Experimental and Quantum Chemical Study of the Mechanism of an Unexpected Intramolecular Reductive Coupling of a Bridging Phosphido Ligand and a C 6F5 Group and the Reversible Oxidative Addition of PPh2C6F5

Article abstract of DOI:10.1021/om034236y

The behavior of intramolecular reductive coupling of a bridging phosphido ligand and a C₆F₅ group was investigated. The geometric and energetic reaction profiles in a vacuum were also discussed at the B3LYP/LANL2DZ level. The molecular structure of the transition metal diphenylphosphido complex was established using X-ray crystallography. Intrinsic reaction paths (IRP) are traced from the various transition structures to make sure that no further intermediates exist.

Full text of DOI:10.1021/om034236y

Organometallics 2004, 23, 1797-1810
1797
Exp er im en ta l a n d Qu a n tu m Ch em ica l Stu d y of th e
Mech a n ism of a n Un exp ected In tr a m olecu la r Red u ctive
Cou p lin g of a Br id gin g P h osp h id o Liga n d a n d a C6F 5
Gr ou p a n d th e Rever sible Oxid a tive Ad d ition of
#,|
P P h 2C6F 5
Naima Chaouche,†,‡ J uan Fornie´s,*,† Consuelo Fortun˜o,† Abdelaziz Kribii,‡
Antonio Mart´ın,† Paraskevas Karipidis,§ Athanassios C. Tsipis, and
Constantinos A. Tsipis*,§
Departamento de Qu´ımica Inorga´nica and Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, De´partement de Chimie,
Faculte´ des Sciences, Universite´ IbnTofail, B. P. 133, Kenitra, Morocco,
Laboratory of Applied Quantum Chemistry, Faculty of Chemistry,
Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece, and
Laboratory of Inorganic Chemistry, Department of Chemistry,
University of Ioannina, 451 10 Ioannina, Greece
Received October 15, 2003
The two-electron oxidation reactions of the [NBu4]2[(C6F5)2M(µ-PPh2)2M′(C6F5)2] (M ) M′
) Pt, 1a ; M ) M′ ) Pd, 1b; M ) Pt, M′ ) Pd, 1c) complexes using I2 as oxidant have been
investigated by experimental and electronic structure calculation methods. The geometric
and energetic reaction profiles in a vacuum have been investigated at the B3LYP/LANL2DZ
level. The first step of the oxidation processes involves the oxidative addition of I2 to one of
the four-coordinate M(II) metal centers yielding a five-coordinate intermediate with square
pyramidal stereochemistry. The oxidized [(I)L2M(µ-PH2)2M′L2]- (L ) CN) complex converts
to a more stable intermediate involving a planar dimetallacycle five-membered ring, which
through a reactant-like transition state surmounting a relatively low barrier of 18.5, 8.7,
and 8.9 kcal/mol for Pt2, PtPd, and Pd2 compounds, respectively, yields the final product. At
this stage an unusual intramolecular reductive coupling of one of the ancillary ligands with
a bridging phosphido ligand takes place, affording the “M(II)(µ-PH2)(µ-I)M′(II)” framework.
This process is exoergic by 11.4, 18.2, and 18.8 kcal/mol for Pt2, PtPd, and Pd2 compounds,
respectively. For the Pt2 and PtPd complexes removal of the iodide ligand from the oxidized
iodo complexes yields the very unusual [L2MIII(µ-PH2)2M′IIIL2] (L ) CN or C6F5) complexes.
In contrast, [L2PdIII(µ-PH2)2PdIIIL2] does not exist as local minima in the PES, but the [L2-
PdII(µ-PH2)PdII(PH2L)L] species was identified, resulting from the intramolecular reductive
coupling promoted by the iodide ligand abstraction. This species could be considered as an
intermediate for the reverse intramolecular oxidative addition of the PH2L ligand promoted
by iodide abstraction with Ag+ ions. Finally, the molecular structure of the [(C6F5)2Pt(µ-
PPh2)(µ-I)Pd(C6F5){PPh2(C6F5)}] complex has been established by X-ray crystallography.
In tr od u ction
cided several years ago to study the reaction of the
[NBu4]2[(C6F5)2M(µ-PPh2)2M′(C6F5)2] (M ) M′ ) Pt, 1a ;
M ) M′ ) Pd, 1b; M ) Pt, M′ ) Pd, 1c) complexes with
AgClO4, aiming to prepare polynuclear complexes of
silver coordinated with the platinate fragment acting
as chelating bidentate ligand (see Scheme 1b).
To our surprise, the reaction of 1a did not afford
the expected trinuclear Pt2Ag derivative, but the bi-
nuclear Pt(III)-Pt(III) complex formulated as [(C6F5)2-
Pt(µ-PPh2)2Pt(C6F5)2], 2, was isolated through an oxi-
dation process with the Ag+ ions acting as the oxidant3
The ability of the anionic perhalophenylplatinate
complexes to act as Lewis bases affording polynuclear
complexes with PtfM donor-acceptor bonds is well
documented1,2 (see Scheme 1a).
In the course of our current research interests on
transition metal diphenylphosphido complexes we de-
#
Dedicated to Professor J ose´ Vicente (Universidad de Murcia) on
the occasion of his 60th birthday.
| Polynuclear Homo- or Heterometallic Palladium(II)-Platinum(II)
Pentafluorophenyl Complexes Containing Bridging Diphenylphosphido
Ligands. 14. For part 13 see ref 58.
* Corresponding authors. E-mail: forniesj@unizar.es (J .F.). Tel:
(+2310) 997851. Fax: (+2310) 997851. E-mail: tsipis@chem.auth.gr
(C.A.T.).
(1) Fornie´s, J .; Mart´ın, A. In Metal Clusters in Chemistry; Braun-
stein, P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim
(Germany), 1999; Vol. 1, pp 417-443.
(2) Uso´n, R.; Fornie´s, J . Inorg. Chim. Acta 1992, 200, 165-177.
(3) Alonso, E.; Casas, J . M.; Cotton, F. A.; Feng, X. J .; Fornie´s, J .;
Fortun˜o, C.; Toma´s, M. Inorg. Chem. 1999, 38, 5034-5040.
† Universidad de Zaragoza-CSIC.
‡ Universite´ IbnTofail.
§ Aristotle University of Thessaloniki.
University of Ioannina.
10.1021/om034236y CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/18/2004

1798 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
Sch em e 1
Sch em e 2
hand, treatment of 1b or 1c with I2 (1:1 molar ratio) at
195 K does not render, as expected, the analogous Pt-
(III)-Pd(III) or Pd(III)-Pd(III) complexes, but the bi-
nuclear derivatives formulated as [NBu4][(C6F5)2M(µ-
PPh2)(µ-I)M′(C6F5)(PPh2C6F5)] (M ) M′ ) Pd, 3b; M )
Pt, M′ ) Pd, 3c) are isolated as yellow solids (Scheme
2b).
(see Scheme 1c). However, reacting the analogues 1b
and 1c with AgClO4, under similar conditions, no
analogous M(III)-M′(III) derivatives ([(C6F5)2M(µ-PPh2)2-
M′(C6F5)2] (M ) M′ ) Pd, 2b; M ) Pd, M′ ) Pt, 2c) could
be isolated, since decomposition of the resulting solu-
tions occurs, yielding dark mixtures from which the
aforementioned complexes were impossible to be sepa-
rated and identified. Complex 2 is very unusual because
it displays the platinum centers in square planar
coordination environments involving a coplanar Pt(III)-
Pt(III) bond (see Scheme 1c). On the other hand,
complexes 1a , 1b, and 1c exhibit a quite similar
electrochemical behavior.3 With this in mind we thought
it would be advisable to study their oxidation reactions
using I2 as an oxidant, aiming to obtain homo- or hetero-
binuclear complexes with the metal centers in formal
oxidation state higher than 2, the stability of the
bridging system “M(µ-PPh2)2M′” maintaining the integ-
rity of the binuclear fragment.
Here we report on the results of a detailed experi-
mental and quantum chemical study of the peculiar
behavior of the [NBu4]2[(C6F5)2M(µ-PPh2)2M′(C6F5)2] (M
) M′ ) Pt, 1a ; M ) M′ ) Pd, 1b; M ) Pt, M′ ) Pd, 1c)
complexes toward the oxidation with iodine, a complex
process accompanied by an unexpected intramolecular
reductive coupling followed by a reversible oxidative
addition reaction. Electronic structure calculations based
on density functional theory (DFT) methods threw light
on the mechanism of the respective reactions in a
vacuum, identifying possible reaction pathways and
providing both the geometric and energetic profile of the
reactions.
Aiming to throw some light on these electron transfer
processes, we have added I2 to CD2Cl2 solutions of 1a
and 1c (1:1 molar ratio) at 213 K and monitored the
processes by 19F NMR spectroscopy. In the first case of
the diplatinum species (Pt2), the colorless solution of 1a
becomes yellow upon addition of I2 and the spectrum
at 213 K shows only signals due to complex 2. By
increasing the temperature in 10-deg intervals up to 313
K, it can be observed that additional signals emerge at
263 K, with a concomitant decrease of the intensity of
the signals assigned to 2. At the end, only the signals
due to [NBu4][(C6F5)2Pt(µ-PPh2)(µ-I)Pt(C6F5)(PPh2C6F5)],
3a , are observed and the solution turns colorless.
Nevertheless, for 1c the spectrum at 213 K already
exhibits a large number of very broad and overlapping
signals, while at 293 K the pattern of the spectrum is
that observed for 3c. Because of the complexity of the
NMR spectra, no information with respect to intermedi-
ate species present at low temperature can be obtained.
Typically, the reaction of 1a involves an oxidative
addition of the I2 molecule to the platinum centers
followed by the elimination of iodide ligands, which
results in the formation of the neutral binuclear Pt(III)
complex 2, without changing the “Pt(µ-PPh2)2Pt” frame-
work. It is worth noting that at 213 K the only species
detected is 2. However, in the presence of iodide anions
and at higher temperatures complex 2 is not stable
enough, and it is immediately transformed to 3a through
the reductive coupling of a phosphido ligand with a
pentafluorophenyl group, affording a coordinated ter-
tiary phosphane (PPh2C6F5) ligand. In 3a the Pt(II)
coordination spheres are completed by the iodide ligand,
thus giving rise to the “Pt(II)(µ-PPh2)(µ-I)Pt(II)” frame-
work (see Scheme 2b). When the binuclear starting
material contains at least one palladium center, the
elusive Pt and/or Pd(III) complexes 2b,c, if formed, could
not be detected, even at low temperature, indicating that
the reductive coupling of the phosphido and pentafluo-
rophenyl groups takes place very fast.
Resu lts a n d Discu ssion
1. Syn th esis. The addition of I2 to a colorless solution
of 1a in CH2Cl2 (1:1 molar ratio) at 195 K renders
instantaneously a yellow solution, from which the
binuclear Pt(III) complex 2 is isolated (75% yield)
(Scheme 2a). It is well known that oxidative addition
of X2 (X ) halogen) to Pd(II) and Pt(II) complexes affords
by a 2e oxidation process halo-M(IV) complexes. Sur-
prisingly, the reaction of 1a with I2 (1:1 molar ratio) at
195 K follows a different pathway involving elimination
of the I- ligands from the platinum centers, thus
affording the neutral binuclear Pt(III) complex [(C6F5)2-
Pt(µ-PPh2)2Pt(C6F5)2], 2, in high yield. On the other
When the yellow solution of 2 is stirred at room
temperature in the presence of iodide anions, the color
fades, and after stirring for 20 h a colorless solution

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1799
results, in line with the NMR information. From this
solution 3a is isolated as a white solid.
established that usually the chemical shift decreases as
the atomic number increases from top to bottom in a
triad.13 In agreement with this the chemical shifts from
3b (Pd2, 3.9 ppm) to 3c (PdPt, -18.7 ppm) and to 3a
(Pt2, -55.3 ppm) show that this P atom is shielded upon
substitution of a palladium by a platinum center, as
observed previously.14
The 19F NMR spectra of 3a -c registered in deu-
teroacetone show in the usual region of o-F atoms of
the C6F5 groups bonded to the metal centers three
signals (intensity ratio 2:2:2) with the expected platinum
satellites. In the usual region of p- and m-F atoms of
the C6F5 groups bonded to the metals appear the
expected six signals (intensity ratio 1:1:2:2:2:1) for 3a .
For 3b,c two of these signals appear overlapped, giving
rise to five signals in the intensity ratio 1:2:1:3:2 and
1:2:1:2:3, respectively (see Experimental Section). All
these data are in agreement with the inequivalence of
the three C6F5 groups bonded to platinum and/or
palladium centers and with the equivalence of both
halves of each C6F5 ring. Moreover, the spectra show
signals due to another type of C6F5 group. The ones due
to o-F (-124.5 (3a ), -124.3 (3b), and -124.4 (3c) ppm
and intensity ratio of 2) and p-F (-152.0 (3a ), -151.9
(3b), and -152.0 (3c) ppm and intensity ratio of 1)
appear in a striking region, while the signals due to m-F
(-162.9 (3a ), -162.5 (3b and 3c) ppm and intensity
ratio of 2) appear only slightly lower field than the m-F
and p-F signals of the other C6F5 groups. These signals
can be assigned unequivocally to the phosphane pen-
tafluorophenyl ring. These upfield and downfield changes
in the chemical shifts of o-F and p-F signals, respec-
tively, have been observed in pentafluorophenyl organic
ligands.4-7 The 31P{1H} NMR spectra of 3a -c show two
doublets (AX spin system) for 3a and 3c. For 3b the
pattern must be analyzed as an AB spin system. In all
cases the signal that appears to more upfield is broad,
as is usual in all our diphenylphosphido derivatives with
C6F5 groups in trans position.8,9 Moreover, in complex
3a this signal shows platinum satellites from which two
values of 1J Pt-P can be extracted, and in complex 3c only
this signal shows platinum satellites (see Experimental
Section). From all these data the unambiguous assign-
ment of this signal to the P atom of the PPh2 ligand
can be made. We have extensively observed that the
chemical shifts of P atoms in bridged phosphido deriva-
tives can be very informative not only if the metal atoms
are joined or not by metal-metal bond but if a single
or two PPh2 groups are bridging the metal centers.4,8,10
In agreement with this, the chemical shifts of the PPh2
ligand in 3a -c (from 4 to -55 ppm) appear at lower
field that those of the M(µ-PPh2)2M′ framework in the
starting materials (from -106 to -147 ppm) and are in
the same range as those observed for other derivatives
that show the M(µ-PPh2)(µ-X)M′ (X ) Cl, Br, OH)
framework without metal-metal bond.11,12 It has been
1
The H NMR spectra of 3a -c have been recorded in
deuteroacetone and show signals due to phenylic H
atoms and those expected for the NBu4+. The intensity
of these signals is in agreement with the monoanionic
nature of the complexes.
In the IR spectra of 3b,c, the X-sensitive modes of
the C6F5 groups appear as one strong and one (3a ,b) or
two (3c) medium absorptions (see Experimental Sec-
tion). Besides the strong signals around 1500 and 950
cm-1 always observed in all pentafluorophenyl deriva-
tives with an M-C6F5 bond, two other absorptions at
higher frequencies, ca. 1520 and 980 cm-1, are observed
at these regions in complexes 3a -c. Analogous absorp-
tions have been observed for two other complexes
prepared by us some years ago, [Pt2Pd2(µ-PPh2)3(C6F5)3-
(PPh2C6F5)(CO)] and [Pt2Pd2(µ-PPh2)3(C6F5)3(PPh2C6F5)].4
Thus we can now establish that the presence of this type
of C6F5 group not metal bonded can be easily detected
by IR spectroscopy.
It is worth noting that in all cases the formation of
the PPh2C6F5 ligand implies the breaking of M-C
bonds. The formation of complexes 3a -c corresponds
to a very unusual and rare process, because it requires
a reductive coupling between a phosphido ligand and a
pentafluorophenyl group to afford a coordinated tertiary
phosphane. Although it is well known15-22 that tertiary
phosphane ligands can be transformed thermally to
bridging phosphido species through a P-C bond activa-
tion and formation of new M-P and M-C bonds, the
reverse process is rare. A cluster-mediated conversion
of M-C6H5 into a P-C6H5 bond, through coupling of a
bridging PPh2 moiety and a phenyl group to give PPh3,
was reported by Braunstein et al.23 On the other hand,
we have reported the elimination process of chloride
ligands from [{(C6F5)2Pt(µ-PPh2)2M(µ-Cl)}2]2- (M ) Pt,
Pd).4,10 Only for the hetero-tetranuclear derivative is a
similar reductive coupling observed, yielding [Pt2Pd2-
(µ-PPh2)3(C6F5)3(PPh2C6F5)]. Considering that the M-
C6F5 bonds are more stable that the corresponding
(13) Carty, A. J .; MacLaughlin, S. A.; Nucciarone, D. Phosphorus-
31 NMR Spectroscopy in Stereochemical Analysis; VCH: 1987.
(14) Alonso, E.; Casas, J . M.; Fornie´s, J .; Fortun˜o, C.; Mart´ın, A.;
Orpen, A. G.; Tsipis, C. A.; Tsipis, A. C. Organometallics 2001, 20,
5571-5582.
(15) Garc´ıa, G.; Garc´ıa, M. E.; Melo´n, S.; Riera, V.; Ruiz, M. A.;
Villafan˜e, F. Organometallics 1997, 16, 624-631.
(4) Falvello, L. R.; Fornie´s, J .; Fortun˜o, C.; Mart´ın, A.; Mart´ınez-
Sarin˜ena, A. P. Organometallics 1997, 16, 5849-5856.
(5) Uso´n, R.; Fornie´s, J .; Espinet, P.; Lalinde, E.; J ones, P. G.;
Sheldrick, G. M. J . Chem. Soc., Dalton Trans. 1982, 2389-2395.
(6) Ara, I.; Fornie´s, J .; Garc´ıa, A.; Go´mez, J .; Lalinde, E.; Moreno,
M. T. Chem. Eur. J . 2002, 8, 3698-3716.
(16) Shiu, K. B.; J ean, S. W.; Wang, S. L.; Liao, F. L.; Wang, J . C.;
Liou, L. S. Organometallics 1997, 16, 114-119.
(17) Dubois, R. A.; Garrou, P. E. Organometallics 1986, 5, 460-
466.
(18) Bender, R.; Braunstein, P.; Dedieu, A.; Ellis, P. D.; Huggins,
B.; Harvey, P. D.; Sappa, E.; Tiripicchio, A. Inorg. Chem. 1996, 35,
1223-1234.
(7) Uso´n, R.; Fornies, J .; Espinet, P.; Lalinde, E. J . Organomet.
Chem. 1983, 254, 371-379.
(8) Falvello, L. R.; Fornie´s, J .; Fortun˜o, C.; Dura´n, F.; Mart´ın, A.
Organometallics 2002, 21, 2226-2234.
(19) Mizuta, T.; Onishi, M.; Nakazono, T.; Nakazawa, H.; Miyoshi,
K. Organometallics 2002, 21, 717-726.
(9) Alonso, E.; Fornie´s, J .; Fortun˜o, C.; Mart´ın, A.; Orpen, A. G.
Organometallics 2001, 20, 850-859.
(20) Geoffroy, G. L.; Rosenberg, S.; Shulman, P. M.; Whittle, R. R.
J . Am. Chem. Soc. 1984, 106, 1519-1521.
(10) Alonso, E.; Fornie´s, J .; Fortun˜o, C.; Mart´ın, A.; Orpen, A. G.
Organometallics 2000, 19, 2690-2697.
(21) Shulman, P. M.; Burkhardt, E. D.; Lundquist, E.; Pilato, R. S.;
Geoffroy, G. L.; Rheingold, A. L. Organometallics 1987, 6, 101-109.
(22) Zhuravel, M. A.; Moncarz, J . R.; Glueck, D. S.; Lam, K.-C.;
Rheingold, A. L. Organometallics 2000, 19, 3447-3454.
(23) Archambault, C.; Bender, R.; Braunstein, P.; Decian, A.;
Fischer, J . Chem. Commun. 1996, 2729-2730.
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Welch, A. J . Inorg. Chem. 1997, 36, 4426-4431.
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Dalton Trans. 1995, 3777-3784.

1800 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
M-C6H5 bonds,24 the reductive coupling should be
expected to be a very unusual and unfavorable process.
Surprisingly, treatment of 3a with AgClO4 in CH2-
Cl2 (molar ratio 1:1) results in the expected precipitation
of AgI and the isolation of a yellow solid, which was
identified (19F and 31P NMR spectroscopy) as the Pt-
(III) binuclear derivative 2 (73% yield). Obviously, this
very unusual reaction corresponds to a formal intramo-
lecular oxidative addition of PPh2C6F5 to a binuclear Pt-
(II)-Pt(II) moiety resulting in the formation of the
binuclear Pt(III)-Pt(III) compound. The reaction is
promoted by eliminating the bridging I- ligand, which
induces the P-C6F5 bond activation and the interme-
tallic Pt(III)-Pt(III) bond formation. Remarkably, the
conversion of [(C6F5)2Pt(µ-PPh2)2Pt(C6F5)2] to [(C6F5)2-
Pt(µ-PPh2)(µ-I)Pt(C6F5)(PPh2C6F5)]-, or vice versa, cor-
responds to a totally reversible process induced by the
presence or absence of I- anions under very mild
conditions.
F igu r e 1. Structure of complex 3b together with the atom-
labeling scheme.
Ta ble 1. Selected Bon d Dista n ces [Å] a n d An gles
[d eg] a n d Th eir Estim a ted Sta n d a r d Devia tion s
for 3b
Bond Distances
Surprisingly, although complexes 3b and 3c react
with AgClO4, they do not render the elusive complexes
2b and 2c. The NMR spectra of the reaction products
indicated that in both cases a complicated mixture of
complexes has been obtained. It is also evident that the
PPh2C6F5 ligand is still present in the mixture. At-
tempts to separate and identify all components of the
mixtures were unsuccessful.
Pd(1)-C(1) 2.007(8) Pd(1)-C(7) 2.099(9) Pd(1)-P(1) 2.306(2)
Pd(1)-I
2.6899(8) Pd(2)-C(13) 2.031(8) Pd(2)-P(1) 2.314(2)
Pd(2)-P(2) 2.377(2) Pd(2)-I
2.6899(8)
Bond Angles
C(1)-Pd(1)-C(7)
C(7)-Pd(1)-P(1)
C(7)-Pd(1)-I
89.7(3)
170.7(2)
92.8(2)
C(1)-Pd(1)-P(1)
C(1)-Pd(1)-I
95.7(2)
167.8(3)
83.43(6)
93.0(2)
171.9(2)
94.08(6)
P(1)-Pd(1)-I
C(13)-Pd(2)-P(1)
P(1)-Pd(2)-P(2)
P(1)-Pd(2)-I
89.4(2)
C(13)-Pd(2)-P(2)
C(13)-Pd(2)-I
P(2)-Pd(2)-I
175.38(8)
83.29(6)
75.27(2)
Despite the well-known activation of the P-C bond
in standard phenyl-containing phosphanes by transition
metals, the corresponding cleavage of the P-C(fluoro-
aryl) bond is a very rare event.25,26 A clean example of
a transition metal-mediated P-ArF cleavage in a ter-
tiary phosphane has recently been reported.27 In this
case the P-C6F5 bond cleavage seems to be a conse-
quence of a formal oxidative addition to a Pd(0) inter-
mediate yielding the Pd(II) dimer [Pd2(C6F5)2{µ-P(C6F5)-
CH2CH2P(C6F5)2}2]. Interestingly, neither the analogous
platinum(0) complex nor a platinum(II) derivative ap-
pears to undergo this type of P-C bond cleavage even
at high temperature.27 In contrast, the transformation
of 3a to 2 corresponds to an oxidative addition process
on Pt(II) metal centers and takes place under very mild
conditions. Obviously, the elimination of a four-electron
ligand from an anionic binuclear Pt(II) complex affords
a coordinatively unsaturated complex in which the
oxidative addition of a tertiary phosphane ligand yields
a diphenylphosphido bridging ligand with concomitant
formation of a Pt-C6F5 bond.
Considering the outlying processes, it seems sensible
to assume that the oxidation of the binuclear [NBu4]2-
[(C6F5)2M(µ-PPh2)2M′(C6F5)2] complexes with I2 results
in the formation of the binuclear M(III)-M′(III) com-
plexes, which in the presence of I- decompose very fast
for complexes containing Pd, through a reductive cou-
pling of PPh2 and C6F5 and forming PPh2C6F5. This
process is sufficiently slow at low temperature for the
diplatinum complex, thereby allowing its isolation. Of,
perhaps, greater interest is the fact that for the diplati-
Pd(1)-I-Pd(2)
num complex the intramolecular reductive coupling and
the reversible oxidative addition processes are induced
by the addition to or elimination from the iodide ligand.
It is worth noting that Cabeza et al.28 reported the first
demonstration of this type of reversibility in a trinuclear
[Ru3]+ complex. In this case there are P-C(phenyl) bond
breaking and bond formation, while in our complex the
reversible process implies a P-C(perfluorophenyl) bond.
The spectroscopic data of 3a , 3b, and 3c indicate that
all species display similar structures; thereby only the
structure of 3b has been established by an X-ray
diffraction study.
2. Cr ysta l Str u ctu r e of [(C6F 5)2P d (µ-P P h 2)(µ-I)-
P d (C6F 5)(P P h 2C6F 5)]-. The structure of complex 3b
together with the atom-labeling scheme is shown in
Figure 1, while selected bond distances and angles are
listed in Table 1.
The anion of 3b is a dinuclear complex in which two
palladium atoms are linked through an asymmetric
bridging system formed by a diphenylphosphido and an
iodide ligand. Both palladium atoms lie in conventional
square planar environments formed, besides the bridg-
ing ligands, by two pentafluorphenyl groups for Pd(1)
and one C6F5 ligand and the PPh2C6F5, a phosphane
ligand formed during the reaction process as previously
observed.4 The best least squares planes of the two
palladium atoms form an angle of 122.8°.
The intermetallic distance is 3.285(1) Å, which ex-
cludes any Pd‚‚‚Pd interaction. The Pd(1)-P(1)-Pd(2)
angle is 90.66(8)°, and the P(1)-Pd-I angles are
somewhat narrower (ca. 83°, see Table 1) than expected
for an idealized square planar geometry for the coordi-
nation environments. Both Pd-I distances have the
(24) Uso´n, R.; Fornie´s, J .; Toma´s, M. J . Organomet. Chem. 1988,
358, 525-543.
(25) Fahey, D. R.; Mahan, J . E. J . Am. Chem. Soc. 1976, 98, 4499-
4503.
(26) Ang, H. G.; Kwik, W. L.; Leong, W. K.; J ohnson, B. F. G.; Lewis,
J .; Raithby, P. R. J . Organomet. Chem. 1990, 396, C43.
(27) Heyn, R. H.; Gorbitz, C. H. Organometallics 2002, 21, 2781-
2784.
(28) Cabeza, J . A.; del R´ıo, I.; Riera, V.; Garc´ıa-Granda, S.; Sanni,
B. Organometallics 1997, 16, 1743-1748.

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1801
F igu r e 2. Geometric and energetic profile of the oxidative addition of I2 to [(CN)2Pt(µ-PH2)2Pt(CN)2]2-, M1a , yielding the
neutral [(CN)2Pt(µ-PH2)2Pt(CN)2] complex, M6a . Relative energies (in kcal/mol) at 0 K (∆E0). Values in parentheses are
reaction enthalpies at 298 K (∆H°298), and values in square brackets are Gibbs free energies at 298 K (∆G°298).
same value, 2.690(1) Å, which is in the range usually
found.5,29-38 On the other hand, the Pd(1)-I-Pd(2)
angle is only 75.7(2)°, which is one of the smallest values
for these kinds of bridging systems. Finally, to the best
of our knowledge, this is the first complex characterized
by X-ray diffraction in which there is only one iodide
bridging ligand, the common trend being the existence
of a symmetric double iodide bridging system.
In an attempt to learn more about these peculiar
intramolecular reversible processes we searched thor-
oughly the potential energy surfaces of the respective
reactions through electronic structure calculations based
on density functional theory (DFT) methods, and the
geometric and energetic profile of the reactions in a
vacuum will be discussed herein.
3. Mod elin g th e In tr a m olecu la r Red u ctive Cou -
p lin g in th e [(CN)2M(µ-P H2)2M′(CN)2] (M ) M′ ) P t
or P d , M ) P t, M′ ) P d ) Com p lexes P r om oted by
I- An ion s in a Va cu u m . The primary scope of this part
is to furnish details on the mechanism of the intramo-
lecular reductive coupling of a µ-PPh2 bridging ligand
and a coordinated C6F5 group as well as the reversible
intramolecular oxidative addition of a coordinated
PPh2C6F5 ligand in the [(CN)2M(µ-PH2)2M′(CN)2]/[(CN)2-
M(µ-PH2)(µ-Ι)M′ (CN)(PH2CN)]- (M ) M′ ) Pt or Pd,
M ) Pt, M′ ) Pd) redox couples. To accomplish this,
the potential energy surfaces (PES) of the respective
reactions of appropriately selected model compounds
with computationally convenient size were computed by
DFT methods (see Computational Methods), as these
methods have extensively been applied to inorganic
problems, and the accuracy of the calculated structural
and energetic parameters has been demonstrated for a
variety of systems, including organometallic com-
pounds.39,40 To denote the model compounds we will use
labels such as M1a , M1b, etc.
3.1. Oxid a tion of th e [(CN)2M(µ-P H2)2M′(CN)2]2-
(M ) M′ ) P t or P d , M ) P t, M′ ) P d ) Com p lexes
by I2 in a Va cu u m . The energetic and geometric
profiles of the oxidative addition of I2 to one of the metal
centers in the [(CN)2M(µ-PH2)2M′(CN)2]2- (M ) M′ )
Pt, M1a ; M ) Pt, M′ ) Pd, M1b; and M ) M′ ) Pt,
M1c) complexes are presented in Figures 2, 3, and 4.
In general terms, the oxidation processes of the Pt2
(M1a ), PtPd (M1b), and Pd2 (M1c) species follow
analogous reaction pathways, but with a noticeable
difference concerning the final product formed in the
case of the Pd2 species. Thus, in the first step of the
oxidative addition reaction the I2 molecule interacts
with one of the four-coordinate metal centers of M1a
(Figure 2), M1b (Figure 3), and M1c (Figure 4) aligned
perpendicularly to the coordination plane, yielding the
five-coordinate intermediates, M2a , M2b1 or M2b2, and
M2c, respectively, with square pyramidal stereochem-
istry. The intermediates M2a , M2b1, M2b2, and M2c
are stabilized with respect to the reactants by 24.1, 24.9,
21.5, and 22.4 kcal/mol, respectively. It is worth noting
that the coordination of the iodine ligand to Pt(II) in
M1b (Figure 3a) is slightly favored with respect to its
coordination to the Pd(II) metal center (Figure 3b). This
could easily be explained in terms of the orbital interac-
(29) Donde, Y.; Overman, L. E. J . Am. Chem. Soc. 1999, 121, 2933-
2934.
(30) Rheingold, A. L.; Fultz, W. C. Organometallics 1984, 3, 1414-
1417.
(31) Maassarani, F.; Pfeffer, M.; Le Borgne, G. Organometallics
1987, 6, 2043-2053.
(32) Smith, D. C.; Lake, C. H.; Gray, G. M. Chem. Commun. 1998,
2771-2772.
(33) Brunet, L.; Mercier, F.; Ricard, L.; Mathey, F. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 742-745.
(34) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 1890-1901.
(35) Crispini, A.; Ghedini, M.; Neve, F. J . Organomet. Chem. 1993,
448, 241-245.
(36) Marshall, W. J .; Young, R. J .; Grushin, V. V. Organometallics
2001, 20, 523-533.
(37) Neve, F.; Crispini, A.; Francescangeli, O. Inorg. Chem. 2000,
39, 1187-1194.
(38) Ryabov, A. D.; Kuzmina, L. G.; Polyakov, V. A.; Kazankov, G.
M.; Ryabova, E. S.; Pfeffer, M.; Vaneldik, R. J . Chem. Soc., Dalton
Trans. 1995, 999-1006.
(39) Ziegler, T. Chem. Rev. 1991, 91, 651-667.
(40) Mire, L. W.; Wheeler, S. D.; Wagenseller, E.; Marynick, D. S.
Inorg. Chem. 1998, 37, 3099-3106.

1802 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
F igu r e 3. Geometric and energetic profile of the oxidative addition of I2 to [(CN)2Pt(µ-PH2)2Pd(CN)2]2-, M1b, either to Pt
(a) or Pd (b) central atoms, yielding the neutral [(CN)2Pt(µ-PH2)2Pd(CN)2] complex, M4b. Relative energies (in kcal/mol)
at 0 K (∆E0). Values in parentheses are reaction enthalpies at 298 K (∆H°298), and values in square brackets are Gibbs
free energies at 298 K (∆G°298).
tions responsible for the association of the iodine
molecule to the metal center.
potential energy surface, without an intervening transi-
tion state demanding about 6.0, 4.8, 3.9, and 2.7 kcal/
mol in terms of ∆E0 for M2a , M2b1, M2b2, and M2c,
respectively. Notice that the coordinated iodine donor
atom remains coordinated to the metal center of M2a ,
M2b1, M2b2, and M2c as an iodide ligand during the
course of the electron transfer process, yielding the
binuclear complexes M3a , M3b1, M3b2, and M3c,
respectively, with the oxidized metal center being five-
coordinate.
In all binuclear complexes there are favorable orbital
interaction between their HOMO or HOMO-1, which
correspond to nonbonding molecular orbitals (MOs)
localized mainly on one of the metal centers (essentially
2
dz orbitals), and the LUMO of I2, which corresponds to
an antibonding σ*I-I MO. In the M2b1 intermediate the
σ* LUMO of the iodine ligand interacts with the
2
HOMO-1 of M1b, which is an almost pure dz orbital
localized on the Pt(II) metal center, whereas in M2b2
the σ* LUMO of the iodine ligand interacts with the
HOMO-2 of M1b, which corresponds to an antibonding
combination of two dz orbitals localized on both the Pd-
(II) and Pt(II) metal centers with nearly the same
contribution.
The n(Pt) f σ*I-I MO or n(Pd) f σ*I-I MO orbital
interactions favor the transfer of the nonbonding elec-
tron pair belonging to the Pt(II) or Pd(II) metal centers
to the antibonding σ*I-I MO, leading to a two-electron
(2e) oxidation of the metal center and the concomitant
rupture of the I-I bond, because of accumulation of the
transferred electron density on the antibonding σ*I-I
MO. The electron transfer proceeds uphill on the
The equilibrium geometries of all stationary points
involved in the oxidative addition of I2 to M1a , M1b,
and M1c, computed at the B3LYP/LANL2DZ level of
theory, are shown in the Supporting Information (Fig-
ures S1, S2, and S3, respectively). In general terms, the
substitution of one of the two Pt metal centers in M1a
with Pd to afford M1b does not alter significantly the
equilibrium geometries of the species involved in the
oxidation process; therefore only the salient features of
their geometries will be discussed herein.
2
There are no metal-metal interactions in M1a , M1b
(Pt‚‚‚Pt and Pt‚‚‚Pd distances of 3.863 and 3.864 Å,
respectively), and M1c (Pd‚‚‚Pd distance of 3.850 Å);
thereby a 16 valence electron count is obtained for each

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1803
F igu r e 4. Geometric and energetic profile of the oxidative addition of I2 to [(CN)2Pd(µ-PH2)2Pd(CN)2]2-. Relative energies
(in kcal/mol) at 0 K (∆E0). Values in parentheses are reaction enthalpies at 298 K (∆H°298), and values in square brackets
are Gibbs free energies at 298 K (∆G°298).
metal center which adopts a distorted square planar
coordination environment with small P-Pt-P and
P-Pd-P bond angles of about 77°. The calculated
structural parameters of M1a compare favorably to the
experimental and theoretical data of 1a reported previ-
ously.3
Intermediates M2a , M2b1, M2b2, and M2c exhibit
analogous geometries corresponding to a loose end-on
association of the I2 molecule with one of the metal
centers of M1a , M1b, and M1c in a perpendicular way,
thus forcing the metal center to adopt a square pyra-
midal stereochemistry with the I2 ligand in the apical
position. The calculated Pt-I and Pd-I bond lengths
of about 2.91 Å are indicative of the weak interaction,
which is also reflected in the computed interaction
energies of 21-25 kcal/mol in terms of ∆E0. Notice that
the Pt‚‚‚I and Pd‚‚‚I bond distances are much smaller
than the sum of the van der Waals radii of both atoms,
3.9 Å.41
Generally, the association of the I2 molecule to the
metal center does not introduce significant structural
changes except for the characteristic folding of the M(µ-
PΗ2)2M′ (M ) M′ ) Pt or Pd; M ) Pt, M′ ) Pd) cores
with flap angles in the range 146-155°. The folding
along the P‚‚‚P axis imposes a narrowing of the M-P-
M′ bond angles and a shortening of the M‚‚‚M′ separa-
tion distances. Interestingly there is a significant elon-
gation of the I-I bond amounting to about 0.25-0.26 Å
for all species (the computed I-I bond length of the free
I2 ligand is 2.863 Å at the same level of theory). This is
expected in terms of the favorable n(Pt) f σ*I-I and
n(Pd) f σ*I-I MO orbital interactions associated with
a charge transfer (CT) from the metal center to the
coordinated I2 molecule. The stabilization energy ∆E(2)
associated with the charge transfer interactions between
the relevant donor-acceptor orbitals computed from the
second-order perturbative estimates of the Fock matrix
in the natural bond orbital (NBO) analysis42 allows
these specific interactions to be pinpointed. The stabi-
lization energy of 28.4, 28.1, 18.1, and 17.2 kcal/mol
introduced by the favorable n(Pt) f σ*I-I MO or n(Pd)
f σ*I-I MO orbital interactions in M2a , M2b1, M2b2,
and M2c, respectively, accounts well for the elongation
of the σ I-I bond. Noteworthy is the remarkable
lowering of the occupancy of one of the 5d or 4d lone
pairs of the oxidized platinum or palladium centers; the
computed occupancies were 1.710, 1.712, 1.771, and
1.781 for M2a , M2b1, M2b2, and M2c intermediates,
respectively. Moreover, the natural charges on the metal
centers were found in the range of 0.27-0.44 charge
units, while the coordinated and terminal iodine atoms
acquire negative natural charges of -0.01 to -0.06 and
-0.40 charge units, respectively.
The computed Pt-I or Pd-I bond dissociation ener-
gies (BDE) of 72.0, 70.8, 68.3, and 76.9 kcal/mol indicate
strong interactions of the apical iodide ligand with the
Pt or Pd centers of M3a , M3b1, M3b2, and M3c
intermediates, respectively. The natural charges on the
metal centers of M3a , M3b1, M3b2, and M3c interme-
diates were found in the range 0.30-0.48 charge units,
while the iodide ligand acquires a negative natural
charge of -0.11 to -0.22 charge units. The charge
density distribution on the metal centers illustrates
their different oxidation states, with the five-coordinated
one exhibiting the higher oxidation state.
(42) Weinhold, F. In The Encyclopedia of Computational Chemistry;
Schleyer, P. v. R., Ed.; J ohn Wiley & Sons: Chichester, 1998; pp 1792-
1811.
(41) Huheey, J . E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry,
4th ed.; Harper: New York, 1993.

1804 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
Sch em e 3
Sch em e 4
In the oxidized complexes M3a and M3c we further
examined the possibility for the iodide anion resulting
from the rupture of the I-I bond to be coordinated to
either Pt(IV) or Pt(II) and Pd(IV) or Pd(II) metal centers,
thus affording the isomeric species M4a 1, M4a 2 or
M5a , and M4c1, M4c2, or M4c3, respectively. The
coordination of the second iodide ligand to the Pt(IV)
metal center of M3a or to the Pd(IV) metal center of
M3c either in axial or equatorial coordination sites
corresponds to an exothermic process, with a computed
reaction enthalpy of -13.3, -15.7, -4.2, and -5.8 kcal/
mol, respectively. In contrast, the coordination to Pt-
(II) or Pd(II) metal centers corresponds to an endother-
mic process, with a reaction enthalpy of 12.5 and 13.1
kcal/mol. Looking at the frontier molecular orbitals of
M3a (Scheme 3) it can be seen that the LUMO, which
is the acceptor orbital to accommodate the lone pair of
electrons of the incoming iodide ligand, is a σ*Pt-I MO
consuming processes, the reaction enthalpy being equal
to 57.2 kcal/mol. Finally, in the planar dinuclear Pt-
(III)-Pd(III) complex M4b the Pt‚‚‚Pd separation dis-
tance of 2.821 Å is indicative of strong intermetallic
Pt‚‚‚Pd interactions. These interactions are clearly
shown in the respective MOs given in Scheme 4.
It is worth noting that the CN ligands coordinated to
the Pd central atom in M4b are closer to the P donor
atom of the bridged phosphido ligand (P‚‚‚C separation
distance of 2.84 Å) than the CN ligands coordinated to
Pt central atom (P‚‚‚C separation distance of 2.96 Å).
This configuration seems to favor the reductive coupling
between the two ligands to form the new PH2(CN)
ligand coordinated to the Pd metal center. The same
holds true for complex M7 representing the real-life
complex (Figure S2).
Finally, it should be noted that the predicted ther-
modynamic stability of the oxidized products M6a , M4b,
and M5c (by about 50 kcal/mol) is not consistent with
the experimental observation that the 2e oxidation
process takes place at very low temperature (-70 °C).
Control calculations using a larger basis set, e.g.,
LANL2DZ for the metal and iodine atoms and 6-31G-
(d,p) for the rest of the non-metal atoms, hoping to
obtain more reliable energies did not improved the
results obtained at the B3LYP/LANL2DZ level. Thus,
for example the predicted thermodynamic stability of
complex M6a is about 46 kcal/mol using the larger basis
set. The phenomenal inconsistency between theory and
experiment is not surprising considering the poor
modeling of the experimental redox behavior in the gas
phase. Actually, this is the case for analogous multi-
electron redox reactions accompanied by a reversible
structural change, where it was found that the inclusion
of solvation was necessary to correctly predict the
energetic reaction profile.43 Unfortunately, due to our
present computational resources and the extremely high
CPU demands, the investigation of the macroscopic
influence of the solvent on the energetics of the redox
reactions was not possible to perform. The electrostatic
effects of the bulk environment are expected to play an
important role in stabilizing the various charged species
along the reaction pathway.
2
corresponding to the out-of-phase combination of the dz
AO of Pt(IV) and the p AO of the iodide ligand. It is
obvious then why the formation of M4a 1 or M4a 2 is
favored with respect to the formation of M5a .
The salient feature of the optimized structures M4a 1
and M4c2 is the planarity of the cyclic Pt(II)(µ-PH2)2-
Pt(IV) and Pd(II)(µ-PH2)2Pd(IV) frameworks, respec-
tively. The M(II)‚‚‚M(IV) distances of 3.850 and 3.817
Å in M4a 1 and M4c2, respectively, do not support
intermetallic interactions. The M(II) center is four-
coordinated with a square planar stereochemistry, while
the M(IV) one is six-coordinated with a distorted
octahedral stereochemistry.
Although the coordination of the iodide ligands to Pt-
(II) and Pt(IV) or Pd(II) and Pd(IV) centers is respon-
sible for the equalization of the oxidation states of the
two platinum or palladium centers to an intermediate
oxidation state, their removal is a prerequisite to obtain
the final oxidized products M6a and M5c, respectively.
Noteworthy is the different behavior of the dipalladium
complex in this stage of the reaction pathway, as
compared to the Pt2 and PdPt analogues. Despite the
weakening of the Pt-I bonds in M5a , their dissociation
is still an energy-consuming process with a reaction
enthalpy of 59.6 kcal/mol. Complex M6a is a planar
dinuclear Pt(III) complex involving strong intermetallic
Pt‚‚‚Pt interactions with a Pt‚‚‚Pt separation distance
of 2.844 Å. Surprisingly, on the other hand, the final
iodide-free dinuclear Pd(III)-Pd(III) complex corre-
sponds to a saddle point (transition state, MTS), which
readily converts to a more stable Pd(II)-Pd(II) complex
M5c through a reductive coupling of one of the CN
ligands with one of the bridged phosphido ligands.
Complex M5c is stabilized with respect to MTS by 19.7
kcal/mol, while on the whole the Pd-I bond dissociation
and the concomitant reductive coupling are energy-
3.2. In tr a m olecu la r Red u ctive Cou p lin g in th e
[(CN)2M(µ-P H2)2M′(CN)2] (M ) M′ ) P t or P d , M )
(43) Baik, M.-H.; Ziegler, T.; Schauer, C. K. J . Am. Chem. Soc. 2000,
125, 9143-9154, and references therein.

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1805
F igu r e 5. Geometric and energetic profiles of the intramolecular reductive coupling in [(CN)2Pt(µ-PH2)2Pt(CN)2] (a) and
[(CN)2Pt(µ-PH2)2Pd(CN)2] (b) promoted by I- anions. Relative energies (in kcal/mol) at 0 K (∆E0). Values in parentheses
are reaction enthalpies at 298 K (∆H°298), and values in square brackets are Gibbs free energies at 298 K (∆G°298).
F igu r e 6. Geometric and energetic profile of the intramolecular reductive coupling in the course of the oxidation of [(CN)2Pd-
(µ-PH2)2Pd(CN)2]2- with iodine. Relative energies (in kcal/mol) at 0 K (∆E0). Values in parentheses are reaction enthalpies
at 298 K (∆H°298), and values in square brackets are Gibbs free energies at 298 K (∆G°298).
P t, M′ ) P d ) Com p lexes P r om oted by I- An ion s in
a Va cu u m . The energetic and geometric profiles of the
intramolecular reductive coupling in the [(CN)2Pt(µ-
PH2)2Pt(CN)2] (M6a ) and [(CN)2Pt(µ-PH2)2Pd(CN)2]
(M4b) complexes promoted by I- anions are depicted
schematically in Figure 5, while those of the intramo-
lecular reductive coupling taking place in the course of
the oxidative addition of I2 to the [(CN)2Pd(µ-PH2)2Pd-
(CN)2]2- (M1c) complex are shown in Figure 6. The
equilibrium geometries of the species involved in the
intramolecular reductive coupling in M6a and M4b
promoted by I- anions and in the course of the oxidative
addition of I2 to M1c are shown in the Supporting
Information (Figures S4, S5, and S6, respectively).
The initiator of the reductive coupling process for M6a
is complex M3a , formed either by the oxidation of M1a
with I2 or via the coordination of the iodide ligand to
one of the platinum centers of the oxidized complex M6a
of the M1a /M6a redox couple (Figure 2); the latter is a
strongly exothermic process with an exothermicity
amounting to -72.0 kcal/mol. On the other hand, the
initiator of the reductive coupling process for M4b could
be either complex M3b1 or M3b2, formed either by the
oxidation of M1b with I2 or via the coordination of the
iodide ligand to Pt or Pd atoms of the oxidized complex
M4b (Figure 3); the latter is also a strongly exothermic
process with exothermicity of -70.8 and -68.3 kcal/mol,
respectively. Since the energetically more favorable

1806 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
Sch em e 5
pathway is that involving the M3b1 intermediate, we
will discuss only the mechanism of the intramolecular
reductive coupling process following this pathway. The
alternative pathway of the reductive coupling through
intermediate M3b2 is expected to follow the same
mechanism without significant changes in both the
geometric and energetic profile of the reaction.
Complexes M3a , M3b1, and M3c readily convert to
the more stable intermediates M8a , M5b, and M6c
through the transition states MTS1, MTS3, and MTS5,
respectively. Noteworthy are the small negative values
of the activation barriers, which become even more
negative (-3.9, -2.6, and -2.9 kcal/mol for the forma-
tion of M8a , M5b, and M6c, respectively) upon enlarge-
ment of the basis set (LANL2DZ for the metal and
iodine atoms and 6-31G(d,p) for the rest of the non-
metal atoms). It should be noted that the structural
parameters of the transition states are only marginally
affected upon enlargement of the basis set. It is evident
then that a more correlated computational technique
could give more realistic activation barriers for the
respective processes.
In the following stage a reductive coupling of a
bridging µ-PH2 ligand and a CN- group takes place,
yielding the final products M9a , M6b, and M10c
through the transition states MTS2, MTS4, and MTS6
with a relatively low activation barrier of 18.5 (14.4),
8.7 (5.6), and 8.9 (6.1) kcal/mol, respectively. The figures
in parentheses are the computed activation barriers
using the larger basis set. The unexpected intramolecu-
lar reductive coupling in M8a , M5b, and M6c corre-
sponds to an exoergic process by 11.4 (13.0), 18.8 (21.8),
and 18.2 (21.0) kcal/mol, respectively.
phosphane ligand is further shortened by about 0.030
Å. This iodophosphane ligand continues to move toward
the coordinated CN ligand with the P-Pt-C bond angle
being narrowed by about 7°. The most noticeable dif-
ference in the structure of M5b with respect to that of
M8a concerns the nonbridged phosphido ligand, which
is closer to the coordinated CN ligand than to the Pd-
(II) center, since both the Pd-P bond and the P-Pd-C
bond angles are smaller than the corresponding struc-
tural parameters of M8a . The transition states MTS2,
MTS4, and MTS6 exhibit a reactant-like structure
involving the planar five-membered ring, while con-
comitantly the P-CN bond is being formed, yielding a
three-membered PtPC or PdPC ring, respectively. The
respective Pt-CN and Pd-CN bonds participating in
the formation of the three-membered rings have been
dramatically elongated by 0.419, 0.311, and 0.316 Å,
respectively.
Following the reverse reaction pathway complexes
M9a , M6b, and M10c could be converted to intermedi-
ates M8a , M5b, and M6c through the transition states
MTS2, MTS4, and MTS6 with a much higher activation
barrier of about 29.9 (27.4), 27.5 (27.4), and 27.1 (27.1)
kcal/mol, respectively. Removal of the bridged iodide
ligand upon reaction of the final products M9a and M6b
with Ag+ ions yields the iodide-free complexes M11 and
M12 (Scheme 5), which are 4.4 and 26.8 kcal/mol more
stable than the oxidized products M6a and M4b,
respectively. The higher stability of M12 with respect
to M4b strongly suggests that this species should be
the final product of the oxidation process, in full
agreement with experiment.
The equilibrium structure of M11 (Scheme 5) corre-
sponds to a diplatinum complex involving strong
Pt‚‚‚Pt interactions (Pt‚‚‚Pt separation distance of 2.797
Å). In M11 both platinum atoms are four-coordinated.
However, the platinum atom bearing the PH2(CN)
ligand exhibits a square planar stereochemistry, while
the other platinum atom adopts a butterfly configura-
tion. It can be seen that the orientation of the ligands
around the two Pt(II) centers in M11 could support
favorable interactions for the intramolecular oxidative
addition of the PH2(CN) moiety to occur, yielding the
oxidized product M6a . On the other hand, the equilib-
rium structure of the iodide-free complex M12 corre-
sponds to a heterodinuclear complex with the Pt(II) and
Pd(II) metal centers three-coordinated in a T-shaped
configuration. The Pt(II)‚‚‚Pd(II) distance of 3.930 Å
does not indicate metal-metal bonding in M12. More-
over, the Pt(II) and Pd(II) metal centers in M12 acquire
a positive natural charge of 0.52 and 0.41 charge units,
respectively, indicating that both metal centers exhibit
the same oxidation state. Obviously complex M12 could
consist of an alternative intermediate in the reductive
The equilibrium geometries of MTS1, MTS3, and
MTS5 are similar, resulting from the structure of the
initiators M3a , M3b1, and M3c, respectively, upon
rupture of one of the Pt-P or Pd-I bonds of the bridged
phosphido ligands of the five-coordinated metal center.
The Pt-I bond in MTS1 and MTS3 is elongated with
respect to initiators M3a and M3b1 by 0.087 and 0.081
Å, respectively, while the Pd-I bond in MTS5 is
shortened with respect to M3c by 0.045 Å.
The intermediates M8a , M5b, and M6c adopt similar
geometries involving a perfect planar five-membered
dimetallacycle ring consisting of the two metal centers,
two P atoms, and the iodine heteroatoms. Both metal
centers are four-coordinated with square planar coor-
dination environment, which is coplanar with the five-
membered ring. The Pt-I bond length in M8a does not
change significantly, while the Pt-P bond of the iodo-

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1807
Sch em e 6
Sch em e 7
free rotation around the respective Pd-P bond the final
product is formed. Irrespective of which of the two Pd-
(II) centers the iodide ligand would be coordinated to,
the formation of the final product corresponds to a
strongly exothermic process, with an exothermicity
equal to 79.7 kcal/mol.
Noteworthy is the excellent agreement of the com-
puted structure of the final product M14 with the
experimentally determined one by X-ray crystallogra-
phy. Certainly, one would expect small deviations of
some of the computed geometrical parameters with
respect to the experimental ones considering that in
M14 the terminal PPh2(C6F5) ligand was replaced by
the simpler PH2(C6F5) one.
Finally, to get a deeper insight of the role of the iodide
anions in promoting the intramolecular reductive pro-
cess in the homo- and hetero-dinuclear platinum and
palladium complexes we searched the PES of the
mononuclear [M(PH2)2(CN)2] and [M(PH2)2(CN)2(I)] (M
) Pt, Pd) complexes. For both five-coordinate Pt(IV) and
Pd(IV) complexes we located on the PES local minima
corresponding to the square pyramidal structures M15
and M16 (Scheme 7).
The salient differences of the structures of M15 and
M16 refer to the M-I and M-P bond lengths. Thus,
the Pd-I bond is longer than the Pt-I bond by 0.015
Å, while the Pd-P bond is shorter than the Pt-P bond
by 0.014 Å. Upon removing the iodide ligand from M15
and M16, the two complexes responded in a different
way. Complex M15 was transformed to the stable four-
coordinate Pt(II) complex M17 (Scheme 7) with the
metal center in a square planar environment involving
a diphosphane ligand resulting from the reductive
coupling of the two terminal phosphido ligands. On the
other hand, complex M16 was spontaneously trans-
formed to the stable two-coordinate Pd(0) complex M18
(Scheme 7) exhibiting a slightly bending configuration
(P-Pd-P bond angle of 172.6°). Complex M18 involves
two phosphane ligands resulting from the double in-
tramolecular reductive coupling of the CN with the PH2
ligands. The salient feature of structure M18 is the
shortening of the Pd-P bond by 0.119 Å with respect
to the corresponding bond of the dinuclear complex
M10c. It should be noted that two-coordinate Pd(0)
complexes with phosphane ligands are well-known
species extensively reported in the literature. In sum-
mary theory provides guides for further experimental
coupling process, since it can easily react with iodide
ligands to afford the final product M6b.
Considering that both the reductive coupling and the
oxidative addition reactions involve the breaking of a
Pt-CN bond and the formation of a P-CN bond, we
calculated the energies of the respective bonds for the
model [Pt(CN)2(PH2)I]- complex and the PH2(CN) ligand.
The Pt-CN and P-CN bond dissociation energies are
predicted to be 71.9 and 91.6 kcal/mol, respectively.
Obviously, there is an energy gain in the reductive
coupling process involving the breaking of the Pt-CN
and the formation of the P-CN bond.
Searching the PES of the Pd2 complex starting with
the geometry of a real-life complex [(C6F5)2Pd(III)(µ-
PΗ2)2Pd(III)(C6F5)2], we found a local minimum corre-
sponding to complex {(C6F5)2Pd(II)(µ-PΗ2)Pd(II)(C6F5)-
[PH2(C6F5)]}, M13 (Scheme 6). In effect, the formation
of complex M13 could be the result of the intramolecular
reductive coupling of one C6F5 ligand with a bridging
phosphido ligand, taking place in a “transient”, highly
unstable, and therefore not isolable dinuclear Pd(III)-
Pd(III) complex.
In complex M13 both Pd(II) centers are three-
coordinated with a T-shaped geometry (the P-Pd-P
bond angles are about 174°). The Pd(II)‚‚‚Pd(II) distance
of 3.878 Å does not indicate intermetallic interactions.
It is clear that complex M13 could be considered as an
alternative intermediate in the reductive coupling pro-
cess, since it can easily react with iodide ligands to
afford the final product M14 (Scheme 6). Both three-
coordinated Pd(II) atoms acquiring a positive natural
charge of 0.42 and 0.54 charge units are electrophilic
centers; thereby the iodide anions have nearly equal
possibility to be coordinated to each one, and under a

1808 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
work on these very interesting reductive coupling
processes predicted to occur even in mononuclear pal-
ladium complexes. Some experimental examples of this
process have recently been reported.44,45
tally, as well. Instead, the equilibrium structure result-
ing from the intramolecular reductive coupling pro-
moted by the iodide ligand abstraction corresponds to
[L2PdII(µ-PH2)PdII(PH2L)L] species, with the Pd(II)
metal centers bridged by only one phosphido ligand. The
[(C6F5)2PdIII(µ-PPh2)2PdIII(C6F5)2] complex was also im-
possible to isolate experimentally.
Con clu sion s
In this work we have reported a comprehensive study
of the two-electron (2e) oxidation reactions of the
[NBu4]2[(L)2M(µ-PPh2)2M′(L)2] (M ) M′ ) Pt or Pd; M
) Pt, M′ ) Pd; L ) C6F5) complexes using I2 as an
oxidant by means of both experimental and electronic
structure calculation methods. The electronic structure
calculations based on the gradient-corrected (B3LYP)
density functional theory were performed on appropri-
ately selected model compounds with computationally
convenient size. The results can be summarized as
follows.
(vi) The [L2PtII(µ-PH2)PtII(PH2L)L] species could be
considered as intermediates for the reverse intramo-
lecular oxidative addition of the PH2L ligand promoted
by iodide abstraction with Ag+ ions.
(vii) Finally, the molecular structure of the [(C6F5)2-
Pd(µ-PPh2)(µ-I)Pd(C6F5){PPh2(C6F5)}] complex has been
established by X-ray crystallography and compared with
the optimized structure at the B3LYP/LANL2DZ level
of theory.
Exp er im en ta l Section
(i) The oxidation reactions were found to be complex
processes involving some very interesting and mostly
unusual steps determined by the nature of the metal
centers.
Gen er a l Da ta . C, H, and N analyses were performed with
a Perkin-Elmer 240B microanalizer. IR spectra were recorded
on a Perkin-Elmer Spectrum One spectrophotometer (Nujol
mulls between polyethylene plates in the range 4000-350
cm-1). NMR spectra were recorded on a Varian Unity 300
instrument with SiMe4, CFCl3, and 85% H3PO4 as external
references for 1H, 19F, and 31P, respectively. Conductivities
(acetone, c ≈ 5 × 10-4 M) were measured with a Philips PW
9509 conductimeter. Mass spectra were recorded on a VG-
Autospec spectrometer operating at 30 kV, using the standard
Cs-ion FAB gun and 3-nitrobenzyl alcohol as the matrix.
Literature methods were used to prepare the starting com-
plexes [NBu4]2[(C6F5)2M(µ-PPh2)2M′(C6F5)2] (M ) M′ ) Pt, 1a ;
M ) M′ ) Pd, 1b; M ) Pt, M′ ) Pd, 1c).46
(ii) The first step of the oxidation process corresponds
to the oxidative addition of the I2 molecule to one of the
four-coordinated M(II) metal centers, aligned perpen-
dicularly to the coordination plane, thus yielding a five-
coordinated intermediate with square pyramidal ster-
eochemistry. In this intermediate the favorable n(M) f
σ*I-I MO orbital interactions support the transfer of the
nonbonding electron pair belonging to the M(II) metal
center to the antibonding σ*I-I MO, leading to a two-
electron oxidation of the metal center and the concomi-
tant rupture of the I-I bond. The electron transfer in a
vacuum proceeds uphill on the potential energy surface
(PES), without an intervening transition state.
(iii) The oxidized iodo complex [(I)L2M(µ-PH2)2M′L2]-
(L ) CN) easily converts to a more stable intermediate
involving a planar dimetallacycle five-membered ring,
which through a reactant-like transition state with a
relatively low barrier (18.5, 8.7, and 8.9 kcal/mol for Pt2,
PtPd, and Pd2 compounds, respectively) yields the final
product. At this reaction stage an unusual intramolecu-
lar reductive coupling of one of the ancillary ligands L
with a bridging phosphido ligand takes place, resulting
in the formation of a “Pt(II)(µ-PH2)(µ-I)Pt(II)” frame-
work. This step corresponds to an exoergic process by
11.4, 18.2, and 18.8 kcal/mol for Pt2, PtPd, and Pd2
compounds, respectively.
Rea ction of [NBu
4]2[(C6F 5)2P t(µ-P P h 2)2P t(C6F 5)2] w ith
I2. (a) A CH2Cl2 (5 mL) solution of I2 (0.013 g, 0.052 mmol)
was added dropwise to a colorless solution of 1a (0.100 g, 0.052
mmol) in CH2Cl2 (20 mL) at -78 °C. The solution quickly
turned yellow, and after 10 min the solvent was evaporated
to ca. 1.5 mL. A yellow solid began to crystallize and was left
in the freezer for 5 h. The solid was filtered off and washed
with cold CH2Cl2 (2 × 0.5 mL), forming 2 (0.056 g, 75%).
(b) A CH2Cl2 (5 mL) solution of I2 (0.039 g, 0.156 mmol) was
added dropwise to a colorless CH2Cl2 (25 mL) solution of 1a
(0.300 g, 0.156 mmol) at room temperature. The color of the
solution quickly turned to yellow and after 20 h stirring (room
temperature) fades to colorless. The solution was evaporated
to ca. 1 mL, iPrOH (10 mL) was added, and by stirring a white
i
solid crystallized, which was filtered off, washed with PrOH
(2 × 0.5 mL), and vacuum-dried. 3a : 0.200 g, 71%. Anal.
Found (calcd for C64F20H56INP2Pt2): C, 42.7 (42.35); H, 3.1
(2.9); N, 0.8 (0.7). IR (X-sensitive47,48 C6F5, cm-1): 798 (s) and
776 (m). ΛM ) 66 Ω-1 cm2 mol-1. FAB-MS: m/z 1557 [M]-,
1204 [M - (PPh2C6F5)]-. 19F NMR (deuteroacetone, 293 K), δ:
-114.2 (2 o-F, J Pt,F ) 327 Hz), -115.0 (2 o-F, J Pt,F ) 536 Hz),
-115.4 (2 o-F, J Pt,F ) 434 Hz), -124.5 (2 o-F, PPh2C6F5),
-152.0 (1 p-F, PPh2C6F5), -162.9 (2 m-F, PPh2C6F5), -165.4
(1 p-F) -165.6 (1 p-F), -165.7 (2 m-F), -166.1 (2 m-F), -167.3
(2 m-F), -167.4 (1 p-F) ppm. 31P{1H} NMR (deuteroacetone,
293 K, AX spin system, δ): 15.0 (PPh2C6F5 J Pt,P ) 2226 Hz),
(iv) In the case of the Pt2 and PtPd complexes removal
of the iodide ligand from the oxidized intermediates [(I)-
L2M(µ-PH2)2M′L2]- yields the very unusual [L2MIII(µ-
PH2)2M′IIIL2] (L ) CN or C6F5) complexes displaying the
metal centers in square planar coordination environ-
ments involving coplanar Pt(III)-Pt(III) and Pt(III)-
Pd(III) bonds, respectively. The pentafluorophenyl Pt-
(III)-Pt(III) derivative was isolated, although we were
not able to isolate the analogous Pt(III)-Pd(III) com-
pound.
2
-55.3 (PPh2 J Pt,P ) 1981 and 1846 Hz) ppm. J P,P ) 328 Hz.
P r ep a r a tion of [NBu 4][(C6F 5)2P d (µ-P P h 2)(µ-I)P d (C6F 5)-
(P P h 2C6F 5)] (3b). A CH2Cl2 (10 mL) solution of I2 (0.030 g,
0.118 mmol) was added to a yellow CH2Cl2 (15 mL) solution
(v) In contrast, the analogous [L2PdIII(µ-PH2)2PdIIIL2]
compounds could not be identified as local minima in
the PES, and it was impossible to isolate experimen-
(46) Fornie´s, J .; Fortun˜o, C.; Navarro, R.; Mart´ınez, F.; Welch, A.
J . J . Organomet. Chem. 1990, 394, 643-658.
(47) Maslowsky, E. J . Vibrational Spectra of Organometallic Com-
pounds; Wiley: New York, 1997.
(48) Uso´n, R.; Fornie´s, J . J . Adv. Organomet. Chem. 1988, 28, 219-
297.
(44) Moncarz, J . R.; Brunker, T. J .; Glueck, D. S.; Sommer, R. D.;
Rheingold, A. L. J . Am. Chem. Soc. 2003, 125, 1180-1181.
(45) Moncarz, J . R.; Laritcheva, N. F.; Glueck, D. S. J . Am. Chem.
Soc. 2002, 124, 13356-13357.

Unexpected Reversible Intramolecular Oxad Process
Organometallics, Vol. 23, No. 8, 2004 1809
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for [NBu 4][(C6F 5)2P d (µ-P P h 2)(µ-I)P d (C6F 5)-
(P P h 2C6F 5)] (3b)
of 1b (0.200 g, 0.0115 mmol) and stirred at room temperature
for 1.5 h. The solvent was evaporated almost to dryness, CHCl3
(4 mL) was added, and the solution was concentrated to ca. 3
mL and left in the freezer overnight. A small amount of
starting material (0.009 g) was crystallized and was filtered
empirical formula
unit cell dimensions
a (Å)
b (Å)
c (Å)
C64H56F20INP2Pd2
i
off. PrOH (10 mL) was added to the filtrate, and the solution
was evaporated to 8 mL. The mixture was stirred for 3 h and
11.9793(9)
13.6815(10)
22.1965(16)
101.270(1)
99.102(1)
107.689(2)
3305.6(4), 2
0.71073
left in the freezer overnight. A yellow solid, 3b, crystallized
i
R (deg)
and was isolated by filtration, washed with PrOH (2 × 0.5
â (deg)
mL), and vacuum-dried (0.105 g, 56%). Anal. Found (calcd for
γ (deg)
C
64F20H56INP2Pd2): C, 47.0 (47.4); H, 3.45 (3.5); N, 0.9 (0.9).
volume (Å3), Z
wavelength (Å)
temperature (K)
radiation
IR (X-sensitive C6F5, cm-1): 785 (s) and 761 (m). ΛM ) 114
Ω-1 cm2 mol-1. FAB-MS: m/z 1027 ([M - (PPh2C6F5)]-). 19F
NMR (deuteroacetone, 293 K), δ: -110.8 (2 o-F), -111.2 (2 o-F),
-112.5 (2 o-F), -124.3 (2 o-F, PPh2C6F5), -151.9 (1 p-F,
PPh2C6F5), -162.5 (2 m-F, PPh2C6F5), -163.8 (1 p-F), -164.0
(2 m-F), -164.5 (1 p-F), -165.3 (3 m + p-F), -165.8 (2 m-F)
ppm. 31P{1H} NMR (deuteroacetone, 293 K, AB spin system,
293(1)
graphite monochromated
Mo KR
space group
P1h
cryst dimens (mm)
0.45 × 0.25 × 0.15
abs coeff (mm-1
)
1.155
transmission factors
diffractometer
1.000, 0.621
Bruker Smart Apex
2
δ): 9.6 (PPh2C6F5), 3.9 (PPh2) ppm. J P,P ) 316 Hz.
P r ep a r a tion of [NBu 4][(C6F 5)2P t(µ-P P h 2)(µ-I)P d (C6F 5)-
(P P h 2C6F 5)] (3c). Complex 3c (yellow solid) was prepared
similarly to 3a from 1c (0.140 g, 0.077 mmol) and I2 (0.020 g,
0.079 mmol) and 5 h stirring. 3c: 0.038 g, 28%. Anal. Found
(calcd for C64F20H56INP2PdPt): C, 44.5 (44.95); H, 3.2 (3.3); N
0.85 (0.8). IR (X-sensitive C6F5, cm-1): 801 (s), 785 (m), and
773 (m). ΛM ) 112 Ω-1 cm2 mol-1. FAB-MS: m/z 1115 ([M -
(PPh2C6F5)]-). 19F NMR (deuteroacetone, 293 K), δ: -112.2 (2
o-F), -114.2 (2 o-F, J Pt,F ) 337 Hz), -115.1 (2 o-F, J Pt,F ) 533
Hz), -124.4 (2 o-F, PPh2C6F5), -152.0 (1 p-F, PPh2C6F5),
-162.5 (2 m-F, PPh2C6F5), -163.9 (1 p-F), -164.0 (2 m-F),
-165.5 (1 p-F), -166.2 (2 m-F), -167.2 (3 m + p-F) ppm. 31P-
{1H} NMR (deuteroacetone, 293 K, AX spin system, δ): 11.8
2θ range for data collection (deg) 3.2 to 49.6 ((h, (k, (l)
no. of reflns collected
no. of indep reflns
17 682
11 057 [R(int) ) 0.0820]
goodness-of-fit on F2 a
final R indices [I > 2σ(I)]b
R indices (all data)
1.037
R1 ) 0.0721, wR2 ) 0.1754
R1 ) 0.1025, wR2 ) 0.2006
Goodness-of-fit ) [∑w(Fo2 - Fc2)2/Nobs - Nparam
]
.
b R1 ) ∑||Fo|
a
0.5
- |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc2)2/∑wFo
] .
2
4 0.5
C atoms of two n-butyl chains were refined with a common
set of anisotropic parameters each. The C-C distance between
one of the methyl C atoms and the one adjacent was con-
strained to a sensible value. Full-matrix least-squares refine-
ment of this model against F2 converged to final residual
indices given in the Supporting Information (Table S1). The
final difference electron density map showed one feature above
1 e/Å3 (max./min. 1.13/-1.35 e/Å3) located at 0.95 Å from the
iodine atom.
2
(PPh2C6F5), -18.7 (PPh2, J Pt,P ) 1933 Hz) ppm. J P,P ) 325
Hz.
Rea ction of 3a w ith AgClO4. AgClO4 (0.030 g, 0.145
mmol) was added to a colorless CH2Cl2 (15 mL) solution of 3a
(0.150 g, 0.083 mmol), and the mixture was stirred in the dark
for 5 h. After filtration the yellow solution was evaporated to
dryness, and Et2O (15 mL) was added. A white solid was
filtered off (NBu4ClO4) and the solution evaporated to ca. 1
mL. Hexane (8 mL) was added, the solution evaporated to ca.
5 mL, and a yellow solid crystallized. This was identified (19F
and 31P NMR spectroscopy) as 2, 73%.
Cr yst a l St r u ct u r e An a lysis of [NBu 4][(C6F 5)2P d (µ-
P P h 2)(µ-I)P d (C6F 5)(P P h 2C6F 5)] (3b). Crystal data and other
details of the structure analysis are presented in Table 2.
Suitable crystals of 3b were obtained by slow diffusion of
n-hexane into a solution of 0.020 g of [NBu4][(C6F5)2Pd(µ-PPh2)-
(µ-I)Pd(C6F5)(PPh2C6F5)] in acetone (3 mL), and one of them
was mounted at the end of a quartz fiber. Unit cell dimensions
were initially determined from the positions of 256 reflections
in 60 intensity frames measured at 0.3° intervals in ω and
subsequently refined on the basis of positions of 5125 reflec-
tions from the main dataset. A hemisphere of data was
collected based on three ω-scans runs (starting ω ) -28°) at
values æ ) 0, 90, and 180 with the detector at 2θ ) 28°. At
each of these runs, frames (606, 435, and 230, respectively)
were collected at 0.3° intervals and 10 s per frame. The
diffraction frames were integrated using the SAINT package
and corrected for absorption with SADABS.
Com p u ta tion a l Meth od s
All the calculations were performed using the GAUSSIAN
98 program package.50 The equilibrium and transition struc-
tures were fully optimized by Becke’s three-parameter hybrid
functional51,52 combined with the Lee-Yang-Parr53 correlation
functional abbreviated as B3LYP level of density functional
theory, using the LANL2DZ basis set. Control calculations
were also carried out using the LANL2DZ basis set for the
metal and iodine atoms and 6-31G(d,p) for the rest of the non-
metal atoms. Full geometry optimization was performed for
each structure using Schlegel’s analytical gradient method,54
and the attainment of the energy minimum was verified by
calculating the vibrational frequencies that result in the
absence of imaginary eigenvalues. All the stationary points
have been identified for minima (number of imaginary fre-
quencies NIMAG ) 0) or transition states (NIMAG ) 1). The
computed electronic energies, the enthalpies of reactions,
(50) Frisch, M. J .; T., G. W.; Schlegel, H. B.; Scusseria, G. E.; Robb,
M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.;
Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millan, J . M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Orchterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaroni, I.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M.; J ohnson, P.; Chen, W.; Wong, M. W.;
Andres, J . L.; Head-Gordon, M.; Replogle, E. S. Gaussian 98, Revision
A.7; Gaussian Inc.: Pittsburgh, PA, 1998.
The structure was solved by Patterson and Fourier methods.
All refinements were carried out using the program SHELXL-
97.49 All non-hydrogen atoms were assigned anisotropic dis-
placement parameters and refined without positional con-
straints. All hydrogen atoms were constrained to idealized
geometries and assigned isotropic displacement parameters
of 1.2 times the Uiso value of the attached carbon atoms (1.5
+
times for methyl hydrogen atoms). For the NBu4 cation, the
(51) Becke, A. D. J . Chem. Phys. 1992, 96, 215.
(52) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.
(53) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. 1998, B 37, 785.
(54) Schlegel, H. B. J . Comput. Chem. 1982, 3, 214.
(49) Sheldrick, G. M. SHELXL-97 a program for crystal structure
determination; University of Gottingen: Germany, 1997.

1810 Organometallics, Vol. 23, No. 8, 2004
Chaouche et al.
∆RH298, and the activation energies, ∆Gq298, were corrected to
constant pressure and 298 K, for zero-point energy (ZPE)
differences, and for the contributions of the translational,
rotational, and vibrational partition functions. For transition
state geometry determination, quasi-Newton transit-guided
(QSTN) computations were performed.55 Moreover, intrinsic
reaction paths (IRPs)56,57 were traced from the various transi-
tion structures to make sure that no further intermediates
exist.
2.433 Å; θe(P-Pt-P) ) 106.0°, 106.4°, and 105.3°; θe(Pt-P-
Pt) ) 74.0°, 73.6°, and 75.5° for the C6F5, CN, and CF3
derivatives, respectively. Moreover, the coordination environ-
ment of the two platinum centers in the [(CF3)2Pt(µ-PH2)2Pt-
(CF3)2] model is not square planar; the C-P-P-C dihedral
angle is 31.5°.
The use of such models does not alter the description of the
“core” region of the compounds and is ultimately the most
efficient and productive route to modeling the electronic
structure and related properties of relatively big-sized transi-
tion metal coordination compounds.
Although substituents on the bridging phosphido groups are
expected to remotely tune the electronic structure of the
complexes, their effects on the structures of the complexes
would be comparable along the series. This is substantiated
through the comparison of the computed geometrical param-
eters of the [(C6F5)2Pt(µ-PH2)2Pt(C6F5)2] complex reported
previously3 to those of the real [(C6F5)2Pt(µ-PPh2)2Pt(C6F5)2]
complex determined experimentally.
Because of the computational cost due to the relatively big
size of the compounds under consideration, to obtain a com-
putationally convenient size, we used models resulting upon
substitution of the phenyl groups of the phosphido ligands by
H atoms, while the C6F5 group was replaced by the CN group.
The choice of substitution of the C6F5 ligands by the CN ones
was based on electronic structure calculations at the B3LYP/
6-31G(d,p) level of the coordinating ability (σ-donor and
π-acceptor capacity) of the ligands by examining the nature
and energy of their frontier molecular orbitals (FMOs). The
two ligands exhibit the same pattern of FMOs, consisting of a
highest occupied molecular orbital (HOMO), corresponding to
a hybridized orbital localized mainly on the C donor atom, and
a lowest unocuppied molecular orbital (LUMO) corresponding
to a π* ΜÃ delocalized over the entire nuclear framework. We
also performed calculations on model complexes resulting upon
substitution of the C6F5 ligands by the CF3 ones. From an
electronic point of view, the CF3 ligand differs remarkably from
the C6F5 one, since the former is a weak pure σ-donor ligand.
Comparison of the computed geometrical parameters of the
[(C6F5)2Pt(µ-PH2)2Pt(C6F5)2], [(CN)2Pt(µ-PH2)2Pt(CN)2], and
[(CF3)2Pt(µ-PH2)2Pt(CF3)2] complexes strongly justifies our
choice to substitute the C6F5 by the CN ligands. Thus, Re(Pt-
Pt) ) 2.875, 2.844, and 2.980 Å; Re(Pt-P) ) 2.388, 2.403, and
Ack n ow led gm en t. We thank the Ministerio de
Ciencia y Tecnolog´ıa y Fondos FEDER for its financial
support (Project BQU2002-03997-CO2-02) and the Agen-
cia Espan˜ola de Cooperacio´n Internacional (Ministerio
de Asuntos Exteriores) for a grant to N.Ch.
Su p p or tin g In for m a tion Ava ila ble: Tables of full atomic
positional and equivalent isotropic displacement parameters,
anisotropic displacement parameters, full bond distances and
bond angles, and hydrogen coordinates and anisotropic dis-
placement parameters for the crystal structure of complex 3b.
Figures S1-S6 showing the equilibrium geometries of all
species involved in the oxidative addition and intramolecular
reductive coupling reactions. Table S1 summarizing the Car-
tesian coordinates and energies of all stationary points (PDF).
This information is available free of charge via the Internet
at http://pubs.acs.org.
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Organometallics 2003, 22, 5011-5019.
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