Cooperation between cis and trans influences in cis-Pt II(PPh3)2 complexes: Structural, spectroscopic, and computational studies

Article abstract of DOI:10.1021/ic901510m

The relevance of cis and trans influences of some anionic ligands X and Y in cis-[PtX₂(PPh₃)₂] and cis-[PtXY(PPh ₃)₂] complexes have been studied by the X-ray crystal structures of several derivatives (X₂ = (AcO)₂ (3), (NO₃)₂ (5), Br₂ (7), I₂ (11); and XY = Cl(AcO) (2), Cl(NO₃) (4), and Cl(NO₂) (13)), density functional theory (DFT) calculations, and one bond Pt-P coupling constants, JPtp. The latter have allowed an evaluation of the relative magnitude of both influences. It is concluded that such influences act in a cooperative way and that the cis influence is not irrelevant when rationalizing the ¹ JPtp values, as well as the experimental Pt-P bond distances. On the contrary, in the optimized geometries, evaluated through B3LYP/def2-SVP calculations, the cis influence was not observed, except for compounds CIPh (21), Ph₂ (22), and, to a lesser extent, Cl(NO₂) (13) and (NO₂) ₂(14). A natural bond order analysis on the optimized structures, however, has shown how the cis influence can be related to the s-character of the Pt hybrid orbital involved in the Pt-P bonds and the net atomic charge on Pt. We have also found that in the X-ray structures of cis-[PtX ₂(PPh₃)₂] complexes the two Pt-X and the two Pt-P bond lengths are different each other and are related to the conformation of the phosphine groups, rather than to the crystal packing, since this feature is observed also in the optimized geometries.

Full text of DOI:10.1021/ic901510m

Inorg. Chem. 2010, 49, 123–135 123
DOI: 10.1021/ic901510m
Cooperation between Cis and Trans Influences in cis-PtII(PPh3)2 Complexes:
Structural, Spectroscopic, and Computational Studies
Luca Rigamonti,† Alessandra Forni,*,‡ Mario Manassero,*,§ Carlo Manassero,§ and Alessandro Pasini*,†
†
ꢀ
Universita degli Studi di Milano, Dipartimento di Chimica Inorganica, Metallorganica e Analitica “Lamberto
Malatesta”, via Venezian 21, 20133 Milano, Italy, ‡ISTM-CNR, via Golgi 19, 20133 Milano, Italy, and
§
ꢀ
Universita degli Studi di Milano, Dipartimento di Chimica Strutturale e Stereochimica Inorganica, via Venezian
21, 20133 Milano, Italy
Received July 29, 2009
The relevance of cis and trans influences of some anionic ligands X and Y in cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2]
complexes have been studied by the X-ray crystal structures of several derivatives (X2 = (AcO)2 (3), (NO3)2 (5), Br2
(7), I2 (11); and XY = Cl(AcO) (2), Cl(NO3) (4), and Cl(NO2) (13)), density functional theory (DFT) calculations, and
one bond Pt-P coupling constants, 1JPtP. The latter have allowed an evaluation of the relative magnitude of both
influences. It is concluded that such influences act in a cooperative way and that the cis influence is not irrelevant when
rationalizing the 1JPtP values, as well as the experimental Pt-P bond distances. On the contrary, in the optimized
geometries, evaluated through B3LYP/def2-SVP calculations, the cis influence was not observed, except for
compounds ClPh (21), Ph2 (22), and, to a lesser extent, Cl(NO2) (13) and (NO2)2 (14). A natural bond order
analysis on the optimized structures, however, has shown how the cis influence can be related to the s-character of the
Pt hybrid orbital involved in the Pt-P bonds and the net atomic charge on Pt. We have also found that in the X-ray
structures of cis-[PtX2(PPh3)2] complexes the two Pt-X and the two Pt-P bond lengths are different each other and
are related to the conformation of the phosphine groups, rather than to the crystal packing, since this feature is
observed also in the optimized geometries.
Introduction
complex. The cis influence can be defined, similarly, as
the weakening of a metal-ligand bond induced by a cis
ligand. Several techniques have been employed to investigate
these influences,4 the most popular being X-ray crystallogra-
phy2a,4,5 (“structural” trans or cis influence) and NMR
spectroscopy. The latter has been used extensively6 when
both the metal and the donor atomsare NMR active (suchas,
in the present case, 195Pt and 31P) by examining the 1J values,
A fundamental aspect of coordination chemistry is the
understanding of the influence that a ligand L, in an MLLn0
complex, exerts on the properties of the M-L0 bonds,
because a knowledge of these influences can give an insight
into aspects like stability and reactivity of the complexes
under study. In the case of square planar and octahedral
complexes, such influences are termed cis and trans influ-
ences, depending on which bond is influenced by the “influ-
encing” ligand L.1 The comprehensive importance of these
aspects is shown by the fact that, in addition to the multitude
of studies performed on transition metal complexes, there are
examples also in main group chemistry.2
(4) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,
10, 335–422.
(5) (a) Kukushkin, V. Yu.; Belsky, V. K.; Konovalov, V. E.; Kirakosyan,
G. A.; Konovalov, L. V.; Moiseev, A. I.; Tkachuk, V. M. Inorg. Chim. Acta
1991, 185, 143–154. (b) Manojlovic-Muir, L.; Muir, K. W.; Solomun, T.
J. Organomet. Chem. 1977, 142, 265–280. (c) Kapoor, P.; Loevqvist, K.;
Oskarsson, Å. J. Mol. Struct. 1998, 470, 39–47.
(6) Some examples: (a) Michelin, R. A.; Ros, R. J. Chem. Soc., Dalton
Trans. 1989, 1149–1159. (b) Otto, S.; Roodt, A. Inorg. Chim. Acta 2004, 357,
1–10. (c) Roodt, A.; Otto, S.; Steyl, G. Coord. Chem. Rev. 2003, 245, 121–137.
(d) Honeychuck, R. V.; Hersh, W. H. Inorg. Chem. 1987, 26, 1826–1828. (e)
Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2117–2124. (f ) Cairns, M.
A.; Dixon, K. R.; Rivett, G. A. J. Organomet. Chem. 1979, 171, 373–385. (g)
Church, M. J.; Mays, M. J. J. Chem. Soc. (A) 1970, 1938–1941. (h) Appleton,
T. G.; Hall, J. R.; Ralph, S. F. Inorg. Chem. 1985, 24, 4685–4693.
(7) For instance: (a) Steyn, G. J. J.; Roodt, A.; Poletaeva, I.; Varshavsky,
Yu. S. J. Organomet. Chem. 1997, 536-537, 197–205. (b) Mather, G. G.;
Pidcock, A.; Rapsey, G. J. N. J. Chem. Soc., Dalton Trans. 1973, 2095–2099. (c)
Otto, S.; Roodt, A.; Leipoldt, J. G. S. Afr. J. Chem. 1995, 48, 114–119.
According to an original definition,1,3 the trans influence
of a ligand is related to the extent to which such ligand
weakens the bond trans to itself in the ground state of the
*To whom correspondence should be addressed. E-mail: alessandro.
pasini@unimi.it (A.P.), a.forni@istm.cnr.it (A.F.), mario.manassero@
unimi.it (M.M.).
(1) Pidcock, A.; Richards, R. E.; Venanzi, L. M. J. Chem. Soc. (A) 1966,
1707–1710.
(2) (a) Ochiai, M.; Sueda, T.; Miyamoto, K.; Kiprof, P.; Zhdankin, V. V.
Angew. Chem., Int. Ed. 2006, 45, 8203–8206. (b) Shustorovich, E. M.; Buslaev,
Yu. A. Inorg. Chem. 1976, 15, 1142–1147.
(3) Venanzi, L. M. Chem. Brit. 1968, 4, 162–167.
r
2009 American Chemical Society
Published on Web 12/01/2009
pubs.acs.org/IC

124 Inorganic Chemistry, Vol. 49, No. 1, 2010
Rigamonti et al.
Scheme 1
evaporation of
a
diisopropyl ether/chloroform solution,
whereas crystals of 5 suitable for X-ray diffraction were
obtained by slow diffusion of diisopropyl ether into a chloro-
form or a dichloromethane solution. All reactions involving silver
salts were performed in the dark.
Synthetic Procedures: cis-[PtCl(AcO)(PPh3)2] (2). AgAcO
(21.1 mg, 0.13 mmol) was added to a solution of 1 (100.1 mg,
0.13 mmol) in CH2Cl2 (10 mL). The mixture was refluxed for 8 h,
hot filtered, and the filtrate was evaporated to dryness in vacuo
yielding 2 as a white solid (74.2 mg, 70%). Anal. calcd for
which can be considered an estimate of the bond strength
between these atoms. Correlations between the two studies
have also been reported.7
C38H33ClO2P2Pt CH2Cl2 (899.09): C, 52.10; H, 3.92. Found: C,
3
52.51; H, 4.09. IR (KBr): 1628, 1370, and 1311 (νCO). δP 3.1 (1P,
d, JPP 19, JPtP 3560), 18.3 (1P, d, JPP 19, JPtP 3956). ESI-MS: m/z
719 ([Pt(PPh3)2]þ, 100%), 754 ([PtCl(PPh3)2]þ, 60), 778
([Pt(AcO)(PPh3)2]þ, 65), 836 ([M þ Na]þ, 25). Crystals suitable
for X-ray diffraction were obtained by slow diffusion of diiso-
propyl ether into a chloroform solution.
1
In a previous paper,8 we have used the JPtP coupling
constants of a series of trans-[PtX2(PPh3)2] and trans-[PtXY-
(PPh3)2] complexes to investigate the cis influence of some
anionic ligands X and Y. We assumed that in these complexes
the mutual trans influence of the two phosphines can be
considered constant to a good approximation and that the
1JPtP values of the various complexes should depend, essen-
tially, on the cis influence of X and/or Y. The series of cis
influence obtained was the following:8
cis-[PtCl(NO3)(PPh3)2] (4). AgNO3 (21.8 mg, 0.13 mmol) was
added to a solution of 1 (94.6 mg, 0.12 mmol) in CH2Cl2
(10 mL). The mixture was refluxed for 8 h, hot filtered, and
the filtrate was taken to dryness yielding 4 as a white solid (40.1
mg, 41%). Anal. calcd for C36H30NClO3P2Pt CH2Cl2 (902.05):
3
C, 49.27; H, 3.58; N, 1.55. Found: C, 49.60; H, 3.61; N, 1.55. IR
(KBr): 1499 and 1273 (νNO3). δP 2.7 (1P, d, JPP 19, JPtP 3858),
17.6 (1P, d, JPP 19, JPtP 3842). Crystals suitable for X-ray
diffraction were obtained by slow diffusion of diisopropyl ether
into a chloroform solution.
I > Cl > SePh = SPh = SeEt > NO3 > AcO > NO2 > H
> Me > Ph
When we moved to the cis-[PtXY(PPh3)2] derivatives, we
reasoned that the Pt-P bond strengths must depend on both
trans and cis influences. For instance, 1JPtP1 (see Scheme 1)
should depend not only on the trans influence of Y, but also
on the cis influence of X. The latter has beenlittle studied, and
although its contribution is usually believed to be of lower
importance,4,9,10 we wondered whether it could be observed,
evaluated, and compared with the trans influence. This paper
tries and addresses these points. We will discriminate the two
contributions analyzing (i) the Pt-P bond distances of the
seven X-ray crystal structures we have determined, together
with some literature data, (ii) the Pt-P bond distances of the
optimized geometries of some derivatives, and (iii) the one
bond coupling constants of several compounds. We will also
compare the different magnitudes of the cis and trans
influences. The nomenclature used in this paper is summar-
ized in Scheme 1.
Observed Formation of cis-[PtBrCl(PPh3)2] (6). Solid KBr
(174.8 mg, 1.47 mmol) was added to a solution of K2PtCl4 (152.4
mg, 0.37 mmol) in water (5 mL). This solution was added to a
solution of PPh3 (192.6 mg, 0.73 mmol) in ethanol (15 mL), and
the mixture was stirred at room temperature for 4 h, when a very
light yellow solid was recovered by filtration. Its 31P NMR
spectrum showed the presence of cis-[PtCl2(PPh3)2] (1),
cis-[PtBr2(PPh3)2] (7), and a new series of signals, δP 13.6 (d,
JPP 14, JPtP 3665), 15.3 (d, JPP 14, JPtP 3618), assigned to 6, in a
1:1:2 ratio (see the Results and Discussion section).
cis-[PtBr2(PPh3)2] (7). Solid KBr (411.7 mg, 3.46 mmol) was
added to a solution of K2PtCl4 (199.8 mg, 0.48 mmol) in water
(7 mL). The resulting solution was added to a solution of PPh3
(258.2 mg, 0.98 mmol) in ethanol (15 mL), and the mixture was
refluxed for 4 h. The very light yellow product was recovered by
filtration of the cooled mixture (380.1 mg, 90%). Anal. calcd for
C36H30Br2P2Pt (879.47): C, 49.17; H, 3.44. Found: C, 49.40; H,
3.51. δP 14.3 (2P, s, JPtP 3614). ESI-MS: m/z 719 ([Pt(PPh3)2]þ,
80%), 799 ([PtBr(PPh3)2]þ, 100), 902 ([M þ Na]þ, 10). Crystals
suitable for X-ray diffraction were obtained by slow diffusion of
diisopropyl ether into a chloroform/dichloromethane solution.
cis-[PtBr(AcO)(PPh3)2] (8). AgAcO (15.8 mg, 0.095 mmol)
was added to a solution of 7 (80.3 mg, 0.091 mmol) in CH2Cl2
(10 mL). The mixture was stirred at room temperature for 8 h,
filtered hot, and the filtrate was concentrated and precipitated
with diisopropyl ether, yielding a very light yellow solid (65.5
Experimental Section
General. Chemicals were reagent grade and used as received.
Elemental analyses were performed at the Microanalytical
Laboratory of the Universita degli Studi di Milano. 31P{1H}
ꢀ
NMR spectra were recorded in CDCl3 on a Bruker Advance
DRX 300 at 121 MHz; δP values (ppm) are versus external
H3PO4, J coupling constants (Hz) have been found reproducible
to (3 Hz. Electrospray ionization (ESI) mass spectra were
recorded with a LCQ Advantage Thermofluxional Instrument.
Infrared spectra were recorded as KBr disks using a JASCO FT-
IR 410 spectrophotometer with a 2 cm-1 resolution, and main
bands are given in inverse centimeters. cis-[PtCl2(PPh3)2] (1),11
cis-[Pt(AcO)2(PPh3)2] (3),12 and cis-[Pt(NO3)2(PPh3)2] (5)12
were synthesized as described in the literature. Crystals
of 3 suitable for X-ray diffraction were obtained by slow
mg, 80%). Anal. calcd for C38H33BrO2P2Pt 2H2O (894.64): C,
3
51.02; H, 4.25. Found: C, 51.03; H, 3.95. ESI-HRMS: m/z Calcd
for [M þ Na]þ: 881.06748. Found: 881.06687. δP 2.0 (1P, d, JPP
17, JPtP 3538), 18.8 (1P, d, JPP 17, JPtP 3907).
cis-[PtBr(NO3)(PPh3)2] (9). AgNO3 (22.1 mg, 0.13 mmol)
was added to a solution of 7 (111.0 mg, 0.13 mmol) in CH2Cl2
(15 mL). The mixture was refluxed for 4 h, filtered hot, and the
filtrate was concentrated and precipitated with diisopropyl
ether, yielding a very light yellow solid. The crude product was
recrystallized from CH2Cl2-n-hexane (30.1 mg, 27%). Anal.
calcd for C36H30BrNO3P2Pt (861.568): C, 50.19; H, 3.51; N,
1.63. Found: C, 50.34; H, 3.48; N, 1.76. δP 1.5 (1P, d, JPP 17, JPtP
3826), 17.8 (1P, d, JPP 17, JPtP 3790).
(8) Rigamonti, L.; Manassero, C.; Rusconi, M.; Manassero, M.; Pasini,
A. Dalton Trans. 2009, 1206–1213.
(9) Muenzenberg, R.; Rademacher, P.; Boese, R. J. Mol. Struct. 1998,
444, 77–90.
(10) Tau, K. D.; Meek, D. W. Inorg. Chem. 1979, 18, 3574–3580.
(11) Hartley, F. R. Organomet. Chem. Rev. A 1970, 6, 119–137.
(12) Alesi, M.; Fantasia, S.; Manassero, M.; Pasini, A. Eur. J. Inorg.
Chem. 2006, 1429–1435.
cis-[PtClI(PPh3)2] (10). A solution of KI (672.2 mg, 4.48
mmol) in H2O (5 mL) and EtOH (5 mL) was added to a white
suspension of 1 (86.5 mg, 0.11 mmol) in CHCl3 (5 mL) and

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 125
acetone (5 mL). The yellow mixture was left under reflux for 1 h,
and then the yellow solid was filtered, washed with H2O, EtOH,
and diisopropyl ether, and dried. The crude product was re-
crystallized from CH2Cl2-diisopropyl ether, yielding 10 as light
yellow polycrystalline solid (42.1 mg, 43%). Anal. calcd for
C36H30ClIP2Pt (882.02): C, 49.02; H, 3.43. Found: C, 49.29; H,
3.32. δP 11.6 (1P, d, JPP 12, JPtP 3683), 13.7 (1P, d, JPP 12, JPtP
3420).
3.73; N, 3.45. Found: C, 53.36; H, 3.65; N, 3.56. IR (KBr): 1412
and 1336 (νNO2). δP -1.3 (2P, s, JPtP 3148).
X-ray Data Collections and Structure Determinations. Crystal
data are summarized in Table 1. The diffraction experiments
were carried out on a Bruker APEX II CCD area-detector
diffractometer, at 150 K for 7 and 13, and at 296 K for 2,
3 0.25H2O, 4 CHCl3, 5 CH2Cl2, 5 CHCl3, and 11 H2O, using
3
3
3
3
3
˚
Mo KR radiation (λ = 0.71073 A) with a graphite crystal
monochromator in the incident beam. No crystal decay was
observed, so that no time-decay correction was needed. The
collected frames were processed with the software SAINT,14 and
an empirical absorption correction was applied (SADABS)15 to
the collected reflections. The calculations were performed using
the Personal Structure Determination Package16 and the physi-
cal constants tabulated therein.17 The structures were solved
by direct methods (SHELXS)18 and refined by full-matrix
least-squares using all reflections and minimizing the function
[PtI2(cod)]. Solid KI (833.7 mg, 5.02 mmol) was added to a
solution of K2PtCl4 (327.6 mg, 0.79 mmol) in water (10 mL).
After 10 min, acetic acid (10 mL) and cod (390 μL, 3.17 mmol)
were added to the dark solution. The mixture was left at 80 °C
for 2 h. After cooling, the product was recovered as a yellow
solid by filtration, washed with water, ethanol, and diisopropyl
ether, and dried in vacuo (395.2 mg, 90%). Anal. calcd for
C8H12I2Pt (557.07): C, 17.25; H, 2.17. Found: C, 17.40; H, 2.36.
IR (KBr): 1499, 1422, and 1340 (νCC).
cis-[PtI2(PPh3)2] (11). A solution of PPh3 (107.8 mg, 0.41
mmol) in CH2Cl2 (5 mL) was added dropwise to a solution of
[PtI2(cod)] (119.6 mg, 0.21 mmol) in CH2Cl2 (5 mL) and the
mixture was stirred at room temperature for 5 min. The resulting
pale yellow solid was filtered, washed with CH2Cl2 and diiso-
propyl ether, and dried in vacuo (183.9 mg, 87%). Anal. calcd
Σw(Fo - kFc ) (refinement on F2). In 4 CHCl3, the solvent
2
2 2
3
molecule is disordered: the carbon atom (C1) and the first Cl
atom (Cl2) are ordered, whereas the second Cl atom is split into
two peaks (Cl3 and Cl4) having occupancies of 0.50 each, and
the third Cl atom is split into four peaks (Cl5, Cl6, Cl7, and Cl8)
having occupancies of 0.25 each. In spite of this, all the non-
hydrogen atoms of this molecule could be refined with aniso-
tropic thermal factors, whereas the hydrogen atom was ob-
for C36H30I2P2Pt 3H2O (1027.51): C, 42.08; H, 3.53. Found: C,
3
42.11; H, 3.63. 31P NMR showed that this solid consisted of a 1:2
mixture of the cis and trans isomers: δP 12.6 (s, JPtP 2493)
trans-[PtI2(PPh3)2], 11.9 (s, JPtP 3455) cis-[PtI2(PPh3)2] (11).
Refluxing this mixture in ethanol (15 mL) in the presence of
an excess of PPh3 gave complete isomerization to trans-[PtI2-
(PPh3)2] (NMR evidence). Slow crystallization from chloro-
form-diisopropyl ether gave deep orange and yellow crystals.
The former consisted of trans-[PtI2(PPh3)2], whose structure is
already published.13 The yellow crystals were found to be
suitable for X-ray crystallography and consisted of 11.
viously ignored. In 3 0.25H2O, the water oxygen atom has an
3
occupancy factor of 0.50 but could be refined with an aniso-
tropic thermal factor. Surprisingly, its two hydrogen atoms were
detected in the final Fourier maps and included in the structure
factor calculations with occupancies 0.50 each, and not refined.
In 11 H2O, the oxygen atom of the solvent molecule has an
3
occupancy factor of 1.00, but its hydrogen atoms were not
detected in the final Fourier maps and were ignored. In
5 CH2Cl2 and 5 CHCl3, the solvent molecules are ordered
3
3
and were treated normally. In 2 and 3 0.25H2O, the hydrogen
3
cis-[PtI(NO3)(PPh3)2] (12). KI (13.3 mg, 0.080 mmol) was
added to a solution of 5 (130.3 mg, 0.15 mmol) in CH2Cl2 (5 mL)
and CHCl3 (5 mL). The mixture was refluxed for 4 h and then
filtered hot. The filtrate was taken to dryness yielding a yellow-
-orange solid. The crude product was recrystallized twice from
dichloromethane-diisopropyl ether, yielding 12 as a yellow
powder (45.6 mg, 30%). Anal. calcd for C36H30I-
atoms of the CH3 groups belonging to the acetato ligands were
detected in the final Fourier maps and not refined. All the other
hydrogen atoms were placed in their ideal positions (C-H =
˚
0.97 A), with the thermal parameter U being 1.10 times that of
the atom to which they are attached, and not refined. All the
non-hydrogen atoms of the eight compounds were refined with
anisotropic thermal parameters. For chiral 2, full refinement of
NO3P2Pt CHCl3 (1027.95): C, 43.23; H, 3.04; N, 1.36. Found:
the correct structure enantiomorph led to R2=0.028 and R2w
0.043, full refinement of the wrong one led to R2=0.096 and
=
3
C, 42.98; H, 3.33; N, 1.32. IR (KBr): 1497 and 1272 (νNO3). δP
-0.5 (1P, d, JPP 14, JPtP 3819), 15.5 (1P, d, JPP 14, JPtP 3596).
cis-[PtCl(NO2)(PPh3)2] (13). AgNO2 (87.8 mg, 0.57 mmol)
was added to a solution of 1 (100.3 mg, 0.13 mmol) in CHCl3:
CH2Cl2 1:1 (10 mL). The mixture was refluxed for 3 days, hot
filtered, and the filtrate was evaporated to dryness yielding 13 as
R
2w=0.163. For chiral 3 0.25H2O, full refinement of the correct
3
(14) SAINT Reference manual; Siemens Energy and Automation: Madison,
W1, 1994-1996.
(15) Sheldrick, G. M. SADABS, Empirical Absorption Correction Pro-
a
white solid (105.2 mg, 99%). Anal. calcd for
€
€
gram; University of Gottingen, Gottingen, Germany, 1997.
C36H30NClO2P2Pt (801.12): C, 53.97; H, 3.77; N, 1.75. Found:
C, 53.56; H, 4.42; N, 1.86. IR (KBr): 1410 and 1335 (νNO2). δP
-0.5 (1P, d, JPP 20, JPtP 2883), 12.2 (1P, d, JPP 20, JPtP 3949).
ESI-MS: m/z 719 ([Pt(PPh3)2]þ, 50%), 754 ([PtCl(PPh3)2]þ,
100), 765 ([Pt(NO2)(PPh3)2]þ, 90), 824 ([M þ Na]þ, 15), 1625
([2 M þ Na]þ, 50). Crystals suitable for X-ray diffraction were
obtained by slow diffusion of diisopropyl ether into a chloro-
form solution.
(16) Frenz, B. A. Comput. Phys. 1988, 2, 42–48.
(17) Crystallographic Computing 5; Oxford University Press: Oxford, U.K.,
1991; Chapter 11, p 126.
(18) Sheldrick, G. M. SHELXS 86. Program for the solution of crystal
€
€
structures; University of Gottingen, Gottingen, Germany, 1985.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.
cis-[Pt(NO2)2(PPh3)2] (14). The reaction was performed un-
der an argon atmosphere in a Schlenk tube. Solid AgNO2
(230.8 mg, 1.50 mmol) was added to a solution of 1 (94.5 mg,
0.12 mmol) in dried CHCl3 (7 mL), and the mixture was left
under reflux for 15 h. The mixture was filtered hot and the
filtrate was taken almost to dryness, yielding a very light yellow
solid, filtrated and washed with diisopropyl ether (85.6 mg,
88%). Anal. calcd for C36H30N2O4P2Pt (811.67): C, 53.27; H,
(13) Boag, N. M.; Mohan Rao, K.; Terrill, N. J. Acta Cryst. Sect. C 1991,
C47, 1064–1065.

126 Inorganic Chemistry, Vol. 49, No. 1, 2010
Table 1. Crystallographic Data
compound
Rigamonti et al.
2
3 0.25H2O
3
4 CHCl3
3
5 CH2Cl2
3
formula
M
C38H33ClO2P2Pt
814.18
C160H146O17P8Pt4
3369.09
orthorhombic
P212121
10.4372(7)
18.6546(12)
34.9948(22)
90
90
90
6813.6(9)
2
3348
C37H31Cl4NO3P2Pt
936.51
triclinic
P1
11.1572(5)
11.6921(5)
14.6755(7)
92.144(1)
96.166(1)
106.110(1)
1824.1(1)
2
C37H32Cl2N2O6P2Pt
928.62
monoclinic
P21/n
11.9715(7)
20.0334(11)
15.4580(9)
90
95.190(1)
90
crystal system
space group
orthorhombic
P212121
10.1657(6)
17.6477(10)
18.5574(10)
90
90
90
3329.2(3)
4
1608
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
U/A
Z
F(000)
3692.1(4)
4
1832
920
1.705
Dc/g cm-3
crystal dimensions/mm
1.624
0.25 ꢀ 0.35 ꢀ 0.45
1.642
0.05 ꢀ 0.45 ꢀ 0.50
1.671
0.32 ꢀ 0.34 ꢀ 0.37
3
0.12 ꢀ 0.20 ꢀ 0.25
μ (Mo KR)/cm-1
44.65
42.94
43.04
41.16
minimum and maximum transmission factors
T/K
0.677-1.000
296
0.71073
ω
0.50
15
3960
6.00
3.00-29.00
full sphere
112229; 8960
0.0286
0.028, 0.043
0.017
8500
397
0.988
0.556-1.000
296
0.71073
ω
0.50
10
2580
6.50
3.00-28.00
full sphere
131789; 16557
0.0508
0.044, 0.063
0.028
15201
856
1.007
0.594-1.000
296
0.71073
ω
0.50
20
3013
5.00
3.00-29.00
full sphere
60430; 11538
0.0246
0.031, 0.048
0.022
9926
469
1.012
0.822-1.000
296
0.71073
ω
0.50
20
3300
6.00
3.00-28.00
full sphere
104889; 9948
0.0296
0.031, 0.047
0.020
7911
451
1.039
λ (Mo KR)
scan mode
frame width/deg
time per frame/s
no. of frames
detector-sample distance/cm
θ range
reciprocal space explored
no. of reflections (total; independent)
Rint
final R2 and R2w indicesa (F2, all reflections)
conventional R1 index (I > σ(I))
reflections with I > 2σ(I)
no. of variables
goodness of fitb
Compound
5 CHCl3
7
11 H2O
13
3
3
formula
M
C
37H31Cl3N2O6P2Pt
C36H30Br2P2Pt
879.50
monoclinic
P21/c
32.6290(21)
9.6539(6)
19.8452(13)
90
93.810(1)
90
C36H30I2OP2Pt
989.49
triclinic
P1
9.8799(5)
10.5936(6)
17.8664(9)
86.690(1)
78.110(1)
69.880(1)
1718.0(2)
2
C36H30ClNO2P2Pt
801.14
monoclinic
P21/c
9.5838(2)
17.2369(3)
19.3496(3)
90
99.869(1)
90
963.07
monoclinic
P21/n
12.0353(6)
19.9799(10)
15.8339(8)
90
95.330(1)
90
3791.0(3)
4
1896
crystal system
space group
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
U/A
Z
F(000)
6237.4(8)
8
3392
3149.2(1)
4
1576
936
1.913
Dc/g cm-3
crystal dimensions/mm
1.687
0.03 ꢀ 0.03 ꢀ 0.25
1.873
0.08 ꢀ 0.35 ꢀ 0.45
1.690
0.21 ꢀ 0.23 ꢀ 0.48
3
0.20 ꢀ 0.25 ꢀ 0.45
μ (Mo KR)/cm-1
40.81
72.10
60.22
47.20
minimum and maximum transmission factors
T/K
0.571-1.000
296
0.71073
ω
0.50
30
2220
5.00
3.00 - 29.00
full sphere
99490; 13130
0.0719
0.050, 0.057
0.034
0.275-1.000
150
0.71073
ω
0.50
20
2880
6.00
3.00 - 26.00
full sphere
113116; 13475
0.0567
0.069, 0.116
0.048
8896
740
1.002
0.496-1.000
296
0.71073
ω
0.50
20
2400
5.00
3.00 - 29.00
full sphere
48198; 11669
0.0286
0.059, 0.096
0.034
7302
370
0.986
0.499-1.000
150
0.71073
ω
0.40
10
1320
6.00
3.00 - 26.00
full sphere
25153; 7362
0.0550
0.063, 0.086
0.038
4577
388
0.974
λ (Mo KR)
scan mode
frame width/deg
time per frame/s
no. of frames
detector-sample distance/cm
θ range
reciprocal space explored
no. of reflections (total; independent)
Rint
final R2 and R2w indicesa (F2, all reflections)
conventional R1 index (I > σ(I))
reflections with I > 2σ(I)
no. of variables
8373
460
0.976
goodness of fitb
a R2 = [Σ(|Fo2 - kFc2|/ΣFo2], R2w = [Σw(Fo2 - kFc2)2/Σw(Fo2)2]1/2
.
b [Σw(Fo2 - kFc2)2/(No - Nv)]1/2, where w = 4Fo2/σ(Fo2)2, σ(Fo2) = [σ2(Fo2) þ
(pFo2)2]1/2, No is the number of observations, Nv is the number of variables, p = 0.02 for 4 CHCl3, 5 CH2Cl2, 5 CHCl3, and 11 H2O, and p = 0.03 for 2,
3 0.25H2O, 7, and 13.
3
3
3
3
3

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 127
structure enantiomorph led to R2=0.044 and R2w=0.063, full
refinement of the wrong one led to R2=0.096 and R2w=0.153. In
the final Fourier maps, the maximum residuals were 1.94(13)
7 analytically pure. We interpreted these facts as the
observation of the stepwise substitution of Cl by Br,
and consequently assigned the two doublets to
cis-[PtBrCl(PPh3)2] (6) unambiguously. Attempts to iso-
late it failed, because of the very similar solubility of 1, 6,
and 7 (in the Experimental Section we reported the
conditions which give the highest concentration of 6 in
the mixture). cis-[PtBr(NO3)(PPh3)2] (9), cis-[PtClI-
(PPh3)2] (10) and cis-[PtI(NO3)(PPh3)2] (12) were ob-
tained pure only after careful recrystallization, but in
rather low yields. The synthesis of cis-[Pt(NO2)2(PPh3)2]
(14) was found to be rather “tricky”, since this compound
was often impure of the oxidation product cis-[Pt(NO3)2-
(PPh3)2] (5); only performing the reaction under a strict
argon atmosphere we managed to obtain 14 analytically
pure.
The cis configuration of compounds 2, 3, 4, 5, 7, 11, and
13 was unambiguously assigned by X-ray structural
determinations (see below). The cis geometry of the mixed
cis-[PtXY(PPh3)2] complexes was confirmed by the pre-
sence, in their 31P NMR spectra, of two doublets, due to
the nonequivalent P atoms. An IR criterion8,26,28 was also
used: an intense band at around 550 cm-1, probably a PC3
overtone, has been reported to be typical of cis-Pt(PPh3)2
complexes, it being absent, or very weak, in the trans
derivatives. This criterion was found to hold each time we
could compare it with X-ray structures or 31P NMR
evidence.
-3
-3
˚
˚
˚
˚
e A at 0.73 A from Pt, 3.01(26) e A at 0.83 A from Pt,
3
3
-3
-3
˚
1.71(18) e A at 0.63 A from Pt, 1.18(20) e A at 0.08 A from
˚
˚
˚
3
3
-3
-3
˚
˚
˚
˚
Cl2, 1.44(44) e A at 1.03 A from P1, 2.63(78) e A at 0.62 A
3
3
from Pt, 2.42(73) e A at 0.55 A from Pt, and 2.41(50) e A-3 at
-3
˚
˚
˚
3
3
˚
0.78 A from Pt, for 2, 3 0.25H2O, 4 CHCl3, 5 CH2Cl2,
3
3
3
5 CHCl3, 7, 11 H2O, and 13, respectively. Minimum peaks
3
3
(holes), in the same order, were -1.00(13), -2.63(26),
-1.33(18), -0.96(20), -1.28(44), -3.35(78), -2.35(73), and
-1.94(50) e A-3, respectively. CCDC 741800-741807 contain
˚
3
the supplementary crystallographic data for this paper.
Computational Details. Calculations were performed with the
Gaussian 03 program package.19 All geometries were fully
optimized using the B3LYP functional20 and the def2-SVP split
valence basis set with polarization functions on all atoms.21,22 In
particular, an extra set of f functions with exponent 0.66813 has
been used for Pt. I and Pt have been treated as a 25- and an
18-electron system, respectively, with relativistic effective core
potentials (ECP’s) taken from the literature.23,24 The natural
bond orbital (NBO) analysis25 was used for evaluating the s
character of the hybrid orbitals involved in the Pt-P bonds in
the different complexes.
Results and Discussion
Preparation and Characterization. The majority of the
compounds were prepared by metathesis reaction, treat-
ing cis-[PtCl2(PPh3)2] (1) with the appropriate silver or
potassium salt. cis-[PtI2(PPh3)2] (11) was prepared by
reaction of [PtI2(cod)] (cod = cyclooctadiene) with
PPh3. This reaction, however, gave a mixture of cis/trans
derivatives. Also the reported preparation of 11 by treat-
ment of 1 with NaI26 gave, in our hands, a substantial
amount of the trans isomer. Pure samples of the cis isomer
were obtained by careful crystallization of these mixtures.
Interestingly, refluxing an ethanol suspension of these
mixtures with excess PPh3 gave pure trans-[PtI2(PPh3)2],
in contrast with the behavior of the cis/trans dichloro
complexes, for which a similar treatment gives rise to
complete isomerization to 1.27
The synthesis of cis-[PtBr2(PPh3)2] (7) was carried out
following the same synthetic procedure of 1,11 from
K2[PtBr4], prepared in situ by the addition of a large
excess of KBr added to K2[PtCl4]. If the synthesis of 7 was
performed using lower amounts of KBr, we detected an
intermediate. Under these conditions, the 31P NMR
spectra of the crude products showed the presence of
the singlets of 1 and 7 and two doublets of the new
compound. The concentrations of 1 and the latter dimin-
ished, increasing the amount of KBr, and eventually
disappeared when a large excess was used, giving
X-ray Structures. We have performed the X-ray struc-
tural determinations of compounds 2, 3 0.25H2O,
4 CHCl3, 5 CH2Cl2, 5 CHCl3, 7, 11 H2O, and 13. OR-
3
3
3
3
3
TEP views of compounds 3, 11, and 13 are presented in
Figures 1-3; pictures of the other compounds can be
found in the Supporting Information. Relevant bond
lengths and angles, together with those of compound
1,29 are collected in Tables 2-4.
All complexes present a square planar coordination of
the Pt atom, with minor distortions. In 3 the two acetato
and in 5 the two nitrato groups are almost perpendicular
to the coordination plane and in an anticonfigu-
ration with respect to it, giving rise to chirality.30 In
3 0.25H2O the elementary cell contains both enantio-
3
meric arrangements.
Bond lengths and angles are within the normal ranges,
the Pt-P bonds are shorter while the Pt-Cl and Pt-O
(acetato and nitrato) are longer than those found in the
corresponding trans isomers.8,13,31,32
A careful inspection of the structures reveals an inter-
esting feature that to our knowledge has never been
described previously. In fact there is a relationship
(28) (a) Brune, H. A.; Ertl, J.; Grafl, D.; Schmidtberg, G. Chem. Ber.
1982, 115, 1141–1153. (b) Brune, H. A.; Ertl, J. Liebigs Ann. Chem. 1980, 928–
937.
(29) Fun, H.-K.; Chantrapromma, S.; Liu, Y.-C.; Chen, Z.-F.; Liang, H.
Acta Cryst. Sect. E 2006, E62, m1252–m1254.
(30) For a discussion on the chirality of square planar complexes see, for
instance: (a) Gullotti, M.; Pacchioni, G.; Pasini, A.; Ugo, R. Inorg, Chem.
1982, 21, 2006–2014. (b) Pasini, A.; De Giacomo, L. Inorg. Chim. Acta 1996,
248, 225–230. (c) Ano, S. O.; Intini, F. P.; Natile, G.; Marzilli, L. G. Inorg. Chem.
1999, 38, 2989–2999. (d) Chifotides, H. T.; Dunbar, K. R. Chem. Eur. J. 2008,
14, 9902–9913. (e) Ranaldo, R.; Margiotta, N.; Intini, F. P.; Pacifico, C.; Natile, G.
Inorg. Chem. 2008, 47, 2820–2830.
(20) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785–
789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(21) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–
3305.
(22) (a) Feller, D. J. Comput. Chem. 1996, 17, 1571–1586. (b) Schuchardt,
K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.;
Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045–1052.
(23) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M. J. Chem.
Phys. 2003, 119, 11113–11123.
(24) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123–141.
(25) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–
926.
(31) Basato, M.; Biffis, A.; Martinati, G.; Tubaro, C.; Venzo, A.; Ganis,
P.; Benetollo, F. Inorg. Chim. Acta 2003, 355, 399–403.
(32) Johanson, M. H.; Otto, S. Acta Cryst. Sect. C 2000, C56, e12–e15.
(26) Mastin, S. H. Inorg. Chem. 1974, 13, 1003–1005.
(27) Anderson, G. K.; Cross, R. J. Chem. Soc. Rev. 1980, 9, 185–215.

128 Inorganic Chemistry, Vol. 49, No. 1, 2010
Rigamonti et al.
Figure 1. ORTEP view of one independent molecule of cis-[Pt(AcO)2-
(PPh3)2] (3 0.25H2O). Ellipsoids are drawn at the 30% probability;
hydrogen atoms and the water molecule are omitted for clarity.
3
Figure 3. ORTEP view of cis-[PtCl(NO2)(PPh3)2] (13). Ellipsoids are
drawn at the 30% probability; hydrogen atoms are omitted for clarity.
˚
Table 2. Main Distances (A) and Angles (deg) of the X2 Compounds 1, 7, and
11 H2Oa
3
1b, X = Cl
7, X = Br
11 H2O,
3
X = I
molecule 1 molecule 2 molecule 1 molecule 2
Pt-P1
Pt-P2
Pt-X1
Pt-X2
2.252(1)
2.271(1)
2.363(1)
2.329(1)
2.271(1)
2.261(1)
2.351(1)
2.348(1)
99.93(4)
87.35(4)
86.25(4)
87.73(4)
167.58(4)
171.13(4)
34.1(1)
2.264(2)
2.285(2)
2.475(1)
2.451(1)
98.95(6)
84.02(5)
90.01(5)
87.41(3)
175.00(5)
169.77(5)
14.7(2)
2.278(2)
2.294(2)
2.485(1)
2.473(1)
99.19(7)
86.69(5)
87.71(5)
87.36(3)
171.54(6)
169.41(7)
34.1(3)
2.277(1)
2.292(1)
2.656(1)
2.634(1)
97.83(5)
85.17(4)
90.64(3)
86.78(1)
174.87(4)
170.06(3)
-14.3(2)
43.7(2)
P1-Pt-P2 99.12(3)
P2-Pt-X1 83.73(3)
X2-Pt-P1 89.82(3)
X1-Pt-X2 87.56(3)
P1-Pt-X1 175.57(3)
P2-Pt-X2 170.41(3)
Ac
Bd
12.6(1)
-59.5(1)
47.8(1)
60.7(2)
46.7(2)
a P1 trans to X1 for all compounds. b Adapted from ref 29. c A =
lowest X2-Pt-P-C torsion angle of phosphine A. d B = lowest
X1-Pt-P-C torsion angle of phosphine B.
Figure 2. ORTEP view of cis-[PtI2(PPh3)2] (11 H2O). Ellipsoids are
3
drawnatthe 30%probability;hydrogenatomsandthewatermolecule are
omitted for clarity.
Strucural Cis and Trans Influences. In order to study the
“structural” cis and trans influences of ligands X and/or
Y, we must compare the Pt-P bond distances of the
various complexes. To render the discussion easier, this is
presented in a table format (Table 5), where we have
collected the Pt-P bond distances of the ClX complexes,
the Pt-P mean values of the X2 complexes, and three
Pt-P mean values taken from the literature.29,33,34
Although the individual bond lengths display significant
differences, we decided to use the mean values for X2
derivatives in order to compare these results with the
spectroscopic data in solution (one NMR signal); the
different bond lengths in the solid state are due to crystal
between the conformations of the phosphine groups and
Pt-X/Y bond lengths. In the case of the X2 complexes
(with the exception of molecule 2 of 1 and molecule 2 of 7),
the two phosphine groups display different conforma-
tions: the ipso carbon atom C (in bold in Scheme 1) of
phosphine A is almost eclipsed to the X ligand cis to it: the
X-Pt-P-C torsion angles vary from
2
to 14°
(Tables 2-4); the lowest X-Pt-P-C torsion angles of
phosphine B are much larger (34-54°). In the PtX2
complexes which present this feature, the shorter Pt-P
bond is that of phosphine A (i.e., the P atom involved in
such eclipsed conformation), while the longer Pt-X bond
is trans to such phosphine. In the structures of the three
PtClX complexes 2, 4, and 13, phosphine A (the one with
the eclipsed C atom) is always cis to Y. We think that such
adopted molecular conformation allows to optimizing
the π-π interaction between two phenyl rings of the two
phosphines.
(33) Yahav, A.; Goldberg, I.; Vigalok, A. Inorg. Chem. 2005, 44, 1547–
1553.
(34) (a) Hannu-Kuure, M. S.; Komulainen, J.; Oilunkaniemi, R.;
ꢁ
Laitinen, R. S.; Suontamo, R.; Ahlgren, M. J. Organomet. Chem. 2003,
ꢁ
666, 111–120. (b) Hannu, M. S.; Oilunkaniemi, R.; Laitinen, R. S.; Ahlgren, M.
Inorg. Chem. Commun. 2000, 3, 397–399.

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 129
˚
Table 3. Main Distances (A) and Angles (deg) of the X2 Compounds 3 0.25H2O,
Optimized Geometries. We have performed gas-phase
theoretical calculations on the investigated complexes in
order to draw further insight on the structural trans and
cis influence series. Selected results of geometry optimiza-
tion of cis-[PtX2(PPh3)2] and cis-[PtClX(PPh3)2] com-
plexes by B3LYP/def2-SVP calculations are reported in
Tables 6 and 7, respectively. Comparison with the avail-
able corresponding experimental results indicates that the
3
a
5 CH2Cl2, and 5 CHCl3
3
3
3 0.25H2O, X = AcO
3
5, X = NO3
molecule 1 molecule 2 5 CH2Cl2
3
5 CHCl3
3
Pt-P1
2.240(1)
2.243(1)
2.070(3)
2.059(3)
97.90(4)
87.41(8)
2.233(1)
2.250(1)
2.079(3)
2.050(3)
96.44(4)
86.96(8)
89.48(8)
87.16(11)
173.97(9)
174.08(8)
2.239(1)
2.244(1)
2.088(2)
2.092(2)
97.33(2)
86.58(4)
91.17(5)
84.96(6)
174.43(4)
171.49(5)
2.237(1)
2.247(1)
2.092(2)
2.097(2)
96.96(2)
87.41(5)
91.08(6)
84.56(7)
174.65(5)
171.96(6)
1.37(9)
Pt-P2
Pt-O1
˚
Pt-O3/O4
P1-Pt-P2
P2-Pt-O1
adopted method reproduces accurately (within 0.02 A)
the Pt-O distances, in particular for cis-[Pt(NO3)2-
(PPh3)2] (5). All other interatomic distances involving Pt
O3/O4-Pt-P1 89.16(8)
O1-Pt-O3/O4 85.44(11)
174.07(9)
P2-Pt-O3/O4 172.64(8)
˚
are overestimated by 0.02-0.08 A, a typical lengthening
of metal-ligand bonds generally obtained by gradient-
corrected density functional theory (DFT) methods.36 An
exception to such lengthening is for cis-[PtCl(NO2)-
P1-Pt-O1
Ab
Bc
7.30(17)
33.41(18)
-49.06(16) 4.58(11)
-49.06(16) -47.70(9) -49.76(10)
˚
(PPh3)2] (13), in which Pt-P2 is 0.112 A longer than
a P1 trans to O1 for all compounds, P2 trans to O3 for 3 and to O4
the experimental value and Pt-N is underestimated
by 0.034 A.
for 5. b A = lowest O3/O4-Pt-P1-C torsion angle of phosphine A.
c B = lowest O1-Pt-P2-C torsion angle of phosphine B.
˚
Geometry optimization of the X2 complexes allows to
reproduce the experimental finding concerning the rela-
tive orientation of the phenyl rings and the consequent
deviation from symmetry of the coordination geometry of
the metal atom. In all cases, except for X=Ph, we have
obtained molecular conformations where atom X2 is
nearly eclipsed to the ipso C atom bonded to P1
(phosphine A of Scheme 1), while X1 forms much larger
torsion angles with all the ipso C atoms bonded to P2 of
phosphine B (see Table 6, torsions A and B, respectively).
In agreement with the experimental structures, the Pt-P1
bonds are systematically shorter than the Pt-P2 bonds.
The exception is cis-[Pt(Ph)2(PPh3)2] (22), where inter-
ference of the phenyl ligands with the PPh3 groups
stabilizes a conformation in which both Ph ligands are
located nearly midway the two positions assumed by the
substituents in the other complexes.
By looking at the Pt-P1 (or Pt-P2) distances of X2
complexes, we see that the structural trans influence series
reported above is confirmed by our calculations, with the
order Ph > NO2 > I > Br > Cl > AcO g NO3.
As for the ClX complexes, optimization has been
performed starting from cis-[PtCl2(PPh3)2] (1). Because
of the observed nonequivalence of the two Pt-Cl bond
distances, we have substituted alternatively the Cl posi-
tions with an X group. The conformation with X cis to
phosphine A was found more stable by up to 3.9 kcal/mol,
and this is in fact the only one observed in the available
experimental structures of the mixed complexes.
˚
Table 4. Main Distances (A) and Angles (deg) of ClX Compounds 2, 4 CHCl3,
3
and 13a
2, X = AcO 4 CHCl3, X = NO3 13, X = NO2
3
Pt-P1
2.244(1)
2.249(1)
2.345(1)
2.070(2)
98.77(2)
87.58(2)
2.252(1)
2.243(1)
2.362(1)
2.109(1)
98.78(2)
87.30(2)
89.73(4)
84.28(4)
173.65(2)
171.15(4)
-14.89(8)
-51.54(7)
2.260(1)
2.284(1)
2.335(1)
2.110(4)
97.39(5)
88.77(5)
90.28(11)
83.55(11)
173.83(5)
172.12(11)
7.48(21)
-42.70(18)
Pt-P2
Pt-Cl
Pt-O1/N
P1-Pt-P2
P2-Pt-Cl
O1/N-Pt-P1 89.32(5)
Cl-Pt-O1/N
P1-Pt-Cl
84.44(5)
172.36(2)
P2-Pt-O1/N 171.80(5)
Ab
Bc
-2.18(18)
54.50(9)
a P1 trans to Cl for all compounds; P2 trans to O1 for 2 and 4 and to
N for 13. b A = lowest O1/N-Pt-P1-C torsion angle of phosphine A.
c B = lowest Cl-Pt-P2-C torsion angle of phosphine B.
packing effects as well as the phoshine conformations, as
described above, while in solution a mean distance with a
symmetric geometry is expected. Appropriate comments
and the deductions drawn from such values are inserted in
the table.
To begin with, let us consider only the mean Pt-P bond
lengths of the X2 complexes. Following the usually ac-
cepted hypothesis4,9,10 that the cis influence has a lower
relevance, we obtain the “structural” trans influence
series PhSe > I > Br > Cl > NO3 ∼ AcO > F. The
trans influence can also be observed, for the few data
available, from the Pt-P2 bond distances of the mixed
ClX complexes, which decreases in the order 1 > 2 > 4 ∼
13, giving another trans influence series Cl > AcO >
NO3 ∼ NO2. The difference of the position of AcO and
NO3 between the two series demonstrates that the cis
contribution cannot be neglected. The cis influence is in
fact evident if we look at the Pt-P1 bond lengths of
the mixed ClX derivatives, which decrease in the order 1
> 13 > 4 > 2, giving the cis influence order Cl > NO2 >
NO3 > AcO.35
Examination of Table 7 reveals that in such complexes
the Pt-P1 bonds (trans to Cl) are virtually unvaried with
˚
respect to the Pt-P1 bond of the Cl2 complex (2.320 A),
suggesting that the presence of an X ligand cis to P1 has
no substantial influence (the exceptions are for X=Ph,
and, in much lesser extent, NO2). The Pt-P2 distances
(trans to X) are also almost unchanged with respect to the
corresponding Pt-P2 distances in the X2 complexes,
excepted for Ph, highlighting the nonobservable struc-
tural cis influence. Then, the order of the structural trans
influence obtained for the X2 complexes is confirmed also
(35) Comparison with the sequences obtained in the previous paper8
holds only for NO2 > NO3 > AcO; the position of Cl changes according to
the class of complexes considered, suggesting that the actual influences of a
ligand depend on its environment, and this will be discussed later.
(36) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Func-
tional Theory, 2nd ed.; Wiley-VCH: New York, 2000.
(37) De Jong, F.; Bour, J. J.; Schlebos, P. P. J. Inorg. Chim. Acta 1988,
154, 89–93.

130 Inorganic Chemistry, Vol. 49, No. 1, 2010
Rigamonti et al.
Table 5. Summary of the Pt-P Bond Distances
a Mean value.
˚
Table 6. Selected Bond Lengths (A) and Torsion Angles (deg) in Computed
influence is detectable and compare it with the Cl2 (1)
and the Ph2 (22) complexes. Comparison with Cl2 indi-
cates that the Pt-P1 bond length in ClPh is shortened by
B3LYP/def2-SVP cis-[PtX2(PPh3)2] Compounds
X
Pt-P1
Pt-P2
Pt-X1
Pt-X2
Aa
8.8
Bb
˚
0.027 A, showing the lower cis influence of Ph with respect
˚
NO3 (5)
AcO (3)
Cl (1)
2.301
2.306
2.320
2.333
2.353
2.370
2.459
2.312
2.318
2.337
2.354
2.375
2.397
2.459
2.094
2.079
2.393
2.534
2.739
2.107
2.069
2.091
2.072
2.368
2.506
2.707
2.090
2.069
-48.7
49.0
50.7
to Cl. On the other hand, Pt-P2 is elongated by 0.178 A,
-4.7
-5.7
-5.1
-3.8
-13.8
29.7
due to the much higher trans influence of Ph with respect
to Cl. Such elongation shows also the lower magnitude of
the cis over the trans influence of Ph. Comparison with
the Ph2 complex shows that Pt-P1 in ClPh is shortened
Br (7)
I (11)
NO2 (14)
Ph (22)
50.9
51.4
-38.8
30.1
˚
˚
by 0.166 A and Pt-P2 is elongated by 0.056 A, due to the
substitution of a Ph with a lower trans-influent and higher
cis-influent Cl ligand. As a consequence of such different
cis/trans influence of the two ligands, the Pt-P bond
lengths in the ClPh complex present a very large asym-
a Lowest X2-Pt-P1-C torsion angle (phosphine A of Scheme 1).
b Lowest X1-Pt-P2-C torsion angle (phosphine B of Scheme 1).
˚
Table 7. Selected Bond Lengths (A) and Torsion Angles (deg) in Computed
˚
metry, differing by 0.222 A.
B3LYP/def2-SVP cis-[PtClX(PPh3)2] Compounds
Spectroscopic Cis and Trans Influences. Table 8 reports
the 31P{1H} NMR spectra of some cis-[PtX2(PPh3)2] and
cis-[PtXY(PPh3)2] complexes. In the case of the latter,
assignment of the two resonances was made by compar-
ison of their chemical shifts with those of the X2 deriva-
tives. As a proof of correctness, in the case of
cis-[PtCl(AcO)(PPh3)2] (2) we have performed a P-H
HMBC (heteronuclear multibond correlation) experi-
ment which showed a cross-peak between the resonance
of the acetato protons and the P resonance at 3.1 ppm, in
agreement with the assignment of this resonance to P
trans to AcO.
X
Pt-P1 Pt-P2 Pt-O/N/C Pt-Cl
Aa
Bb
NO3 (4)
AcO (2)
Cl (1)
Br (6)
I (10)
2.322
2.318
2.320
2.319
2.318
2.314
2.293
2.311
2.319
2.337
2.352
2.374
2.396
2.515
2.092
2.072
2.368
2.503
2.699
2.076
2.052
2.383
2.388
2.393
2.394
2.398
2.393
2.399
-3.3
-2.7
-5.7
-2.7
-1.5
-11.1 -48.6
11.2 -48.3
52.4
48.7
50.7
50.7
50.8
NO2 (13)
Ph (21)
a Lowest Cl-Pt-P2-C torsion angle (phosphine B of Scheme 1).
b Lowest X-Pt-P1-C torsion angle (phosphine A of Scheme 1).
considering the mixed ClX complexes: Ph > NO2 > I >
Br > Cl > AcO g NO3.
It is interesting to analyze the case of the ClPh complex
(21), essentially the only one where the structural cis
If we shall follow, again, a first approach considering
only the X2 complexes, and neglecting, in the first
approximation, the cis influence, or assuming that it has
an irrelevant weight, the 1JPtP values of these complexes

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 131
give the “apparent” order of trans influence (increasing
coupling constants, decreasing trans influence):
The contributions of Cl cannot be obtained with the
present treatment, but they can be used as references,
thus d and δ are the cis and trans contributions of NO3
relative to Cl. Performing similar calculations for all
complexes, we obtain the contributions of the two influ-
ences of the various ligands. The equations considered
and the results are reported in Table 8.
Ph > Me > SPh > SePh > NO2 > I > N3 > Br > Cl
> AcO > CF3COO > F > NO3
However, inspection of the data of the mixed XY
complexes shows that the cis influence must not be
neglected. For instance, let us consider cis-[PtCl(NO3)-
Note that we obtain two similar but different values for
each contribution, depending on the subtraction consid-
ered. For instance, for d, 1 - 2=-169 Hz, whereas 3 - 4=
-155 Hz. These differences are too large to be only due to
the reproducibility of the measure of the coupling con-
stants ((3 Hz). We believe that these differences arise
mainly from the fact that both subtractions reduce to
b - d, eliminating N and especially a, whose constancy is a
working hypothesis, useful for the present treatment. In
particular, the value of N can be safely considered con-
stant as explained above, while, on the contrary, the
values of a (and also R) may depend on the overall
coordination set and can vary according to the nature
of the other ligands (X and Y). In fact, while d obtained
from 1 - 2 is referred to the Cl2 complex, subtracting
3 - 4 gives d referred to the mixed Cl(NO3) complex. Thus
the contributions of the phoshines, a, and of the ligand
X/Y depend not only on their nature, but also on that of
all their neighbors. The additivity scheme must therefore
be applied with great care in the present case, where both
the cis and trans influences are taken into account.
Despite this drawback, however, some consistency is
obtained if we refer to a homogeneous class of com-
pounds, such as the ClX derivatives. In this case, the
values of c and χ, d and δ, etc. discriminate between the cis
and trans influences and give a sequence of both and,
more importantly, an insight into their relative weights.
Since we have subtracted 1 - 2, 3 - 4, etc., negative values
mean higher contributions to 1JPtP of X compared to Cl
and then a lower influence.
1
(PPh3)2] (4): while JPtP2 is higher than that of 1, in
agreement with the fact that the nitrato ligand displays
a trans influence lower than that of Cl, 1JPtP1 is also higher
than that of 1, suggesting that the low cis influence8 of
NO3 is operative.
In our previous paper on trans-[PtXY(PPh3)2],8 we con-
firmed the already noticed6a,38,45 additivity of the contribu-
tions of the cis influences to 1JPtP. We tried and apply an
additive scheme in this case to both cis and trans influences.
1
The coupling constants JPtP of each P nucleus in
compounds cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] can
be considered as the sum of all the contributions given by
the cis and trans ligands PPh3, X and/or Y to a basic value
(N), considered as a constant starting point for this class
of square planar, neutral Pt(II) compounds, and which
represents the energy of the interaction between the
nuclear spins of Pt and P. This value is perturbed by the
presence of the ligands. If we call a, b, c, etc. the cis
contributions and R, β, χ, etc. the trans contributions
(Here, a, R for PPh3; b, β, for Cl; c, χ, for AcO; d, δ, for
NO3; e, ε for Br; f, φ for I; g, γ for NO2; h, η for PhS; i, ι for
F3CS; l, λ, Me; m, μ, Ph. See Table 8.), the Pt-P coupling
constant of 1 is
N þ a þ b þ β ¼ 3673 Hz
ð1Þ
ð2Þ
ð3Þ
the 1JPtP values of cis-[PtCl(NO3)(PPh3)2] (4) are
Under the above-discussed approximations, we obtain
the following series:
N þ a þ d þ β ¼ 3842 Hz ðfor P1Þ
and
cis
N þ a þ b þ δ ¼ 3858 Hz ðfor P2Þ
f < e < b < i < d≈h < g < c < l, m
whereas the Pt-P coupling constant of both P nuclei of
cis-[Pt(NO3)2(PPh3)2] (5) is
I > Br > Cl > SCF3 > NO3≈SPh > NO2≈AcO > Me > Ph
N þ a þ d þ δ ¼ 4013 Hz
ð4Þ
Subtracting 1 - 2, or 3 - 4, will reduce to b - d, the
difference between the cis contributions of NO3 and Cl.
trans
(38) Yamashita, F.; Kuniyasu, H.; Terao, J.; Kambe, N. Inorg. Chem.
2006, 45, 1399–1404.
λ, μ < η < γ < ι < β≈ε < φ < χ < δ
(39) Kirij, N. V.; Tyrra, W.; Naumann, D.; Pantenburg, I.; Yagupolskii,
Yu. L. Z. Anorg. Allg. Chem. 2006, 632, 284–288.
(40) Cobley, C. J.; Pringle, P. G. Inorg. Chim. Acta 1997, 265, 107–115.
(41) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.;
Giering, W. P. Organometallics 1999, 18, 474–479.
(42) Eaborn, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc., Dalton Trans.
1978, 357–368.
(43) Braterman, P. S.; Cross, R. J.; Young, G. B. J. Chem. Soc., Dalton
Trans. 1977, 1892–1897.
(44) Kreutzer, P. H.; Schorpp, K. T.; Beck, W. Z. Naturforsch. 1975, 30b,
544–549.
Ph, Me > SPh > NO2 > SCF3 > AcO > Br≈Cl > I > NO3
Some points are worthy of discussion:
(i) There is a span of approximately 2000 and 1000
Hz for the trans and cis influences, respectively.
The latter, therefore, has a lower weight, but our
data show that it is by no means irrelevant (in
some cases it appears to be even higher).
(ii) We confirm the almost opposite trend of the cis
and trans influence series.4
(45) Additivity has been noted also for the kinetic cis effect, for example:
Jaganyi, D.; Hofmann, A.; van Eldik, R. Angew. Chem., Int. Ed. 2001, 40,
1680–1683.

132 Inorganic Chemistry, Vol. 49, No. 1, 2010
Rigamonti et al.
Table 8. 31P NMR Data for Some cis-[PtX2(PPh3)2] and cis-[PtXY(PPh3)2] Complexesa

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 133
Table 8. Continued
a CDCl3 solutions, unless otherwise stated; δP values in parts per million versus aqueous H3PO4 (for compounds 21, 22, and 24, different references
were used, and here, the δP values have been recalculated to agree with all other data); 2JPP are in the 12-20 Hz range, see the Experimental Section. b a, b,
c, etc. are the contributions of the cis ligands; R, β, χ, etc. are the contributions of the trans ligands (a, R = PPh3; b, β = Cl; c, χ = AcO; d, δ = NO3; e, ε =
Br; f, φ = I; g, γ = NO2; h, η = PhS; i, ι = F3CS; l, λ = Me; m, μ = Ph); and N is the basic value; such contributions are given mainly as the difference
from Cl2. c C6D6. d CD2Cl2. e CH2Cl2. f C6H6.
(iii) The cis influence series is similar to that obtained
in the previous work,8 the positions of NO2 and
AcO are inverted, but the contributions of these
ligands are very similar in both cis and trans
derivatives.
(iv) The values of the cis contributions obtained here
are about 30% higher than those calculated in
the previous paper8 from the Pt-P coupling
constants of the trans-[PtXY(PPh3)2] isomers,
showing that the actual contribution of a given
ligand depends on the system under study.
(v) The trans influence series parallels that obtained
above considering only the X2 complexes, with
the notable exception of the position of AcO,
which becomes higher in the series when the cis
influence is taken into account, clear-cut evi-
dence that the cis influence cannot be neglected.
(vi) The trans sequence of the halides (Br g Cl g I) is
unexpected, but their contributions are similar
and low and can be affected by some uncertainty.
The sequence I > Br > Cl seems well established
in the literature,46 but such reports do not take
into account the contribution of the cis influence.
While the order (I > Br > Cl) of the kinetic trans
effect is well-established,47,48 the fact that, on the
contrary and according to our results, their trans
influences are similar stresses the different sig-
nificance and origin (ground state versus transi-
tion state stabilization) of the two phenomena.49
(vii) The cis contributions of the halides are higher
than their trans contributions. As argued in the
previous paper,8 the cis influence of a ligand is
related to the lowering of the positive charge on
Pt induced by that ligand. The halides display a
high cis influence because, being both σ and π
donor,50 they decrease the positive charge on Pt
efficaciously (as shown below), decreasing the
P f Pt donation.51
(viii) There is no net parallelism between the struc-
tural and spectroscopic series, as a confir-
mation of the well-known dependence on
the physical method used to establish such
series.4,46
In order to check the general validity of the additivity of
the contributions of the various X ligands calculated with
respect to cis-[PtClX(PPh3)2] (Table 8), let us take into
consideration a compound without Cl as a ligand,
cis-[PtBr(AcO)(PPh3)2] (8). Referring to compounds 3
and 7, we obtain ε - χ=76 or 81 (confirming the higher
trans influence of Br than AcO) and e - c=-293 or -288
(confirming the lower cis influence of Br than AcO).
However, if the contributions are calculated using com-
pound 1 as reference, we obtain ε - χ=(β - χ) - (β - ε)=
(130) - (8)=122 Hz and e - c=(b - c) - (b - e)=(-266)
- (55)=-321 Hz. Although the calculated values of χ and
c reflect the general trend, they are rather different. The
same discrepancies are obtained with the other com-
pounds without Cl, 9 and 12. This fact clearly confirms
that the actual contributions of the ligands depend from
the nature of its neighbor Y, indicating that there is
cooperation between the two cis ligands.
ꢁ
(46) See, for instance, Bennett, M. A.; Bhargava, S. K.; Priver, S. H.;
Willis, A. C. Eur. J. Inorg. Chem. 2008, 3467–3481, and references therein.
(47) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry,
3rd ed.; John Wiley and Sons: New York, 1995.
(48) The rather higher trans effect of I compared with Cl is the basis for a
stereoselective synthesis of cisplatin (cis-[PtCl2(NH3)2]): [PtCl4]2- is first
allowed to react with KI to give [PtI4]2- which, upon treatment with
ammonia, gives cis-[PtI2(NH3)2] in very high isomeric purity. cis-[PtCl2-
(NH3)2] is then obtained by metathesis reactions. See: Dhara, S. C. Indian J.
Chem. 1970, 8, 193–194. The direct reaction of [PtCl4]2- with ammonia gives
cisplatin contaminated by the trans isomer.
Natural Bond Orbital (NBO) Analysis. To give a pos-
sible explanation of the different trans/cis influence of the
(50) Bridgeman, A. J.; Gerloch, M. J. Chem. Soc., Dalton Trans. 1995,
197–204.
(51) Strong donor ligands display also a strong (kinetic) cis effect because,
lowering the positive charge on Pt, they reduce its electrophilicity. See: (a)
Hofmann, A.; Dahlernburg, L.; van Eldik, R. Inorg. Chem. 2003, 42, 6528–
6538. (b) Jaganyi, D.; Reddy, D.; Gertenbach, J. A.; Hofmann, A.; van Eldik, R.
Dalton Trans. 2004, 299–304.
(49) To our knowledge there is at least one example in which the order of
the trans influence is opposite to that of the trans effect, see: Wendt, O. F.;
Elding, L. I. J. Chem. Soc., Dalton Trans. 1997, 4725–4731.

134 Inorganic Chemistry, Vol. 49, No. 1, 2010
Rigamonti et al.
Table 9. Pt and P Natural Orbitals Involved in the Pt-P Bonds and Natural Charges on Pt, qPt (in e), in Computed B3LYP/def2-SVP cis-[PtX2(PPh3)2] Compounds
bond orbital
X
Pt-P1
Pt-P2
qPt
I (11)
Br (7)
Cl (1)
AcO (3)
NO3 (5)
NO2 (14)
Ph (22)
0.06
0.14
0.20
0.45
0.44
0.27
0.19
33.72%(sd1.06
33.51%(sd1.06
31.47%(sd1.08
32.44%(sd1.07
30.46%(sd1.12
)
)
)
)
)
Pt þ 66.28%(sp2.94
Pt þ 66.49%(sp2.93
Pt þ 68.53%(sp2.88
Pt þ 67.56%(sp3.01
Pt þ 69.54%(sp3.08
)
)
)
)
)
32.98%(sd1.08
32.84%(sd1.10
30.82%(sd1.10
32.01%(sd1.11
29.45%(sd1.10
)
)
)
)
)
Pt þ 67.02%(sp2.96
Pt þ 67.16%(sp2.95
Pt þ 69.18%(sp2.91
Pt þ 67.99%(sp3.05
Pt þ 70.55%(sp3.10
)
)
)
)
)
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
Table 10. Pt and P Natural Orbitals Involved in the Pt-P Bonds and Natural Charges on Pt, qPt (in e), in Computed B3LYP/def2-SVP cis-[PtClX(PPh3)2] Compounds
bond orbital
X
Pt-P1
Pt-P2
qPt
I (10)
Br (6)
Cl (1)
AcO (2)
NO3 (4)
NO2 (13)
Ph (21)
33.13%(sd1.10
33.51%(sd1.10
33.51%(sd1.06
32.98%(sd1.00
33.24%(sd0.99
31.92%(sd0.85
31.71%(sd0.85
)
)
)
)
)
)
)
Pt þ 66.87%(sp2.83
Pt þ 66.49%(sp2.88
Pt þ 66.49%(sp2.93
Pt þ 67.02%(sp2.98
Pt þ 66.76%(sp3.06
Pt þ 68.08%(sp3.00
Pt þ 68.29%(sp2.69
)
)
)
)
)
)
)
0.13
0.17
0.20
0.34
0.32
0.25
0.21
P1
P1
P1
P1
P1
P1
P1
33.06%(sd1.04
32.84%(sd1.10
31.56%(sd1.18
32.60%(sd1.19
)
)
)
)
Pt þ 66.94%(sp3.00
Pt þ 67.16%(sp2.95
Pt þ 68.44%(sp2.88
Pt þ 67.40%(sp2.94
)
)
)
)
P2
P2
P2
P2
ligands, we have performed a natural bond orbital (NBO)
analysis25 on the optimized structures discussed in a
previous section. We were interested, in particular, to
analyze the bonding orbitals used by Pt and P in the Pt-P
bonds. One bond coupling constants are in fact supposed
to be dominated by the Fermi contact term,52 which takes
into account the s-character of the bonding orbitals and
the electron densities of the ns valence orbitals at the
nuclei. In the complexes studied in this paper, both the
s-character of the hybrid orbitals and the electron density
of the 3sp3 orbital of phosphorus and sd of platinum have
been considered to be rather similar throughout,3,4,7b,7c,53
but we wanted to have a deeper look at these points, since
these factors can be related to the different trans and cis
influences of the ligands in the complexes examined.
The results, reported in Tables 9 and 10 for cis-[PtX2-
(PPh3)2] and cis-[PtClX(PPh3)2] complexes, respectively,
are far to be conclusive, also because in the cases of the
long Pt-P bonds, the involved orbitals are interpreted by
NBO analysis as lone pairs rather than bonding orbitals,
so they have not been included in Tables 9 and 10. Some
observations are, however, noteworthy:
(ii) In cis-[PtClX(PPh3)2] complexes (Table 10), on
the other hand, the s-character of the Pt-P1
bond orbitals shows a clear trend, increasing in
the order I = Br < Cl < AcO < NO3 < NO2 =
Ph for the sdx orbitals of Pt and decreasing in the
same order (with the exception of NO2 and Ph)
for the spy orbitals of P. As a result, the total
s-contribution to the Pt-P1 bond is nearly con-
stant and equal to 33% for all ligands, except for
NO2 (34%) and Ph (36%). The slight increase of
the s-character of Pt-P1 for X =NO2 and Ph
explains its shortening with respect to the value
of the other complexes, as obtained from geo-
metry optimization calculations. It is to be noted
that the sequence of increasing s-character of the
sdx orbitals of Pt parallels approximately the
order of cis influence obtained by NMR mea-
surements reported here and in the previous
work8 (the order of AcO and NO3 is inverted):
the greater the s-character of such orbitals, the
greater the 1JPtP1 value. It therefore appears that
the cis influence is principally related with the
s-character of the Pt valence orbitals. It is also to
be stressed that, while the cis influence cannot be
completely revealed by looking at the Pt-P1
bond length of the optimized structures of the
mixed complexes, it manifests itself clearly in the
nature of the associated bonding orbitals.
(i) For cis-[PtX2(PPh3)2] compounds (Table 9), the
s-character of both P and Pt orbitals involved in
the Pt-P bonds is almost the same. Averaging on
Pt-P1 and Pt-P2 bonds, the hybridization de-
grees of sdx and spy orbitals vary from x=1.07 to
1.11 and from y=2.90 to 3.09, respectively, and in
particular, they do not show any clear correlation
with the structural and spectroscopic influences
reported above. It therefore seems that for such
complexes the s-character of Pt and P orbitals
involved in the Pt-P bonds does not represent a
relevant factor in explaining the trans influence
of the ligands.
(iii) As for the Pt-P2 bond orbitals of cis-[PtClX-
(PPh3)2] complexes (Table 10), where the trans
influence of the ligands comes into play, the
s-character displays an opposite trend of the
sdx orbitals, as it decreases in the order Br >
Cl > AcO > NO3, while the spy orbitals on the
P2 atom do not show a clear trend. As a result,
the total s-contribution to the Pt-P2 bond
shows only a slight decrease from Br (33%) to
NO3 (32%). Such an outcome, as discussed in
previous point i, confirms that other factors
(52) Jameson, C. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New
York, 1987; pp 89-131.
(53) For instance: (a) Allen, F. H.; Pidcock, A. J. Chem. Soc. (A) 1968,
2700–2704. (b) Allen, F. H.; Sze, S. N. J. Chem. Soc. (A) 1971, 2054–2056.

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 135
seem principally responsible of the trans influ-
ence of the ligands.
this approximation, the Pt-P bonds become weaker as the
metal atom becomes less positively charged,8 a trend con-
firmed by natural bond orbital analysis on the B3LYP/def2-
SVP optimized structures. Such a computational study has
also pointed out that the s-character of Pt and P hybrid
orbitals involved in the Pt-P bonds can be related to the cis
influence of the ligands, while it does not represent a relevant
factor in explaining their trans influence.
Further information derived from the natural orbital
analysis of electron density concerns the atomic charges,
whose values for Pt, qPt, are also reported in Tables 9 and
10 for cis-[PtX2(PPh3)2] and cis-[PtClX(PPh3)2] com-
plexes, respectively. These quantities, which reflect the
total electron density associated with the Pt atom show an
interesting trend, which is in good agreement with what
argued on the basis of the NMR measurements. The
lowest positive charges are in fact obtained with the
halides, in the order I < Br < Cl, i.e. for the ligands
which display the highest cis influences, while the largest
qPt values correspond to the low cis-influencing ligands
AcO and NO3. Low positive charges on Pt are also
obtained in the complexes with NO2 and Ph, for which
a larger covalence degree of the Pt-X bond can be
expected. All these changes in qPt point out the relative
importance of also the electrostatic contribution to the
Pt-P interaction. In the case of halides and anionic
ligands (NO3, AcO), such contribution cannot be ne-
glected, while it appears less relevant for complexes with
metal-ligand bonds with a higher degree of covalency
(ligand=Ph, NO2).
It is interesting to compare the results here presented with
those reported by our group in the previous paper,8 where we
have evaluated the cis influences of some anionic ligands
1
analyzing the JPtP values of the trans isomers of some
compounds of the present study. Although the cis influence
series are similar, the magnitudes calculated from the cis
isomers are about 30% higher than those obtained pre-
viously. Clearly these weights depend on the system under
study; for instance in the X2 complexes, a ligand, say AcO, is
cis to a phosphine in the trans isomer, but cis to another AcO
in the cis derivative.
A related point is worth noting. As stated above, the
weights of both influences of a given ligand depend not only
on its nature but also on its neighborhood; for instance, the
weight of AcO in a mixed complex X(AcO) varies according
to the ligand X, suggesting a synergism between the two cis
ligands. One can speculate that, since these weights are in Hz,
they describe how a ligand X perturbs (modifies) the energy
of the interaction between the nuclear spins of Pt and P.
Clearly such perturbation must depend not only on the
nature of X but also on its environment, the charge on Pt,
etc., ultimately on the overall coordination set.
Conclusions
We have shown that in compounds of the type cis-[PtXY-
(PPh3)2] the cis and trans influences cooperate in determining
the experimental bond lengths and one bond coupling con-
1
stants JPtP of the Pt-P bonds. We have also obtained an
Despite this limitation, however, both cis and trans influ-
ences have been clearly detected, although precise quanti-
tative evaluation of their magnitudes requires further studies,
which we aim to undertake in the near future.
insight on the magnitude of both influences from the values
of such constants and found that the weight of the cis
influence is not irrelevant and must be taken into account
in establishing the trans influence scale.
A commonly accepted interpretation of the trans influence
is that two ligands trans to each other compete for a same
hybrid orbital, which spans across the metal atom. As for the
cis influence, it has been rationalized assuming that, in Pt(II),
Pt-P back-donation can be considered little relevant,54
therefore, the Pt-P bonds can be described mainly as dona-
tion from an sp3 orbital of P to an hybrid orbital of Pt. Under
Acknowledgment. This work has been supported by the
ꢀ
Ministero dell’Universita e della Ricerca Scientifica
e Tecnologica.
Supporting Information Available: X-ray crystallographic
data of compounds 2, 3 0.25H2O, 4 CHCl3, 5 CH2Cl2,
5 CHCl3, 7, 11 H2O, and 13 in CIF format and figures of the
3
3
3
3
3
(54) In Pt(II) complexes, there is a neat correlation between 1JPtP and the
donor power of aryl phosphines, suggesting that π back-donation to P is
negligible, see ref 40.
crystal structures of the compounds 2, 4 CHCl3, 5 CH2Cl2,
5 CHCl3, and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
3
3
3