry, and paramagnetic. While on one side it resembles a
prolate bisphenoid (the angle N-Ni-N = 83 deg), on the second side it mimics an oblate bisphenoid
(Br-Ni-Br = 126 deg). It exhibits a zero-field splitting of the ground term 3A2 into three crystal-field
multiplets that can be described by D and E parameters within the spin Hamiltonian formalism. The
ab initio calculations confirm this interpretation; however, the evaluation of the spin-Hamiltonian
parameters meets difficulties owing to the quasi-degeneracy of the electronic terms.
Key words
Ni(II) complexes, P-donor ligands, N-donor ligands, (pseudo)halide ligands, paramagnetism,
diamagnetism.
1. Introduction
Magnetic anisotropy of transition metal complexes attracts much attention in the recent period
because this is a key factor that induces the slow magnetic relaxation and consequently a behavior
of the metal complexes as single molecule (single ion) magnets – SMMs, SIMs. The SIM behavior
has been observed among several first-row transition metal complexes such as V(IV), Cr(III), low-
spin Mn(IV), Mn(III), Fe(III), Fe(II), Fe(I), Co(II), Ni(II), Ni(I), and Cu(II) [1-11]. Though some
families are extremely rich, such as tri-, tetra-, penta-, hexa-, hepta- and octacoordinate Co(II)
complexes, some are represented only by a few or a single example [12, 13]. The class of Ni(II)
SIMs is limited only to three compounds so far of which [Ni(pydca)(dmpy)]·H2O and
[Ni(NCS)2(nqu)2(H2O)2]·2nqu are hexacoordinate, and [Ni(mdabco)2Cl3]ClO4 is pentacoordinate
[14, 15]. All of them possess negative axial zero-field splitting parameter D < 0 (13.7, 5.9, and
311 cm-1) as a prerequisite of the barrier to spin reversal that facilitates the SIM behavior. The
class of hexacoordinate Ni(II) complexes is a widely studied and well understood case where the
magnetic anisotropy parameter can be predicted and tuned on the basis of the magnetostructural D-
correlation [16].
The class of tetracoordinate Ni(II) complexes is studied in a lower extent. There is one obstacle:
3
owing to the Jahn-Teller effect the ground electronic term T1 in the tetrahedral geometry is
unstable and the geometry is stabilized along the e-mode of distortion leading either to the 3A2 or 3E
state of the D2d geometry. The latter is also unstable and the distortion continues to the stable
3
3
3
geometry of the C2v symmetry (or lower) with A1, B1, B2 terms. Another obstacle originates in
weaker crystal-field strength of the tetrahedral systems when compared to the octahedral ones: the
quasi-degeneracy is a common feature since the energy gap ax between the ground and the first
excited terms is rather low. This causes that the evaluation of the traditional spin-Hamiltonian
parameters (D and E) could violate because the perturbation theory tends to diverge.
1

Herein, we are reporting about structural, spectral, and magnetic characterization of
tetracoordinate Ni(II) complexes with various P-donor ligands, N-donor ligands and (pseudo)halide
coligands: [Ni(PPh3)2(NCS)2] 1, [Ni(dppm)Br2] 2, [Ni(biqu)Br2] 3 and [Ni(dppp)(NCS)2] 4. Our
primary goal was to explore the possibilities of preparing pseudotetrahedral Ni(II) SIMs containing
P-donor ligands. However, we managed to isolate diamagnetic square-planar systems as a
consequence of the generated strong ligand field. Complex 3 is an analog of complex 2 containing
N-donor neutral ligand with weaker field. As expected, this complex is pseudotetrahedral and hence
paramagnetic.
2. Experimental
2.1. Chemicals and handling
All chemicals were purchased and used as received in reagent grade. The inorganic Ni(II) salts
used in reactions were available as freshly synthesized Ni(SCN)2 by changing reaction of KSCN
with NiCl2·6H2O or in a form of anhydrous NiCl2. All solids and solvents (ethanol, acetonitrile)
were used without any further purification or drying. The manipulations were made under air
conditions. The products were filtered over the ashless paper and fritted glass with porosity no. 4.
2.2. Physical Measurements
Elemental analyses were measured by Flash 2000 CHNS apparatus (Thermo Scientific). The
samples for FT-IR measurements was not dried prior to its using and were used as freshly growth
crystals. The KBr powder for FT-IR measurements was kept against absorption of moisture in the
oven. The Nujol´s absorption UV-Vis spectra (190 - 1100 nm) for solid samples were measured by
Specord 250 Plus (Analytica Jena) with the DAD detector at room temperature. Melting points were
studied with thermo-microscopy by Kofler hot-stage microscope at 4°C·min-1 and reported without
corrections.
2.3. Preparation of complexes
[Ni(PPh3)2(NCS)2], 1. A 100 cm3 round bottom flask was charged with triphenylphosphine (525
mg, 2 mmol) acetonitrile (15 cm3). Ligand was dissolved in 5 minutes at 60 °C. 1 cm3 of freshly
synthesized Ni(NCS)2 in half of molar equivalent was added. The reaction mixture changed color
gradually to dark red. The reaction was heated for 20 hours at 80 °C. Red needles were obtained
after standing for two days and filtration of mother liquid. Yield: 0.068 g. Melting point: 228 -
230°C. Anal. Calc. for C38H30N2P2NiS2 (699.41g·mol-1): C, 65.25; H, 4.32; N, 4.01; S, 9.17. Found:
C, 64.91; H, 4.25; N, 4.01; S, 9.17. Selected IR bands (1.4 mg/250 mg KBr) /cm-1: 3060(w),
2085(s) νas(CN from NCS), 1479(m), 1434(s), 1308(w), 1178(w), 1096(s), 1024(w), 997(w), 867(m),
750(s), 744(w), 707(w), 693(s), 524(s), 512(m), 497(m), 457(w), 414(w) (s = strong, m = medium,
w = weak). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): 24.04 (1.112), 19.46 (0.666).
[Ni(dppm)Br2], 2.
A
100 cm3 round bottom flask was charged with
bis(diphenylphosphino)methane (150 mg, 0.39 mmol), and dissolved in acetonitrile (20 cm3). NiBr2
(85.2 mg, 0.39 mmol) was added into the solution and color changed immediately to dark red.
Mixture was stirred for 4 hours with intensive reflux. Red-block crystals were isolated after 3 days
by filtration. Melting point: > 200°C. Anal. Calc. for C25H22Br2NiP2 (602.89 g·mol-1): C, 49.80; H,
3.68. Found: C, 49.74; H, 3.59. IR bands /cm-1: 3350(w), 3051(w), 2953(w), 2681(w), 2581(w),
2322(w), 2198(w), 1583(w), 1568(w), 1479(m), 1432(s), 1335(m), 1307(m), 1182(m), 1157(m),
1097(s), 1075(m), 1024(m), 997(s), 924(w), 843(w), 740(m), 730(s), 712(m), 685(s), 653(m),
616(w), 539(s), 497(s), 476(m), 438(m)
(s = strong, m = medium, w = weak). UV/Vis (Nujol)
νmax/103 cm-1 (relat. absorb.): 16.86 (0.532), 20.33 (0.809).
[Ni(biqu)Br2], 3. A mixture consisting of 0.109 g of NiBr2 (0.5 mmol) and 0.128 g of biqu (0.5
mmol) in 15 cm3 of ethanol was placed into teflon-lined steel autoclave and heated to 120 °C for 16
h. Red crystals formed in the solution upon slow cooling of the mixture to the room temperature.
2

Crystals were separated by filtration, rinsed with cold ethanol and dried in air. Anal. Calc. for
C18H12Br2N2Ni (474.80 g·mol-1): C, 45.53; H, 2.55; N, 5.90. Found: C, 45.25; H, 2.51; N, 5.86. IR
bands /cm-1: 3086(w), 3055(w), 1965(w), 1934(w), 1822(w), 1586(m), 1508(s), 1434(m),
1380(m), 1366(w), 1341(w), 1302(w), 1214(m), 1145(s), 1106(m), 978(w), 957(w), 870(m), 829(s),
780(s), 746(s), 699(w), 659(w), 637(w), 526(w), 499(w), 487(s) (s = strong, m = medium, w =
weak). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): 9.86 (0.282), 11.05 (0.246), 11.42 (0.247),
15.63 (0.225), 19.19 (0.481).
[Ni(dppp)(NCS)2], 4.
A
50 cm3 round bottom flask was charged with
bis(diphenylphosphino)propane (177 mg, 0.43 mmol) and acetonitrile (20 cm3) as solvent.
Ni(SCN)2 (75 mg, 0.43 mmol) in ethanolic solution was added into the clear solution and color
changed immediately to yellow-brown. Mixture was stirred for 3 hours with intensive reflux.
Yellow crystals were isolated after 2 days of controlled evaporation at room temperature. Selected
IR bands /cm-1: 3159,(w), 3053(w), 2926(w), 2686(w), 2074(s) νas(CN from NCS), 1965(w), 1815(w),
1678(w), 1585(w), 1483(m), 1435(s), 1408(m), 1315(w), 1306(w), 1101(s), 996(m), 944(m),
832(m), 747(m), 719(m), 704(s), 686(s), 559(m), 554(s), 477(m), 457(s), 439(m) (s = strong, m =
medium, w = weak). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): 9.86 (0.282), 11.05 (0.246),
11.42 (0.247), 15.63 (0.225), 19.19 (0.481). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): 23.75
(0.779)
2.4. X-ray crystal structure determination
Data collection and cell refinement of suitable single crystals of 1, 2 and 4 were made by Stoe
StadiVari diffractometer using PILATUS3R 300K HPAD detector and micro-focused source
Xenocs Genix3D with CuKα at 100K. Corrections to Lorentz, polarization and multi-scan
absorption effects were applied. The structure was solved by charge-flipping method and refined
anisotropically by common least-squares methods. The programs SUPERFLIP or SHELXT,
SHELXL (vers. 2014/7 or 2018/3), OLEX2 and MERCURY have been used for structure
determination, refinement and drawing [17-19]. The hydrogen atoms were refined with fixed
distances from the parent carbon atoms. Selected crystal data are presented in Table 1.
One of the phenyl groups [C1–C6/C1A–C6A] of 2 is disorderd with occupancy factors 0.537(7)
and 0.463(7), respectively.
3

Table 1. Crystal data and structure refinement for complexes under study.
[Ni(PPh3)2(NCS)2], 1 [Ni(dppm)Br2], 2 [Ni(dppp)(NCS)2], 4
Empirical formula
C38H30N2P2NiS2
699.41
C25H22Br2NiP2
602.89
C29H26N2P2NiS2
587.29
Formula weight /g mol-1
Crystal system
Space group
Temperature /K
Crystal size /mm
Z
triclinic
P-1
100
0.21 × 0.12 × 0.06
1
monoclinic
I2
100
0.38 × 0.35 × 0.25
2
monoclinic
C2/c
100
0.26 × 0.17 × 0.08
4
a / Å
b / Å
c / Å
 /°
β /°
γ /°
7.8813(5)
10.3849(7)
11.4155(8)
68.742(5)
74.235(5)
88.129(5)
835.73(10)
1.390
11.9829(3)
7.4780(2)
13.6265(4)
90
106.522(2)
90
1170.63(6)
1.710
6.557
24.1817(7)
9.0253(3)
14.6344(4)
90
120.762(2)
90
2744.53(15)
1.421
3.706
V /Å3
ρcalc /g cm-3
μ /mm-1
F(000)
3.140
362.0
600.0
1216.0
Radiation
CuKα
CuKα
CuKα
(λ = 1.54186)
8.656 to 141.354
(λ = 1.54186)
8.686 to 142.508
(λ = 1.54186)
8.510 to 142.762
2Θ range for data
collection/°
Index ranges
-9 ≤ h ≤ 9,
-10 ≤ k ≤ 12,
-7 ≤ l ≤ 13
-12 ≤ h ≤ 14,
-9 ≤ k ≤ 6,
-16 ≤ l ≤ 16
1883/188/133
1.050
R1 = 0.0306,
wR2 = 0.0805
R1 = 0.0308,
wR2 = 0.0807
red
-24 ≤ h ≤ 29,
-11 ≤ k ≤ 9,
-15 ≤ l ≤ 17
2620/0/73
1.053
R1 = 0.0290,
wR2 = 0.0751
R1 = 0.0317,
wR2 = 0.0771
yellow
Data/restraints/parameters 3072/0/206
Goodness-of-fit on F2
Final R indexes [I>=2σ
(I)]
1.070
R1 = 0.0400,
wR2 = 0.1118
R1 = 0.0439,
wR2 = 0.1144
red
R indices (all data)
color
CCDC No.
1585156
1585157
1844141
2.5. Magnetic data collection
The magnetic data was collected by SQUID apparatus (MPMS-XL7/evercool, Quantum
Design) with samples encapsulated in the diamagnetic gelatin holder. The DC data was acquired in
the RSO mode of detection at BDC = 0.1 T. Raw magnetic data was corrected for underlying
magnetism and presented either as the effective magnetic moment. The magnetization data was
taken at two temperatures, T = 2.0 and 4.6 K, up to B = 7 T.
2.6. Quantum-chemical calculations
Ab initio calculations were performed with ORCA 4.0.0 computational package at the
experimental geometries [20]. The relativistic effects were included in the calculations with zero
order regular approximation (ZORA) together with the scalar relativistic contracted version of def2-
TZVPP basis functions for the Ni atom and def2-SV(P) basis functions for other elements. The
calculations were based on state average complete active space self-consistent field (SA-CASSCF)
wave functions complemented by N-electron valence second order perturbation theory (NEVPT2)
[21]. The active space comprised of eight electrons in ten metal-based d-orbitals (including the
double-shell effect). The state averaged approach was used, in which 10 triplet and 15 singlet states
were equally weighted. The calculations utilized the RI approximation with appropriate
decontracted auxiliary basis set and the chain-of-spheres (RIJCOSX) approximation to exact
4

exchange. Increased integration grids (Grid4 and GridX5) and tight SCF convergence criteria were
used. Energies of multiplets were calculated through quasi-degenerate perturbation theory in which
an approximation to the Breit-Pauli form of the spin-orbit coupling operator (SOMF) was utilized
[22]. The ZFS parameters were calculated using the effective Hamiltonian theory.
3. Results and discussion
3.1. Structural data
The complex 1 crystallized in the triclinic system with the space group P–1. Perspective view
on the molecular structure of 1 is displayed in Fig. 1. The chromophore {NiP2N2} belongs to square
planar shape formed by central Ni(II) atom surrounded by two nitrogen atoms of isothiocyanato
groups (-NCS) and two phosphorus atoms of two triphenylphospine ligands in trans positions. The
Ni1 atom lies in special position of centre of symmetry. The bond distances between Ni–P are
identical (2.24 Å) and much shorter than Ni–N by 0.41 Å (Table 2). One type of intermolecular
contact was identified as C∙∙∙H-C. Considering of the square planar geometry, two bond angles P1i-
Ni1-P1 and N1-Ni1-N1i lies on 180°, whereas other angles have 92.13° and 87.87°, respectively
(Table 2).
The complex 2 crystallized in the monoclinic system with the non-centrosymmetric space group
I2. Perspective view on the molecular structure of 2 is displayed in Fig. 1. Red color of this
complex has been well predicted its square planar geometry. The chromophore {NiP2Br2} is
composed by two atoms of phosphorus from one chelating bis(diphenylphosphino)methane ligand
and two bromine atoms. The Ni1 atom and methylene carbon atom (C13) of dppm lie on 2-fold
axis. The bond distances between Ni–P are 2.14 Å and between Ni–Br are 2.35 Å. The angle Br1-
Ni1-Br1ii lies on 99.8°, in comparison of P1-Ni1-P1ii which is 75.9° (Table 3).
The crystal structure of 3 has been determined by Butcher & Sinn [23]. The complex
crystallizes in the monoclinic system with the centrosymmetric space group P21/c. The complex 3 is
quasi-tetrahedral, with the geometry close to C2v symmetry. The Ni–Br bond distances are 2.33 and
2.35 Å, and both Ni–N bond lenghts are 1.99 Å (Table 4).
The complex 4 crystallized in the monoclinic system with the centrosymmetric space group
C2/c. Molecular structure of 4 is drawn in Fig. 1. The chromophore {NiP2N2} belongs to square
planar shape formed by central Ni(II) atom surrounded by two nitrogen atoms of isothiocyanato
groups (-NCS) and two phosphorus atoms of bis(diphenylphosphino)propane ligand. The Ni1 atom
and one methylene carbon atom of propylene group of dppp (C13) lie on 2-fold axis. The bond
distances between Ni–P are identical (2.26 Å) and between Ni–N are also identical (1.88 Å, Table
2). Considering of the square planar geometry in cis configuration, two bond angles P1iii-Ni1-P1
and N1-Ni1-N1iii are 89.9° and 93.1°, respectively (Table 5).
[Ni(PPh3)2(NCS)2], 1
[Ni(dppm)Br2], 2
5

[Ni(biqu)Br2], 3
[Ni(dppp)(NCS)2], 4
(refcode NIBRBQ)
Fig. 1. Perspective views of the Ni(II) complexes 1-4.
Table 2. Selected bond lengths (Å) and bond angles (°) in chromophore of 1.
Ni1-P1
Ni1-P1i
Ni1-N1i
Ni1-N1
2.2421(5)
2.2421(5)
1.829(2)
1.829(2)
P1i–Ni1–P1
N1–Ni1–P1i
N1–Ni1–P1
N1i–Ni1–P1
N1i–Ni1–P1i
N1–Ni1–N1i
180.0
92.13(5)
87.87(5)
92.13(5)
87.87(5)
180.0
Symmetry code: (i) 1–x, 1–y, 2–z
Table 3. Selected bond lengths (Å) and bond angles (°) in chromophore of 2.
Ni1–Br1
Ni1–Br1ii
Ni1–P1
2.3454(9)
2.3454(9)
2.1423(16)
2.1423(16)
Br1ii–Ni1–Br1
P1–Ni1–Br1
P1–Ni1–Br1ii
P1ii–Ni1–P1
P1ii–Ni1–Br1
P1ii–Ni1–Br1ii
99.77(5)
92.44(4)
166.74(5)
75.95(8)
166.74(5)
92.44(4)
Ni1–P1ii
Symmetry code: (ii) –x, y, 1–z
Table 4. Selected bond lengths (Å) and bond angles (°) in chromophore of 3.
Ni1–Br1
Ni1–Br2
Ni1–N1
Ni1–N2
2.331(1)
2.351(1)
1.991(5)
1.993(5)
Br1-Ni1-Br2
N1-Ni1-Br1
N1-Ni1-Br2
N1-Ni1-N2
N2-Ni1-Br1
N2-Ni1-Br2
124.94(4)
109.5(2)
111.8(2)
82.6(2)
115.5(2)
104.5(2)
Table 5. Selected bond lengths (Å) and bond angles (°) in chromophore of 4.
Ni1–P1
2.1594(5)
2.1594(5)
1.885(1)
1.885(1)
P1–Ni1–P1iii
N1–Ni1–P1
89.92(2)
173.98(4)
88.77(4)
93.12(8)
88.77(4)
173.98(4)
Ni1–P1iii
Ni1–N1
Ni1–N1iii
N1–Ni1–P1iii
N1–Ni1–N1iii
N1iii–Ni1–P1
N1iii–Ni1–P1iii
Symmetry code: (iii) 1–x, y, 3/2–z
6

3.2. Electronic spectra
Solid state electronic spectra (measured in Nujol mull) of the studied complexes are shown in
Fig. 2. Red tetracoordinate complexes [Ni(PPh3)2(NCS)2], 1; [Ni(dppm)Br2], 2; [Ni(biqu)Br2], 3 and
[Ni(dppp)(NCS)2], 4 show relatively intense d-d bands in the range of 10 000 – 26 000 cm-1 (Table
1
6). In square-planar complexes 1, 2 and 4 a diamagnetic ground term A1g is stabilized as a
consequence of increased ligand-field strength to the detriment of vacant axial positions. Following
1
1
transitions can be identified: 19 462 cm-1 (1A2g ← A1g), 24 047 cm-1 (1B1g ← A1g) in the complex
1, 20 332 cm-1 (1A2g ← 1A1g) in the complex 2 and 23 753 (1B1g ← 1A1g) in the complex 4.
The complex 3 possess a nearly tetrahedral structure (close to D2d or C2v) showing the d-d
transitions at 9 863 cm-1 (3A2 ← 3T1), 15 637 cm-1 and 19 193 cm-1 (3T1(P) ← 3T1). The splitting of
the second band is a consequence of the symmetry lowering. Intense charge transfer bands are
located above 26 000 cm-1.
1,2
[Ni(PPh3)2(NCS)2], 1
3T1(9)
[Ni(dppm)Br2], 2
1,0
[Ni(biqu)Br2], 3
1Eg
[Ni(dppp)(NCS)2], 4
0,8
1B1g
1A2g
1A1g
3A2(3)
18Dq
8Dq
0,6
0,4
0,2
0,0
3T2(9)
3T1(9)
1 2 3
1 2 3
10 12 14 16 18 20 22 24 26
wavenumber /103 cm-1
a)
b)
Fig. 2. a) Solid state electronic spectra of complexes 1-4. b) General view of spin allowed d-d
transitions in tetrahedral (left) and square-planar (right) Ni(II) complexes – not in scale.
Table 6. Electronic transitions assignment in complexes 1 through 4
Compound
Electronic transitions /cm-1
1b
19 462 (1A2g ← 1A1g), 24 047 (1B1g ← 1A1g)
20 332 (1A2g ← 1A1g)
2b
3a
9 863 (3A2 ← 3T1), 15 637, 19 193 (3T1(P) ← 3T1)
23 753 (1B1g ← 1A1g)
4b
[NiCl4] a,c
E0(3T1); E1(3T2) 3 500; E2(3A2) 6 650 ( = 21); E3(3T1(P)) 14 250  15 240 ( =
160) with multiplet splitting; E(1E) 11 630 ( = 6); E(1T2) 19 800; 10Dq = T =
3850 cm, B = 725 cm, C = 3262 cm
E0(1A1g); E2(1B1g) 31 100, E3(2Eg) 31 650
[Ni(CN)4] b,d
a Assignments in Td symmetry. b Assignments in D4h symmetry. c [24]. d [25].
3.3. Modeling by the generalized crystal field theory
3
A regular tetrahedral Ni(II) complex possesses the ground electronic term T1 that is orbitally
three-fold degenerate. Owing to the Jahn-Teller effect this geometry is unstable and it is a subject of
the distortion along the e+t2 vibrational modes. Distortions along the e-mode lead to the bisphenoid
of the D2d symmetry that can be either prolate (elongated, two angles L-Ni-L < 109.5 deg) or oblate
(compressed, two angles L-Ni-L > 109.5 deg; in the limiting case the square geometry is obtained)
3
– Fig. 3. The prolate bisphenoid possesses the orbitally non-degenerate ground term A2 and the
7

first excited term 3E; the spin-orbit coupling then yields the crystal field multiplets 1 (ground) and
5 (excited) separated by a gap 1 = D, that is the traditional zero-field splitting within the double
group D2’. On the contrary, the oblate bisphenoind possesses the orbitally degenerate ground term
3E that is further split by the spin-orbit coupling to a set of multiplets: 1, 2, 5, 3, and 4; this
situation is outside the applicability of the spin Hamiltonian formalism so that nothing like D and E
parameters are meaningful.
5(2)
3A2(3)
4 (1)
1(1)
5 (2)
3 (1)
1 (1)
ax|
3E(6)
ax

4(1)
3(1)
 (1)
5(2)
2(1)

3E(6)
5 (2)
3A2(3)
D2d(p)

D
1(1)
1 (1)
D2d(o)
D2d'
D2'
Fig. 3. Ground electronic terms and multiplets for the prolate (left) and oblate (right) geometries of
a homoleptic tetracoordinate Ni(II) complex of the D2d symmetry as modelled by the GCFT.
The above considerations result from the quantitative calculations using the Generalized
Crystal-Field Theory (GCFT); its principles are described elsewhere [26, 27]. A modelling of the
temperature dependence of the magnetic susceptibility (converted to the effective magnetic
moment) and the field dependence of the magnetization per formula unit is displayed in Fig. 4.
5
4
3
2
1
0
5
4
3
2
1
0
5
4
3
2
1
0



weak crystal field
strong crystal field
weak crystal field
strong crystal field
0
100
200
300
0
100
200
300
0
100
200
300
T/K
T/K
T/K
2
1
0
2
1
0
2
1
0
weak crystal field
strong crystal field
weak crystal field
strong crystal field



0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7
B/T
B/T
B/T
Prolate bisphenoid,
L-M-L = 100 o
Oblate bisphenoid,
L-M-L = 120 o
Experimental geometry of 3
Fig. 4. Modeling of the magnetic functions for a homoleptic tetracoordinate Ni(II) complex. For the
weak crystal field the pole strengths were F4 = 5000 cm-1; for the strong crystal field F4 = 15000
cm-1; F2 = 1.5 F4 are used. Magnetization per formula unit is calculated for T = 2.0 K.
In both cases the magnetic functions resemble the behaviour of a zero-field splitting system: on
cooling the effective magnetic moment slightly decreases from its room-temperature value and then
8

it drops down to the zero. The magnetization per formula unit is almost linear until B = 7 T and
much lower than the spin-only value of M1 = Mmol/NA = 2.0 B. The calculated magnetic parameters
are collected in Table 7 though only the first case of the D2d(p) allows an evaluation of them by
means the perturbation theory. Because the first excitation energy is low, ax = 1400 cm-1, this case
need be considered as quasidegenarate. Therefore it is not surprising that the calculated value of
D/hc = 284 cm-1 is much different from the exact multiplet splitting 1/hc = 181 cm-1.
Table 7. Calculated magnetic parameters
Geometry
Spin Hamiltonian parameters
Excitation energy Multiplet gap

TIM /10-9 |ax|/hc
1/hc
/cm-1
181
D/hc
E/hc gx, gy gz
/cm-1
/cm-1
m3 mol-1 /cm-1
D2d(p), GCFT a 284
-
3.803 2.0
12.5
1400
1600
3308
4308
2438
3450
D2d(o), GCFT a undefined -
2.852 2.546 7.78
2.638 2.401 5.94
2.682
126
3, GCFT b
3, ab initio
3, fitting
40.8
3.5
27.6
35.0
20.7
59.2
10
2.271 2.697
2.440
49
16.5
5.3
2.321 2.0
2.417
1.3
a A homoleptic complex in a weak crystal field. b Combined weak (Br) and intermediate (N) crystal
field.
The complex 3 in one side resembles the prolate disphenoid (angle N-Ni-N = 82 deg, a
compression by 27 deg), however, on the second side it matches the oblate disphenoid (Br-Ni-Br =
126 deg, an expansion by 17 deg). The experimental geometry of the {NiN2Br2} chromophore has
been used in the GCFT calculations with the pole strengths F4(Br) = 5000 cm-1 and F4(N) = 8000
cm-1. On the symmetry descent to the real C1 symmetry the mother ground term T1(Td) splits into
three orbitally non-degenerate daughter terms; their energies are at {0, 3308, 4308} cm-1 which
3
3
resembles the situation close to the prolate bisphenoid with the ground A2 and excited E terms.
Therefore it can be expected that the spin-Hamiltonian formalism is applicable, predicting D > 0.
3.4. Ab initio calculations
The ab initio calculation at the CASSCF/NEVPT2 level predict the lowest excitation energies
1 = 2438 and 2 = 3450 cm-1. These arise from the splitting of the mother tetrahedral ground term
3
3T1 into daughter terms (3A2, E) on symmetry lowering to D2d and further to C2v. Three daughter
terms {0, 2438, 3450 cm-1} can be considered as quasi degenerate so that an application of the
perturbation theory in evaluating the spin-Hamiltonian parameter might be problematic. The lowest
spin-orbit corrected energy levels lie at {0, 20.7 (1), 59.2 (2)}, {2910, 2933, 3104}, and {4170,
4173, 4252} cm-1 where the square brackets collect the crystal field multiplets arising from the
crystal field terms. By applying 1 = D – E and 2 = D + E one gets D = 39.9 and E = 19.3 cm-1
(Fig. 5). However, a reverse situation, 1 = 2E and 2 = –D – E yields D = –69.6 and E = 10.4 cm-1
(E/D = 0.18). Effective Hamiltonian spin-orbit coupling contribution gave the zero-field splitting
parameters D = –48.8 cm-1, E/D = 0.21 and the g-factors {2.271, 2.440, 2.697}.
The calculated ab initio value of D = –48.8 cm-1, contradicts the theoretical expectations based
upon GCFT method that predicts positive value for D-parameter. An inspection to the contributions
to the zero-field splitting parameters from individual excitations (see Table S1) confirms that D-
value is dominated by the first excitation energy 1 = 2438 cm-1 with the increment 128 cm-1. This
is an overestimated value due to the failure of the perturbation theory.
9

3E(6)
2 = D + E
1 = D - E
5(2)
D > 0
1(1)
D2d'
3A2(3)
D2d
C2v'
Fig. 5. The lowest energy levels on symmetry lowering from elongated (prolate) bisphenoid for d8
systems.
5
4
3
2
1
0
2
1
0


0
100
200
300
0
1
2
3
4
5
6
7
T/K
B/T
Fig. 6. Modeling of the magnetic functions for 3 at the CASSCF/NEVPT2 level. Magnetization per
formula unit is calculated for T = 2.0 K.
3.5. Magnetic data
Three tetracoordinate Ni(II) complexes are almost planar and thus diamagnetic. These are:
[Ni(PPh3)2(NCS)2], 1; [Ni(dppm)Br2], 2; and [Ni(dppp)(NCS)2], 4. The available magnetic data are
deposited in ESI.
The quasi-tetrahedral complex [Ni(biqu)Br2], 3, is a paramagnetic species (Fig. 7). It possesses
the room-temperature value of the effective magnetic moment eff = 3.26 B which decreases only
slightly down to 50 K; on further cooling a sharp drop occurs and eff = 1.35 B at T = 1.9 K. This
indicates positive zero-field splitting, D > 0. Magnetization per formula unit adopts a value of M1 =
1.12 at T = 2.0 K and B = 7.0 T which also confirms large value of the zero-field splitting.
The DC magnetic data was fitted by the standard model of the zero-field splitting
r2
2
2
2
2
2
ˆ
ˆ
ˆ
ˆ
Hk,l  D(Sz  S / 3)h  E(Sx  Sy )h
(1)
1
ˆ
ˆ
ˆ
BBm (gx sink cosl Sx  gy sink sinl Sy  gz cosk Sz )h
Simultaneous fitting of the susceptibility and magnetization data gave the following set of
magnetic parameters referring to the spin-Hamiltonian formalism: gz = 2.002, gx = 2.321, gy =
2.417, D/hc = +16.5 cm-1, E/hc = 5.3 cm-1, zj/hc = 0.15 cm-1 (molecular-field corrections) and
temperature-independent magnetism TIM = 1.3 × 10-9 m3 mol-1 [discrepancy factors of the fit R()
= 0.032, R(M) = 0.093]. The magnetic parameters qualitatively fulfill the consistency criterion
according to which D ~ (gz – gx)/2, so that gz < gx implies D > 0 (/hc = –315 cm-1). The fitting
with D < 0 was not successful (see Fig. S3 in ESI). The fitted value of the axial zero-field splitting
parameter D matches the range expected on the basis of the GCFT model.
10

4
3
2
1
0
B = 0.1 T
2
1
0
2


T = 2.0 K
T = 4.6 K

1
0

0
10
20
200
30
250
40
300
0
1
2
3
4
5 6 7
0
50
100
150
T/K
B/T
Fig. 7. Magnetic functions for 3. Left – temperature dependence of the effective magnetic moment
(inset: molar magnetic susceptibility); right – field dependence of the magnetization per formula
unit. Lines – fitted.
4. Conclusions
Four tetracoordinate Ni(II) complexes, 1 [Ni(PPh3)2(NCS)2], 2 [Ni(dppm)Br2], 3 [Ni(biqu)Br2]
and 4 [Ni(dppp)(NCS)2], have been structurally, spectrally and magnetically characterized.
Complexes 1, 2 and 4 are planar diamagnetic, while the complex 3 is quasi-tetrahedral
paramagnetic. Electronic structure and magnetic properties of the paramagnetic complex 3 have
been subjected to deep experimental and theoretical analysis. It exhibits a zero-field splitting of the
ground term quantified by a sizeable positive D-value. The GCFT and ab initio calculations confirm
3
presence of the nondegenerate A2 ground term as a consequence of the significantly distorted
structure (prolate bisphenoid on one side and oblate bisphenoid on the second side).
Acknowledgments
The Slovak grant agencies (VEGA 1/0534/16 and APVV-14-0073) are acknowledged for the
financial support. We thank the UCM-FPV staff for their support during measurements. C.R. thanks
the grant from Fund to Support of Research at UCM. This article was created with the support of
the MŠVVaŠ of the Slovak Republic within the Research and Development Operation Programme
for the project "University Science Park of STU Bratislava" (ITMS project no. 26240220084)
cofounded by the European Regional Development Fund.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/. CCDC nos. 1585156-1585157, and 1844141 contains the crystallographic data
for
1-2
and
4.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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12

Highlights:
 Four tetracoordinate Ni(II) complexes have been prepared.
 The complexes [Ni(PPh3)2(NCS)2], [Ni(dppp)(NCS)2], and [Ni(dppm)Br2] are planar and
diamagnetic.
 The complex [Ni(biqu)Br2] is pseudotetrahedral and paramagnetic.
3
 It exhibits a zero-field splitting of the ground term A2 into three crystal-field multiplets that
can be described by D and E parameters within the spin Hamiltonian formalism.
 The GCFT and ab initio calculations confirm this interpretation.
13

Four tetracoordinate Ni(II) complexes have been prepared, structurally characterized, and subjected
to magnetometric studies. The complexes [Ni(PPh3)2(NCS)2], [Ni(dppp)(NCS)2], and
[Ni(dppm)Br2] are planar and thus diamagnetic. The complex [Ni(biqu)Br2] is pseudotetrahedral,
with the geometry close to C2v symmetry, and paramagnetic.
14