Syntheses and luminescent properties of tetrahedral nickel(0) complexes

Article abstract of DOI:10.1016/S0020-1693(96)05335-2

Tetrahedral nickel(0) complexes [NiL₄], [Ni(dppe)₂] and [Ni(CO)₂(SbPh₃)₂] (L=AsPh₃, SbPh₃, P(OPh)₃, dppe=1,2-bis(diphenylphosphino)ethane) were prepared by reduction of NiCl₂·6H₂O with NaBH₄ under N₂ or CO atmosphere in the presence of the ligand. The complex [Ni(SbPh₃)₄] was also obtained by electrolysis at -1.3 V (Ag/Ag⁺), under a platinum gauze, of the system NiCl₂·6H₂O/SbPh₃ (molar ratio=1:4). These complexes, both in the solid state and in solution, show an orange emission at room temperature, when excited with UV radiation. A qualitative molecular orbital diagram for the [NiL₄] complexes is proposed. Electronic absorption spectra of the complexes show bands near 400 nm assigned as MLCT π*_2e←d_2t2. A ¹A₁←³T₁ transition is suggested for the emission observed in these systems. Lifetimes in microsecond range were estimated from time-resolved emission spectra. Spectroscopic properties of the free ligands have also been investigated.

Full text of DOI:10.1016/S0020-1693(96)05335-2

Inorganica Chimica Acta 255 (1997) 53–58
Syntheses and luminescent properties of tetrahedral nickel(0) complexes
R.C.G. Frem U, A.C. Massabni, A.M.G. Massabni, A.E. Mauro
Departamento de Qu´ımica Geral e Inorgaˆnica, Instituto de Qu´ımica de Araraquara, UNESP, C.P. 355, 14800-900 Araraquara, SP, Brazil
Received 1 March 1995; revised 8 May 1996
Abstract
Tetrahedral nickel(0) complexes [NiL4], [Ni(dppe)2] and [Ni(CO)2(SbPh3)2]L( sAsPh3, SbPh3, P(OPh)3, dppes1,2-
bis(diphenylphosphino)ethane) were prepared by reduction of NiCl2P6H2O with NaBH4 under N2 or CO atmosphere in the presence of the
ligand. The complex [Ni(SbPh3)4] was also obtained by electrolysis at y1.3 V (Ag/Agq), under a platinum gauze, of the system
NiCl2P6H2O/SbPh3 (molar ratios1:4). These complexes, both in the solid state and in solution, show an orangeemission atroomtemperature,
when excited with UV radiation. A qualitative molecular orbital diagram for the [NiL4] complexes is proposed. Electronic absorption spectra
of the complexes show bands near 400 nm assigned as MLCT pU2e§d2t2.A 1A1§ T1 transition is suggested for the emission observed in
3
these systems. Lifetimes in microsecond range were estimated from time-resolved emission spectra. Spectroscopic properties of the free
ligands have also been investigated.
Keywords: Nickel(0) complexes; Electronic absorption; Photophysics
1. Introduction
As to the metal complexes of the nickel group, very few
articles [10–12] deal with photophysical properties of these
compounds. This fact motivated the investigation of the
nickel(0) complexes which show a broad emission band
at room temperature: [Ni(AsPh3)4], [Ni(SbPh3)4], [Ni-
(CO)2(SbPh3)2], [Ni(dppe)2] (dppes1,2-bis(diphenyl-
phosphino)ethane) and [Ni{P(OPh)3}4].
Studies of transition metal complexes of tertiary phos-
phines, arsines and stibines are carried out in many labora-
tories, because of their rich chemistry and catalytic activity
[1]. However, interest in the photophysics and photochem-
ical behavior of organometallic and coordination compounds
[1,2] and, in particular, of d10 metal complexes [3] has
grown only in the last decade.
Investigations of the excited state properties of coordina-
tion compounds containing a d10 metal center have provided
important information as to the origin of orbitals, energies,
electronic structures and lifetimes which, in their turn, form
the basis for the comprehension of the reactivity of excited
species [4]. The excited states of transition metal complexes
may beregarded asnewreagents,oftendifferingconsiderably
in reactivity from the corresponding ground-state precursors
[5].
2. Experimental
2.1. Materials
All the synthetic procedures were carried out under a puri-
fied nitrogen atmosphere using Schlenk techniques, except
for the complex [Ni(CO)2(SbPh3)2] which was prepared
under a carbon monoxide atmosphere. The yields were
around 80% with small variations from one synthesis to
another, except for the complex [Ni{P(OPh)3}4] obtained
with a yield of 46%.
Most of the studies on luminescent properties of d10 sys-
tems involve complexes containing the Cu(I) ion with ter-
tiary phosphine and/or heteroaromatic ligands including
interlocked macrocyclic molecules (catenands) [6]. Lumi-
nescence can also be observed for some compounds of Ag(I)
[7], Au(I) [8], Zn(II) or Cd(II) [9], and a number of
electronic excited states have been assigned depending on
the nature of the ligand.
Ethanol (Merck) was purified by conventional methods
and stored over 3 A molecular sieves. The ligands AsPh3,
˚
SbPh3 and dppe (Aldrich) were recrystallized from hot eth-
anol solutions. The ligand P(OPh)3 (Aldrich), NiCl2P6H2O
(CAAL), Ni(NO3)2P6H2O (Carlo Erba), NaBH4 (Merck)
and the solvents acetonitrile and methanol (Merck) were
used as supplied.
U Corresponding author.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05335-2

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R.C.G. Frem et al. / Inorganica Chimica Acta 255 (1997) 53–58
2.2. Syntheses
trode, an Ag/AgClO4 electrode as the referenceelectrodeand
a platinum spiral coil as the auxiliary electrode. The synthesis
was carried out in acetonitrile solution containing 0.1000 g
of nickel chloride and 0.9600 g of SbPh3 (molar ratio 1:3.5),
at 25 8C underN2 usingtetrabutylammoniumperchlorate(0.2
M) as supporting electrolyte. Two minutes after the begin-
ning of the electrolysis at y1.3 V on platinum, precipitation
of a yellow compound occurred. No metallic nickel traces
were detected. The compound was isolated after 1 h, purified
and identified as the complex [Ni(SbPh3)4].
2.2.1. Preparation of [NiL4] (LsAsPh3, SbPh3)
A mixture containing 0.5000 g of NiCl2P6H2O and the
particular ligand in a 1:4 molar ratio in ethanol was prepared
under N2. The resulting solution was stirred at room temper-
ature for 5 min and a freshly prepared suspension containing
0.2388 g of NaBH4 (1 salt:3 reducing agent) in ethanol was
added dropwise to the solution. The evolution of H2 was
immediately observed and a suspension containing fine solid
was formed. After2hof
stirring, the complex was filtered
off, washed with ethanol, and dried in vacuo.
2.3. Instrumentation
Compound [Ni(AsPh3)4]: m.p. 70.0–70.9 8C (dec.).
Anal. Calc. for C72H60As4Ni: C, 67.36; H, 4.72; Ni, 4.57.
Found: C, 63.22; H, 4.61; Ni, 4.57%.
Compound [Ni(SbPh3)4]: m.p. 148.5–149.2 8C. Anal.
Calc. for C72H60Sb4Ni: C, 58.78; H, 4.12; Ni, 3.99. Found:
C, 58.62; H, 4.32; Ni, 3.99%.
Carbon, hydrogen and nitrogen were determined using a
Perkin-Elmer model 240 automatic elemental analyzer.
Room-temperature absorption spectra were recordedusing
a Zeiss-Jena SPECORD M40 spectrophotometer. Solid sam-
ples of the nickel(0) complexes and ligands in Nujol were
placed in a 1 mm quartz cell. MgO in Nujol was used as a
reference sample.
2.2.2. Preparation of [Ni(dppe)2]
The procedure followed for the preparationofthiscomplex
was the same as described in Section 2.2.1, except for the 1
salt:3 ligand molar ratio which was utilized in this case.
Compound [Ni(dppe)2]: m.p. 263.6–265.4 8C (dec.).
Anal. Calc. for C52H48P4Ni: C, 72.99; H, 5.67; Ni, 6.86.
Found: C, 71.87; H, 5.49; Ni, 6.68%.
The luminescence and excitation spectra were measured at
room temperature on a SPEX FLUOROLOG F212I spectro-
fluorimeter using a 450 W xenon lamp, Spectramate 1680B
double monochromator, and a thermoelectrically cooled
Hamamatsu R928 photomultiplier tube. Grounded samples
were placed in a quartz tube and the emission signal was
collected at a 24.58 angle from the incident beam.
2.2.3. Preparation of [Ni{P(OPh)3}4]
Excited state lifetimes in microsecond range for the
nickel(0) complexes were estimated from the time-resolved
emission spectra. A pulsed xenon lamp (EG&G FX-265, 5
J/pulse, 3 ms halfwidth) and a SPEX 1934 phosphorimeter
were coupled to the SPEX FLUOROLOG F212I.
To a solution containing 0.5000 g of Ni(NO3)2P6H2Oin
acetonitrile and 2.1 ml of triphenylphosphite, a suspension of
0.1301 g of NaBH4 in acetonitrile was added dropwise under
N2 (1 salt:4.6 ligand:2 reducing agent). The formation of a
white solid and the evolution of H2 were soon observed. After
2 h the complex was filtered off, washed with methanol, and
dried in vacuo.
3. Results and discussion
Compound [Ni{P(OPh)3}4]: m.p. 109.4–109.7 8C. Anal.
Calc. for C72H60O12P4Ni: C, 66.52; H, 4.66; Ni, 4.52. Found:
C, 66.35; H, 4.58; Ni, 4.39%.
3.1. Synthesis
Sodium tetrahydroborate, NaBH4, was shown to be a suit-
able reducing agent for the preparation of this class of
nickel(0) complexes. The reaction is instantaneous, it uses
common materials (nickel salts) as starting materialsandcan
be carried out at room temperature. This method has already
been used for the preparation of the complexes [Ni(dppe)2]
[13] and [Ni{P(OPh)3}4] [14], but this work reports for
the first time the usage of tetrahydroborate for the synthesis
of the complexes [Ni(EPh3)4]E( sAs, Sb). These
nickel(0) compounds with triphenylarsine and triphenylsti-
bine have already been prepared by the Et3Al reduction of
[Ni(acac)2] in the presence of the ligand [15] or by the
replacement of cyclooctadiene from [Ni(COD)2] [16].
Reduction by NaBH4 was also efficient for the synthesis
of [Ni(CO)2(SbPh3)2], avoiding the use of the toxic
carbonyl [Ni(CO)4] as starting compound [17].
2.2.4. Preparation of [Ni(CO)2(SbPh3)2]
[Ni(CO)2(SbPh3)2] was synthesized under a CO atmos-
phere using 0.2000 g of nickel chloride, 0.8912 g of the ligand
and 0.0955 g of NaBH4 (1:3:3 molar ratio) as starting mate-
rials, following the same experimental procedure as reported
in Section 2.2.1.
Compound [Ni(CO)2(SbPh3)2]: m.p. 194.0–195.5 8C.
Anal. Calc. for C38H30O2Sb2Ni: C, 55.60; H,3.69. Found: C,
51.01; H, 3.64%.
2.2.5. Preparation of [Ni(SbPh3)4] by electrochemical
reduction
The controlled-potential electrolysis was carried out using
an FAC 200-A potentiostat-galvanostat, combined with an
FAC 201 triangle wave generator and a Houston Instrument
X-Y recorder. The electrochemical cell was of the three-
electrode type with a platinum gauze as the working elec-
The complex [Ni(SbPh3)4] was also obtained for the first
time by cathodic reduction of the nickel(II) salt in the pres-

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55
Table 1
ence of the ligand under N2. The major virtue of the electro-
analytical techniques for the synthesis of coordination
compounds is its selectivity. That iswhyithasbeenemployed
by some research groups [18].
Solid-state absorption, excitation and emission data for the free ligands (at
room temperature)
Ligand
Absorption
l (nm)
Excitation
lmax (nm)
(under 436 nm
em.)
Emission
lmax (nm)
(under 350 nm
exc.)
The nickel(0) complexes were precipitated as non-crys-
talline solids, slightly soluble in organic solvents such as
benzene or dichloromethane. The complexes are diamagnetic
with the typical tetrahedral structure of d10 systems.
The stabilities of these complexes to atmospheric oxygen
in the solid state change substantially. The complex
[Ni(SbPh3)4] is the most stable within the studied series.
Samples kept in stoppered vialsforthree yearsgavenovisible
evidence of decomposition. [Ni{P(OPh)3}4] is relatively
stable, but over a period of a few days the originally colorless
compound gradually becomes dark, suggesting the presence
of metallic nickel. Other complexes, however, are not so
stable; samples of the yellow [Ni(AsPh3)4] for instance
started to decompose giving a green Ni2q complex only after
a few hours in air. It is noteworthy that solutions containing
these complexes, except that of the complex [Ni(SbPh3)4],
showed instantaneous decomposition when exposed to air. A
yellow solution of this complex in benzene or dicholorome-
thane remained unchanged for a few minutes (and then
became discolored).
AsPh3
SbPh3
dppe
236, 253–271
237, 251–269
234, 255–272
226, 264
339
366
376
351
421
421
454
398
P(OPh)3
the lone pair localized on the ligand atom, an absorption shift
should be observed in the structured system near 265 nm for
different ligands. However, the observation of this group of
bands in the same spectral region for all the ligands led us to
question the pU§n assignment for this transition. For the
first time, we obtained the excitation spectra of these com-
pounds to clarify this point. A broad, structureless excitation
band was observed near 350 nm showing some displacement
according to the nature of the ligand (Table 1, Fig. 2). We
suggest then that this excited state can be associated with the
pU§n transition, which was not observed in the absorption
spectra since it is a forbidden transition. We believe that the
system near 265 nm is related to another pU§p type
transition.
3.2. Electronic spectroscopy
Solid samples of these ligands exhibit strong emission in
the 380–480 nm region when excited at 350 nm (Table 1,
Fig. 2) and the excited state n,pU should be the emissive one
of this class of ligands.
3.2.1. Free ligands
The solid-state absorption spectrum of the ligand AsPh3 is
shown in Fig. 1, as representative of the series herein studied,
and shows a very intense band at 236 nm and a structured
system, whose maximum is centered at 265 nm (Table 1).
The absorption spectra of aromatic phosphine and arsine
solutions were interpreted previously. The highest energy
band (235 nm) was assigned as a pU§p transitionlocalized
on the aromatic rings and the structured system was associ-
ated with a pU§n transition [19]. As this transitioninvolves
3.2.2. Nickel(0) complexes
Fig. 3 shows the electronic absorption spectra of the
nickel(0) complexes in the solid state at room temperature.
The absorption spectra of these compounds are very similar
Fig. 2. Excitation (∆) and emission (——) spectra of AsPh3 (A), SbPh3
(B), dppe (C) and P(OPh)3 (D) at room temperature in the solid state.
Fig. 1. Solid-state absorption spectrum of AsPh3 (room temperature).

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R.C.G. Frem et al. / Inorganica Chimica Acta 255 (1997) 53–58
molecular orbital diagram for Td d10 [NiL4] complexes is
proposed (Fig. 4). The electronic configuration of the fun-
damental state corresponds to a 1A1 state in close agreement
with the diamagnetism of the zerovalent nickel complexes.
We suggest the assignment of this visible absorption band to
1
the spin-allowed 1T2 § A1 transition derived from
pU2e§d2t2 excitations. The higher energy bands of the
mixed-complex [Ni(CO)2(SbPh3)2] can be classified as
pU§p intraligand (SbPh3)ro
pUCO§d (239 nm in
[Ni(CO)4] [20]), while the absorption at 378 nm can be
assigned as a pUSbPh3§d MLCT transition.
The absorption spectrum of the more stable complex
[Ni(SbPh3)4] in benzene and dichloromethane solutions
revealed no solvatochromism and the integrity of the tetrakis-
complex in solution. We believe that this fact is related to a
stronger Ni–SbPh3 bond responsible for the reduction of the
reactivity of [Ni(SbPh3)4] when compared to the other com-
plexes of this series [21]. There are no published UV–Vis
data for solutions containing any other compound of the stud-
ied series, probably because of the low stability of such sys-
tems. The spectrum of an analogous complex [Ni(PPh3)4]
in solution shows absorption bands at 393 and 530 nm while
MLCT bands in the region of 333 nm are observed for the
complex [Ni{P(OEt)2Ph}4] [22]. The presence of absorp-
tion bands in longer wavelengths in the first case is due to a
substantial dissociation of the tetrakis-complex in solution
giving [Ni(PPh3)3] and PPh3. So, reddish solutions of
[NiL4] having high ligand dissociation constants exhibit
absorption spectra very different from the spectra of the
tetrakis-complexes previously obtained in the solid state.
Strong orange emissions were observed in solid samples
of the nickel(0) complexes (Table 2). The luminescence
spectra of these compounds are broad, unstructured and show
a shoulder shifted to lower energy (1600–1700 cmy1) of the
band maximum (Fig. 5). Similar emission spectra resulted
when [Ni(SbPh3)4] was dissolved in benzene or
dichloromethane.
Fig. 3. Solid-state absorption spectra of the complexes [Ni(AsPh3)4] (A),
[Ni(SbPh3)4] (B), [Ni(CO)2(SbPh3)2] (C), [Ni(dppe)2] (D) and
[Ni{P(OPh)3}4] (E) at room temperature. Expanded scales: 2.00:1.00 (A)
and 2.00:0.200 (D).
(except the one for [Ni{P(OPh)3}4]) and exhibit an absorp-
tion system in regions of high energy and a broad band near
the visible region which is shifted to the UV region in the
case of the complex [Ni{P(OPh)3}4]. Absorption energies
are given in Table 2.
The similarity of the system seen below 272 nm to that
observed in the absorption spectra of the free ligands seems
to indicate pU§p intraligand transitions. In order to fix the
assignment of the absorption band near 400 nm, a qualitative
The large Stokes shift (;11 000 cmy1) suggests the exis-
tence of forbidden transitions in the 400–500 nm region. In
fact, the excitation spectra of these zerovalent complexes
show a broad band near 460 nm (Fig. 5). The excited state
1
1
3
1
can be associated with the MLCT T1§ A1 or T2§ A1
transitions. The absence of an MLCT excitation band in this
Table 2
Solid-state absorption, emission and lifetime data for the nickel(0) complexes (at room temperature)
Complex
Color
Absorption
Emission
t (ms)
l (nm)
lmax (nm)
(under 460 nm exc.)
[Ni(AsPh3)4]
[Ni(SbPh3)4]
[Ni(CO)2(SbPh3)2]
[Ni(dppe)2]
[Ni{P(OPh)3}4]
mustard yellow
lemon yellow
greenish yellow
gold yellow
white
235, 250–271, 407
237, 371
239–261, 378
234, 253–272, 400
244, 270
620
634
639
613
634
20.2
9.7
a
7.6
32.4
a Not determined.

R.C.G. Frem et al. / Inorganica Chimica Acta 255 (1997) 53–58
57
Fig. 5. Excitation (∆) and emission (———) spectra of [Ni(AsPh3)4]
(A), [Ni(SbPh3)4] (B), [Ni(dppe)2] (C) and [Ni{P(OPh)3}4] (D) at
room temperature in the solid state.
Fig. 4. Molecular orbital diagram for the tetrahedral complexes [NiL4].
region for the complex [Ni{P(OPh)3}4] can be related to
the destabilization of the pU orbital of the phosphite due to
the presence of the oxygen atom. Therefore we suggest that
the luminescence of these systems can originate from the
Cu(I) ion (474–537 nm) was reported [23]. A destabiliza-
tion of the Cu(I) compound excited states when compared
with those of the M(0) (MsNi, Pd, Pt) complexes is then
observed and may be related to the poorer retrodonor ability
of the Cu(I) ion [24]. Therefore, the nature of the lowest
energy excited state for this class of compounds can be mod-
ulated by chemical modifications of the ligands and/or by
judicious choice of the metallic center.
Emission lifetimes of the complexes were estimated from
the time-resolved emission spectra. The 3 ms halfwidth for
the excitation source prevented lifetime measurements by
single-exponential decay fitting (except for [Ni{P-
(OPh)3}4]). However, usage of the time-resolved emission
spectra allowed an evaluation of the time required to lower
the intensity to a certain I0/e value (I0sintensity in the emis-
3
1A1§ T1 transition derived from the metal–ligand excita-
tion. The presence of the 3MLCT emission bandscorrespond-
ing to almost the same energy levels can be associated with
the strong p-acceptor character of the ligands used in this
study. Emission bands at lower energy for the analogous
complex [Ni(PPh3)4] (740–800 nm) were observed insolu-
tion [10] and this fact must be associated with the formation
of tri-coordinated species [22].
3MLCT emission bands in the orange region are also
observed in tetrahedral complexes of zerovalent Pd and Pt
with aromatic phosphines [11]. However, a shift to higher
energies of the emission bands in related complexes of the
Fig. 6. Time-resolved emission spectrum of [Ni(dppe)2] in the solid state at room temperature (lexcs400 nm). Delay time (ms): 0 (A); 10 (B); 20 (C);
30 (D).

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R.C.G. Frem et al. / Inorganica Chimica Acta 255 (1997) 53–58
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complexes are listed in Table 2. The [Ni(SbPh3)4] emission
lifetime is shorter than those measured for the other [NiL4]
tetrahedral complexes and can be associated with a consid-
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4. Conclusions
This is the first communication reporting the luminescent
properties of some zerovalent nickel complexes with p-
acceptor ligands at room temperature. We deal with an inter-
esting class of inorganic compounds exhibiting a broad-band
luminescence in the visible region. The above results show
that long-lived MLCT excited states can be efficiently gen-
erated by irradiation of these d10 complexes. The study indi-
cates that the redox chemistry of the excited states of the
mononuclear Ni(0) systems should be a rich area for explo-
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This work was partially supported by Conselho Nacional
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