Organometallic complexes for nonlinear optics. 11.1 molecular quadratic and cubic hyperpolarizabilities of systematically varied (cyclopentadienyl) (triphenylphosphine)nickel σ-arylacetylides

Article abstract of DOI:10.1021/om961066z

The complexes Ni(C≡CR)(PPh₃)(η-C₅H₅) (R = 4-C₆H₄NO₂ (3), 4,4′-C₆H₄C₆H₄NO₂ (4), (E)4,4′-C₆H₄CH=CHC₆H₄NO ₂ (5), (Z)-4,4′-C₆H₄CH=CHC₆H₄NO ₂ (6), 4,4′-C₆H₄C≡CC₆H₄NO ₂ (7), 4,4′-C₆H₄N=CHC₆H₄NO ₂ (8)) have been prepared. Electrochemical data for the series of complexes NiCl(PPh₃)(η-C₅H₅) (1), Ni(C≡CPh)(PPh₃)(η-C₅H₅) (2), and 3-8 have been determined. Introduction of nitro substituent (in progressing from 2 to 3) results in a substantial increase in the Ni^II/III oxidation potential, all of which is lost on progressing from a one-ring (3) to two-ring (4-8) chromophore. The molecular quadratic and cubic optical nonlinearities of the series of complexes have been determined by hyper-Rayleigh scattering (HRS) and Z-scan techniques, respectively. HRS measurements at 1064 nm are consistent with an increase in β upon chromophore chain-lengthening (in progressing from 3 to 7 to 5), replacing Z by E stereochemistry (in progressing from 6 to 5), and in replacing N by CH (in progressing from 8 to 5), general trends that are maintained with the two-level-corrected data. Nonlinearities for the 18-electron nickel complexes are larger than those for related 14-electron (triphenylphosphine)gold acetylides, but smaller than those for the more easily oxidizable 18-electron (cyclopentadienyl)bis(phosphine)ruthenium acetylide analogues. Z-scan data at 800 nm reveal a negative γ for the nitro-containing complexes, consistent with the dispersion effect of two-photon states contributing to the nonlinearities. An increase in γ upon chromophore chain-lengthening (in progressing from 3 to 5, 4 and 7) and replacing Z by E stereochemistry (in progressing from 6 to 5) is observed. Nonlinearities for the extended-chromophore acetylide complexes 5 and 7 are significantly less than those for the (triphenylphosphine)gold acetylide analogues.

Full text of DOI:10.1021/om961066z

Organometallics 1997, 16, 2631-2637
2631
Or ga n om eta llic Com p lexes for Non lin ea r Op tics. 11.1
Molecu la r Qu a d r a tic a n d Cu bic Hyp er p ola r iza bilities of
System a tica lly Va r ied
(Cyclop en ta d ien yl)(tr ip h en ylp h osp h in e)n ick el
σ-Ar yla cetylid es
Ian R. Whittall, Marie P. Cifuentes, and Mark G. Humphrey*
Department of Chemistry, Australian National University, Canberra, ACT 0200, Australia
Barry Luther-Davies and Marek Samoc
Australian Photonics Cooperative Research Centre, Laser Physics Centre,
Research School of Physical Sciences and Engineering, Australian National University,
Canberra, ACT 0200, Australia
Stephan Houbrechts and Andre´ Persoons
Centre for Research on Molecular Electronics and Photonics, Laboratory of Chemical and
Biological Dynamics, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Graham A. Heath and Denes Bogsa´nyi
Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
Received December 17, 1996X
The complexes Ni(CtCR)(PPh3)(η-C5H5) (R ) 4-C6H4NO2 (3), 4,4′-C6H4C6H4NO2 (4), (E)-
4,4′-C6H4CHdCHC6H4NO2 (5), (Z)-4,4′-C6H4CHdCHC6H4NO2 (6), 4,4′-C6H4CtCC6H4NO2 (7),
4,4′-C6H4NdCHC6H4NO2 (8)) have been prepared. Electrochemical data for the series of
complexes NiCl(PPh3)(η-C5H5) (1), Ni(CtCPh)(PPh3)(η-C5H5) (2), and 3-8 have been
determined. Introduction of nitro substituent (in progressing from 2 to 3) results in a
substantial increase in the NiII/III oxidation potential, all of which is lost on progressing from
a one-ring (3) to two-ring (4-8) chromophore. The molecular quadratic and cubic optical
nonlinearities of the series of complexes have been determined by hyper-Rayleigh scattering
(HRS) and Z-scan techniques, respectively. HRS measurements at 1064 nm are consistent
with an increase in â upon chromophore chain-lengthening (in progressing from 3 to 7 to 5),
replacing Z by E stereochemistry (in progressing from 6 to 5), and in replacing N by CH (in
progressing from 8 to 5), general trends that are maintained with the two-level-corrected
data. Nonlinearities for the 18-electron nickel complexes are larger than those for related
14-electron (triphenylphosphine)gold acetylides, but smaller than those for the more easily
oxidizable 18-electron (cyclopentadienyl)bis(phosphine)ruthenium acetylide analogues. Z-
scan data at 800 nm reveal a negative γ for the nitro-containing complexes, consistent with
the dispersion effect of two-photon states contributing to the nonlinearities. An increase in
γ upon chromophore chain-lengthening (in progressing from 3 to 5, 4 and 7) and replacing
Z by E stereochemistry (in progressing from 6 to 5) is observed. Nonlinearities for the
extended-chromophore acetylide complexes 5 and 7 are significantly less than those for the
(triphenylphosphine)gold acetylide analogues.
In tr od u ction
cal (NLO) responses have a donor-bridge-acceptor
composition. Organometallic complexes can be con-
structed similarly, but with additional flexibility due to
the potential of a variable d electron count, oxidation
state, and ligand environment, all of which may impact
significantly on the NLO performance. However, few
investigations correlating systematic changes in these
parameters against NLO merit have been reported. Our
studies have focussed on metal σ-acetylide complexes,1,3
which are frequently accessible by modification or
extension of established synthetic methodologies and are
in general thermally and oxidatively stable. They also
satisfy a suggested organometallic NLO chromophore
design criterion (incorporation of the metal into the
The optical nonlinearities of organometallic complexes
have been the subject of intense investigation recently.2
Many organic molecules with enhanced nonlinear opti-
* To whom correspondence should be addressed. E-mail:
Mark.Humphrey@anu.edu.au.
X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) Part 10: Whittall, I. R.; Cifuentes, M. P.; Humphrey, M. G.;
Luther-Davies, B.; Samoc, M.; Houbrechts, S.; Persoons, A.; Heath,
G. A.; Hockless, D. C. R. J . Organomet. Chem., submitted for publica-
tion.
(2) (a) Nalwa, H. S. Appl. Organomet. Chem. 1991, 5, 349. (b)
Marder, S. R. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.;
Wiley: Chichester, England, 1992; p 115. (c) Long, N. J . Angew.
Chem., Int. Ed. Engl. 1995, 34, 21.
S0276-7333(96)01066-7 CCC: $14.00 © 1997 American Chemical Society

2632 Organometallics, Vol. 16, No. 12, 1997
Whittall et al.
plane of the π-system) not fulfilled by, for example, the
extensively investigated ferrocenyl system.4 We have
recently reported optical nonlinearities for 18-electron
(cyclopentadienyl)bis(phosphine)ruthenium
acetyl-
ides,1,3a-c,3i for which electrochemical studies confirmed
facile oxidizability of the metal, and 14-electron (tri-
phenylphosphine)gold acetylides,3g-i for which difficulty
in metal oxidation was found. The 18-electron, more
easily oxidizable, ruthenium complexes exhibit uni-
formly larger quadratic nonlinearities â than the 14-
electron, less readily oxidizable, gold compounds. In
contrast, cubic nonlinearities γ for metal acetylides
bearing “extended chromophore” ligands (4,4′-
F igu r e 1. Numbering scheme for NMR spectral assign-
ment of 5. Analogous scheme for 2-4 and 6-8.
CtCC6H4CtCC6H4NO2
and
(E)-4,4′-CtCC6H4-
using a Cary 5 spectrophotometer. Electrochemical measure-
ments were carried out using a Princeton Applied Research
Model 170 Potentiostat. The supporting electrolyte was
[NBun4][PF6] in distilled deoxygenated CH2Cl2. Solutions (1
× 10-3 M) were made under a purge of nitrogen and measured
versus a Ag/AgCl reference electrode at -50 °C, such that the
ferrocene/ferrocenium redox couple was located at 0.55 V.
1H, 13C, and 31P NMR spectra were recorded using a Varian
Gemini-300 FT NMR spectrometer and are referenced to
residual CHCl3, 7.24 ppm and CDCl3, 77.0 ppm, and external
85% H3PO4, 0.0 ppm, respectively. NMR spectral assignments
for 5 follow the numbering scheme shown in Figure 1.
Compounds 2-4 and 6-8 are numbered analogously.
CHdCHC6H4NO2) are larger for the gold complexes
than their ruthenium analogues. Clearly, further stud-
ies examining the impact on optical nonlinearities of
modifying the valence electron count and the ease of
oxidation are required. We have now extended our
investigations to embrace 18-electron (cyclopentadi-
enyl)(triphenylphosphine)nickel acetylide complexes,
which are expected to be less oxidizable than their
ruthenium analogues. This paper reports the synthesis
and spectroscopic and electrochemical characterization
of these complexes, their molecular quadratic optical
nonlinearities at 1064 nm and two-level-corrected val-
ues, as determined by hyper-Rayleigh scattering (HRS),
and their cubic NLO data at 800 nm, as measured by
Z-scan, together with comparisons to the relevant data
for the analogous ruthenium and gold complexes.
Ch ar acter ization of Ni(CtCP h )(P P h 3)(η-C5H5) (2). Com-
plex 2 was prepared following the literature method.6 IR:
(cyclohexane) ν(CtC) 2107 cm-1; (CH2Cl2) ν(CtC) 2098 cm-1
.
1H NMR (δ, 300 MHz, CDCl3): 5.24 (s, 5H, C5H5), 6.64 (d, J HH
) 8 Hz, 2H, H4), 6.90 (m, 3H, H5, H6), 7.38 (m, 9H, Hm, Hp),
7.73 (dd, J HH ) 8 Hz, J HP ) 12 Hz, 6H, Ho). 13C NMR (δ, 75
MHz, CDCl3): 86.1 (d, J CP ) 48 Hz, C1), 92.6 (C5H5), 119.7
(C2), 124.6 (C6), 127.2 (C5), 128.1 (d, J CP ) 10 Hz, Cm), 129.6
(C3), 130.2 (Cp), 130.8 (C4), 133.7 (d, J CP ) 11 Hz, Co), 133.9
(d, J CP ) 48 Hz, Ci). 31P NMR (δ, 121 MHz, CDCl3): 41.6.
FAB MS m/z (fragment, relative intensity): 486 ([M]+, 100),
385 ([M - CtCPh]+, 41), 320 ([M - CtCPh - C5H5]+, 31).
Syn t h esis of Ni(4-CtCC6H 4NO2)(P P h 3)(η-C5H 5) (3).
NiCl(PPh3)(η-C5H5) (200 mg, 0.47 mmol) and 4-HCtCC6H4NO2
(77 mg, 0.52 mmol) were stirred in NEt3 (20 mL) with CuI (5
mg, 0.03 mmol) for 16 h. The solvent was removed under
reduced pressure, and the residue was extracted with dichlo-
romethane (50 mL) and then filtered through a plug of silica.
Petroleum ether (10 mL) and ethanol (2 mL) were added to
the eluate, and then the dichloromethane was removed under
reduced pressure, precipitating the microcrystalline product
which was collected by filtration (152 mg, 60%). Anal. Calcd
for C31H24NNiO2P: C, 69.95; H, 4.55; N, 2.63. Found: C,
Exp er im en ta l Section
All organometallic reactions were carried out under an
atmosphere of nitrogen with the use of standard Schlenk
techniques; no attempt was made to exclude air during work-
up of the products. Petroleum ether refers to a fraction of
boiling range 60-80 °C. Copper(I) iodide (Aldrich) and tri-
ethylamine (BDH) were used as received. NiCl(PPh3)(η-
C5H5),5 Ni(CtCPh)(PPh3)(η-C5H5),6 4-HCtCC6H4NO2,7
4-HCtCC6H4C6H4NO2,1 (E)-4,4′-HCtCC6H4CHdCHC6H4NO2,3b
4,4′-HCtCC6H4CtCC6H4NO2,1 and 4,4′-HCtCC6H4NdCH-
3b
C6H4NO2 were prepared following the literature methods.
Column chromatography was carried out using 7734 Kieselgel
60 silica (Merck). Microanalyses were performed at the
Research School of Chemistry, Australian National University.
Infrared spectra were recorded using a Perkin-Elmer System
2000 FT-IR spectrometer. UV-vis spectra were recorded
69.54; H, 4.58; N, 2.60. IR: (cyclohexane) ν(CtC) 2101 cm-1
(CH2Cl2) ν(CtC) 2091 cm-1 1H NMR (δ, 300 MHz, CDCl3):
;
.
(3) (a) Whittall, I. R.; Humphrey, M. G.; Hockless, D. C. R.; Skelton,
B. W.; White, A. H. Organometallics 1995, 14, 3970. (b) Whittall, I.
R.; Humphrey, M. G.; Persoons, A.; Houbrechts, S. Organometallics
1996, 15, 1935. (c) Whittall, I. R.; Humphrey, M. G.; Samoc, M.;
Swiatkiewicz, J .; Luther-Davies, B. Organometallics 1995, 14, 5493.
(d) McDonagh, A. M.; Whittall, I. R.; Humphrey, M. G.; Skelton, B.
W.; White, A. H. J . Organomet. Chem. 1996, 519, 229. (e) McDonagh,
A. M.; Whittall, I. R.; Humphrey, M. G.; Hockless, D. C. R.; Skelton,
B. W.; White, A. H. J . Organomet. Chem. 1996, 523, 33. (f) McDonagh,
A. M.; Cifuentes, M. P.; Whittall, I. R.; Humphrey, M. G.; Samoc, M.;
Luther-Davies, B.; Hockless, D. C. R. J . Organomet. Chem. 1996, 526,
99. (g) Whittall, I. R.; Humphrey, M. G.; Houbrechts, S.; Persoons,
A.; Hockless, D. C. R. Organometallics 1996, 15, 5738. (h) Whittall, I.
R.; Humphrey, M. G.; Samoc, M.; Luther-Davies, B. Angew. Chem.
1997, 109, 386; Angew. Chem., Int. Ed. Engl. 1997, 36, 370. (i)
Houbrechts, S.; Clays, K.; Persoons, A.; Cadierno, V.; Gamasa, M. P.;
Gimeno, J .; Whittall, I. R.; Humphrey, M. G. Proc. SPIE-Int. Soc. Opt.
Eng. 1996, 2852, 98. (j) Humphrey, M. G. Chem. Aust. 1996, 63, 442.
(4) Calabrese, J . C.; Cheng, L.; Green, J . C.; Marder, S. R.; Tam,
W. J . Am. Chem. Soc. 1991, 113, 7227.
5.25 (s, 5H, C5H5), 6.60 (d, J HH ) 9 Hz, 2H, H4), 7.40 (m, 9H,
Hm, Hp), 7.69 (dd, J HH ) 9 Hz, J HP ) 13 Hz, 6H, Ho), 7.79 (d,
J HH ) 9 Hz, 2H, H5). 13C NMR (δ, 75 MHz, CDCl3): 92.8
(C5H5), 103.6 (d, J CP ) 47 Hz, C1), 119.3 (C2), 123.0 (C5), 128.3
(d, J CP ) 10 Hz, Cm), 130.4 (Cp), 131.0 (C4), 133.5 (d, J CP ) 48
Hz, Ci), 133.7 (d, J CP ) 11 Hz, Co), 134.8 (C3), 144.1 (C6). 31P
NMR (δ, 121 MHz, CDCl3): 42.5. FAB MS m/z (fragment,
relative intensity): 531 ([M]+, 100), 385 ([M - CtCC6H4NO2]+,
55), 320 ([M - CtCC6H4NO2 - C5H5]+, 56).
Syn th esis of Ni(4,4′-CtCC6H4C6H4NO2)(P P h 3)(η-C5H5)
(4). Following the method for 3, NiCl(PPh3)(η-C5H5) (300 mg,
0.71 mmol), 4,4′-HCtCC6H4C6H4NO2 (170 mg, 0.76 mmol), and
CuI (5 mg, 0.03 mmol) afforded 4 as a brown microcrystalline
solid (300 mg, 69%). Anal. Calcd for C37H28NNiO2P: C, 73.06;
H, 4.65; N, 2.30. Found: C, 72.75; H, 4.51; N, 2.19. IR:
(cyclohexane) ν(CtC) 2103 cm-1; (CH2Cl2) ν(CtC) 2094 cm-1
.
1H NMR (δ, 300 MHz, CDCl3): 5.25 (s, 5H, C5H5), 6.70 (d, J HH
) 8 Hz, 2H, H4), 7.22 (d, J HH ) 8 Hz, 2H, H5), 7.40 (m, 9H,
Hm, Hp), 7.58 (d, J HH ) 9 Hz, 2H, H10), 7.69 (dd, J HH ) 8 Hz,
J HP ) 12 Hz, 6H, Ho), 8.18 (d, J HH ) 9 Hz, 2H, H11). 13C NMR
(5) Barnett, K. W. J . Chem. Educ. 1974, 51, 422.
(6) Bruce, M. I.; Humphrey, M. G.; Matisons, J . G.; Roy, S. K.;
Swincer, A. G. Aust. J . Chem. 1984, 37, 1955.
(7) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627.

Organometallic Complexes for Nonlinear Optics
Organometallics, Vol. 16, No. 12, 1997 2633
(δ, 75 MHz, CDCl3): 92.0 (d, J CP ) 48 Hz, C1), 92.7 (C5H5),
0.72 mmol), and CuI (5 mg, 0.03 mmol) afforded 8 as a brown
powder (210 mg, 47%). Anal. Calcd for C38H29NNiO2P: C,
71.83; H, 4.61; N, 4.41. Found: C, 71.29; H, 4.40; N, 4.31. IR:
(cyclohexane) ν(CtC) (not sufficiently soluble); (CH2Cl2) ν(CtC)
119.3 (C2), 123.9 (C11), 126.3, 127.0 (C5, C10), 128.2 (d, J CP
)
10 Hz, Cm), 129.0 (C3), 129.7 (C6), 130.2 (Cp), 130.2 (C4), 133.8
(d, J CP ) 11 Hz, Co), 133.8 (d, J CP ) 48 Hz, Ci), 146.4 (C9),
147.3 (C12). 31P NMR (δ, 121 MHz, CDCl3): 42.0. FAB MS
m/z (fragment, relative intensity): 607 ([M]+, 100), 385 ([M -
2094 cm-1 1H NMR (δ, 300 MHz, CDCl3): 5.24 (s, 5H, C5H5),
.
6.65 (d, J HH ) 8 Hz, 2H, H4), 6.91 (d, J HH ) 8 Hz, 2H, H5),
7.38 (m, 9H, Hm, Hp), 7.73 (dd, J HH ) 8 Hz, J HP ) 13 Hz, 6H,
Ho), 7.96 (d, J HH ) 9 Hz, 2H, H10), 8.26 (d, J HH ) 9 Hz, 2H,
CtCC6H4C6H4NO2]+, 82), 320 ([M - CtCC6H4C6H4NO2
-
C5H5]+, 62).
H
11), 8.42 (s, 1H, H16). 13C NMR (δ, 75 MHz, CDCl3): 90.0 (d,
Syn t h esis of Ni((E)-4,4′-CtCC6H 4CHdCH C6H 4NO2)-
(P P h 3)(η-C5H5) (5). Following the method for 3, NiCl(PPh3)(η-
C5H5) (130 mg, 0.31 mmol), (E)-4,4′-HCtCC6H4CHdCHC6-
H4NO2 (80 mg, 0.32 mmol), and CuI (5 mg, 0.03 mmol) afforded
5 as a brown microcrystalline solid (110 mg, 56%). Anal.
Calcd for C39H30NNiO2P: C, 73.84; H, 4.77; N, 2.21. Found:
C, 73.47; H, 4.47; N, 2.22. IR: (cyclohexane) ν(CtC) 2100
J CP ) 47 Hz, C1), 92.6 (C5H5), 119.6 (C2), 120.5 (C5), 123.9 (C11),
127.5 (C3), 128.2 (d, J CP ) 10 Hz, Cm), 129.0 (C10), 130.2 (Cp),
131.8 (C4), 133.8 (d, J CP ) 11 Hz, Co), 133.9 (d, J CP ) 48 Hz,
Ci), 141.8 (C9), 146.8 (C6), 148.9 (C12), 155.1 (C16). 31P NMR
(δ, 121 MHz, CDCl3): 41.8. FAB MS m/z (fragment, relative
intensity): 634 ([M]+, 100), 385 ([M
-
CtCC6H4-
NdCHC6H4NO2]+, 88), 320 ([M - CtCC6H4NdCHC6H4NO2
cm-1; (CH2Cl2) ν(CtC) 2093 cm-1
.
1H NMR (δ, 300 MHz,
- C5H5]+, 47).
CDCl3): 5.24 (s, 5H, C5H5), 6.61 (d, J HH ) 8 Hz, 2H, H4), 6.92
(d, J HH ) 16 Hz, 1H, H15), 7.07 (d, J HH ) 16 Hz, 1H, H16), 7.12
HRS Mea su r em en ts. An injection-seeded Nd:YAG laser
(Q-switched Nd:YAG Quanta Ray GCR5, 1064 nm, 8 ns pulses,
10 Hz) was focussed into a cylindrical cell (7 mL) containing
the sample. The intensity of the incident beam was varied by
rotation of a half-wave plate placed between crossed polarizers.
Part of the laser pulse was sampled by a photodiode to
measure the vertically polarized incident light intensity. The
frequency-doubled light was collected by an efficient condenser
system and detected by a photomultiplier. The harmonic
scattering and linear scattering were distinguished by ap-
propriate filters; gated integrators were used to obtain intensi-
ties of the incident and harmonic scattered light. Further
details of the experimental procedure have been reported
elsewhere.8
(d, J HH ) 8 Hz, 2H, H5), 7.38 (m, 9H, Hm, Hp), 7.51 (d, J HH
)
9 Hz, 2H, H10), 7.73 (dd, J HH ) 8 Hz, J HP ) 13 Hz, 6H, Ho),
8.15 (d, J HH ) 9 Hz, 2H, H11). 13C NMR (δ, 75 MHz, CDCl3):
91.7 (d, J CP ) 48 Hz, C1), 92.7 (C5H5), 120.0 (C2), 124.1 (C11),
124.6 (C15), 126.2, 126.5 (C5, C10), 128.2 (d, J CP ) 10 Hz, Cm),
128.8 (C3), 130.2 (Cp), 131.3 (C4), 132.2 (C6), 133.3 (C16), 133.8
(d, J CP ) 11 Hz, Co), 133.9 (d, J CP ) 48 Hz, Ci), 144.2 (C9),
146.3 (C12). 31P NMR (δ, 121 MHz, CDCl3): 41.8. FAB MS
m/z (fragment, relative intensity): 633 ([M]+, 100), 385
([M
-
CtCC6H4CHdCHC6H4NO2]+, 84), 320 ([M
-
CtCC6H4CHdCHC6H4NO2 - C5H5]+, 52).
Syn t h esis of Ni((Z)-4,4′-CtCC6H 4CHdCH C6H 4NO2)-
(P P h 3)(η-C5H5) (6). Following the method for 3, NiCl(PPh3)(η-
C5H5) (200
mg, 0.47
mmol), (Z)-4,4′-HCtCC6H4-
Z-Sca n Mea su r em en ts. Measurements were performed
at 800 nm, using a system consisting of a Coherent Mira Ar-
pumped Ti-sapphire laser generating a mode-locked train of
approximately 100 fs pulses and a home-built Ti-sapphire
CHdCHC6H4NO2 (120 mg, 0.48 mmol), and CuI (5 mg, 0.03
mmol) afforded 6 as a light brown powder (180 mg, 60%).
Anal. Calcd for C39H30NNiO2P: C, 73.84; H, 4.77; N, 2.21.
Found: C, 72.94; H, 4.37; N, 2.35. IR: (cyclohexane) ν(CtC)
regenerative amplifier pumped with
a frequency-doubled
2103 cm-1; (CH2Cl2) ν(CtC) 2094 cm-1
.
1H NMR (δ, 300 MHz,
Q-switched pulsed YAG laser (Spectra Physics GCR) at 30 Hz
and employing chirped pulse amplification. THF solutions
were examined in a glass cell with a 0.1 cm path length. The
Z-scans were recorded at two concentrations for each com-
pound, and the real and imaginary part of the nonlinear phase
change was determined by numerical fitting.9 The real and
imaginary parts of the hyperpolarizability of the solute were
then calculated by assuming linear concentration dependencies
of the solution susceptibility. The nonlinearities and light
intensities were calibrated using measurements of a 1 mm
thick silica plate for which the nonlinear refractive index n2
) 3 × 10-16 cm2 W-1 was assumed.
CDCl3): 5.23 (s, 5H, C5H5), 6.41 (d, J HH ) 12 Hz, 1H, H15),
6.51 (d, J HH ) 8 Hz, 2H, H4), 6.60 (d, J HH ) 12 Hz, 1H, H16),
6.78 (d, J HH ) 8 Hz, 2H, H5), 7.27 (d, J HH ) 9 Hz, 2H, H10),
7.37 (m, 9H, Hm, Hp), 7.70 (dd, J HH ) 8 Hz, J HP ) 13 Hz, 6H,
Ho), 7.99 (d, J HH ) 9 Hz, 2H, H11). 13C NMR (δ, 75 MHz,
CDCl3): 90.3 (d, J CP ) 47 Hz, C1), 92.8 (C5H5), 119.8 (C2), 123.4
(C11), 126.8 (C15), 127.6 (C3), 127.9, 129.5 (C5, C10), 128.2 (d,
J CP ) 10 Hz, Cm), 130.2 (Cp), 130.9 (C4), 132.3 (C6), 133.8 (d,
J CP ) 11 Hz, Co), 133.8 (d, J CP ) 48 Hz, Ci), 134.0 (C16), 144.3
(C9), 146.2 (C12). 31P NMR (δ, 121 MHz, CDCl3): 41.8. FAB
MS m/z (fragment, relative intensity): 633 ([M]+, 100), 385
([M
-
CtCC6H4CHdCHC6H4NO2]+, 83), 320 ([M
-
CtCC6H4CHdCHC6H4NO2 - C5H5]+, 47).
Resu lts a n d Discu ssion
Syn th esis of Ni(4,4′-CtCC6H4CtCC6H4NO2)(P P h 3)(η-
C5H5) (7). Following the method for 3, NiCl(PPh3)(η-C5H5)
(100 mg, 0.24 mmol), 4,4′-HCtCC6H4CtCC6H4NO2 (60 mg,
0.24 mmol), and CuI (5 mg, 0.03 mmol) afforded 7 as a brown
powder (100 mg, 66%). Anal. Calcd for C39H28NNiO2P: C,
74.07; H, 4.47; N, 2.22. Found: C, 73.31; H, 4.47; N, 2.19. IR:
(cyclohexane) ν(CtC) 2101, 2213 cm-1; (CH2Cl2) ν(tC) 2096,
Syn th eses a n d Ch a r a cter iza tion of σ-Acetylid e
Com p lexes. The synthetic methodology employed for
the preparation of 2-8 (Scheme 1) has been successfully
utilized for the preparation of 2 by Bruce and co-
workers;6 applying it to the nitro-containing acetylenes
afforded the corresponding acetylides in 46-69% yields.
2212 cm-1 1H NMR (δ, 300 MHz, CDCl3): 5.26 (s, 5H, C5H5),
.
1
Complexes 3-8 were characterized by IR, H, 13C, and
6.59 (d, J HH ) 8 Hz, 2H, H4), 7.12 (d, J HH ) 8 Hz, 2H, H5),
7.40 (m, 9H, Hm, Hp), 7.55 (d, J HH ) 9 Hz, 2H, H10), 7.71 (dd,
J HH ) 8 Hz, J HP ) 13 Hz, 6H, Ho), 8.16 (d, J HH ) 9 Hz, 2H,
31P NMR spectroscopies, FAB mass spectrometry, and
satisfactory microanalyses. Characteristic ν(CtC) in
the IR move to lower energy on increasing solvent
polarity (a difference of 5-10 cm-1 in moving from
cyclohexane to dichloromethane), suggestive of in-
H
11). 13C NMR (δ, 75 MHz, CDCl3): 88.1 (C15), 92.8 (C5H5),
94.0 (d, J CP ) 47 Hz, C1), 95.7 (C16), 117.6 (C6), 119.8 (C2), 123.6
(C11), 128.3 (d, J CP ) 10 Hz, Cm), 129.1 (C3), 130.3 (Cp), 130.6
(C9), 130.9, 131.0, 131.9 (C4, C5, C10), 133.8 (d, J CP ) 11 Hz,
Co), 133.8 (d, J CP ) 48 Hz, Ci), 146.6 (C12). 31P NMR (δ, 121
MHz, CDCl3): 42.0. FAB MS m/z (fragment, relative inten-
sity): 631 ([M]+, 87), 385 ([M - CtCC6H4CtCC6H4NO2]+,
100), 320 ([M - CtCC6H4CtCC6H4NO2 - C5H5]+, 73).
Syn th esis of Ni(4,4′-CtCC6H4NdCHC6H4NO2)(P P h 3)(η-
C5H5) (8). Following the method for 3, NiCl(PPh3)(η-C5H5)
(300 mg, 0.71 mmol), 4,4′-HCtCC6H4NdCHC6H4NO2 (179 mg,
(8) (a) Clays, K.; Persoons, A. Rev. Sci. Instrum. 1992, 63, 3285. (b)
Hendrickx, E.; Dehu, C.; Clays, K.; Bre´das, J . L.; Persoons, A. In
Polymers for Second-Order Nonlinear Optics; Lindsay, G. A., Singer,
K. D., Eds.; ACS Symposium Series 601; American Chemical Society:
Washington, DC, 1995, 82. (c) Houbrechts, S.; Clays, K.; Persoons,
A.; Pikramenou, Z.; Lehn, J .-M. Chem. Phys. Lett. 1996, 258, 485.
(9) Sheikh-bahae, M.; Said, A. A.; Wei, T.; Hagan, D. J .; van
Stryland, E. W. IEEE J . Quantum Electron. 1990, 26, 760.

2634 Organometallics, Vol. 16, No. 12, 1997
Whittall et al.
Sch em e 1
ring nitrophenylacetylide ligand in 3 by the two-ring
nitro-containing ligands in 4-8 leads to a decreased
II/III
E°Ni , with the value for the latter similar to that
of the phenylacetylide complex 2. These trends repro-
duce those observed with (cyclopentadienyl)bis(tri-
phenylphosphine)ruthenium complexes.1,3a,b Complexes
3-8 undergo a one-electron reduction between -0.80
(7) and -1.05 V (4), assigned to reduction of the nitro
substituent. The oxidation waves are associated with
nickel-centered HOMOs and the reduction waves with
nitro-centered LUMOs. The ease of electron removal
from the donor and addition to the acceptor group may
be relevant to the nonlinear optical response. The nitro
substituents have similar reduction potentials in analo-
gous ruthenium and nickel complexes, but the nickel
complexes are 0.14-0.27 V more difficult to oxidize than
the ruthenium analogues, suggesting that polarization
of the metal-centered electrons toward the nitro acceptor
may be less favorable for nickel than ruthenium.
Qu a d r a tic Hyp er p ola r iza bilities. We have previ-
ously determined the molecular quadratic optical non-
linearities of (cyclopentadienyl)bis(phosphine)ruthenium
and (triphenylphosphine)gold σ-arylacetylide com-
plexes1,3b,g and have now extended these studies to the
nickel complexes 1-8. Experimentally obtained data
at 1064 nm by hyper-Rayleigh scattering and two-level-
corrected values are collected in Table 2, together with
related data for the ruthenium and gold complexes.
Earlier studies with related ruthenium complexes
suggested that replacing L ) PMe3 by PPh3 in Ru(4-
CtCC6H4NO2)(L)2(η-C5H5), extending the nitro-contain-
ing chromophore in progressing from R ) 4-C6H4NO2
to 4,4′-C6H4C6H4NO2, 4,4′-C6H4CtCC6H4NO2, and then
(E)-4,4′-C6H4CHdCHC6H4NO2 in Ru(CtCR)(PPh3)2(η-
C5H5), and replacing bridging N by CH in progressing
from Ru(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)2(η-C5H5)
creased contribution of a vinylidene form (Ni-CtC T
NidCdC). Cyclopentadienyl (1H NMR, 5.23-5.26 ppm;
13C NMR, 92.6-92.8 ppm), NiCtC (119.3-120.0 ppm),
and 31P NMR phosphine resonances (41.6-42.5 ppm)
are insensitive to changes in the acetylide ligand,
whereas NiC moves downfield on introduction of a nitro-
substituent (2, 86.1 ppm; 3, 103.5 ppm) and then upfield
on progressing from a one-ring to two-ring acetylide
ligand (4-8, 90.0-94.0 ppm). We have confirmed the
identities of 2-5 and 7-8 by carrying out single-crystal
X-ray diffraction studies, complete details of which will
be reported elsewhere.10 Importantly, no significant
increase in the quinoidal vinylidene contribution to the
acetylide ground state structure is apparent on intro-
duction of the nitro substituent.
to Ru((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)2(η-
1,3b
C5H5) all lead to an increase in âHRS
.
These data
were substantially resonance enhanced, although the
relative orderings were maintained with two-level-
corrected values. Subsequent work with analogous gold
complexes which have λmax much further removed from
2ω (ω ) irradiating frequency) confirmed these relative
trends with both experimentally obtained and two-level-
corrected data and also suggested that the E stereo-
chemistry was more efficient than the Z stereo-
chemistry.3g The two-level-corrected data for ruthenium
and gold complexes suggested that the 18-electron, more
easily oxidizable ruthenium is a better donor than the
14-electron, less easily oxidizable gold in these donor-
bridge-acceptor complexes. The present studies extend
our earlier work by affording data for an 18-electron,
but less easily oxidizable metal center. The trends
observed in chromophore variation with the ruthenium
and gold complexes are maintained with these nickel
complexes. Although the absolute value of âHRS for the
biphenyl-linked 4 is less than that for the one-ring
chromophore 3, this relative ordering is reversed with
two-level-corrected values. All of the nickel data are
greater than the â values for the corresponding gold
complexes, emphasizing the importance of the valence
electron count in determining quadratic optical nonlin-
earity. Conversely, all of the nickel data are less than
the â values for the related ruthenium complexes (with
the exception of two-level-corrected values for the imino-
Electr och em ica l Stu d ies. The results of the cyclic
voltammetric investigations of complexes 1-8 are sum-
marized in Table 1; to the best of our knowledge, the
results for 2-8 are the first such data for nickel
acetylide complexes. All complexes undergo a one-
electron oxidation assigned to the NiII/III couple. Re-
placement of the electron withdrawing Cl by CtCPh
in progressing from 1 to 2 leads to a decrease in the
oxidation potential, and replacing phenylacetylide by
the more electron withdrawing 4-nitrophenylacetylide
in progressing from 2 to 3 results in the expected
increase in the oxidation potential. Replacing the one-
(10) Whittall, I. R.; Humphrey, M. G.; Hockless, D. C. R. Manuscript
in preparation.

Organometallic Complexes for Nonlinear Optics
Organometallics, Vol. 16, No. 12, 1997 2635
Ta ble 1. Cyclic Volta m m etr ic Da ta for Com p lexes 1-8a
E°Ni (V)
ipc/ipa
E°NO (V)
ipa/ipc
2
NiCl(PPh3)(η-C5H5) (1)
0.86
0.81
0.90
0.81
0.78
0.75
0.77
0.81
0.5
0.4
0.4
0.4
0.5
0.4
0.3
0.2
Ni(CtCPh)(PPh3)(η-C5H5) (2)
Ni(4-CtCC6H4NO2)(PPh3)(η-C5H5) (3)
-0.97
-1.05
-0.85
-0.96
-0.80
-0.79
0.7
0.6
0.4
0.6
0.7
1.0
Ni(4,4′-CtCC6H4C6H4NO2)(PPh3)(η-C5H5) (4)
Ni((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (5)
Ni((Z)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (6)
Ni(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)(η-C5H5) (7)
Ni(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)(η-C5H5) (8)
a
At -50 °C vs Ag/AgCl; 0.5 M [NBun4][PF6] in CH2Cl2 with a rate of 100 mV s-1 and a switching potential of 1.0 V.
Ta ble 2. Exp er im en ta l Lin ea r Op tica l Sp ectr oscop ic a n d Qu a d r a tic Non lin ea r Op tica l Resp on se
P a r a m eter sa
λ (nm) (ꢀ (104 M-1 cm-1))
â1064
â1064, corr
ref
b
c
compound
NiCl(PPh3)(η-C5H5) (1)
330 (0.6), 263 (1.1)
307 f (2.5)
89
45
15
59
65
120
47
106
93
<4
10
96
38
134
232
212
86
4
12
20
49
this work
this work
this work
this work
this work
this work
this work
this work
Ni(CtCPh)(PPh3)(η-C5H5) (2)
24d
Ni(4-CtCC6H4NO2)(PPh3)(η-C5H5) (3)
Ni(4,4′-CtCC6H4C6H4NO2)(PPh3)(η-C5H5) (4)
Ni((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (5)
Ni((Z)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (6)
Ni(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)(η-C5H5) (7)
Ni(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)(η-C5H5) (8)
RuCl(PPh3)2(η-C5H5)
439 (0.9), 368 f (1.3)
413 (1.6), 310 (2.3), 263 f (2.4)
437 (2.8), 313 (2.3)
417 (1.0), 307 (2.8)
417 (1.5), 313 (2.3)
448 (1.4), 282 (2.8)
357 (0.3) br, 290 (0.5) sh
310 (2.0)
460 (1.1), 382 (1.1)
477 (1.7), 279 (1.0)
448 (1.6), 310 (2.3)
476 (2.6), 341 (2.4)
446 (1.9), 340 (2.8)
496 (1.3), 298 (2.6)
296 (1.3), 282 (3.0), 268 (2.7)
338 (2.5)
221
193
445
145
326
387
<7
1
Ru(CtCPh)(PPh3)2(η-C5H5)
16
3b
3b
3b
1
3b
1
3b
3g
3g
3g
3g
3g
3g
3g
Ru(4-CtCC6H4NO2)(PPh3)2(η-C5H5)
468
248
560
1455
865
840
6
22
39
120
58
59
Ru(4-CtCC6H4NO2)(PMe3)2(η-C5H5)
Ru(4,4′-CtCC6H4C6H4NO2)(PPh3)2(η-C5H5)
Ru((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)2(η-C5H5)
Ru(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)2(η-C5H5)
Ru(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)2(η-C5H5)
Au(CtCPh)(PPh3)
Au(4-CtCC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4C6H4NO2)(PPh3)
Au((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)
Au((Z)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)
350 (2.9), 287 (1.8), 274 (1.9)
386 (3.8), 303 (2.0)
362 (2.0), 298 (2.8)
362 (3.6), 301 (3.2)
392 (2.1), 297 (3.2)
28
28
34
85
a
b
All measurements in THF solvent. All complexes are optically transparent at 1064 nm. HRS at 1064 nm; values (10%. c HRS at
1064 nm corrected for resonance enhancement at 532 nm, using the two-level model with âo ) â[1 - (2λmax/1064)2][1 - (λmax/1064)2];
damping factors not included. Value (20%.
d
linked complexes), emphasizing the importance of oxi-
dizability in determining NLO merit. We have previ-
ously discussed problems in utilizing the two-level-
correction; it is likely that â1064,corr for Ru(4,4′-
CtCC6H4NdCHC6H4NO2)(PPh3)2(η-C5H5) is an under-
estimation.1
Figure 2 correlates quadratic optical nonlinearities for
the nickel complexes with those of the precursor termi-
nal alkynes, with the slopes of the graphs (7.1 (R ) 0.93)
for uncorrected; 2.8 (R ) 0.80) for two-level-corrected)
defining a “figure of merit” for the (cyclopentadienyl)-
(triphenylphosphine)nickel moiety compared to hydro-
gen. We had earlier analyzed the related (cyclopenta-
dienyl)bis(triphenylphosphine)ruthenium (24 (R ) 0.99)
for uncorrected; 6.7 (R ) 0.69) for two-level-corrected)
and (triphenylphosphine)gold (2.4 (R ) 0.99) for uncor-
rected; 2.0 (R ) 0.99) for two-level-corrected) complexes.
F igu r e 2. Correlation of molecular quadratic hyperpolar-
Although the correlation coefficients for the two-level-
izabilities of complexes 3-8 with those of the corresponding
corrected values in particular are low (perhaps reflecting
the lack of applicability of this model to the ruthenium
and nickel complexes), the figures of merit provide a
terminal alkynes (10-30 cm5 esu-1); b uncorrected; O two-
level-corrected. 10% uncertainty.
useful comparison of the donor strength of these ligated
metal centers, with the Ru(PPh3)2(η-C5H5) unit three
times as efficient as the Ni(PPh3)(η-C5H5) moiety.
Cu bic Hyp er p ola r iza bilities. Third-order nonlin-
earities for the nickel complexes were determined by
Z-scan at 800 nm. An electronic origin for cubic non-
linearities in these metal acetylide complexes has been
demonstrated earlier by degenerate four-wave mixing
measurements on the related ruthenium complexes.3c
Data for the nickel acetylides, together with those from
the ruthenium and gold complexes, are collected in
Table 3.
The real component of the γ values for the nitro-
containing nickel complexes are large, negative, and
incorporate substantial error margins; the negative real
components and presence of significant imaginary com-

2636 Organometallics, Vol. 16, No. 12, 1997
Whittall et al.
Ta ble 3. Exp er im en ta l Lin ea r Op tica l Sp ectr oscop ic a n d Cu bic Non lin ea r Op tica l Resp on se P a r a m eter sa
γ (10-36 esu)b
λ (nm) (ꢀ (104 M-1 cm-1))
real
imaginary
ref
NiCl(PPh3)(η-C5H5) (1)
330 (0.6), 263 (1.1)
307 f (2.5)
c
this work
this work
this work
this work
this work
this work
this work
this work
3c
3c
3c
3c
3c
3c
1
3c
3c
1
3h
3h
3h
3h
3h
3h
3h
3h
Ni(CtCPh)(PPh3)(η-C5H5) (2)
15 ( 10
< 10
Ni(4-CtCC6H4NO2)(PPh3)(η-C5H5) (3)
Ni(4,4′-CtCC6H4C6H4NO2)(PPh3)(η-C5H5) (4)
Ni((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (5)
Ni((Z)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)(η-C5H5) (6)
Ni(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)(η-C5H5) (7)
Ni(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)(η-C5H5) (8)
RuCl(PPh3)2(η-C5H5)
439 (0.9), 368 f (1.3)
413 (1.6), 310 (2.3), 263 f (2.4)
437 (2.8), 313 (2.3)
417 (1.0), 307 (2.8)
417 (1.5), 313 (2.3)
448 (1.4), 282 (2.8)
357 (0.3) br, 290 (0.5) sh
347 (0.1)
-270 ( 100
-580 ( 200
-420 ( 100
-230 ( 50
-640 ( 300
<120
70 ( 50
300 ( 60
480 ( 150
160 ( 80
720 ( 300
360 ( 100
150 ( 100
e80
e150
RuCl(PMe3)2(η-C5H5)
Ru(CtCPh)(PPh3)2(η-C5H5)
310 (2.0)
325
Ru(CtCC6H4Br-4)(PPh3)2(η-C5H5)
e150
Ru(4-CtCC6H4NO2)(PPh3)2(η-C5H5)
Ru(4-CtCC6H4NO2)(PMe3)2(η-C5H5)
Ru(4,4′-CtCC6H4C6H4NO2)(PPh3)2(η-C5H5)
Ru((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)2(η-C5H5)
Ru(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)2(η-C5H5)
Ru(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)2(η-C5H5)
AuCl(PPh3)
460 (1.1), 382 (1.1)
477 (1.7), 279 (1.0)
448 (1.6), 310 (2.3)
476 (2.6), 341 (2.4)
446 (1.9), 340 (2.8)
496 (1.3), 298 (2.6)
275 (0.2), 268 (0.3)
296 (1.3), 282 (3.0), 268 (2.7)
338 (2.5)
350 (2.9), 287 (1.8), 274 (1.9)
386 (3.8), 303 (2.0)
362 (2.0), 298 (2.8)
362 (3.6), 301 (3.2)
392 (2.1), 297 (3.2)
-210 ( 50
-230 ( 70
-380 ( 200
-450 ( 100
-450 ( 100
-850 ( 300
180 ( 100
39 ( 20
120 ( 40
540 ( 150
1200 ( 200
420 ( 150
1300 ( 400
130 ( 30
e10
74 ( 30
320 ( 160
210 ( 60
e20
360 ( 200
Au(CtCPh)(PPh3)
Au(4-CtCC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4C6H4NO2)(PPh3)
Au((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)
Au((Z)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4CtCC6H4NO2)(PPh3)
Au(4,4′-CtCC6H4NdCHC6H4NO2)(PPh3)
20 ( 15
120 ( 50
470 ( 150
92 ( 30
560 ( 150
330 ( 60
a
b
All measurements as THF solutions (all complexes are optically transparent at 800 nm).
All results are referenced to the
hyperpolarizability of THF γ ) 1.6 × 10-36 esu. c Insufficiently soluble.
ponents of the nonlinearities indicate two-photon dis-
persion is contributing to the observed responses. As
noted with our previous data from the ruthenium and
gold systems, these two-photon states become important
for an 800 nm irradiating wavelength when complexes
contain λmax > 400 nm, and real components for the
nickel complexes considered here become negative when
the optical absorption maximum fulfills this criterion.
Despite the large error margins, an increase in the real
component of the nonlinearity upon chain-lengthening
is evident and an increase in efficiency upon replacing
Z by E stereochemistry in progressing from 5 to 6 is
suggested. The γreal values for these nickel complexes
are the same within the error margins to those of the
ruthenium complexes, ease of oxidation making no
significant difference to cubic NLO merit. Nonlineari-
ties for Au(4,4′-CtCC6H4CtCC6H4NO2)(PPh3) and
Au((E)-4,4′-CtCC6H4CHdCHC6H4NO2)(PPh3) are the
largest values for any of the complexes in the ruthe-
nium, gold, and nickel systems, despite the gold com-
plexes having the lower valence electron count.
with attenuation of this electron depletion as the nitro
group becomes further removed from the metal, on
replacement of one-ring acetylide groups by two-ring
acetylides to afford 4-8. These trends in the spectro-
scopic and electrochemical data mirror those observed
for the ruthenium complexes. Experimentally obtained
and two-level-corrected quadratic nonlinearities are
consistent with an increase in â upon chain-lengthening
from a one-ring to two-ring chromophore, replacing Z
by
E
stereochemistry in the 4,4′-CtCC6H4-
CHdCHC6H4NO2 chromophores, replacing N by CH in
the 4,4′-CtCC6H4XdCHC6H4NO2 chromophores, and
progressing from a biphenyl to yne-linked and then
E-ene-linked chromophore. Where data exist, these
trends are also followed by the related ruthenium and
gold complexes. Figures of merit for the ligated metal
centers defined by comparison to the parent acetylenes
as the standard are a necessarily crude approximation.
Nevertheless, decreases by factors of three in proceeding
from Ru(PPh3)2(η-C5H5) to Ni(PPh3)(η-C5H5) and then
to Au(PPh3) provide strong support for the importance
of valence electron count and ease of oxidation in
enhancing quadratic NLO merit. In contrast, these
variables do not influence cubic NLO response in a
rational fashion. Data for the nickel complexes are,
within the error margins, equivalent to those of the
ruthenium complexes, although the ruthenium com-
plexes possess a more easily oxidizable metal and
greater delocalization possibilities with the second
triphenylphosphine ligand. Data for Au(4,4′-CtCC6-
H4CtCC6H4NO2)(PPh3) and Au((E)-4,4′-CtCC6H4-
CHdCHC6H4NO2)(PPh3) are substantially larger than
those for the nickel and ruthenium analogues (and
indeed are the largest values thus far for monomeric
organometallic complexes), despite the lower valence
electron count, decreased co-ligand delocalization pos-
sibilities, and less easily oxidizable metal. Further
Con clu sion
The series of donor-bridge-acceptor (cyclopentadi-
enyl)(triphenylphosphine)nickel σ-acetylide complexes
with an 18-electron, less oxidizable metal fill the niche
existing between the 18-electron, more oxidizable (cyclo-
pentadienyl)bis(triphenylphosphine)ruthenium acetyl-
ides and the 14-electron (triphenylphosphine)gold acetyl-
ides and, thus, permit cautious comment on the effect
of the ease of oxidation and valence electron count on
quadratic and cubic nonlinearities. The 13C NMR
chemical shifts of the metal bound R-carbon and NiII/III
redox couple (by cyclic voltammetry) are consistent with
the NiC unit becoming less electron rich upon replace-
ment of CtCPh in 2 by 4-CtCC6H4NO2 to afford 3, but

Organometallic Complexes for Nonlinear Optics
Organometallics, Vol. 16, No. 12, 1997 2637
studies with systematically varied molecules are cur-
rently underway.
work and J ohnson-Matthey Technology Centre (M.G.H.)
for the loan of ruthenium salts. I.R.W. is the recipient
of an Australian Postgraduate Research Award (Indus-
try), M.P.C. holds an ARC Australian Postdoctoral
Research Fellowship, M.G.H. holds an ARC Australian
Research Fellowship, and S.H. is a Research Assistant
of the Belgian National Fund for Scientific Research.
Ack n ow led gm en t. We thank the Australian Re-
search Council (M.G.H.), Telstra (M.G.H.), the Belgian
Government (Grant No. IUAP-16) (A.P.), the Belgian
National Science Foundation (Grant No. G.2103.93,
9.0012.92, and 9.0011.92) (A.P.), and the University of
Leuven (Grant No. GOA 95/1) (A.P.) for support of this
OM961066Z