Structural assignment of the stable carbonylhydridotris-(triphenylphosphine)iridium(II) cation and spectroscopic and voltammetric identification of the transient Ir(III) dication and its decomposition pathway

Article abstract of DOI:10.1016/S0020-1693(99)00596-4

Detailed studies on the electrochemical oxidation of the 18-electron IrH(CO)(PPh₃)₃ complex have been undertaken in dichloromethane. Under voltammetric conditions, the process IrH(CO)(PPh₃)₃ ? [IrH(CO)(PPh₃)₃]⁺ + e^-, which leads to the formation of the 17-electron [IrH(CO)(PPh₃)₃]⁺ cation, is chemically and electrochemically reversible. In contrast, the 16-electron dication [IrH(CO)(PPh₃)₃]²⁺, formed by a further one-electron oxidation process, is very reactive and undergoes a rapid internal redox reaction, [IrH(CO)(PPh₃)₃]²⁺ ? [Ir(CO)(PPh₃)₃]⁺ + H⁺, to form the stable 16-electron [Ir(CO)(PPh₃)₃]⁺ species. In situ infrared (IR) spectroelectrochemical studies at low temperature, enable the v(CO) and v(IrH) IR bands to be obtained for [IrH(CO)(PPh₃)₃]⁺ as well as for transiently formed [IrH(CO)(PPh₃)₃]²⁺. EPR spectra obtained from frozen solutions of electrochemically generated [IrH(CO)(PPh₃)₃]⁺ have been simulated. In agreement with results of density functional calculations on related IrX(CO)(PPh)₃ (X = H, Cl) complexes, the EPR data are consistent with [IrH(CO)(PPh₃)₃]⁺ having a square pyramidal structure. Data are compared with those available for oxidation of the analogous RhH(CO)(PPh₃)₃ complex. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00596-4

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Inorganica Chimica Acta 300–302 (2000) 565–571
Structural assignment of the stable
carbonylhydridotris-(triphenylphosphine)iridium(II) cation and
spectroscopic and voltammetric identification of the transient
Ir(III) dication and its decomposition pathway
Alan M. Bond *, David G. Humphrey, Dmitri Menglet, Georgii G. Lazarev,
Ron S. Dickson, Truc Vu
Department of Chemistry, Monash Uni6ersity, Clayton, Vic. 3168, Australia
Received 9 September 1999; accepted 12 November 1999
Abstract
Detailed studies on the electrochemical oxidation of the 18-electron IrH(CO)(PPh3)3 complex have been undertaken in
dichloromethane. Under voltammetric conditions, the process IrH(CO)(PPh3)3 X [IrH(CO)(PPh3)3]++e−, which leads to the
formation of the 17-electron [IrH(CO)(PPh3)3]+ cation, is chemically and electrochemically reversible. In contrast, the 16-electron
dication [IrH(CO)(PPh3)3]2+, formed by a further one-electron oxidation process, is very reactive and undergoes a rapid internal
redox reaction, [IrH(CO)(PPh3)3]2+ X [Ir(CO)(PPh3)3]++H+, to form the stable 16-electron [Ir(CO)(PPh3)3]+ species. In situ
infrared (IR) spectroelectrochemical studies at low temperature, enable the w(CO) and w(IrH) IR bands to be obtained for
[IrH(CO)(PPh3)3]+ as well as for transiently formed [IrH(CO)(PPh3)3]2+. EPR spectra obtained from frozen solutions of
electrochemically generated [IrH(CO)(PPh3)3]+ have been simulated. In agreement with results of density functional calculations
on related IrX(CO)(PPh)3 (X=H, Cl) complexes, the EPR data are consistent with [IrH(CO)(PPh3)3]+ having a square pyramidal
structure. Data are compared with those available for oxidation of the analogous RhH(CO)(PPh3)3 complex. © 2000 Elsevier
Science S.A. All rights reserved.
Keywords: Iridium(II) cation; Transient Ir(III) dication; Electrochemical oxidation; Voltammetric oxidation; 18-electron complex
[RhH(CO)(PPh3)3]+ is an example of an uncommon
paramagnetic monomeric rhodium(II) complex in
1. Introduction
The 18-electron MIH(CO)(PPh3)3 complexes, where
M=Rh or Ir, are important precursors to homoge-
neous catalysts [1,2]. Catalytic reactions involving these
species are believed to occur via coordinatively unsatu-
rated 16- or 14-electron species, which have been iden-
tified as reaction intermediates. However, we have
which a structural change from trigonal bipyramidal to
square pyramidal occurs upon oxidation of the rhodi-
um(I) to the rhodium(II) complex. In this paper, we
have extended our studies to the iridium analogue,
where the 17-electron complex is stable and even the
much more reactive 16-electron [IrH(CO)(PPh3)]2+ di-
cation is voltammetrically and spectroscopically de-
tectable. EPR and IR spectra of the 17-electron
[IrH(CO)(PPh3)]+ cation and IR spectra of
[IrH(CO)(PPh3)]2+ are reported. Results of density
functional calculations on closely related Ir and Rh
17-electron species are consistent with the interpretation
made on the basis of EPR spectra that a square pyrami-
dal geometry is favoured for the one-electron oxidised
form of the carbonyl hydrides. Finally, voltammetric
characterised
a
moderately stable 17-electron
monomeric [RhH(CO)(PPh3)3]+ species, which can be
obtained by chemical or electrochemical oxidation of
the 18-electron precursor [3]. An analysis of the EPR
spectrum of this 17-electron species implied that
* Corresponding author. Tel.: +61-3-9905 4593; fax: +61-3-9905
4597.
E-mail address: alan.bond@sci.monash.edu.au (A.M. Bond)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00596-4

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A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
and spectroscopic studies have been undertaken to con-
firm that the transient 16-electron Ir(III) dication is ra-
pidly converted to the Ir(I) complex [Ir(CO)(PPh3)3]+,
via an internal redox reaction. The results extend
voltammetric and other data available on IrH(CO)-
(PPh3)3 in acetonitrile–toluene and dichloroethane
solutions [4,5].
lar reflectance accessory located in the sample compart-
ment of a Bruker IFS 55 FTIR spectrometer. The
electrode arrangement consisted of a platinum wire
working electrode, platinum gauze auxiliary electrode,
and a silver wire pseudo reference electrode. Electroly-
ses were carried out by stepping the potential of the
working electrode from a rest potential to a potential
sufficient to cause electrolysis, typically 200 mV past
the peak potential, and single-scan IR spectra (resolu-
tion 1.0 cm−1) were collected as a function of time.
Electrolyses in IRRAS experiments were performed
with a Princeton Applied Research (PAR) (Princeton,
NJ) model 174A Polarographic Analyser. The
dichloromethane (0.2 M nBu4NPF6) electrolyte solu-
tions were prepared as for the voltammetric experi-
ments, and solutions were deoxygenated before
syringing into the IRRAS cell. The cell itself was thor-
oughly flushed with nitrogen prior to addition of the
samples and maintained under a nitrogen atmosphere
throughout the experiments.
2. Experimental
2.1. Materials
IrH(CO)(PPh3)3 was prepared as described in the
literature [6]. Dichloromethane was dried over CaH2
and freshly distilled under a nitrogen atmosphere prior
to use.
2.2. Electrochemical methods
Conventional voltametric measurements were carried
out in dichloromethane solution (0.2 M nBu4NPF6,
nBu4NBF4 or nBu4NClO4) using a Cypress System
model CS-1090 computer-controlled electroanalytical
system or a BAS-100 electrochemical analyser. The
working electrode was a platinum (d, 1.0 mm) or glassy
carbon (GC) (d, 3.0 mm or 1.0 mm), the auxiliary
electrode was a platinum wire and the reference elec-
trode was Ag/AgCl (0.1 M nBu4NBF4, dichloro-
methane-saturated LiCl) separated from the test solu-
tion by a salt bridge. The reversible voltammetry of an
approximately 1.0 mM ferrocence (Fc) solution in the
same solvent was used as a reference redox couple, and
all potentials are quoted relative to Fc/Fc+. The elec-
trolyte solutions were purged with nitrogen gas, and the
cell was constantly maintained under an inert atmo-
sphere. Rotating disc electrode voltammetry employed
a variable speed rotator (Metrohm 628-10). Bulk elec-
trolysis was carried out using a Bioanalytical System
100 electrochemical analyser. The bulk electrolysis cell
contained two platinum baskets, which served as the
working electrode and counter electrode and separated
by a glass cylinder with porous glass frits in the base
[7]. The reference electrode (Ag/Ag+) was positioned as
close as possible to the working electrode in order to
maximise the uniformity of the potential over its sur-
face. Digital simulation of cyclic voltammograms was
performed with the simulation package DIGISIM V 2.1
(BAS, West Lafayette, IN) [8], run on a 150 MHz
Pentium PC.
3. Results and discussion
3.1. Electrochemistry of IrH(CO)(PPh3)3
Cyclic voltammograms of the IrH(CO)(PPh3)3 in
dichloromethane solution (0.2 M Bu4NPF6) at a GC
n
disc macro-electrode (d, 1.0 mm) over the scan rates of
1.0–0.05 V s−1 at 2092°C, reveal the presence of an
initial reversible one-electron oxidation process, which
has a reversible potential (Er1/2-value) of −0.47 V
versus Fc/Fc+. The analogous process obtained from
oxidation of the Rh complex has a reversible potential
of −0.48 V versus Fc/Fc+ [3]. Data obtained for this
process as a function of scan rate are summarised in
Table 1. Comparison of experimental and simulated
responses (Fig. 1), confirms that the first process is
reversible in both the chemical and electrochemical
senses. Importantly, the voltammetry is unaffected by
the presence of up to a 20-fold concentration excess of
triphenylphosphine. In regard to thermodynamics, this
result implies that the equilibrium position for the
reaction shown in Eq. (1), and the analogous reaction
for the oxidised form of the compound, must lie a long
way to the left in dichloromethane. The voltammetry
also remains unchanged in the presence of up to 30-fold
molar excess of AsPh3, which indicates that no ligand
exchange is taking place.
IrH(CO)(PPh3)3 X IrH(CO)(PPh3)2+PPh3
(1)
2.3. Spectroscopic methods
The initial voltammetric oxidation process therefore
can be assigned to the reaction given in Eq. (2).
Infrared spectroelectrochemical experiments were
carried out using a modified IR reflection–absorption
spectroscopy (IRRAS) cell [9,10], mounted on a specu-
IrIH(CO)(PPh3)3 X [IrIIH(CO)(PPh3)3]++e−
(2)

A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
567
Table 1
Cyclic voltammetric data obtained for the oxidation of IrH(CO)(PPh3)3 (4 mM) in dichloromethane (0.2 M nBu4NPF6) at a GC macro-disc
electrode (d, 3.0 mm)
a
b
Scan rate (mV s−1
)
Epa (V)
Epc (V)
Er1/2 (V)
ipa (mA)
ipa/ipc
First oxidation process
50
−0.43
−0.42
−0.41
−0.39
−0.36
−0.27
−0.52
−0.52
−0.53
−0.56
−0.58
−0.67
−0.47
−0.47
−0.47
−0.47
−0.47
−0.47
42
59
82
125
170
330
0.53
0.56
0.75
0.93
0.96
0.98
100
200
500
1000
5000
Second oxidation process
50
0.02
0.03
0.05
0.08
0.12
0.23
−0.08
−0.08
−0.08
−0.10
−0.11
−0.17
−0.03
−0.03
−0.01
−0.01
−0.01
−0.01
36
55
70
100
143
270
0.32
0.45
0.70
0.94
0.95
0.96
100
200
500
1000
5000
a The assumption that Er1/2=(Epa+Epc)/2 is only strictly correct for the second process for data obtained at fast scan rates when this process
is chemically as well as electrochemically reversible.
b This ratio is close to 1.0 for the first oxidation process if the potential is switched after the first oxidation process. Data reported refer to results
obtained when the potential is switched after the second process.
n
whilst with 0.2 M Bu4NClO4 as the electrolyte, greater
chemical reversibility is seen at slow scan rates than at
fast scan rates. The degree of reversibility of the second
oxidation process with respect to electrolyte dependence
is in the order: nBu4NPF6\nBu4NBF4\nBu4NClO4.
Finally, it was noted that voltammetry at the platinum
electrode is more complicated than at the glassy carbon
because of the presence of H+ and the low over-poten-
tial of the 2H+“H2+2e− process at platinum.
If the potential of the glassy carbon electrode is
scanned to the negative potential region on the reverse
scan, switching the potential just after the initial one-
electron oxidation process, no additional electrochemi-
cal processes are observed. However, if the potential is
scanned beyond the first process, a second one-electron
oxidation process is observed at about −0.01 V versus
Fc/Fc+ (Fig. 2). Now, on the reverse scan, a reversible
two-electron process is observed at −1.57 V versus
Fc/Fc+. This process may be assigned to formation and
then two electron reduction of the [IrI(CO)(PPh3)3]+
species [11]. The electrochemical behaviour for the sec-
ond oxidation step is therefore readily shown to be of
the EC type as described in Eq. (3).
Clearly, the formally 16-electron [IrIIIH(CO)-
(PPh3)3]2+ dicationic species undergoes overall re-
ductive elimination of a proton on long time scale
voltammetric reduction experiments according to the
reaction shown in Eq. (4). This fact is evidenced by the
generation of a reduction process (Eq. (5)) at very
negative potentials after scanning of the potential is
(3)
The voltammetric data obtained in dichloromethane
are summarised in Table 1 and are generally consistent
with earlier studies which were undertaken in acetoni-
trile/toluene and dichloroethane media [4,5]. However,
unlike the case in dichloroethane, in dichloromethane
this second process is not fully reversible in the chemi-
cal sense at slow scan rates (cyclic voltammetry) or slow
rotation rates (rotating disc electrode voltammetry) at
20°C. Furthermore, the nature of the electrolyte anion
(PF6−, BF4− and ClO4−) is crucial to the degree of
chemical reversibility as well as scan rate and tempera-
ture. Maximum chemical reversibility for the second
process was detected at fast scan rates or at low temper-
Fig. 1. Comparison of (a) experimental and (b) simulated cyclic
voltammograms at 20°C for the initial one-electron oxidation process
of IrH(CO)(PPh3)3 (4 mM) in dichloromethane (0.2 M nBu4NPF6) at
a GC macro-disc electrode (d, 1.0 mm) using a scan rate of 100 mV
s
−1. Simulations were undertaken with a diffusion coefficient value
of 5×10−6 cm2
s F
−1, a double-layer capacitance of 1×10−7
n
atures with 0.2 M Bu4NPF6 (Fig. 2) as the electrolyte,
cm−2 and uncompensated resistance of 1200 V.

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A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
electrons per mole of complex. However, this experiment
results in about an equimolar mixture of [IrH(CO)-
(PPh3)3]+ and [Ir(CO)(PPh3)3]+ as calculated on the
basic of voltammetric peak heights obtained after elec-
trolysis. Electro-regeneration of IrH(CO)(PPh3)3 is ef-
fected by holding the potential at −1.9 V versus Fc/Fc+,
but only about 40% of the starting material is recovered.
Bulk electrolysis on the plateau of the second oxidation
process (0.5 V versus Fc/Fc+) requires 2 equiv. of
electrons per mole of complex and [Ir(CO)(PPh3)3]+ is
the final product. These data also imply that
[IrH(CO)(PPh3)3]+ is not completely stable and that on
long time scales, the reaction given in Eq. (6) occurs. The
presence of other minor side reactions is also indicated
[4,5].
[IrIIH(CO)(PPh3)3]+“[IrI(CO)(PPh3)3]++1/2H2
(6)
3.2. IR spectroelectrochemistry
The IR spectrum of [IrH(CO)(PPh3)3] contains two
bands at 1924 and 2071 cm−1, which can be assigned to
w(CO) and w(IrH), respectively [4,12] (Table 2). Upon
oxidation in an IRRAS cell at −0.22 V versus Fc/Fc+
at 20°C, the spectral changes shown in Fig. 3(a) occur.
In this and other IR spectroelectrochemical experiments,
the absorbance, relative to that of the parent complex,
is plotted as a function of wavenumber (i.e. the difference
spectrum). Those bands that appear to grow in Fig. 3(a)
by giving negative absorbance differences at 1924 and
Fig. 2. Voltammograms over a wide potential range for oxidation of
IrH(CO)(PPh3)3 (4 mM) in dichloromethane (0.2 M nBu4NPF6) at a
GC macro-electrode (d, 3.0 mm). (a) Cyclic voltammogram at a scan
rate of 200 mV s−1 and (b) rotated disc voltammogram at a scan rate
of 10 mV s−1 and rotation rate of 500 rpm.
Table 2
IR spectroscopic data a
Complex
w(CO) (cm−1
)
w(IrH) (cm−1
)
[IrH(CO) (PPh3)3]
[IrH(CO)(PPh3)3]+
[IrH(CO)(PPh3)3]2+
[Ir(CO)(PPh3)3]+
1924 b
1988 b
2068 c
2017 b
2071 b
2104 b
2176 c
a Recorded in an IRRAS cell in dichloromethane (0.2
nBu4NPF6).
M
b Electrogenerated at 20°C.
c Electrogenerated at −45°C.
reversed at more positive values than for the second
process under conditions when the second process is
chemically irreversible (Fig. 2).
k
[IrIIIH(CO)(PPh3)3]2+ “1 [IrI(CO)(PPh3)3]++H+
(4)
[IrH(CO)(PPh3)3]2+ +2e−
k
2
X [Ir(CO)(PPh3)3]−+H+ X IrH(CO)(PPh3)3
(5)
k
−2
The iridium 16-electron dication is considerably more
stable on the voltammetric time scale than its rhodium
analogue [3], where no reduction component is associ-
ated with the second oxidation process, even at fast scan
rates and low temperatures.
Bulk electrolysis of IrH(CO)(PPh3)3 in dichloro-
methane (0.2 M nBu4NBF4 or 0.2 M nBu4NClO4) at 20°C
at potentials between the first and second oxidation
processes (−0.22 V versus Fc/Fc+) requires 1 equiv. of
Fig. 3. Changes in the IR difference spectra accompanying the
oxidation of
2 mM IrH(CO)(PPh3)3 in an IRRAS cell in
dichloromethane (0.2 M nBu4NPF6). (a) First oxidation at 20°C and
(b) second oxidation process at −45°C.

A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
569
2071 cm−1 signify that depletion of [IrH(CO)(PPh3)3]
occurs within the thin-layer, whilst those that grow with
a positive absorbance difference at 1988 and 2104 cm−1
are attributed to the formation of the product,
[IrH(CO)(PPh3)3]+. The changes in the IR spectra oc-
cur with retention of isobestic points and re-reduction
quantitatively regenerates the starting spectrum in its
entirety, such that the one-electron [IrH(CO)(PPh3)3]+/0
process is fully reversible under the experimental condi-
tions and on the time scale of the IR spectroelectro-
chemical experiment. Reactions that occur on very long
time scales (10–20 min) of bulk electrolysis experiments
(Eq. (6)) do not influence either voltammetric or rapid
thin-layer electrolysis experiments.
literature [12]. However, the details of the spectrum
have not been assigned, via comparison with simulated
spectra.
The [IrIIH(CO)(PPh3)3]+ cation was electrogenerated
by bulk electrolysis at a platinum basket electrode in
dichloromethane for the EPR measurements at a poten-
tial of −0.22 V versus Fc+/Fc, which was approxi-
mately 0.25 V more positive than the reversible
potential of the first oxidation process but still well
removed from the potential of the second oxidation
process. The electrogeneration was carried out at both
low (approximately −45°C) and ambient temperature
in dichloromethane, but this had no effect on the EPR
spectrum of the product. Fortunately, any diamagne-
tic [IrI(CO)(PPh3)3]+ formed as in Eq. (6) does not
cause a problem with respect to the measurement and
interpretation of EPR spectra of the paramagnetic
[IrIIH(CO)(PPh3)3]+.
If, after electrogeneration of [IrH(CO)(PPh3)3]+ at
ambient temperature (20°C), the potential of the work-
ing electrode is stepped to +0.1 V versus Fc/Fc+ in
order to cause the second oxidation step to occur, the
bands at 1988 and 2104 cm−1 collapse and they are
replaced by a single band at 2017 cm−1. In this case
returning the potential of the working electrode to
−0.22 V does not cause any reductive current to flow
through the cell, nor do any further spectral changes
occur. Thus, the one-electron oxidation of [IrH(CO)-
(PPh3)3]+ is chemically irreversible on the time scale of
IR spectroelectrochemical experiments conducted at
20°C. The product formed from an overall two-electron
oxidation is [Ir(CO)(PPh3)3]+, as identified by the
wavenumber of 2017 cm−1 for the w(CO) band [13].
This result is consistent with the conclusion made on
the basis of voltammetric data (Fig. 2).
n
A frozen dichloromethane (0.2 M Bu4NBF4) EPR
spectrum at 77 K obtained from the electrochemically
generated [IrIIH(CO)(PPh3)3]+ species is presented in
Fig. 4(a) and is consistent with the literature [12]. The
EPR signal consists of several lines, which is expected
from the strong anisotropy of the g-value and the
presence of hyperfine structure. The signal manifold
exhibits some resemblance to that obtained with the
rhodium analogue [3], even though it appears to be
apparently less complex. Comparisons with simulated
spectra suggest that the EPR response of
[IrIIH(CO)(PPh3)3]+ is constructed from two similar
sets of lines (A, B, C and D, E, F in Fig. 4), each set
corresponding to a significant anisotropy of the g-
value. However, for the Ir complex, the lines B and D
partly overlap, as do E and C. As in the case with the
rhodium analogue, the splitting between the two sets of
lines is attributed to strong hyperfine interaction of the
unpaired electron in a low spin Ir(II) complex with
either a P or H nucleus, possessing a nuclear spin of
1/2. Further hyperfine splitting can be discerned in the
quartets C and F, which must arise from the interaction
of the unpaired electron with the Ir metal atoms having
nuclear spin of 3/2.
If the oxidation of [IrH(CO)(PPh3)3]+ is carried out
at low temperature (−45°C), the spectral changes
shown in Fig. 3(b) occur, and they are very different to
those observed at 20°C. In this low-temperature study,
the bands due to [IrH(CO)(PPh3)3]+ at 1988 and 2104
cm−1 collapse with the growth of new bands at 2068
and 2176 cm−1. If the electrolysis is stopped after
oxidation of approximately 20% of [IrH(CO)(PPh3)3]+,
and the potential of the working electrode returned to
−0.22 V, then [IrH(CO)(PPh3)3]+ is quantitatively re-
generated. Conversely, if the oxidation is allowed to
continue past this point, isobestic points are lost and
the starting spectrum cannot be fully regenerated. The
bands at 2068 and 2176 cm−1 are attributed to the
formation of the transient dication, [IrH(CO)-
(PPh3)3]2+, which is moderately stable at low tempera-
ture, but which is rapidly deprotonated at room tem-
perature. This appears to be the first time the IR
spectrum of [IrH(CO)(PPh3)3]2+ has been observed.
The anisotropy of the g-value for the
[RhIIH(CO)(PPh3)3]+ complex [3], may be accounted
for by assuming a pseudo square pyramidal structure
with a phosphorus atom at the apex (Cs symmetry).
With this geometry, the splitting between the two sets
of lines (A, B, C and D, E, F) is caused by the hyperfine
coupling with the apical phosphorus, which is then
amplified by the mixing of the singly occupied rhodium
orbital and the apical phosphorus s orbital.
3.3. EPR spectroscopy
Computer simulation of the EPR spectrum using the
Bruker ‘SIMFONIA’ program was undertaken in order to
confirm that a similar interpretation applies to the
An EPR spectrum of the one-electron oxidised
[IrII(H)(CO)(PPh3)3]+ cation has been reported in the

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A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
Fig. 6. Computed[17] pseudo square pyramidal structure for
[IrIIH(CO)(PH3)3]+
.
spectrum of [IrIIH(CO)(PPh3)3]+ and to quantify the
values of the EPR parameters. Fig. 4(a(ii)) illustrates the
result of the simulation with the following parameters:
g1=2.219, g2=2.147, g3=1.975; A(Pap)1=140.0 G;
A(Pap)2=147.0 G; A(Pap)3=200.0 G (P nucleus, spin
1/2); A(Ir)1=8.0 G; A(Ir)2=7.0 G; A(Ir)3=39.0 G (Ir
nucleus, spin 3/2); line-widths for one, two and three
components are 15.0, 10.0 and 10.0 G, respectively. A
Gaussian line-shape was used in the simulation and the
comparison with experimental results is excellent.
A frozen solution EPR spectrum of the species pre-
pared chemically by oxidation with the ferrocenium
cation in dichloromethane, rather than by controlled
potential electrolysis, represents the superimposition of
the above spectrum and residual ferrocenium cation.
An EPR spectrum of the related [IrIICl(CO)(PPh3)3]+
cation has also been reported in the literature (Fig. 4(b))
[14]. The spectrum bears a strong resemblance to that of
the hydride analogue. However, an important difference
is that the lines C and F now appear to be more like
triplets rather than quartets, which may be explained if
hyperfine interaction with the two in-plane phosphorus
atoms takes precedence over iridium coupling. The
original description of the EPR signal for
[IrIICl(CO)(PPh3)3]+ solely in terms of two g-values of
2.128 and 2.00 therefore seems to represent an over
simplification. Fig. 4(b(ii)) presents the result of a com-
puter simulation with the following parameters: g1=
2.3300, g2=2.2370, g3=2.0640; A(Pap)1=180.0 G;
A(Pap)2=182.0 G; A(Pap)3=235.0 G; A(Peq)1=18.0 G;
A(Peq)2=8.0 G; A(Peq)3=33.0 G (two nuclei); line-
widths for one, two and three components are 15.0, 10.0
and 10.0 G, respectively. Again, Gaussian line-shape was
used in the simulation and the comparison with experi-
mental results is excellent.
Fig. 4. (a) (i) Experiment and (ii) simulated EPR spectra of (a)
[IrIIH(CO)(PPh3)3]+ and (b) [IrIICl(CO)(PPh3)3]+
.
Fig. 5. Predicted [17] changes in the geometric structure and frontier
orbital domain accompanying the oxidation of the IrH(CO)(PPh3)3 to
the [IrIIH(CO)(PPh3)3]+ complex: widening of the angle a lifts the
degeneration of the two highest occupied d orbitals. In the new
3.4. Density functional calculations
Results [17] of density functional calculations
geometry, the HOMO, by convention, is assigned as the dz orbital.
on the [IrIIX(CO)(PH3)3]+ (X=H or Cl) and the
2

A.M. Bond et al. / Inorganica Chimica Acta 300–302 (2000) 565–571
571
[RhIIH(CO)(PH3)3]+ cations also suggest that
[IrIIX(CO)(PPh3)3]+ complexes are likely to have square
pyramidal structures with a phosphorus atom in the apex
and the other two P atoms trans to each other. In
reaching this conclusion, it is assumed that the more
[2] G.W. Parshall, Homogeneous Catalysis: the Applications and
Chemistry of Catalysis by Soluble Transition Metal Complexes,
Wiley, New York, 1992.
[3] D. Menglet, A.M. Bond, K. Coutinho, R.S. Dickson, G.
Lazarev, S.A. Olsen, J.R. Pilbrow, J. Am. Chem. Soc. 120 (1998)
2086.
[4] G. Pilloni, G. Schiavon, G. Zotti, S. Zecchin, J. Organomet.
Chem. 134 (1977) 305.
[5] S. Valcher, G. Pilloni, M. Martelli, J. Electroanal. Chem. 42
(1973) 5.
[6] N. Ahmad, S.D. Robinson, M.F. Uttley, J. Chem. Soc., Dalton
Trans. (1972) 843.
[7] C. Moinet, D. Peltier, Bull. Soc. Chim. Fr. (1969) 690.
[8] M. Rudolph, D.P. Reddy, S.W. Feldberg, Anal. Chem. 66 (1994)
589.
complex,
from
the
calculation
perspective,
[IrIIX(CO)(PPh3)3]+, would produce analogous results to
those undertaken with [IrIIX(CO)(PH3)3]+. Indeed, all d7
trigonal bipyramidal complexes would be Jahn–Teller
unstable and may therefore be expected to rearrange by
widening the PꢀIrꢀP or PꢀRhꢀP angle [15,16] (Fig. 5),
which results in the calculated pseudo square pyramidal
structure shown in Fig. 6 [17].
[9] S.P. Best, C.A. Ciniawasky, D.G. Humphrey, J. Chem. Soc.,
Dalton Trans. (1996) 2945.
[10] S.P. Best, R.J.H. Clark, R.P. Cooney, R.C.S. McQueen, Rev.
Sci. Instrum. 58 (1987) 2071.
[11] G. Zotti, S. Zecchin, G. Pilloni, J. Organomet. Chem. 246 (1983)
61.
[12] S. Zecchin, G. Zotti, G. Pilloni, J. Organomet. Chem. 294 (1985)
379.
[13] J. Peone Jr., L. Vaska, Angew. Chem., Int. Ed. Engl. 10 (1971)
511.
Acknowledgements
The authors express their appreciation to Dr Stuart
MacGregor for provision of results of the density func-
tional calculations. Financial assistance from the Aus-
tralian Research Council also is gratefully acknowledged.
[14] M. Kubota, M.K. Chan, D.C. Boyd, K.R. Mann, Inorg. Chem.
26 (1987) 3261.
[15] J.F. Reihl, Y. Jean, O. Eisenstein, M. Pe´lissier, Organometallics
11 (1992) 729.
References
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[17] S.A. Macgregor, Heriot–Watt University, Edinburgh, UK, un-
published data, 1999.
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.