The trans effect and trans influence of triphenyl arsine in platinum(II) complexes. A comparative mechanistic and structural study

Article abstract of DOI:10.1039/b202100p

The kinetics and mechanism of the reaction between [PtI₃(AsPh₃)]^- and pyridine in acetonitrile solvent has been studied by use of stopped-flow spectrophotometry. Substitution of iodide trans to arsine is irreversible under the conditions used and takes place via parallel direct and solvolytic pathways. By comparing the rate constants of the direct pathway with literature values of the corresponding phosphine and stibine complexes the following trans effect series can be obtained: Ph₃Sb > Ph₃P > Ph₃As. The crystal and molecular structures of reactant and product have been determined. Based on Pt-I distances for the investigated arsine complex and literature data for the corresponding phosphine and stibine complexes the following trans influence series was derived: Ph₃P ≥ Ph₃As > Ph₃Sb. To the best of our knowledge this is the first comparison of all the three heavier pnictogens as donor atoms using strictly analogous complexes with identical cis-ligands.

Full text of DOI:10.1039/b202100p

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The trans effect and trans influence of triphenyl arsine in
platinum(II) complexes. A comparative mechanistic and
structural study†
Nikodem Kuznika,b and Ola F. Wendt*a
a Inorganic Chemistry, Department of Chemistry, Lund University, P.O. Box 124,
S-221 00 Lund, Sweden. E-mail: ola.wendt@inorg.lu.se
b Chemistry Department, Silesian University of Technology, Krzywoustego 6, 44-101 Gliwice,
Poland
Received 27th February 2002, Accepted 5th June 2002
First published as an Advance Article on the web 9th July 2002
The kinetics and mechanism of the reaction between [PtI3(AsPh3)]Ϫ and pyridine in acetonitrile solvent has been
studied by use of stopped-flow spectrophotometry. Substitution of iodide trans to arsine is irreversible under the
conditions used and takes place via parallel direct and solvolytic pathways. By comparing the rate constants of the
direct pathway with literature values of the corresponding phosphine and stibine complexes the following trans effect
series can be obtained: Ph3Sb > Ph3P > Ph3As. The crystal and molecular structures of reactant and product have
been determined. Based on Pt–I distances for the investigated arsine complex and literature data for the
corresponding phosphine and stibine complexes the following trans influence series was derived: Ph3P ≥ Ph3As >
Ph3Sb. To the best of our knowledge this is the first comparison of all the three heavier pnictogens as donor atoms
using strictly analogous complexes with identical cis-ligands.
use. All other reagents and solvents were purchased and used
Introduction
without further purification. Elemental analysis was performed
Pnictogen donor ligands and especially phosphines are ubiqui-
by Mikro Kemi AB, Uppsala, Sweden. NMR spectra were
tous in the coordination and organometallic chemistry of
recorded on a Varian Unity 300 spectrometer. Chemical shifts
platinum().1 However, the number of complexes investigated
are given in ppm downfield from TMS using residual solvent
become more scarce as you proceed down Group 15. There are
peaks as internal standard.
a few examples of crystal structures of stibine complexes of the
platinum group metals, and crystal structures of platinum()
Synthesis
stibine complexes were only recently published.2–4 trans
Influences and trans effects and their origin continue to intrigue
chemists and is still an active area of research.5 To probe the
trans effect of different ligands the reactions of heterocyclic
amines and other nucleophiles with complexes of the type
[PtCl3(L)]Ϫ have been investigated for neutral ligands L.6
Previous work has also aimed at subdividing the trans effect
into its σ- and π-components by means of an analysis of the
nucleophilic discrimination. To our knowledge there is still
no strictly analogous series of complexes with the heavier
pnictogen donor atoms (P, As, Sb) that has been investigated
with respect to structure and reactivity in this sense and
where the only factor being changed is the nature of the trans
donor. We therefore thought it timely to complete our earlier
investigation on [PtI3(EPh3)]Ϫ (E = P, Sb) complexes with
data on the corresponding arsine complex.3b Here we report on
the kinetics and mechanism for the reaction of [PtI3(AsPh3)]Ϫ
with pyridine together with the crystal structures of (Bu4N)-
[PtI3(AsPh3)] and [PtI2(py)(AsPh3)].
(Bu4N)[PtI3(AsPh3)]([Bu4N][1]). A sample of (Bu4N)2[Pt2I6]
(0.191 g, 0.117 mmol) was dissolved in 10 cm3 CH2Cl2. A
solution of AsPh3 (0.069 g, 0.225 mmol) in 15 cm3 CH2Cl2 was
added dropwise and the resulting solution was stirred at room
temperature overnight. The solvent was evaporated and the
crude product recrystallised from CH2Cl2–diethyl ether (1 : 1)
yielding 0.209 g (83%) of 1. Anal. calc. for C34H51AsI3NPt: C
36.3; H 4.6. Found: C 36.6; H 4.6%. 1H NMR (CDCl3, 299.79
MHz): δ 1.02 (t, J = 7.3 Hz, 12H), 1.44–1.55 (m, 8H), 1.63–1.76
(m, 8H), 3.31–3.41 (m, 8H), 7.30–7.45 (m, 9H), 7.63–7.80
(m, 6H).
trans-[PtI2(AsPh3)(py)] (2). A sample of [Bu4N][1] (23.7 mg,
0.021 mmol) was dissolved in 10 cm3 MeCN. Pyridine (py, 0.017
cm3) was added and the solution was stirred for 1 h. The solvent
was evaporated to 1/3 of the original volume and the solution
was kept in a freezer overnight. The solution was decanted and
the orange crystals formed were washed twice with cold MeCN
and dried in vacuo. Yield: 8 mg (46%). Anal. calc. for C23H20As-
I2NPt: C 33.1; H 2.4. Found: C 32.9; H 2.6%. 1H NMR (CDCl3,
299.79 MHz): δ 7.34–7.36 (m, 12 H), 7.72–7.78 (m, 6 H),
9.03–9.07 (m, 2 H, 3JPt–H = 36 Hz, o-H).
Experimental
General procedures and materials
(Bu4N)2[Pt2I6] was prepared according to the literature.7
Pyridine was refluxed over KOH pellets and distilled prior to
Structure determination
Single crystals of [Bu4N][1] and 2 suitable for X-ray diffraction
were obtained by slow evaporation at room temperature of the
solvent from a dichloromethane–hexane solution (ca. 1 : 1) of
[Bu4N][1] and an acetonitrile solution of complex 2.
Crystal data and data collection details are given in Table 1.
The intensity data sets for [Bu4N][1] and 2 were collected at
† Electronic supplementary information (ESI) available: observed
pseudo-first order rate constants for reaction (4) in acetonitrile at dif-
ferent temperatures and ligand concentrations. Equilibrium absorb-
ances for the solvolysis of complex 1 and 2 at 25 ЊC in acetonitrile at
different iodide and pyridine concentrations, respectively. See http://
www.rsc.org/suppdata/dt/b2/b202100p/
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J. Chem. Soc., Dalton Trans., 2002, 3074–3078
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202100p

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Table 1 Details of data collection and refinement
Compound
[Bu4N][1]
2
Chemical formula
C34H51AsI3NPt
1124.5
C23H20AsI2NPt
834.21
MW/g molϪ1
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
a/Å
b/Å
c/Å
9.788(2)
18.474(4)
21.956(4)
98.67(3)
3925(2)
4
6.79
12.869(3)
11.283(2)
16.842(3)
101.83(3)
2393.6(8)
4
β/Њ
V/Å3
Z
µ/mmϪ1
9.822
Collected reflections
Unique reflections
No. of parameters
R(F), wR(F 2) (I > 2σ(I ))
wR(F 2) (all data)
S
38886
12004 (Rint = 0.0906)
361
0.0453, 0.0939
0.1178
0.939
23939
7407 (Rint = 0.0672)
253
0.0481, 0.1128
0.1302
0.944
293 K with a Bruker SMART CCD system with a rotating
anode (Mo-Kα radiation, λ = 0.71073 Å) using ω-scans.8 The
intensities were corrected for Lorentz, polarisation and
absorption effects using SADABS.9 In both data sets the first
50 frames were collected again at the end to check for decay
and none was observed. All reflections were integrated using
SAINT.10 Both structures were solved by Patterson methods
and refined by full-matrix least-squares calculations on F 2
using SHELXTL5.1.11 Non-H atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were
constrained to parent sites, using a riding model.
or two-electron donors.6d,13 This method is sometimes
inconvenient due to the decreasing thermal stability of the
dinuclear complexes going down Group 15 and the general
difficulties in preparing and purifying them. Instead, using a
procedure that was earlier used for the analogous phosphine
and stibine complexes,3b the [Pt2I6]2Ϫ ion was cleaved by use of a
stoichiometric amount of AsPh3 giving 1 as the only product in
high yield. Compound 2 was made from 1 with an excess of
pyridine.
Crystal structures
Both models contain residual peaks with ∆ρ > 1 e ÅϪ3, but
they are situated less than 1.15 Å from an iodide or platinum
atom.
The anionic complex 1 is isostructural with the corresponding
phosphine and stibine analogues.3b Its structure is given in Fig.
1. The molecular structure of 2 is given in Fig. 2. In both cases
CCDC reference numbers 180736 and 180737.
See http://www.rsc.org/suppdata/dt/b2/b202100p/ for crystal-
lographic data in CIF or other electronic format.
Kinetics
The stopped-flow experiments were performed on an Applied
Photophysics Bio Sequential SX-17 MX, stopped-flow
spectrofluorimeter. The substitution of iodide in 1 by pyridine
was studied in acetonitrile solvent by observing the decrease in
absorbance at 366 nm. The complex solution (5× 10Ϫ5 mol
dmϪ3) containing excess iodide (5–20 × 10Ϫ3 mol dmϪ3) was
mixed directly in the stopped-flow instrument with an equal
volume of pyridine solution, resulting in a large excess of
entering ligand (1–10 × 10Ϫ2 mol dmϪ3), assuring pseudo
first-order conditions. The kinetic traces were fitted to single
exponentials using the software provided by Applied Photo-
physics,12 resulting in observed rate constants at different con-
centrations of leaving and incoming ligand. Rate constants are
given as an average of at least five runs. Time-resolved spectra
were also recorded on the Applied Photophysics instruments.
Variable-temperature experiments were performed between 288
and 318 K. Complete data are reported in ESI Table S1.
Fig. 1 Diamond drawing with atomic numbering of the structure of
the anionic complex 1. The ellipsoids denote 30% probability. The
tetrabutylammonium cation was omitted for clarity.
the coordination geometry around platinum is distorted square-
planar with angles ranging from 87.8 to 93.6Њ in 1 and from 87.9
to 92.6Њ in 2. The largest deviations from a least-squares plane
through PtI2XAs is 0.14 Å (I1 and I2) in 1 (X = I) and 0.057 Å
(I1 and I2) in 2 (X = N). The plane of the pyridine is almost
perpendicular (72.9Њ) to the coordination plane. Selected bond
distances and angles are given in Table 2.
UV/VIS equilibrium measurements
UV/VIS spectra were recorded on a Milton Roy 3000 diode
array spectrophotometer. The equilibrium absorbance in the
solvolysis of 1 and 2 according to reaction (1) below was
measured for different [IϪ] and [py], respectively. Complete data
are reported in ESI Table S2.
Solvolysis
Results
The UV/VIS spectrum of 1 dissolved in MeCN changes when
free iodide is added and one peak in the visible region at 363 nm
grows in. The spectral changes are best explained by a solvolytic
equilibrium according to eqn. (1).
Synthesis
Mixed platinum arsine complexes have earlier been prepared by
cleaving the dinuclear [PtCl(AsR3)(µ-Cl)]2 with neutral solvents
J. Chem. Soc., Dalton Trans., 2002, 3074–3078
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Table 2 Selected crystallographic distances (Å) and angles (Њ) with
estimated standard deviations
1 (X = I3)
2 (X = N1)
Pt1–I1
Pt1–I2
Pt1–X
Pt1–As1
As1–C11
As1–C21
As1–C31
2.6112(9)
2.6181(9)
2.6585(7)
2.3753(8)
1.965(7)
1.954(7)
1.964(6)
2.5855(8)
2.6062(8)
2.109(5)
2.3567(8)
1.946(6)
1.935(7)
1.941(6)
I1–Pt1–X
I2–Pt1–X
I1–Pt1–As1
I2–Pt1–As1
C11–As1–C21
C11–As1–C31
C21–As1–C31
90.27(2)
88.96(2)
93.60(2)
87.79(2)
106.8(3)
100.6(3)
99.7(3)
87.94(16)
88.34(16)
92.60(4)
91.15(4)
107.1(3)
100.3(3)
102.2(3)
Fig. 3 Equilibrium absorbance at 363 nm for the solvolytic reaction
(1) as a function of iodide concentration. T = 298.2 K. [Pt] = 1 × 10Ϫ4
mol dmϪ3
.
Reaction with pyridine
Reaction (4) was studied in MeCN:
(4)
Time-resolved spectra of the reaction are given in Fig. 4,
showing the presence of an isosbestic point at 338 nm.
Fig. 2 Diamond drawing with atomic numbering of the molecular
structure of complex 2. The ellipsoids denote 30% probability.
(1)
This is a fairly fast equilibrium (after a few minutes there are no
spectral changes with time) and its equilibrium constant, K1,
was determined by measuring the absorbance at different iodide
concentrations. Eqn. (2), where A∞ (the absorbance at infinite
[IϪ]), εM (the extinction coefficient of 3 at the path length used)
and K1 are the adjustable parameters, was fitted to the data.
(2)
Fig. 4 Time-resolved spectra for reaction (4). T = 298.8 K. [Pt] = 5 ×
10Ϫ5 mol dmϪ3, [py] = 0.100 mol dmϪ3, [IϪ] = 5 × 10Ϫ3 mol dmϪ3. Scans
obtained at 1.2, 3.6, 6.0, 8.4, 12 and 36 s.
The fit (Fig. 3) gave K1 = (3.4 0.4) × 10Ϫ3 mol dmϪ3
.
Reaction (4) is an equilibrium reaction with an approximate
equilibrium constant (as determined from the ratio of solvolysis
constants) of 13. During the kinetic experiments high iodide
and pyridine concentrations ([py] ≥ [IϪ] > 100[Pt]) were used to
avoid solvolysis and shift the equilibrium to the right. Thus,
there was no observable variation in absorbance amplitude with
pyridine concentration, indicating that the position of the equi-
librium does not change and is far to the right already at the
lowest pyridine concentrations. Plots of kobs vs. [py] at different
[IϪ] are linear with non-zero intercepts, cf. Fig. 5. The usual two-
term rate law, eqn. (5), was fitted to the data.
The initial UV/VIS spectrum of 2 in MeCN changes substan-
tially upon addition of pyridine with the largest absorbance
changes at 296 and 371 nm. This suggests a similar solvolytic
equilibrium, although complex 2 is less prone to solvolysis:
(3)
As described above, eqn. (2) was fitted to the data at the two
different wavelengths giving K3 = (2.6 0.5) × 10Ϫ4 mol dmϪ3
.
kobs = a ϩ k2[py]
(5)
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J. Chem. Soc., Dalton Trans., 2002, 3074–3078

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Table 3 Rate constants and activation parameters for reaction (4) at 25 ЊC in acetonitrile solvent
k (298.8 K)/MϪ1 sϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
k2-path
1.10 0.06
0.012 0.005
9.2 0.2
43.7 1.3
32.0 1.8
34.0 0.4
11.6 1.0
Ϫ98
Ϫ174
Ϫ112.6 1.2
Ϫ165
4
6
a-path (kϪ1
)
PtI3(PPh3)Ϫ (k2)a
PtI3(SbPh3)Ϫ (k2)a
145 10
4
a Ref. 3b.
kobs in (5) can most certainly be interpreted in a direct
associative attack at the metal centre (k2-term in (6)). This is
also supported by the activation parameters. The origin of the
intercept is somewhat less clear. One possible explanation is a
reversible reaction (kϪ2) but this can be ruled out on the
grounds that the intercept seems to decrease rather than
increase with increasing iodide concentration. Also, using the
value of the equilibrium constant K2, it is obvious that the
reverse reaction would give a negligible contribution to kobs
under the experimental conditions used. Thus the intercept
arises from a parallel solvent attack at the metal centre. Usually
the third term in eqn. (6) reduces to k1. If this were the case also
in the current system it would require the different intercepts in
Fig. 5 to be explained by experimental error. This seems rather
unlikely and probably the other rate constants in the third term
of (6) also make a contribution. An attempt to fit the full
expression in the rewritten form (7) gives large errors in all
constants but k2, which is not significantly altered as compared
to the linear fit.
Fig. 5 Observed pseudo first-order rate constants for reaction (4) as a
function of pyridine concentration at different iodide concentrations. T
= 298.8 K. (᭺) [IϪ] = 5 × 10Ϫ3 mol dmϪ3 (ᮀ) [IϪ] = 20 × 10Ϫ3 mol dmϪ3
.
(7)
Activation parameters were obtained by fitting the Eyring
equation to values of a and k2 at different temperatures. Rate
constants and activation parameters are given in Table 3
together with some literature values.
To conclude, the constant a can be ascribed to the solvent
path, but its full mechanistic significance cannot be resolved
with the current data. This also means that the activation
parameters associated with this pathway should be treated with
caution.
Discussion
Rate laws and mechanisms
Reactivity and trans effect
Scheme 1 depicts a substitution reaction involving a solvento
The trans effect is a well established concept in platinum()
chemistry and a lot of ligands have been investigated with
respect to this property. It is generally subdivided into a σ- and
a π-contribution.14 Strong σ-donation from a ligand, giving a
weaker trans bond (i.e. a high ground-state trans influence),
increases the rate of substitution. Charge delocalization in the
transition state helps to stabilise it and thus to increase the rate.
Strong π-acceptors typically give this kind of stabilisation, and
thus have a high trans effect.15
Complex
1 reacts with the same mechanism as the
corresponding phosphine and arsine complexes. Using the
k2-values allows for the first direct comparison of the trans
effect of the heavier pnictogens as donor atoms, cf. Table 3.
The reactivity of the AsPh3-complex is clearly lower than that
of the phosphine and stibine complexes, the order of reactivity
being:
Scheme 1
species and with full reversibility in all steps. The rate law
derived from this scheme, using the steady-state approximation
for the solvento species, is given in eqn. (6). The use of this
approximation is validated by the use of high [IϪ] together with
the fact that Fig. 4 shows a well-defined isosbestic point,
although the solvolysis constants show that 3 can exist in quite
high concentrations at low nucleophile concentrations.
Ph3Sb > Ph3P > Ph3As
This is in agreement with earlier indirect comparisons and
confirms the absence of a periodicity in the trans effects
down Group 15.3b,6d,16 The explanation is most likely a balance
between σ- and π-contributions.
(6)
Before continuing this discussion it is enlightening to look
at the trans influence sequence of the ligands. It has been
proposed that the trans influence be used as a measure of
Clearly the experimental rate law (5) must be some simpli-
fication of eqn. (6). The pyridine-dependent contribution to
J. Chem. Soc., Dalton Trans., 2002, 3074–3078
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the σ component of the trans effect thus separating the two
contributions.3b The classical way of distinguishing between
these two components is to use nucleophilic discrimination as a
criterion but the mechanistic ambivalence of the a value makes
this approach uninformative.17
To the best of our knowledge the present results complete the
first crystallographic comparison of all three heavy pnictogens
and gives the following sequence of trans influence with the
Pt–I distances in Å given in parentheses,3b based on Pt–I
distances trans to PPh3 and SbPh3 of 2.662(3) and 2.637(2) Å,
respectively.
Acknowledgements
We thank Prof. Åke Oskarsson for his help with the X-ray
crystallography. We are also grateful to Prof. Lars I. Elding
for a generous loan of the stopped-flow instrument and for
valuable discussions. Financial support from the Swedish
Research Council, the Crafoord Foundation and the Royal
Physiographic Society in Lund is gratefully acknowledged. N.
K. thanks the Swedish Institute for a fellowship.
References and notes
1 F. R. Hartley, The chemistry of platinum and palladium, Applied
Science Publishers, Barking, Essex, England, 1973.
Ph3P (2.662(3)) ≥ Ph3As (2.6585(7)) > Ph3Sb (2.637(2))
2 For a review on metal stibine complexes see: N. R. Champness and
W. Levason, Coord. Chem. Rev., 1994, 133, 115.
This is in reasonable agreement with the assessment made
using NMR coupling constants which gives a monotonic
decrease of trans influence on going down the group from
phosphorus.18 It can thus be concluded that the arsine is a
stronger σ-donor than SbPh3 but probably a weaker donor
than PPh3. Some results deviating from this picture have
been reported but usually cis effects as well as packing
effects cannot be ruled out in these cases. Thus, a recent
crystallographic comparison using the cis-PtCl2L2 series have
put AsPh3 and SbPh3 on par with each other3a,4d and also
stibines and phosphines have been considered to have equal
trans influences.3g
3 For crystal structures of metal-stibine complexes see: (a) O. F.
Wendt, A. Scodinu and L. I. Elding, Inorg. Chim. Acta, 1998, 277,
237; (b) O. F. Wendt and L. I. Elding, J. Chem. Soc., Dalton Trans.,
1997, 4725; (c) P. Sharma, A. Cabrera, M. Sharma, C. Alvarez,
J. L. Arias, R. M. Gomez and S. Hernandez, Z. Anorg. Allg. Chem.,
2000, 626, 2330; (d ) M. Mathew, G. J. Palenik and C. A. McAuliffe,
Acta Crystallogr., Sect. C, 1987, 43, 21; (e) R. Cini, G. Giorgi
and L. Pasquini, Inorg. Chim. Acta, 1992, 196, 7; ( f ) N. J. Holmes,
W. Levason and M. Webster, J. Chem. Soc., Dalton Trans., 1998,
3457; (g) T. Even, A. R. J. Genge, A. M. Hill, N. J. Holmes,
W. Levason and M. Webster, J. Chem. Soc., Dalton Trans., 2000, 655;
(h) A. Mentes, R. D. W. Kemmitt, J. Fawcett and D. R. Russell,
J. Organomet. Chem., 1997, 528, 59.
The lower reactivity of 1 is thus probably a ground state
effect based on the lower donating ability of the arsine
ligand as compared to the phosphine. In the arsine case
there seems to be no significant π-stabilisation of the transi-
tion state, as is the case of the stibine having a high
trans effect despite its inferior trans influence. This line of
reasoning is in agreement with a recent proposal by Elding
and co-workers to use the contribution of T ∆S‡ to ∆G‡ as
a measure of how well-ordered the transition state is.15 It
seems that a high entropy contribution is associated with
substitutions trans to ligands which derive their trans
effect mainly from π-acidity; thus substitution trans to
triphenylstibine has an entropy contribution at room temper-
ature of around 80%, whereas the corresponding value for
1 is 40%, similar to what is found in the triphenylphosphine
case.
4 For platinum arsine complexes see: (a) M. K. Cooper, P. J. Guerney
and M. McPartlin, J. Chem. Soc., Dalton Trans., 1980, 349;
(b) M. H. Johansson, S. Otto, A. Roodt and Å. Oskarsson, Acta
Crystallogr., Sect. B, 2000, 56, 226; (c) S. Otto and A. J. Muller,
Acta Crystallogr., Sect. C, 2001, 57, 1405; (d ) S. Otto and
M. H. Johansson, Inorg. Chim. Acta, 2002, 329, 135.
5 See for example: (a) T. G. Appleton, H. C. Clark and L. E. Manzer,
Coord. Chem. Rev., 1973, 10, 335; (b) A. Fischer and O. F. Wendt,
J. Chem. Soc., Dalton Trans., 2001, 1266; (c) S. Otto and L. I. Elding,
J. Chem. Soc., Dalton Trans., 2002, 2354; (d ) K. M. Anderson and
A. G. Orpen, Chem. Commun., 2001, 2682; (e) M. H. Johansson,
Å. Oskarsson, K. Lövqvist, F. Kiriakidou and P. Kapoor, Acta
Crystallogr., Sect. C, 2001, 57, 1053.
6 (a) R. Romeo and M. L. Tobe, Inorg. Chem., 1974, 13, 1991;
(b) B. P. Kennedy, R. Gosling and M. L. Tobe, Inorg. Chem., 1977,
16, 1744; (c) R. Gosling and M. L. Tobe, Inorg. Chim. Acta, 1980, 42,
223; (d ) R. Gosling and M. L. Tobe, Inorg. Chem., 1983, 22, 1235;
(e) M. L. Tobe, A. T. Treadgold and L. Cattalini, J. Chem. Soc.,
Dalton Trans., 1988, 2347.
To measure σ and π contributions to metal–P bonds of PR3
complexes in the ground state also the P–C distances and C–P–
C angles have been successfully used.19,20 As shown by Orpen
and co-workers19 these two effects almost cancel for Pt–PPh3
complexes and the average distances and angles in the com-
plexes are approximately the same as in free PPh3. Present data
for AsPh3 reveal an average As–C distance of 1.961(7) and
1.941(1) Å for 1 and 2, respectively, to be compared with the
value for free triphenylarsine of 1.957(9) Å.21 This could be
taken as further evidence for a similar σ/π ratio in the Pt–As
bond as compared to the Pt–P bond. Clearly the fact that the
more electron rich complex 1 has longer As–C bonds speaks in
favour of the σ* orbitals being involved in π-acceptance. It has
been shown earlier for all stibine complexes examined that
upon coordination the Sb–C distances decrease and C–Sb–C
angles increase, suggesting a different hybridisation scheme in
stibine complexes.3a,b,f,g One explanation offered is that stibines
are weaker π acceptors,3f but this was disputed by one of us
based on the higher trans effect of the stibine. We instead
suggest that the π acceptor function on the stibine is mainly of
5d-character.3b
7 P. L. Goggin, J. Chem. Soc., Dalton Trans., 1974, 1483.
8 BrukerAXS, SMART, Area Detector Control Software, Bruker
Analytical X-Ray System, Madison, Wisconsin, USA, 1995.
9 G. M. Sheldrick, SADABS, Program for absorption correction,
University of Göttingen, Germany, 1996.
10 BrukerAXS, SAINT, Integration Software, Bruker Analytical
X-Ray System, Madison, Wisconsin, USA, 1995.
11 G. M. Sheldrick, SHELXTL5.1, Program for structure solution and
least squares refinement, University of Göttingen, Germany, 1998.
12 Applied Photophysics Bio Sequential SX-17MV Stopped Flow ASVD
Spectrofluorimeter, software manual, Applied Photophysics Ltd.,
203/205 Kingston Road, Leatherhead, UK KT22 7PB.
13 R. J. Goodfellow and L. M. Venanzi, J. Chem. Soc., 1965, 7533.
14 (a) F. Basolo, J. Chatt, H. B. Gray, R. G. Pearson and B. L. Shaw,
J. Chem. Soc., 1961, 2207; (b) C. H. Langford and H. B. Gray,
Ligand Substitution Processes, W. A. Benjamin Inc., New York,
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15 M. R. Plutino, S. Otto, A. Roodt and L. I. Elding, Inorg. Chem.,
1999, 38, 1233.
16 T. P. Cheeseman, A. L. Odell and H. A. Raethel, Chem. Commun.,
1968, 1496.
17 See ref. 3b for a discussion of the drawbacks of the nucleophilic
discrimination approach in acetonitrile.
18 T. G. Appleton and M. A. Bennett, Inorg. Chem., 1978, 17, 738.
19 (a) B. J. Dunne, R. B. Morris and A. G. Orpen, J. Chem. Soc., Dalton
Trans., 1991, 653; (b) D. S. Marynick, J. Am. Chem. Soc., 1984, 106,
4064; (c) A. G. Orpen and N. G. Connelly, Organometallics, 1990, 9,
1206.
In conclusion it therefore seems that the lower trans effect
of arsines as compared to phosphines is explained by their
lower trans influence, but it cannot be ruled out that the
higher trans effect of the phosphine to some extent is due to
transition state effects. On the other hand, the lower trans effect
of arsine as compared to stibine is clearly an effect of the better
π-stabilisation of the transition state that the latter exerts.
20 As proposed in ref. 19 the P–C σ* orbital is involved in the
π acceptor function on phosphorus. See also discussion in ref 3b.
21 A. N. Sobolev, V. K. Belsky, N. Yu. Chernikova and F. Yu.
Akhmadulina, J. Organomet. Chem., 1983, 244, 129.
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