Organometallic nickel(II) complexes with substituted benzonitrile ligands. Synthesis, electrochemical studies and non-linear optical properties. The X-ray crystal structure of [Ni(η5-C5H5){P(C6H 5)3}(NCC6H4NH2)][PF 6]

Article abstract of DOI:10.1016/S0022-328X(97)00645-1

A new family of organometallic Ni(II) benzonitrile derivatives bearing strong donor substituents of general formula [Ni(η⁵-C₅H₅){P(C₆H ₅)₃}NCC₆H₄R][PF₆] (R=NH₂, N(CH₃)₂, C₆H₅, OCH₃, H, F) has been synthesized. A structural study by X-ray diffraction of the compound with R=NH₂ showed crystallization on the centrosymmetric monoclinic space group, P2₁/n, with a quasi planar structure of the coordinated nitrile. First and second hyperpolarizabilities evaluated by hyper-Rayleigh scattering and Maker fringe techniques showed essentially the same values for the complexes and the uncoordinated chromophores. Theoretical studies by the extended Hueckel method found a small amount of π delocalization due to the cancelling effects on the interaction of the metal fragment orbitals with π and π^* nitrile orbitals.

Full text of DOI:10.1016/S0022-328X(97)00645-1

Ž
.
Journal of Organometallic Chemistry 553 1998 115–128
ž /
Organometallic nickel II complexes with substituted benzonitrile
ligands. Synthesis, electrochemical studies and non-linear optical
properties. The X-ray crystal structure of
5
w ž
/
½
ž
/
5
ž
/xw x
Ni h -C5H5 P C6 H5 3 NCC6 H4 NH2 PF6
a
a,b
a,b
Alberto R. Dias , M. Helena Garcia a,b,), Paulo Mendes , M. Fatima M. Piedade ,
´
a
b,c
d
e
M. Teresa Duarte , Maria J. Calhorda , Carlo Mealli , W. Wenseleers ,
e
e
A.W. Gerbrandij , Etienne Goovaerts
a
Centro de Quımica Estrutural, Instituto Superior Tecnico, AÕ. RoÕisco Pais, 1096 Lisboa Codex, Portugal
´
´
b
Faculdade de Ciencias, R. Ernesto de Vasconcelos, Ed. C1, 1700 Lisboa, Portugal
ˆ
c
ITQB, Apart. 127, 2780 Oeiras, Portugal
d
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, Via Nardi 39, 50132 Florence, Italy
e
Physics Department, UniÕersity of Antwerp, Antwerp, Belgium
Received 5 May 1997; received in revised form 13 August 1997
Abstract
5
Ž .
w Ž
A new family of organometallic Ni II benzonitrile derivatives bearing strong donor substituents of general formula Ni h -
.Ä Ž
. 4
xw x Ž
Ž
.
.
C5H5 P C6H5 NCC6H4R PF6 RsNH2, N CH3 2, C6H5, OCH3, H, F has been synthesized. A structural study by X-ray
3
diffraction of the compound with RsNH2 showed crystallization on the centrosymmetric monoclinic space group, P21rn, with a quasi
planar structure of the coordinated nitrile. First and second hyperpolarizabilities evaluated by hyper-Rayleigh scattering and Maker fringe
techniques showed essentially the same values for the complexes and the uncoordinated chromophores. Theoretical studies by the
extended Huckel method found a small amount of p delocalization due to the cancelling effects on the interaction of the metal fragment
¨
orbitals with p and p ) nitrile orbitals. q 1998 Elsevier Science S.A.
Keywords: Organometallic Ni; Delocalization; Nitrile orbitals
1. Introduction
In the field of organometallic chemistry, much interest is currently devoted to the synthesis of new materials
possessing large second-order nonlinearities, especially because of their potential use in the area of integrated optics
w x
w x
1 . In particular, after the 1987 report of Green et al. 2 on the second harmonic generation efficiency of
w
Ž
.
x
w
x
cis- 1-ferrocenyl-2- 4-nitrophenyl ethylene the related research 3–8 in this area has increased constantly.
Ž
.
Non-linear optical NLO properties have been associated, in organic and in organometallic materials, with highly
polarized p-electronic systems which form under an applied electric field associated with a laser beam. Under this
perspective, molecules possessing conjugated donor-acceptor systems, are seen as potentially good candidates for the
manifestation of the properties. Recently, we published some results on the chemistry and NLO properties, namely
5
Ž
.Ž
. Ž
Ž .
Ž ..
SHG efficiencies, of compounds formed by the unit M h -C5H5 P PPP P MsFe II , Ru II and p-substituted
w
x
Ž
.
Ž .
benzonitriles containing ligands 9,10 . Also, we reported on the optical third-harmonic generation THG of Ru II
compounds 11 . This work has been motivated by the reports on the relatively high second harmonic generated
signals for some para-substituted benzonitriles such as p-MeOC6 H4CN, p-H2 NC6 H4CN 12 and p-
w
x
w
x
)
Corresponding author.
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

(
)
116
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
w
x
MeOC6 H4C6 H4CN 13 . We have explored the possibility of enhancing the signals by introducing a NC acceptor
group to given organometallic moieties. It is known that normal s coordination of the NC functional group to a metal
center takes place through the nitrogen lone pair. The two sets of orthogonal p and p ) orbitals more localized on N
Ž
.
and C atoms, respectively may be also involved in interactions with d metal orbitals having the right p symmetry.
Therefore, the nitrile group could play an attractive role as its p electron system may be alternatively attracted toward
the metal center or toward a p acceptor group located away from the metal. This effect was studied for
5
5
w
Ž
.Ž
.
x
w
Ž
.Ž
.
x
Ž
.
Fe h -C5H5 P PPP P - and Ru h -C5H5 P PPP P -fragments where P PPP Psbidentate phosphine attached to
Ž
.
the ligand p-NCC6 H4R Rsdonor or acceptor substituent . In particular, it was found that, in the case of
p-NCC6 H4 NO2 , a significant p backdonation from the metal extends through the whole p system and toward the
NO2 acceptor group. The large electron polarization, in some of the systems investigated, reflects in the Kurtz powder
5
w
Ž
and SHG values which were 38 times more efficient than the urea standard in the case of Fe h -
5
.Ž
.Ž
xw x w x
w
Ž
.Ž
. Ž
xw
x
C H5 qDiop NCC6 H4 NO2 PF6 9 , and 10 times in the case Ru h -C5H5y3q3 Diop NCC6 H4 NO2 CF3SO3
w
5 x
Ž
.
10 . The value of the second hyperpolarizability g y3v;v,v,v of 2.3=10
esu, measured by maker fringe
5
w
Ž
.Ž
. Ž
xw x w x
technique, was found for the ruthenium analogue Ru h -C5H5 dppe NCC6 H4C6 H4 NO2 PF6 11 .
5
Ž
.
Our continuing studies in this field prompted us to explore the chemistry of other M h -C5H5 based 18 electron
complexes and we present here the synthesis and characterization of a new family of nickel II derivatives as well as
Ž .
their second and third order NLO properties determined by hyper-Rayleigh scattering and Maker fringe technique,
respectively.
2. Results and discussion
2.1. Chemical studies
5
w
Ž
New nitrile complexes were prepared by chloride abstraction from the parent neutral compound Ni h -
.
Ä
Ž
.
4
x
Ž
C5H5 P C6 H5 Cl , by TlPF6 in the presence of a large excess of the NCC6 H4 R adequate nitrile RsNH2 ,
3
Ž
.
.
N CH3 2 , C6 H5, OCH3, H, F , in diethyl ether at room temperature. A brown greenish microcrystalline precipitate of
5
w
Ž
.
Ä
Ž
.
4
xw
x
Ni h -C5H5 P C6 H5 NCC6 H4R PF6 was obtained with yield in the range of 78–98%, depending on the nitrile,
3
this yield being based on the 1H NMR spectra of the crude reaction products. The compounds are fairly stable towards
oxidation, in air and to moisture, in solid state, but they are not so stable in solution. Special care had to be taken
5
w
Ž
.
Ä
Ž
.
342 xw
x
during recrystallization owing to the easy formation of Ni h -C5H5 P C6 H5
PF6 which was also identified by
1H NMR as being the major by-product of the studied reaction. Formulation of the new compounds was supported by
analytical data, IR and H, 31P, C NMR spectra Tables 1 and 2 as discussed below. The molar conductivities of
1
13
Ž
.
Table 1
1
w
Ž
.xw x
H NMR data in chloroform-d for complexes NiCpPPh3 p-NCC6 H4R PF6

(
)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
117
Table 2
13
w
Ž
.xw x
C NMR data, in chloroform-d, for complexes NiCpPPh3 p-NCC6 H4R PF6
10y3 M solutions of the complexes in nitromethane, in the range 70–90 Vy1 cm2 moly1, were consistent with values
w
x
reported for 1:1 electrolytes 14 .
Typical IR bands confirm the presence of the cyclopentadienyl ligand 3060 cm
y1
y
Ž
.
Ž
and the PF6 anion 840 and
560 cmy1 . A shift of the NC stretching frequency upon coordination is observed in the positive range 20–45 cm
This implies a s-coordinated nitrile in which the overall N[C bond is strengthened upon coordination. At variance
Ž . Ž .
.
y1
.
with what is found for the previously mentioned for the Fe II and Ru II analogs, the p back-donation seems non
Ž .
existent in these Ni II complexes.
13
Ž
.
The C NMR data show that amongst the atoms of the nitrile benzene ring only that bound to CN C1 is not
deshielded. All the others are slightly deshielded as expected when the benzonitrile group is involved in a normal s
Ž
Ž
.
coordination. The effects seem to depend on the donor capabilities of the p-substituent NH2 (N CH3 2 )C6 H5 )
.
Ž
.
OCH3 )H(F . The highest shielding of the C1 atom comparing the free and the coordinated donor nitrile ligands
1
has a value of 3.9 ppm and it is observed for the NCC6 H4 NH2 substituent. Considering the H NMR data, the
protons attached to the carbon atoms are shielded, especially so the ortho bonded H2 , H6 protons. In particular for
the NH2 case, the shielding was up to 0.69 ppm. A possible explanation can be found by assuming a electronic
Ž .
w x
interaction of the protons with the counter ion, as observed before in our studies of Ru II compounds 10 . It was also
observed that H and C nmr data for h5-C5H5 together with 31 P data for PPh3 ligand were only slightly influenced
1
13
by the p-substituted nitrile group and followed the same general trend of the shielding, presented above. These
observations, altogether, suggest some electronic relationship between the p-substituted donor group and the metal
5
w
Ž
.
Ä
Ž
.
34xq
fragment: in particular, the trends associated with Ni h -C5H5 P C6 H5
seem to revert the situation previously
Ž .
Ž .
observed for the Ru II and Fe II complexes containing the same nitrile ligand. In the latter, the metal essentially
behaves as a s acceptor.
2.2. Crystallographic studies
5
w
Ž
.
Ž
.
xq
w
xy
cations and PF6 anions. A view of
The crystal structure contains discrete Ni h -C5H5 PPh3 NCC6 H4 NH2
the crystal packing, in the centrosymetric space group P21rn, is shown in Fig. 1. The complex cation is depicted in
Fig. 2 and selected bond lengths and bond angles are listed in Table 3.
5
Ž
.
The nickel atom is coordinated to one h -cyclopentadienyl ring, one triphenylphosphine, and one NC C6 H4 NH2
Ž .
ligand. By assuming that Cp occupies three coordination positions, the Ni II species can be considered five-coordi-
nated and it obeys the 18 electron rule. The metal atom deviates only 0.062 4 A from the plane formed by the
centroid of the Cp ligand and by the donor atoms N 1 and P 1 . The coordination angles around the nickel atom,
P–Ni–N 95.4 1 8, P–Ni–Cp 132.9 1 8, and N–Ni–Cp 131.2 2 8, are comparable with those in the related Ni h -
C5H5 PPh3 ON2C6 H4 NO2-p 17 , 96.1 1 8, 133.88 and 129.28, respectively.
˚
Ž .
Ž . Ž .
5
Ž .
Ž .
x
Ž .
w
Ž
.Ž .Ž x w Ž .
.

(
)
118
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
5
w
Ž
.
Ž
.
xw
x
w x
Fig. 1. Molecular diagram showing the crystal packing of Ni h -C5H5 PPh3 NCC6 H4 NH2 PF6 , using SCHAKAL 15 .
˚
˚
x
w
Ž .
x
w
The Ni–P bond length 2.174 1 A is near the upper limit of the range 2.183–2.139 A found in the Cambridge
w
x
Ž
.
Crystallographic Data Base 18 from ten complexes of the type CpNi PPh3 L. The distance between Ni and the Cp
centroid of 1.737 1 A is similar to that of related triphenylphosphine complexes 1.717–1.763 A . The Cp ring is
practically planar. The Ni–N bond length in Ni h -C5H5 PPh3 NCC H4 NH2
ones in other Ni-nitrile complexes, such as Ni C6 H 5 3 P NCC6 H 5
˚
˚
x
Ž .
w
5
xq
w
,
Ž .
˚
x
w
Ž
.Ž
.Ž
.
1.855 3 A is shorter than the
˚
A
w
Ž
.
x6
Ž
.
w
Ž
.
x
w x
19 and
1.889 11
3
˚
w
Ž
. Ž
.
x
w
Ž .
Ž .
Ž .
Ž . x w
x
Ni NCC6 H5 P C6 H5 3 P2C7 H8, 1.87 2 , 1.91 2 , 1.89 2 , 1.91 3 A 20 .
4
Ž . .
Ž
.
Ž .
Ž
.
Ž
The arrangement Ni–N–C–C deviates somewhat from linearity, the angles Ni–N 1 –C 21 and N 1 –C 21 –C 22
Ž . Ž . Ž . Ž .
Ž
.
being 176.2 4 8 and 177.5 6 , respectively. Also the Ni 1 , N 1 and C 21 atoms are out of the plane of the phenyl
˚
Ž
.
Ž
.
Ž .
ring with deviations of 0.366 13 , 0.155 10 and 0.074 8 A, in the order. A non perfectly linear but coplanar
geometry has been observed also for the free ligand at room temperature 21 . However, at lower temperature the
w
x
w
x
amino group becomes pyramidal 22 .
˚
˚
Ž .
Ž .
The N–C distance of 1.124 5 A appears somewhat shorter than the value 1.15 2 A, found in the complex
˚
x
w
Ž
.
x
Ž
.
w
x
w
Ž .
Ni C6 H5 3P NCC6 H5 19 and in the uncoordinated molecule of p-aminobenzonitrile 1.143 5 A . Nevertheless,
3
5
w
x
w
Ž
.
Ž
.
xq
Fig. 2. ORTEP II 16 molecular diagram of the cation Ni h -C5H5 PPh3 NCC6 H4 NH2 , anisotropic displacement parameters depicted at
Ž .
50% ? probability level.

(
)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
119
Table 3
5
w
Ž
.Ä Ž
. 4 Ž x
.
.xw
Selected bond lengths and bond angles for Ni h -C5H5 P C6 H5
NCC6 H4 NH2 PF6
3
˚
Ž .
Bond lengths A
Ž . Ž .
Ž .
1.855 3
2.073 5
Ž .
Ž
.
.
.
Ž .
2.064 4
Ni 1 –N 1
Ni 1 –C 111
Ž .
Ž
.
.
Ž .
Ž .
Ž
Ž .
2.091 9
Ni 1 –C 113
Ni 1 –C 115
Ž .
Ž
Ž .
Ž .
Ž
Ž .
2.133 5
Ni 1 –C 112
2.126 5
Ni 1 –C 114
Ž . Ž .
Ž
.
Ž .
Ž
.
Ž .
1.809 4
Ni 1 –P 1
2.1737 11
P 1 –C 13
Ž . Ž .
Ž .
Ž . Ž .
Ž .
1.825 4
P 1 –C 1
1.816 4
P 1 –C 7
Ž .
Ž
.
Ž .
Ž
.
Ž
.
.
.
.
Ž .
1.422 6
N 1 –C 21
1.124 5
C 21 –C 22
Ž . Ž
Ž
.
Ž
Ž
.
.
Ž .
Ž .
1.384 7
C 22 –C 23
1.383 6
Ž .
C 22 –C 27
Ž
.
Ž
.
Ž
Ž .
1.382 7
C 23 –C 24
1.371 6
C 24 –C 25
Ž
.
Ž .
Ž .
Ž . Ž
Ž .
1.392 7
C 25 –N 2
1.362 6
C 25 –C 26
Ž
.
Ž
.
.
Ž .
Ž .
Ž
.
Ž .
0.92 2
C 26 –C 27
1.366 7
N 2 –H 2A
Ni–Cp
Ž .
Ž
Ž .
Ž .
1.737 1
N 2 –H 2B
0.894 11
Ž .
Bond angles 8
N 1 –Ni 1 –P 1
Ž .
Ž . Ž .
Ž .
95.4 7
131.2 2
Ž
.
Ž .
Ž .
Ž .
176.2 4
C 21 –N 1 –Ni 1
Ž . Ž .
Ž . Ž .
N 1 –C 21 –C 22
Ž .
Ž .
132.9 1
N 1 –Ni 1 –Cp
P 1 –Ni 1 –Cp
Ž .
Ž
.
Ž
.
Ž .
Ž . Ž . Ž
.
.
.
Ž .
119.1 4
177.5 6
C 23 –C 22 –C 27
Ž
.
Ž
.
Ž
Ž
.
.
Ž .
Ž
.
.
Ž
.
Ž
Ž .
119.3 4
C 23 –C 22 –C 21
121.6 4
C 27 –C 22 –C 21
Ž
.
Ž
.
Ž .
Ž
Ž
.
Ž
Ž .
120.9 4
C 24 –C 23 –C 22
120.2 4
C 23 –C 24 –C 25
Ž .
Ž
.
Ž
.
Ž .
Ž .
Ž
.
Ž
.
Ž .
120.4 5
N 2 –C 25 –C 24
120.9 5
Ž .
N 2 –C 25 –C 26
Ž
.
Ž
Ž
Ž .
.
.
Ž
Ž
.
.
Ž . Ž . Ž
.
.
Ž .
120.3 5
C 24 –C 25 –C 26
118.7 4
C 27 –C 26 –C 25
Ž
.
Ž .
112 4
Ž
.
Ž .
Ž
Ž .
113 5
C 26 –C 27 –C 22
120.7 5
C 25 –N 2 –H 2A
Ž .
Ž
.
Ž
.
Ž .
Ž . Ž .
H 2A –N 2 –H 2B
Ž .
134 6
C 25 –N 2 –H 2B
˚
Ž
.
Ž
.
Ž .
the C–C bond connecting the aromatic ring and the nitrile group, C 21 –C 22 , 1.422 6 A, is shorter than that in the
˚
w
Ž . x w .
x
w
Ž
.
x
Ž
free p-aminobenzonitrile 1.430 5 A 21 and than the corresponding bond length in Ni C6 H5 3P NCC6 H5
3
˚
w
Ž . x w Ž .
x
w
Ž
.
Ž
.
Ž .
Ž
.
Ž
.
1.44 2 A 19 . The bonds between the ortho and meta carbons, C 23 –C 24 1.371 6 and C 26 –C 27 1.366 7
˚
˚
Ž .
x
Ž .
A , are slightly shorter than the other C–C bonds in the phenyl ring, which range from 1.382 7 to 1.392 7 A. The
˚
Ž .
Ž .
N 2 –phenyl bond length of 1.362 6 A in the complex is analogous to that in the free ligand at room temperature,
˚
Ž .
pyramidalized 22 .
The crystal structure exhibits short contacts between one hydrogen atom of NH2 and the PF6y anion H 2B PPP F 1
Ž .
w x
1.360 5 A 21 and shorter than the one found in the free ligand at lower temperature, where the amino group is
w
x
w
Ž
.
˚
Ž
.
Ž .
x
2.692 and H 2B PPP F 2 2.394 A, see Fig. 1 . This also causes the two hydrogen atoms to be distinct. Intermolecular
interactions between the hydrogens of one phenyl ring of the triphenylphosphine and fluorines of the PF6y anion are
˚
x
w
Ž
.
Ž .
Ž .
Ž .
also observed H 15 PPP F 4 2.525, H 5 PPP F 3 2.710 A .
2.3. NLO properties
Ž
.
The values of first hyperpolarizability b measured in methanol solutions by the hyper-Rayleigh scattering HRS
w
x
technique 23 for two compounds of this family showed no important increase of b as compared to the b value of
5
w
Ž
.
Ä
Ž
.
4
Ž
Ž
.2 .xw
x
the free nitrile ligands. Both Ni h -C5H5 P C6 H5 3 p-NCC6 H4 N CH3
PF6 and the free ligand
NCC6 H4 N CH3 itself have a value of b 18=10y30 esu, although a small increase of b with respect to the free
Ž
.
2
5
y30
w
Ž
.
Ä
Ž
.
34
Ž
.
xw
x
ligand is found for Ni h -C5H5 P C6 H5
twice the value of the free ligand p-NCC6 H4C6 H5 8.7=10
p-NCC6 H4C6 H5 PF6 which showed a b value of 17=10
esu,
esu . For both complexes and free nitrile ligands, a
y30
Ž
.
Ž
.
depolarization ratio of 0.20"0.02 was found, as would be expected for a linear molecule with only one diagonal b
tensor component. This suggests that b originates mainly from the conjugated path from metal to nitrile ligand and
not for example in the PPh3 ligand.
It would be expected that the increase of extension through the p system would rise the values of the second
Ž
.
hyperpolarizability g y3v;v,v,v , at least for the compound possessing the NCC H4C6 H5 coordinated ligand.
6 w
Ž
.
Ä
Ž
.
34
Ž
5
Nevertheless this was not the case and it was found that all compounds of this family Ni h -C5H5 P C6 H5
p-
.
xw
x
Ž
Ž
.
.
NCC6 H4 R PF6 RsNH2 , N CH3 2 , C6 H5, OCH3, H, F measured at ls1064 nm, displayed a THG signal only
slightly higher than the quartz background. This might be not surprising since experimental data did not give any
Ž .
evidence for such an extension as found for Ru II related compounds, where the metal centre was also involved on
the extension of that p system through a back-donation mechanism 9 , and the THG signal was quite significant
g y3v;v,v,v of 2.3=10
w x
y33
Ž Ž
.
. w x
esu 11 .

(
)
120
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
2.4. Molecular orbital calculations
Extended Huckel molecular orbital calculations were performed on
a
model complex
having Cs symmetry, in order to understand the bonding mode of the nitrile ligand to
the metallic fragment and its consequences upon the structural features of the complex. The interaction diagram
¨
w
Ž
.
Ž
.
xq
Cp PH3 Ni N[CC6 H4 NH2
q
Ž
.
between the nitrile ligand and the Cp PH3 Ni cation is shown in Fig. 3.
Before analysing the diagram in detail, we can start by looking at the frontier orbitals of N[CC6 H4 NH2 , drawn in
Ž
.
Fig. 4. These can be grouped in one set of cylindrically symmetric s orbitals with respect to the Ni–N bond and
two sets of p orbitals in orthogonal planes. The latter descend from the interactions of the N[C p and p ) orbitals
Ž
.
with the p and s orbitals of the arene p H and p 5 MOs, respectively . The p H set is orthogonal to the plane of the
arene and therefore combines with the arene p orbitals, giving rise to combinations having bonding, non-bonding and
Ž
antibonding characters relative to the C–C junction between the arene and the nitrile groups notice the relevant
orbitals p H , p H nb, and p H) in Fig. 4 . The N[C p 5 parallel set can only combine with arene s orbitals p5 and p5
)
.
Ž
.
in Fig. 4, for instance .
q
Ž
.
Fig. 3. Interaction diagram between the Cp PH3 Ni cation and the nitrile N[CC6 H4 NH2.

(
)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
121
Fig. 4. CACAO three-dimensional representation of the frontier orbitals of N[CC6 H4 NH2.
Ž
Finally, two orbitals having s character on the nitrile nitrogen atom are represented in Fig. 4 s1 and s2 ,
.
respectively . As a matter of fact, the most important bonding component between the ligand and the nickel fragment
X
is the combined s donation mainly from the latter s1 and s2 nitrile components into the empty 3a orbital of the d8
Ž .
Ž
.
Ni II center see Fig. 3 . This interaction is represented by the thin solid line in the diagram and is responsible for the
greatest part of the stabilization of the molecule relative to the two isolated fragments. While it is evident from the
Fig. 4 that both the s1 and s2 orbitals are bonding between the atoms C and N of the ligand, it must be mentioned
here some involvement of a third ligand orbital of s symmetry not shown , namely the low and filled antibonding
combination between the carbon and nitrogen s orbitals. Importantly, the latter contribution nicely account for the
strengthening of the N[C bond upon coordination to the metal atom. This trend, curiously pointed out at the
experimental level see above , is nicely reflected in the increase of N[C overlap population when the free and the
coordinated ligand molecules are compared 1.675 vs. 1.744 . A careful analysis indicates that the effect does not
have a prevailing p-nature vide infra but can be traced to a certain depopulation of the low lying NC s level also
involved into the s donation to the metal atom.
Ž
.
Ž
.
Ž
.
)
Ž
.
Still the major p interactions need to be underlined. Some of the occupied d levels are clearly interacting with
both the p H and the p 5 sets of nitrile ligand. In any case, both the p and p ) NC combinations are simultaneously
involved. The resulting MOs are given by a linear combination of aY ql1p 5 ql2p 5), in such a way that the final
Ž
.
molecular orbital is virtually nonbonding between the N and C atoms Fig. 5 . However, the interaction with p 5 is
slightly stronger than with p 5), making the global interaction a very weak p donation from the nitrile to the metal,
instead of backdonation.
The interesting feature of the frontier p MOs is that they are not totally delocalized from the nickel atom to the
NH2 end, as either the C or the N atom of the nitrile does not contribute to them. This may be important in connection
to the NLO properties.

(
)
122
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
q
Fig. 5. The bonding and ‘nonbonding’ orbital resulting from the interaction between p and p ) and one orbital of the Cp PH3 Ni fragment.
Ž
.
w
x
Recently calculations by Whitaker et al. 24 suggested a correlation between the hyperpolarizability and the
w
Ž
.
Ž
.
xq
HOMO–LUMO gap in a given molecule. The complex Cp PH3 Ni N[CC6 H4–NH2
exhibits a large HOMO–
1.5 eV and even more to
Ž
.
w
Ž
.
Ž
.
xq
Ž
.
LUMO gap 3.0 eV , which decreases when going to Cp PH3 Ni N[CC6 H4C6 H5
w
Ž
.
Ž
.
xq
Ž
.
Cp PH3 Ni N[CC6 H4 NO2
0.4 eV , according to our calculations, though the nature of both HOMO and
LUMO remains similar. In the latter two complexes, the LUMO has a lower energy as in comparison with the p H)
orbital, reported in Fig. 4, the arene’s contribution is larger. Moreover the presence of the nitro group is stabilizing
Ž
.
notice by contrast the antibonding and destabilizing effect of the NH2 substituent . Ultimately, we might expect that
the nitro derivative, with the smallest HOMO–LUMO gap, could be the best candidate for second harmonic
generation. In this respect, also the unsubstituted phenyl derivative is expected to be better than the amino derivative.
2.5. Electrochemical studies
The electrochemical behaviour of this new family of compounds was investigated in acetonitrile and
dichloromethane by cyclic voltammetry, with tetrabutylammonium hexafluorofosphate salt as supporting electrolyte,
5
w
Ž
and showed both oxidative and reductive processes. The electrochemical studies discussed below for Ni h -
.
Ä
Ž
.
34
Ž
.
xw
x
C5H5 P C6 H5 NCC6 H4 NH2 PF6 , typify the general behaviour of these nitrile nickel derivatives. The oxidative
Ž
Ž
.
Ž
..
electrochemistry presented one single anodic wave Epa ;q1.25 V acetonitrile , Epa ;q0.94 V dichloromethane ,
Ž
.
without the cathodic counterpart, assigned to the irreversible oxidation of coordinated P C6 H5 and NCC6 H4 NH2
3
ligands, by comparison with their electrochemical behaviour as uncoordinated compounds. The reductive electrochem-
istry was somehow more complex and a set of experiments had to be carried out in order to understand the overall
process. In fact, when the solvent was CH2Cl2 , only one cathodic wave at Epc ;y1.1 V was observed without the
anodic counterpart, which slightly emerged, as the temperature went down to fy308C. However, equivalent studies
in acetonitrile showed a more complex set of results associated to the reductive electrochemistry of these compounds
Ž
.
as shown on Fig. 6 line a .
At room temperature, a broad cathodic process suggesting overlap of two waves with maxima at Epc ;y0.83 V
Ž .
Ž .
A was found, followed by a second cathodic wave at Epc ;y1.04 V B . Both processes presented very broad
anodic counterparts. An additional, quasi-reversible, weak redox wave C at Ep1r2 s0.08 V was also observed in all
voltammograms independently of the potential scan direction, but its intensity was found to be higher immediately
after reversing the potential at y1.5 V. This wave C was assigned to a small amount of nickelocene, formed in situ,
according to the mechanism discussed below. Experiments with decrease of temperature showed that when
temperature went down to y308C the first cathodic wave A became sharper, the wave B became relatively less
intensive, and simultaneously both corresponding anodic counterparts emerged clearer as can be observed on line b of
Fig. 6. These observations suggested that the broadness of wave A might be related to any possible decomposition
5
w
IŽ
.
Ä
Ž
.
34
Ž
.x
product of the reduced Ni h -C5H5 P C6 H5 NCC6 H4 NH2 nineteen electron species, this process being slower
Ž
at lower temperatures, as would be expected and was found at y308C. Addition of excess of NCC6 H4R 10 times the
.
initial nickel compound concentration did not show any significant difference on the voltammogram. However, if the
same excess of PPh3 was added to the parent solution, only a new quasi-reversible redox wave was found in the
5
Ž
.
w
Ž
voltammogram, at Ep1r2 sy0.66 V D Es90 mV , and it was assigned to the reduction of the Ni h -

(
)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
123
5
y3
Fig. 6. Cyclic voltammogram of compound Ni h -C5H5 P C6 H5 NCC6 H4 NH2 PF6 , 10 M in acetonitrile, at scan rate of 200 mV sy1
w
Ž
.Ä Ž
. 4Ž
.xw
x
Ž3 .
Ž .
n
wŽ
.
xw
x
on 0.1 M Bu 4 N PF6 rCH3CN solution vs. wire Ag reference. a room temperature; b y308C.
.
Ä
Ž
.
342 xq
new species, formed in the electrochemical cell by a metathesis chemical reaction, due to the
C5H5 P C6 H5
large excess of phosphine, as was proved in a separate chemical reaction carried out on the bench. Therefore,
5
w
Ž
.
w
Ž
.
342 xw
x
electrochemical studies on compound Ni h -C5H5 P C6 H5
same experimental conditions, an analogous pattern was found for the voltammogram of this compound and again the
PF6 were found to be necessary. In fact, under the
very first irreversible reduction broad wave showed up at Epc ;y0.83 V. Nevertheless, with the presence of large
Ž
.
excess of P C6 H5 in solution, only a new quasi-reversible redox process at Ep1r2 sy0.68 V was found. This
3
Ž
.
observation suggested that the nineteen unstable reduced species, led to the decoordination of one P C6 H5 ligand in
3
the present case, and to the decoordination of the more labile NCC6 H4 NH2 ligand, in our nickel nitrile derivative
5
)
w
IŽ
.
Ä
Ž
.
34xq
Ž .
with formation in both cases, of a Ni h -C5H5 P C6 H5
new species A on the scheme below , whose
reduction would be responsible for the broadness on the first reduction wave. In addition, other decomposition
products might be formed leading to the presence of Ni0 and NiII derivatives, responsible for wave B and the
increased amount of nickelocene associated to wave C.
w
x
Our results are in good agreement with other studies published in the literature 25 according to which the
following reactions are suggested:
3 CpNiI PPh NCC H NH
3 . Ž
™ Ni0 PPh
NCC H NH
2
2
B
q Cp2 NiII
Ž
.
Ž
3 .2Ž
.
6
4
2
6
4
A
C
x
NiI h5 yC H P C H
qqNCC6 H4 NH2
.
Ž
Ž
.
5
5
6
5
3
A)
According to our calculations, reduction of the complex should lead to population of the Ni–N antibonding LUMO,
resulting in loss of the nitrile ligand.
2.6. Electronic spectra
5
Electronic spectra of complexes Ni h -C5H5 P C6 H5 NCC6 H4R PF6 were recorded in 1.7=10y5 M in
w
Ž
.
Ä
Ž
.
x
Ž
.
xw
x
3
dichloromethane, methanol and chloroform solutions, in the range of wavelengths 200–1000 nm. All the compounds

(
)
124
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
5
y5
w
Ž
.Ä Ž
. 4Ž
.xw x Ž
.
Fig. 7. Electronic spectra of compound Ni h -C5H5 P C6 H5 NCC6 H4 NH2 PF6 1.7=10 M in dichloromethane .
3
presented two intense absorption bands on the UV region attributed to electronic transitions occurring in the aromatic
ligands, and also one band in the visible region where d–d transitions are expected to occur. Fig. 7 recorded for
5
w
Ž
.
w
Ž
.
34
Ž
.
xw
x
compound Ni h -C5H5 P C6 H5 NCC6 H4 NH2 PF6 typifies the electronic spectra, obtained in solutions of
dichloromethane.
Band at 228–235 nm ´s1.8=10 l moly1 cm was attributed to transitions occurring in the fragment
4
1
Ž
.
5
5
w
Ž
.
Ä
Ž
.
4
x
w
Ž
.
Ä
Ž
.
342 xw
x
- Ni h -C5H5 P C6 H5 - by comparison with the electronic spectra of Ni h -C5H5 P C6 H5
PF6 . The
3
absorption band at 296 nm ´s3.2=10 l moly1 cmy1 was attributed to the coordinated NCC6 H4 NH2 nitrile,
4
Ž
.
which in the free ligand occurs at 269 ´s2.2=10 l mol cmy1 in the same solvent. The band in the visible,
4
y1
Ž
.
occurring at 451 nm ´s1.3=10 l mol cmy1 was attributed to d–d transitions for some compounds of this
3
y1
Ž
.
Ž
.
family a shoulder at lower wavelength was also observed both in dichloromethane and methanol . The presence of this
5
5
w
Ž
.
Ä
Ž
.
342 xw
x
w
Ž
.
Ä
Ž
.
4
x
band on the electronic spectra of Ni h -C5H5 P C6 H5
assignment to any nitrile–metal charge transfer. Therefore, the CT transition relative to the nitrile might be the
responsible for the quadratic response observed on the complexes.
PF6 and Ni h -C5H5 P C6 H5 Cl precludes its
3
3. Concluding remarks
5
w
Ž
.
Ä
Ž
.
4
xw
x
Ž
Ž
.
The new compounds Ni h -C5H5 P C6 H5 NCC6 H4R PF6 where R is a donor group NH2 , N CH3 2 , C6 H5,
3
.
OCH3, H, F showed no interesting SHG or THG signals, though they apparently obeyed the required structural
conditions, exhibiting an Acceptor-p system-Donor arrangement. In fact, crystal structure determination of the NH2
derivative showed an almost planar geometry for the substituted nitrile and our studies by the hyper-Rayleigh
scattering technique suggest that b originates mainly from the conjugated path from metal to nitrile ligand. A
reasonable explanation for the absence of NLO properties was provided by our theoretical studies which suggested the
s donation of nitrile to nickel to be the most significant component of the bonding and that p interaction had a net
negligible effect on the delocalization.

(
)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
125
4. Experiment
All experiments were carried out under nitrogen by the use of standard Schlenk-tube techniques. Solvents were
w
x
purified according to the usual methods 26 . Solid state IR spectra were measured on a Perkin-Elmer 457
1
spectrophotometer with KBr pellets; only significant bands are cited in the text. H and 31 P NMR spectra were
recorded on a Bruker CXP 300 spectrometer, and 13C NMR spectra were recorded on a Varian Unity 300
spectrometer at probe temperature. Microanalyses were performed, in our laboratories, using a Fisons Instruments
EA1108 system. Data acquisitions, integration and handling were performed using a PC with the software package
Eager-200 Carlo Erba Instruments . Melting points were obtained using a Reichert Thermovar. Electronic spectra
were recorded using dried solvents at room temperature, on a Shimadzu UV-1202.
Ž
.
1
13
Ž
.
Ž
.
The H acetone-d6 and C chloroform-d chemical shifts are reported in parts per million downfield from
internal Me4Si and 31 P NMR spectra are reported in parts per million downfield from external 85% H3PO4.
5
[
(
)
{
(
)
}
]
4.1. Preparation of Ni h -C5 H5 P C6 H5 Cl
3
w
x
This starting material was prepared following the process described in the literature 27 .
5
[
(
)
{
(
)
}
( )][ ]
NCC6 H4 R PF6
4.2. Preparation of Ni h -C5 H5 P C6 H5
3
5
w
Ž
.
Ä
Ž
.
4
x Ž
All the complexes were prepared by the process described as follows. A solution of Ni h -C5H5 P C6 H5 Cl 1
3
.
Ž
.
Ž
.
mmol in diethyl ether ;150 ml , was added dropwise, to a solution of nitrile NCC6 H4R f10 mmol in the same
Ž
.
Ž
.
solvent 20 ml containing TlPF6 1.5 mmol in suspension, at room temperature with stirring. A brown solid
precipitates immediately together with simultaneous precipitation of TlCl. The mixture was allowed to stir for 2 h.
After filtration, the solid residue was washed several times with diethyl ether to remove the excess of unreacted
nitrile. The residue was then dissolved in chloroform and filtrated, leaving behind the insoluble TlCl. After
concentration of this solution and addition of ether, brown–green crystals of the new compound nicely precipitate.
Crystals for X-ray measurements can be grown by slow diffusion vapor of ether in a concentrated dichloromethane
solution of the required compound.
5
w
Ž
.
w
Ž
.
34
Ž
.
xw
x
Ni h -C5H5 P C6 H5 NCC6 H4 NH2 PF : Yield:70%; Anal. Found: C, 55.4; H, 4.1; N, 4.3. C30 H26 N2 P2 F6 Ni;
Calc. C, 55.5; H, 4.0; N, 4.3. M. p.: 1738C d . Ni h -C5H5 P C6 H5 NCC6 H4 N CH3 PF6 : Yield: 95%; Anal.
Ž . w6
Ž
.
Ä
Ž
.
34
Ž
Ž
.
xw
x
5
2
5
Ž .
w
Ž
Found: C, 57.0; H, 4.5; N, 4.2. C H30 N2 P2 F6 Ni; Calc. C, 56.8; H, 4.5; N, 4.1. M. p.: 1678C d . Ni h -
.
w
Ž
.
34
Ž
.
xw 32 x
C5H5 P C6 H5 NCC6 H4C6 H5 PF6 : Yield: 85%; Anal. Found: C, 59.6; H, 4.0; N, 2.0. C36 H29 NP2 F6 Ni; Calc.
5
w
Ž
.
Ä
Ž
.
xŽ
.
xw
x
C, 60.5; H, 4.1; N, 2.0. M. p.: 1048C. Ni h -C5H5 P C6 H5 NCC6 H4OCH3 PF6 : Yield: 95%; Anal. Found: C,
3
5
w
Ž
55.8; H, 4.1; N, 2.2. C H NOP2 F6 Ni; Calc. C, 56.1; H, 4.1; N, 2.1. M. p.: 1708C. Ni h -
.
Ä
Ž
.
34
Ž
.
xw31 x27
C5H5 P C6 H5 NCC6 H5 PF6 : Yield: 95%; Anal. Found: C, 56.5; H, 3.9; N, 2.1. C30 H25NP2 F6 Ni; Calc. C,
5
w
Ž
.
w
Ž
.
34
Ž
.
xw
x
56.8; H, 4.0; N, 2.2. M. p.: 1598C. Ni h -C5H5 P C6 H5 NCC6 H4 F PF6 : Yield: 90%; Anal. Found: C, 54.9; H,
3.8; N, 2.1. C30 H24 NP2 F7 Ni; Calc. C, 55.2; H, 3.7; N, 2.1. M. p.: 1648C d .
Ž .
4.3. X-ray measurements
Intensities were measured on an Enraf-Nonius CAD4 diffractometer using graphite monochromated MoK a
˚
Ž
.
radiation 0.71069 A . Cell dimensions were determined from the measured u values for 25 intense reflections with
12FuF15.58. 8260 reflection intensities, of which 6305 were independent, were measured by the v-2u scan mode,
in the range 1.7FuF27.08. Data were corrected for Lorentz and polarization effects. The compound
Ž .
C30 H26 N2 P2 F6 Ni; Mrs649.2, crystallizes in the monoclinic space group; P21rn; with cell dimensions as9.354 2
3
y3
˚
˚
˚
˚
Ž
.
Ž .
Ž .
Ž
.
Ž
.
A, bs14.600 3 A, cs21.589 6 A, bs101.31 2 8; Vs2891.1 12 A ; Zs4; Dc s1.491 g cm and m MoK a
s8.44 cmy1
The structure was solved by a combination Patterson and difference Fourier syntheses using programs SHELX86
.
w
x
w x
28 and SHELX93 29 . All non hydrogen atoms and the two amino hydrogen atoms were found in the difference
maps. The non-hydrogen atoms were refined anisotropically. The localized hydrogen atoms were refined isotropically
˚
with a restrained N–H distance of 0.86 A. The remaining hydrogen atoms were placed in calculated positions and
refined with group isotropic thermal parameters one Uiso for the Ph ring of the phosphine, one Uiso for the Cp ring,
and one Uiso for the Ph ring of the nitrile ligand . In the final refinement the R1 and wR2 obtained were 0.063 and
Ž
.
Ž .
0.157 for I)2s I and 378 refined parameters. The R indices using all data were R1 s0.089 and wR2 s0.1897
with a goodness of fit of ss1.058.
Final atomic positional coordinates for the non-hydrogen atoms are given in Table 4. Anisotropic thermal

(
)
126
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
Table 4
4
2
3
5
˚
Ž
.
Ž
.
w
Ž
.Ä Ž
. 4Ž
.
.xw
x
Atomic coordinates =10 , and equivalent isotropic displacement parameters A =10 , for Ni h -C5H5 P C6 H5 NCC6 H4 NH2 PF6 .
3
U , is defined as one third of the trace of the orthogonalized U tensor
eq
i j
x
y
z
U
eq
Ž .
Ni 1
P 1
Ž .
13 1
229 1
Ž .
1990 1
1162 1
Ž .
2199 1
1386 1
Ž .
37 1
Ž .
Ž .
Ž .
Ž .
Ž .
33 1
Ž .
Ž .
Ž .
Ž .
Ž .
39 1
C 1
y830 4
1642 3
664 2
Ž .
Ž .
Ž .
Ž .
Ž .
48 1
C 2
y1849 4
1142 3
253 2
Ž .
Ž .
y2581 5
Ž .
Ž .
Ž .
66 1
C 3
1563 4
y297 2
Ž .
Ž .
Ž .
Ž .
Ž .
66 1
C 4
y2272 6
2444 4
y436 2
Ž .
Ž .
y1268 6
Ž .
Ž .
y27 2
Ž .
61 1
C 5
2937 3
Ž .
Ž .
Ž .
Ž .
Ž .
51 1
C 6
y559 5
2537 3
526 2
Ž .
Ž .
2031 4
Ž .
Ž .
Ž .
38 1
C 7
965 3
1206 2
Ž .
Ž .
Ž .
Ž .
Ž .
52 1
C 8
3065 5
517 3
1640 2
Ž .
C 10
Ž .
Ž .
Ž .
Ž .
64 1
C 9
Ž
4447 5
369 4
1527 2
.
.
.
.
.
.
.
.
.
Ž .
Ž .
Ž .
Ž .
72 2
4776 5
647 4
969 3
Ž
Ž .
Ž .
Ž .
Ž .
77 2
C 11
3771 6
1089 4
531 3
Ž
Ž .
Ž .
Ž .
Ž .
57 1
C 12
2379 5
1247 3
640 2
Ž
Ž .
Ž .
Ž .
Ž .
37 1
C 13
y482 4
24 3
1464 2
Ž
Ž .
Ž .
Ž .
Ž .
42 1
C 14
y1413 4
y116 3
1882 2
Ž
Ž .
y1956 5
Ž .
y971 3
Ž .
Ž .
52 1
C 15
1957 2
Ž
Ž .
Ž .
Ž .
Ž .
60 1
C 16
y1590 5
y1695 3
1611 3
Ž
Ž .
y657 6
Ž .
y1569 3
Ž .
Ž .
67 1
C 17
1201 3
Ž
Ž .
Ž .
Ž .
Ž .
57 1
C 18
y117 5
y706 3
1124 2
Ž .
Ž .
y1834 4
Ž .
2426 3
Ž .
Ž .
51 1
N 1
1855 2
Ž
.
.
.
.
.
.
.
Ž .
Ž .
Ž .
Ž .
52 1
C 21
y2919 5
2725 3
1628 2
Ž
Ž .
y4258 4
Ž .
Ž .
Ž .
51 1
C 22
3129 3
1326 2
Ž
Ž .
Ž .
Ž .
Ž .
C 23
y5380 5
3278 4
Ž .
1646 2
Ž .
57 1
Ž
Ž .
y6630 5
Ž .
C 24
3710 4
1351 2
59 1
Ž
Ž .
Ž .
Ž .
Ž .
C 25
y6797 4
3997 3
731 2
55 1
Ž .
Ž
Ž .
y5674 5
Ž .
Ž .
C 26
3832 4
407 2
71 2
Ž
Ž .
Ž .
Ž .
Ž .
C 27
y4429 5
3402 4
Ž .
702 2
Ž .
71 2
Ž .
Ž .
y8034 5
Ž .
N 2
4434 4
440 3
83 2
Ž
.
.
.
.
.
Ž .
Ž .
Ž .
Ž .
C 111
2111 5
2009 4
2722 2
62 1
Ž .
Ž
Ž .
Ž .
Ž .
C 112
1512 6
2842 3
2808 2
63 1
Ž
Ž .
Ž .
Ž .
Ž .
C 113
292 6
2668 4
Ž .
3058 2
Ž .
71 2
Ž
Ž .
Ž .
C 114
213 6
1722 5
3184 2
71 2
Ž
Ž .
Ž .
Ž .
Ž .
C 115
1311 6
1324 4
2966 2
65 1
Ž .
Ž .
Ž .
Ž .
Ž .
P 2
9075 1
5403 1
1504 1
52 1
Ž .
Ž .
Ž .
Ž .
Ž .
155 2
Ž .
172 3
Ž .
184 3
Ž .
115 1
Ž .
158 2
Ž .
120 2
F 1
Ž .
7845 6
5979 4
Ž .
1124 2
Ž .
Ž .
F 3
10,134 7
5956 4
1183 3
Ž .
Ž .
Ž .
Ž .
F 4
8110 7
4853 4
1838 3
Ž .
Ž .
Ž .
Ž .
F 2
8949 5
4698 3
952 2
Ž .
Ž .
Ž .
Ž .
F 5
Ž .
10,425 6
4886 4
1878 2
Ž .
Ž .
Ž .
F 6
9259 6
6123 3
2052 2
parameters for the non-hydrogen atoms, atomic positional and thermal parameters for hydrogen atoms and a list of
observed and final calculated structure factors are available as supplementary material. Drawings were made with the
w
x
w x
program ORTEPH 16 and SCHAKAL 15 . Atomic scattering values were taken from the International Tables of
w
x
X-ray Crystallography 30 .
4.4. b measurements
Ž
.
Because the usual EFISHG electric field induced second harmonic generation technique for determination of mb
is not applicable for these compounds, the hyper-Rayleigh scattering technique was used, which does not require any
static field to be applied.
Ž
HRS measurements with a fundamental wavelength of 1.064 mm were performed on dilute solutions in the order
.
of 1 mgrml in methanol. The solutions were systematically passed through 500 nm microporous filters. Laser pulses
Ž
.
energy: ;20 mJ, width: 70 ps, repetition rate 2 kHz from a Nd:YAG regenerative amplifier were focused into a
rectangular glass cell by a 100 mm lens. The scattered harmonic light was collected at 908 and filtered by a
monochromator with 1 nm bandwidth. Single photon pulses from a photomultiplier were detected in a 5 ns time gate

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)
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
127
around the laser pulse. The count rates were corrected for pile-up errors at increasing count gates. Systematic scanning
of a narrow region around 532 nm showed no significant photoluminescense background. In a reference arm a fraction
of the laser light was frequency-doubled and this intensity was used to correct for slow laser fluctuations. Using the
internal reference method we obtain the ratio bHRSŽsolute.rbHRSŽsolvent., where bHRS is given in terms of the oriental
2
2
2
²
:
²
: ² :
bZZZ q bXZZ . Applying the previously used
averages bIJK of the molecular hyperpolarizability by
(
Ž
.
w
x
approximation 23 that the b tensor of the solvent molecules is also dominated by bZZZ and adopting b values of
y30
0.49=10y30 esu for chloroform 31 and 0.23=10
esu for methanol 31 , the b value for the solute s bZZZ
w
x
w
x
Ž
<
<.
was determined. We measured this b value of methanol by external reference relative to chloroform, using the usual
Lorentz local field factors and correcting for the different refraction by both solvents. The incident power had to be
Ž
.
reduced to less than 20 mW 10 mJ per pulse during the external reference measurements to minimize thermal
lensing effects in methanol.
4.5. THG measurements
w
x
THG measurements of the thin films were carried out using the Maker fringe technique 32 employing a 10 pps
Q-switched ND:YAG laser operating at 1064 nm with a pulse duration of 10 ns and a pulse energy at the sample of at
most 140 mJ. The fundamental beam was directed through filters and polarizing optics before it was focused on the
sample by a 300 mm lens. The sample was mounted on a rotation stage so that the angle of incidence of the laser
beam could be varied, with the film side of the sample facing the detector. The polarization of the electrical field was
parallel to the rotation axis and care was taken that the incident beam and the rotation axis coincided. The third
harmonic beam at 355 nm was detected in a transmission geometry using a photomultiplier connected to a photon
counter. In order to block other wavelengths, a monochromator was used. A personal computer was used to control
the rotation stage and collect the data. Each data point was obtained by sampling signals from 300 laser pulses.
Maker fringes were recorded in the range y308 to q308 with respect to normal incidence of the laser beam. The
fringe patterns of a sample consisting of a thin film on a fused silica substrate were first recorded, then after removal
of the thin film, in situ, Maker fringes of the bare substrate were recorded in the same range.
4.6. Molecular orbital calculations
w
x
All the molecular orbital calculations were done using the extended Huckel method 33–35 with modified H ’s
¨
i j
w
x
Ž
.
36 . The basis set for the metal atom consisted of ns, np, and ny1 d orbitals. The s and p orbitals were described
by single Slater-type wave functions, and the d orbitals were taken as contracted linear combinations of two
Slater-type wave functions. Standard parameters were used for H, C, N, and O, while those for Ni were the following
Ž
.
Ž
.
w
Ž
.
Ž
.
HiireV, z : 4s, y9.11, 1.825; 4p, y5.15, 1.125; 3d, y13.40, 5.75, 2.000 z2 , 0.5683 C1 , 0.6292 C2 .
x
Three-dimensional representations of orbitals were drawn using the program CACAO 37 .
An idealized model having Cs symmetry was built, based on the structure of the nickel complex described in this
˚
Ž .
Ž .
work. The following distances A and angles 8 were used: Ni–Cp 1.74, C–C 1.40, C–H 1.08, Ni–P 2.17, P–H 1.4,
Ni–N 1.65, NC 1.13, N–H 1.0, C–N 1.35, Cp–Ni–P 133.0. The position of the nitrile ligand relative to the plane
bisecting the Cp–Ni–P plane was optimized.
4.7. Electrochemical apparatus
The electrochemistry instrumentation consisted of a Potentiostate EG&A, Princeton Applied Research, Model
273A, and experiments were monitored in a personal computer loaded with Model 270 Electrochemical Analysis
Software 3.00 of EG&A, Princeton Applied Research. Potentials were referred to a silver electrode which was
checked with a 1.0=10y3 M solution of ferrocene in acetonitrile containing 0.10 M LiClO4 for which the
w
x
ferroceniumrferrocene potential was in good agreement with the literature 38 . The working electrode was a 2-mm
piece of platinum wire. The secondary electrode was a platinum wire coil. Cyclic voltammetry experiments were
performed at room temperature in a PAR polarographic cell. Solutions studied were 1 mM in solute and 0.1 M in
tetrabutylammonium hexafluorophosphate as supporting electrolyte.
Ž
.
The electrolyte was prepared from Bu4 NBr and KPF6 both purchased from Aldrich Chemical , recrystallized from
ethanol, washed with diethyl ether, and dried in vacuum at room temperature for 24 h. Reagent grade acetonitrile and
dichloromethane were dried over P2O5 and CaH2 , respectively, and distilled before use under an argon atmosphere.
An argon atmosphere was maintained over the solution during the experiment.

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)
128
A.R. Dias et al.rJournal of Organometallic Chemistry 553 1998 115–128
Acknowledgements
Financial support of this work was provided by Project STRDArPrCENr512r92 and EU Human Capital and
Mobility Programme. Paulo Mendes thanks Junta Nacional de Investigac¸ao Cientfica e Tecnologica for Grant
˜
´
Ž
.
BDr2168r92-IC and support of his stay at the Danish Institute of Fundamental Metrology DFM at Copenhagen,
Denmark. Authors deeply thank Dr. Jan Petersen at DFM, Dr. Thomas Bjornholm at the Center for Interdisciplinary
Studies of Molecular Interactions CISMI at Copenhagen, and Dr. Tommy Geisler at Institut of Physics, Aalborg
University, Denmark, for THG measurements and fruitful discussions. E.G. is Research Director of the Fund for
Scientific Research of Flanders FWO . The work in Antwerp is partly funded by the Flanders Government in it’s
action for the promotion of participation in EU-research programmes.
Ž
.
Ž
.
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