Relative M-X bond dissociation energies in 16-, 17- And 18-electron organotransition-metal complexes (X = halide, H)

Article abstract of DOI:10.1039/a800742j

The Fe-halide bonds in Cp*Fe(dppe)X (X = F, Cl, Br, I) complexes are weakened as a consequence of one and two-electron oxidations; the bond weakening decreases in the order F ? Cl < Br < I and is much less pronounced for F than the other halides, indicating a pronounced effect of apparent fluoride-to-metal backbonding as a consequence of the removal of electrons.

Full text of DOI:10.1039/a800742j

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Relative M–X bond dissociation energies in 16-, 17- and 18-electron
organotransition-metal complexes (X = halide, H)
Mats Tilset,*a† Jean-Rene´ Hamon*b and Paul Hamonb
a Department of Chemistry, University of Oslo, PO Box 1033 Blindern, N-0315 Oslo, Norway
b UMR CNRS 6509, ‘Organome´talliques et Catalyse: Chimie et Electrochimie Mole´culaires’, Universite´ de Rennes 1, Campus de
Rennes-Beaulieu, F-35042 Rennes Cedex, France
The Fe–halide bonds in Cp*Fe(dppe)X (X = F, Cl, Br, I)
complexes are weakened as a consequence of one and two-
electron oxidations; the bond weakening decreases in the
order F < < Cl < Br < I and is much less pronounced for
F than the other halides, indicating a pronounced effect of
apparent fluoride-to-metal backbonding as a consequence of
the removal of electrons.
Scheme 1 is quite general and eqn. (1) can therefore be applied
for any other X, and also to evaluate relative BDEs between
multiply oxidized species.§ The pertinent 16- and 17-electron
species M+ amd MX+ are usually so short-lived that electrode
potential data, and thereby BDE data, are inaccessible.
However, it has been recently demonstrated that the sterically
crowded and electron-rich Cp*Fe(dppe) moiety¶ supports metal
complexes in a great number of oxidation states, and com-
pounds have been isolated with electron counts ranging from 16
to 19.16 The persistent 15-electron Cp*Fe(dppe)2+ has even
been generated in solution.16d The Cp*Fe(dppe) derivatives
therefore are particularly well suited for the application of
thermochemical cycles.
Table 1 summarizes electrochemical data obtained by cyclic
voltammetry for the Cp*Fe(dppe)X–Cp*Fe(dppe)X+ and
Cp*Fe(dppe)X+–Cp*Fe(dppe)X2+ couples for X = H, F, Cl,
Br, I. All neutral–monocation redox couples in Table 1 were
chemically reversible, near-Nernstian processes (DEp = 67–75
mV). Remarkably, except for X = H, the cation–dication
couples were also reversible.
The electrochemical data for these halides constitute the first
example of reversible electrode potentials for a complete
organometallic LnMX series (X = F, Cl, Br, I). Interestingly,
the reversible oxidation to monocation occurs most readily for
the most electronegative halide and becomes progressively
more difficult in the series F < Cl < Br < I.17,18 The trend is
the opposite of that predicted on the basis of halide electro-
negativities alone, and most likely is a manifestation of the
importance of apparent p donation from halide to metal. The
particularly large jump, > 0.2 V, in the E° value for F relative
to Cl and the other halides support the idea that F acts as an
exceptionally good donor.‡
The electrochemical data are used in conjunction with eqn.
(1) to give the differences in M–X BDEs between Cp*Fe(dp-
pe)Xn+ complexes when n changes from 0 to 1 to 2. The results
are summarized in Table 2. It is important to keep in mind that
the data show only the net change in the bond energies caused
by oxidation state changes. The data carry no information about
Knowledge of the nature and energetics of metal–ligand
bonding in organotransition-metal complexes is crucial to the
understanding of organometallic reactions and catalysis.1
Recently, the nature of the bonding between organotransition-
metal centers and electronegative s-bonded ligands such as
halide, alkoxide, and amido groups has received considerable
attention.2 In addition to forming covalent M–X bonds, these
ligands are capable of acting as p donors towards the metal. A
ligand pp to metal dp electron-pair donation generally serves to
destabilize electronically saturated complexes via filled–filled
repulsive interactions, whereas coordinatively unsaturated spe-
cies may achieve considerable stabilization through partial p
bond formation.3–9 A substantial line of evidence,2 including IR
nCO spectroscopy data,4,5 electrode potentials,6,10 chemical
reactivity,4,7 and theoretical calculations,3,4,8 suggests that
among the halides, it is the fluoride ligand that is the most
efficient electron-pair donor towards the metal.‡
The presence of significant p bonding from halide to metal
should be reflected in bond dissociation energy (BDE) changes
when this bonding is ‘switched on’3a by the generation of
coordinative unsaturation. To the best of our knowledge, there
exist no quantitative data on the homolytic M–X BDEs of
metal–halide bonds in closely related pairs of coordinatively
saturated (18 electron) and unsaturated (16 electron) species.
Here we present the first data concerning the relative M–X bond
strengths in 18-, 17- and 16-electron complexes which differ
only in their number of valence electrons and thence charge of
the central atom. The data establish a pronounced ‘fluorine
effect’, and suggest that a specific electronic or ionic effect of
the fluorine is evident already in the 17-electron systems.
Thermochemical cycles that incorporate electrode potential
data, introduced by Breslow and Chu,11 have been frequently
used in organic12 and organometallic13–15 chemistry to obtain
bond-energy data that are not available by direct methods. We
have employed this technique to establish a bond-weakening
effect of ca. 25–33 kJ mol21 when 18-electron metal hydrides
were oxidized to their 17-electron cation radicals.13c,d Eqn. (1)
derived from the thermochemical cycle in Scheme 1, was used
to quantify this effect for X = H.
Table 1 Cyclic voltammetry data for the oxidation of Cp*Fe(dppe)
derivativesa
Compound M
E1 (M/M+)b
E2 (M+/M2+ b
)
Cp*Fe(dppe)c
Cp*Fe(dppe)H
Cp*Fe(dppe)Fe
Cp*Fe(dppe)Cl
Cp*Fe(dppe)Br
Cp*Fe(dppe)I
21.272
20.747
20.824
20.618
20.582
20.540
20.290
0.75d
0.688
0.823
0.811
0.780
BDE(MX) 2 BDE(MX+) = F[Eox(MX) 2 Eox(M)] (1)
a
THF–0.2 m NBu4+PF62, T = 20 °C, Pt disk electrode (d = 0.4 mm),
BDE(MX)
LnM
X
LnM + X •
voltage sweep rate v = 1.0 s21
The voltammograms were reversible unless otherwise stated. Measure-
ments were done on Cp*Fe(dppe)+PF62 which, contrary to Cp*Fe(dppe)·,
is stable in THF at room temperature. Peak potential for irreversible
process. Measurements were performed on Cp*Fe(dppe)F·+PF6 (ref.
.
b Oxidation potential, V vs. Cp2Fe–Cp2Fe+.
c
FEox(MX)
LnM X +
–FEox(M)
LnM+ + X •
BDE(MX +)
d
e
Scheme 1
19).
Chem. Commun., 1998
765

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Table 2 Relative bond dissociation energies for Cp*Fe(dppe)Xn+ com-
plexes (kJ mol21 a
might anticipate MX and M to have nearly identical solvation properties.
)
The same applies to MX+ vs. M+.
5
2
¶ Abbreviations: Cp* = h -C5Me5; dppe = h -Ph2PCH2CH2PPh2.
∑ The X-ray crystal structures of Cp*Fe(dppe)n+ (n = 0, 1) have been
reported. The cation does not bind THF or counter anion. There was no
indication of stabilization by agostic interactions with ligand C–H
bonds.16d
DBDE
DBDE
DBDE
Compound M–X
(MX–MX+)
(MX+–MX2+
)
(MX–MX2+
)
Cp*Fe(dppe)H
Cp*Fe(dppe)F
Cp*Fe(dppe)Cl
Cp*Fe(dppe)Br
Cp*Fe(dppe)I
51
43
63
67
71
100b
94
107
106
103
151b
138
171
173
174
** Preliminary DFT calculations on the halide series indicate that the MX
and MX+ BDEs defined in Scheme 1 include, in addition to the energy
necessary to break the M–X bond, a significant component associated with
electronic reorganization of the LnM fragment. However, the reorganization
energy barely varies in the halogen series, so that the big jump observed in
the DBDE values from F to the other halogens is essentially associated with
variations on the M–X bond.
a
b
Obtained using the data in Table 1 and eqn. (1). Minimum value. The
corresponding value for Eox(MX+–MX2+) is a minimum value due to the
unknown kinetic potential shift that is imposed by the irreversible nature of
this electrode process for X = H.
1 J. A. M. Simo˜es and J. L. Beauchamp, Chem. Rev., 1990, 90, 629;
Energetics of Organometallic Species, ed. J. A. M. Simo˜es, NATO ASI
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American Chemical Society, Washington, DC, 1990.
2 For recent reviews, see: K. G. Caulton, New J. Chem., 1994, 18, 25;
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absolute BDEs. The observed bond energy changes result from
the combined s and p effects.‡ For all X, overall bond
weakening occurs as a consequence of oxidation of the neutral
Cp*Fe(dppe)X to their monocations. A further bond weaken-
ing, almost twice as large, results when the monocations are
oxidized to dications. Thus, the data unambiguously demon-
strate that for all X studied, Fe–X bond energies decrease in the
order Fe–X > Fe–X·+ > Fe–X2+.
For both oxidation processes, there is a very interesting and
obvious trend in the bond activation for the halides. The
oxidatively induced bond weakening decreases in the order I >
Br > Cl > F and is particularly less pronounced for F than for
the other halides. In particular, for the overall two-electron
oxidation (which in principle corresponds to the generation of a
vacant coordination site) the difference between F and the other
halides is > 30 kJ mol21. It is tempting to attribute this
phenomenon to a more efficient donation from F to the metal.
This quantity may be viewed as an extra stabilization of the
unsaturated 16-electron complex Cp*Fe(dppe)X2+ that is
provided by F, relative to the other halides.
9 M. D. Butts, B. L. Scott and G. J. Kubas, J. Am. Chem. Soc., 1996, 118,
11831.
10 S. S. P. R. Almeida and A. J. L. Pombeiro, Organometallics, 1997, 16,
4469.
Interestingly, whereas the bond weakening is less pro-
nounced for X = F than for H, a pure s donor, the opposite is
true when Cl, Br and I are compared to H. For a pure s donor,
Eox(MX) should be more positive than for Eox(M) when X is
more electronegative than M, and eqn. (1) shows that an
oxidation in this case should lead to a weakening of the s bond.
In particular, this situation applies to X = H. For X = F, p
donation to the metal is enhanced by the oxidation, and this in
part compensates for the s bond weakening. On the other hand,
for X = Cl, Br, and I, a combination of a greater s bond
weakening and a poorer p donation to the metal leads to an
overall bond weakening that exceeds even that found for
X = H. As noted in the introduction, metal–halide bonding can
be rather complex, and we plan to further develop these issues
in an extended study that includes theoretical aspects of the Fe–
X bonding.**
M. T. gratefully acknowledges support from Statoil under the
VISTA program, administered by the Norwegian Academy of
Science and Letters, from the Norwegian Research Council, and
from Universite´ de Rennes 1 during a sabbatical (1996/97).
J.-R. H. is deeply grateful to Mrs M. H. Lorrilleux for her
generous and valuable assistance in reading articles. We thank
Professor J.-Y. Saillard and K. Costuas for their assistance and
helpful discussions.
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17 This trend has been commonly found for the heavier halides (Cl, Br, I).
See for example: R. Poli, J. Coord. Chem., 1993, 29, 121.
18 A similar trend for the heat of protonation of a series of CpOs(PR3)2X
complexes (decrease in 2DH in the order Cl > Br > I) has been
reported: M. K. Rottink and R. J. Angelici, J. Am. Chem. Soc., 1993,
115, 7267.
Notes and References
† E-mail: mats.tilset@kjemi.uio.no
‡ The theoretical results3b,4,8 imply that p-effects, s-effects and the ionicity
of the M–X bond must all be taken into account when trends in nCO and
other observable parameters are to be explained.
19 The compound Cp*Fe(dppe)F·+PF6 is isolated after oxidation of the
2
16-electron Cp*Fe(dppe)+PF6 with Cp2Fe+PF6 in THF at 290 °C:
2
2
§ The DBDE data obtained from eqn. (1) are in reality free energy based.
However, the enthalpic DBDE values will be identical to the free-energy
ones if DS for the top and bottom homolytic processes in Scheme 1 cancel.
This will be the case here since M and MX have the same charges and the
different X groups are small in comparison to the M fragment. Thus, one
C. P. Hamon, J.-R. Hamon and C. Lapinte, unpublished work.
Received in Basel, Switzerland 27th October 1997; revised manuscript
received 21st January 1998; 8/00742J
766
Chem. Commun., 1998