EPR, electronic spectra, and electron transfer properties of the 17 electron carbonylhydrotris(triphenylphosphine)rhodium(II) cation

Article abstract of DOI:10.1021/ja973164x

An unusually stable five-coordinate monomeric divalent rhodium complex, [Rh(II)(H)(CO)(PPh₃)₃]⁺, is produced by bulk oxidative electrolysis or chemical oxidation of Rh(I)(H)(CO)(PPh₃)₃ in dichloromethane. Consequently, odd as well as even electronic configurations are available in this well- known catalytic system. The EPR and electronic spectra of electrogenerated paramagnetic 17-electron cation [Rh(II)(H)(CO)(PPh₃)₃]⁺ have been obtained at low temperatures as has the EPR spectrum of the deuterated analogue. Computer simulation of the EPR spectra of the hydride and deuteride complexes reveals three g-values and anisotropic coupling constants for hydrogen, phosphorus, and rhodium. One of the phosphorus coupling constants is very large (A₁ = 175.0 G; A₂ = 176.0 G; A₃ = 230.0 G). This may be accounted for if [Rh(II)(H)(CO)(PPh₃)₃⁺ has the square pyramidal structure, and substantial mixing of the singly occupied metal orbital and the apical phosphorus s-orbital are considered. NMR measurements on mixture of Rh(I)(H)(CO)(PPh₃)₃ and [Rh(II)(H)-(CO)(PPh₃)]⁺ are consistent with a very fast electron self-exchange reaction and the heterogeneous charge- transfer rate constant for the [Rh(II)(H)(CO)(PPhh₃)₃](+/0) redox couple also is very fast. One electron electrochemical oxidation of [Rh(II)(H)(CO)(PPh)₃]⁺ to [Rh(III)(H)(CO)(PPh₃)₃]²⁺ is followed by a very fast reductive elimination reaction (loss of proton) which generates [Rh(I)(CO)(PPh₃)₃]⁺.

Full text of DOI:10.1021/ja973164x

2086
J. Am. Chem. Soc. 1998, 120, 2086-2089
EPR, Electronic Spectra, and Electron Transfer Properties of the 17
Electron Carbonylhydrotris(triphenylphosphine)rhodium(II) Cation
Dmitri Menglet,† Alan M. Bond,*,† Kevin Coutinho,† Ron S. Dickson,† Georgii G. Lazarev,†
Scott A. Olsen,† and John R. Pilbrow‡
Contribution from the Departments of Chemistry and Physics, Monash UniVersity, Clayton,
Victoria 3168, Australia
ReceiVed September 9, 1997
Abstract: An unusually stable five-coordinate monomeric divalent rhodium complex, [RhII(H)(CO)(PPh3)3]+,
is produced by bulk oxidative electrolysis or chemical oxidation of RhI(H)(CO)(PPh3)3 in dichloromethane.
Consequently, odd as well as even electronic configurations are available in this well-known catalytic system.
The EPR and electronic spectra of electrogenerated paramagnetic 17-electron cation [RhII(H)(CO)(PPh3)3]+
have been obtained at low temperatures as has the EPR spectrum of the deuterated analogue. Computer
simulation of the EPR spectra of the hydride and deuteride complexes reveals three g-values and anisotropic
coupling constants for hydrogen, phosphorus, and rhodium. One of the phosphorus coupling constants is very
large (A1 ) 175.0 G; A2 ) 176.0 G; A3 ) 230.0 G). This may be accounted for if [RhII(H)(CO)(PPh3)3]+ has
the square pyramidal structure, and substantial mixing of the singly occupied metal orbital and the apical
phosphorus s-orbital are considered. NMR measurements on mixtures of RhI(H)(CO)(PPh3)3 and [RhII(H)-
(CO)(PPh3)]+ are consistent with a very fast electron self-exchange reaction and the heterogeneous charge-
transfer rate constant for the [RhII/I(H)(CO)(PPh3)3]+/0 redox couple also is very fast. One electron
electrochemical oxidation of [RhII(H)(CO)(PPh)3]+ to [RhIII(H)(CO)(PPh3)3]2+ is followed by a very fast
reductive elimination reaction (loss of proton) which generates [RhI(CO)(PPh3)3]+.
Introduction
spectroscopic study in dichloromethane which provides direct
evidence that a moderately stable, but rare, monomeric 17
electron Rh(II) compound is in fact formed by bulk electrolysis
of the important Rh(I) catalyst. The product of the second one-
electron process also is identified and other features associated
with electron-transfer reactions in this well-known catalytic
system also are established in this investigation.
The 18 electron RhI(H)(CO)(PPh3)3 complex is one of the
most commercially important homogeneous catalysts.1,2 Con-
sequently, the chemistry of this rhodium(I) compound has been
widely investigated in order to understand mechanisms associ-
ated with its reactivity. Of particular importance has been the
dissociation of the triphenylphosphine to generate a 16 electron
RhI(H)(CO)(PPh3)2 moiety.3,4
Experimental Section
An electrochemical study of the oxidation of the Rh(H)(CO)-
(PPh3)3 complex in acetonitrile was published in 1977.5,6 The
data revealed that a reversible one electron oxidation process
was available on the voltammetric time scale and that further
oxidation also was possible. However, no spectroscopic
evidence for existence of the presumed 17 electron monomeric
[Rh(H)(CO)(PPh3)3]+ species, containing the uncommon diva-
lent rhodium(II) oxidation state, was presented, nor were details
of the products of the second process characterized. In this
paper we wish to report the results of an electrochemical and
Rh(H)(CO)(PPh3)3 was prepared as described in the literature.7 The
deuterated sample used for electrooxidation and EPR measurements
was prepared in situ in both CH2Cl2 and CD2Cl2 solution by H-D
exchange3 using deuterium gas (Matheson). The completion of the
exchange reaction was confirmed by IR spectroscopy.3,8 The oxidized
form was prepared (but not isolated) by slow addition of a CH2Cl2
solution containing equivalent amount of ferrocenium hexafluorophos-
phate to the CH2Cl2 solution of the Rh(I) complex under nitrogen gas.
[RhI(CO)(PPh3)3]BF4 was synthesized according to the literature
procedure.9
Electrochemical measurements and electrosynthesis was carried out
in 0.2 M [Bun N]BF4 solution in CH2Cl2. The electrolyte, [Bun N]-
† Department of Chemistry.
4
4
BF4, was prepared by neutralizing aqueous [Bun N]OH (Aldrich) with
‡ Department of Physics.
4
(1) Jardine, F. H. Polyhedron 1982, 1, 569-605.
(2) Dickson, R. S. Homogeneous Catalysis with Compounds of Rhodium
and Iridium; D. Reidel Publishing Company, Dordrecht, 1985. Parshall,
G. W.; Ittel, S. D. Homogeneous Catalysis: the Applications and Chemistry
of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; John Wiley
& Sons: New York, 1992.
HBF4 (Aldrich). The product was washed by warm deionized water
and subsequently recrystallized three times from methanol/water (4:1)
and dried in vacuo for 4 h. HPLC purity grade CH2Cl2 (Mallinckrodt)
was used for the electrochemical measurements. Dichloromethane used
for the spectroelectrochemical experiments was predried over KOH
pellets before distilling from CaH2 immediately prior to an electro-
chemical experiment. Cyclic (CV) and rotating disk voltammetry
(3) Evans, D.; Yagupsky, G.; Wilkinson, G. J. Chem. Soc. (A) 1968,
2660-2665.
(4) Brown, J. M.; Canning, L. R.; Kent, A. G.; Sidebottom, P. J. J. Chem.
Soc., Chem. Commun. 1982, 721-723.
(7) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg. Synth.
1974, 15, 59-60.
(8) Vaska, L J. Am. Chem. Soc. 1966, 88, 4100-4101.
(9) Legzdins, P.; Mitchel, R. W.; Rempel, G. L.; Fuddick, J. D.;
Wilkinson, G. J. Chem. Soc. (A) 1970, 3322-3326.
(5) Valcher, S.; Pilloni, G.; Martelli, M. J. Electroanal. Chem. 1973,
42, App.5-App.6.
(6) Pilloni, G.; Schiavon, G.; Zotti, G.; Zecchin, S. J. Organomet. Chem.
1977, 134, 305-318.
S0002-7863(97)03164-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/24/1998

Properties of [RhII(H)(CO)(PPh3)3]+
J. Am. Chem. Soc., Vol. 120, No. 9, 1998 2087
the square root of scan rate was linear, and the peak-to-peak
separation of the oxidation and reduction peak potentials was
identical to those for the known reversible oxidation of ferrocene
for which the heterogeneous charge-transfer rate constant10 is
g1 cm s-1 and therefore reversible under conditions used for
the cyclic voltammetric experiments. Voltammograms for the
first oxidation process in dichloromethane are consistent with
literature data reported in acetonitrile.5,6 Importantly, the
voltammetry is unaffected by the presence of up to a 5-fold
concentration excess of triphenylphosphine in the solution. In
the thermodynamic sense, this result implies that the equilibrium
position for the reaction
Figure 1. Cyclic voltammograms of 1 × 10-3 M RhI(H)(CO)(PPh3)3
using a platinum macrodisk electrode in a dichloromethane solution at
298 K. Insert is a voltammogram for the reduction of 1 × 10-3 M of
[RhI(CO)(PPh3)3]+ which verifies that this is the product of the second
process for oxidation of [RhI(H)(CO)(PPh3)3]+.
Rh(H)(CO)(PPh3)3 h Rh(H)(CO)(PPh3)2 + PPh3 (1)
and the analogous reaction for the oxidized form of the
compound must lie a long way to the left in dichloromethane.
The initial oxidation process therefore can be assigned to the
chemically and electrochemically reversible reaction
measurements at platinum macrodisk working Pt auxiliary and Ag/
AgCl reference electrodes were carried out using a BAS-100 Electro-
chemical Analyzer or a Cypress System (CYSY-1R). A Metrohm 628-
10 assembly was used for rotating disk electrode experiments. The
potential of this electrode was calibrated against that of the reversible
Fc/Fc+ redox couple (Fc ) ferrocene) from voltammetric data obtained
from the oxidation of 5 × 10-4 M ferrocene in dichloromethane (0.1
RhI(H)(CO)(PPh3)3 h [RhII(H)(CO)(PPh3)3]+ + e- (2)
Rotating disk voltammetry confirms that the first oxidation
process is diffusion controlled, since the limiting current (iL) is
linearly dependent on the square root of rotation rate (rotation
rate 500-3000 rpm). Additionally, a plot of E vs ln[(iL - i)/
iL] is linear with a slope of RT/F at 20 °C at all rotation rates
examined. The reversible half wave potential is -0.48 V vs
Fc/Fc+ which is identical to the value obtained from cyclic
voltammetry.
A second chemically irreversible one electron oxidation
process (Figure 1) is observed at about 0 V vs Fc/Fc+ under
both cyclic voltammetric and rotating disk electrode conditions.
On the reverse scan, two reversible processes are observed at
-1.57 and -1.74 V vs Fc/Fc+ (Figure 1). These two processes
are identical to those observed when voltammetric reduction
of the compound [RhI(CO)(PPh3)3]+ is studied (Figure 1). This
process is therefore readily shown to be of the EC type, in which
the Rh(II/III) oxidation step is followed by rapid reductive
elimination of H+ resulting in the formation of [RhI(CO)-
(PPh3)3]+ (eqs 3 and 4):
M [Bun N]BF4). The electrolyte solutions were purged with nitrogen
4
gas, and the cell was constantly maintained under an inert atmosphere.
Bulk electrolysis was performed using a two-compartment cell, with
a platinum mesh basket working electrode, a double-fritted Ag/AgCl
reference electrode, and the second platinum mesh basket counter
electrode separated from the bulk solution by a glass frit. Dry ice/
acetone bath was used to achieve low temperatures (ca. 210 K).
Electronic spectra were recorded using a Perkin-Elmer λ9 double-
beam UV/vis/near-IR spectrophotometer with digital background
subtraction capability. The spectra of electrogenerated species were
collected in situ, by the use of an optically transparent thin layer-
electrochemical (OTTLE) quartz cell, with a fine platinum minigrid
as working electrode (ca. 70% transmittance), mounted within the
sample compartment of the spectrophotometer. The cell placed in the
reference beam was of similar profile and contained a matching section
of platinum minigrid. The deoxygenated sample solution was prepared
and transferred via syringe into the sample cell. The working, auxiliary,
and reference electrodes were added to the sample cell and connected
to a Thompson E-series Ministat potentiostat. During an experiment
the cells (sample and reference) were cryostatted in gastight, double-
glazed PTFE cell blocks, enabling both the cells and their contents to
be cooled by cold N2 gas. The N2 gas was chilled by passing it through
a copper coil immersed in liquid nitrogen. The electrolysis was
continued until the spectrum ceased to change and the current decayed
to a constant minimum. After completion, the potential was reset and
the spectrum of the starting complex regenerated.
[RhII(H)(CO)(PPh3)3]+ h
[RhIII(H)(CO)(PPh3)3]2+ + e- (3)
[RhIII(H)(CO)(PPh3)3]2+ f [RhI(CO)(PPh3)3]+ + H+
(4)
X-band EPR spectra of frozen solution (glass) in CH2Cl2 containing
0.2 M [Bun4N]BF4 electrolyte were recorded in deoxygenated EPR tubes
at 77 K, using Bruker ESP-300 E and Varian E-12 spectrometers.
1H NMR experiments on samples of partially electrolyzed solution
were obtained at 20 °C using a Bruker DRX 400 (MHz) spectrometer.
Chemical shifts are referenced against tetramethylsilane.
Such a reaction sequence has been observed for other
transition metal hydrides.11-14 After conversion to a common
reference potential scale the values observed for reduction of
[RhI(CO)(PPh3)3]+ are close to the published -0.88 and -1.15
V vs SCE, respectively, measured in THF15 and -1.31 and
Results and Discussion
(10) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Mann, T. F.;
Thormann, W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878-1882, and
references therein.
(11) Marken, F.; Bond, A. M.; Colton, R. Inorg. Chem. 1995, 34, 1705-
1710.
Electrochemistry and Electronic Spectroscopy. Cyclic
voltammograms of the Rh(H)(CO)(PPh3)3 complex at a platinum
disk electrode over the scan rate range of 50-2000 mV s-1 at
(12) Bianchini, C.; Peruzzini, M.; Ceccanti, A.; Laschi, F.; Zanello, P.
Inorg. Chim. Acta 1997, 259, 61-70.
room temperature in the presence of 0.2 M [Bun N]BF4 as the
4
electrolyte in dichloromethane solution reveal the presence of
a chemically and electrochemically reversible oxidation process
(Figure 1) which has a reversible potential of -0.48 V vs Fc/
Fc+ as calculated from the average of the oxidation and
reduction peak potentials. Thus, a plot of peak height versus
(13) Guedes da Silva, M. F. C. Port. Electrochim. Acta 1996, 14, 31-
43.
(14) Smith, K.-T.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 1993,
115, 8681-8689, and references therein.
(15) Lahuerta, P.; Soto, J.; Mugnier, Y.; Mo¨ıse, C.; Laviron, E. New J.
Chem. 1987, 11, 411-414.

2088 J. Am. Chem. Soc., Vol. 120, No. 9, 1998
Menglet et al.
Figure 3. (A) Experimental EPR spectrum (Bruker Instrument) of the
electrogenerated [RhII(H)(CO)(PPh3)3]+ complex at 77 K and (B)
simulated spectrum.
Figure 2. (A) Spectral changes accompanying electrochemical oxida-
tion of Rh(H)(CO)(PPh3)3 in a dichloromethane solution at 223 K. (B)
As for (A) but with 4-fold molar excess of triphenylphosphine. Arrows
indicate the direction of change. Estimated values of ꢀ for RhI(H)-
(CO)(PPh3)3 are 3.6 × 104 M-1 cm-1 (λ ) 34 500 cm-1), 1.8 × 104
M-1 cm-1 (λ ) 26 500 cm-1); and for [RhI(H)(CO)(PPh3)3]+ is 2.7 ×
104 M-1 cm-1 (λ ) 30 500 cm-1).
complete chemical and electrochemical reversibility of the first
oxidation process.
1
At 20 °C, the H NMR signal18 resulting from the hydridic
proton of the Rh(H)(CO)(PPh3)3 was observed before electroly-
sis as a singlet at ca. -9.8 ppm. The fine structure expected
from coupling to rhodium and phosphorus cannot be resolved
at ambient temperature.18 1H NMR spectra of the hydride
resonance obtained from solutions containing different ratios
of the Rh(I) and Rh(II) species were recorded during the course
of oxidative electrolysis. Substantial broadening of the signal
was observed for electrolyzed samples containing ca. 10 and
20% of the oxidized form, and the signal completely disappeared
for solutions containing greater than 40% of the Rh(II) complex.
This result confirms that [RhII(H)(CO)(PPh3)3]+ is a paramag-
netic 17 electron system and that the electron self-exchange
reaction between the RhI(H)(CO)(PPh3)3 and [RhII(H)(CO)-
(PPh3)3]+ complexes is very fast. A fast electron self-exchange
process also is consistent with the reversible diffusion controlled
process observed in cyclic and rotating disk voltammetric
experiments.
-1.56 vs Ag/Ag+, respectively, measured in CH3CN.16 The
second oxidation process observed in voltammetric studies in
acetonitrile5,6 is complicated by interaction with the solvent.
The [RhII(H)(CO)(PPh3)3]+ cation was initially electrogen-
erated under a nitrogen atmosphere in situ in a platinum OTTLE
cell at a potential which was 250 mV more positive than the
reversible potential of the first oxidation process. The OTTLE
experiments were carried out at low temperature (210 K) in
order to achieve complete chemical stability on the synthetic
time scale of the product of the first oxidation process of the
bulk electrolysis. Electrooxidation was performed both with
and without triphenylphosphine present in the solution to give
the electronic spectra reported in Figure 2 (parts A and B,
respectively). Triphenylphosphine, when present in a 4-fold
excess (Figure 2A), obscures the far UV region of the spectrum,
but otherwise both experiments give identical results. The well-
defined isosbestic points are consistent with the reaction given
in eq 2 being valid on the synthetic time scale at low temperature
and in the absence of oxygen. In bulk electrooxidation
experiments conducted on the hour time scale at 20 °C, slow
decomposition of [RhII(H)(CO)(PPh3)3]+ occurs to give [RhI-
(CO)(PPh3)3]+ as identified by IR measurements3,8,16 and
comparison of data obtained from an authentic sample of [RhI-
(CO)(PPh3)3]BF4. IR carbonyl bands are 1917 cm-1 for RhI-
(H)(CO)(PPh3)3 and 1977 cm-1 for [RhI(CO)(PPh3)3]+ in
dichloromethane solution. Small quantities of other unidentified
product(s) may be related to those formed by reaction of the
hydride complex with adventitious oxygen.17
An EPR spectrum of the electrochemically generated cationic
[RhII(H)(CO)(PPh3)3]+ species in frozen dichloromethane (0.2
M [Bun N]BF4 solution (77 K)) is presented in Figure 3a. The
4
complex signal consists of several lines, which indicates strong
anisotropy of the g-value and the presence of hyperfine structure.
The signal manifold comprises two similar sets of lines (A, B,
C and D, E, F), each set corresponding to a significant anisotropy
of the g-value. However, the C and D lines partly overlap.
Splitting between the two sets of lines is caused by strong
hyperfine interaction of the unpaired electron in a low spin Rh-
(II) complex with a nucleus, possessing the nuclear spin of 1/2.
Further hyperfine splitting can be discerned in the components
C and F.
If the original trigonal bipyramidal structure19 (C3V symmetry)
were retained for the cationic species [RhII(H)(CO)(PPh3)3]+,
one would expect to observe a uniaxial EPR signal showing
only two g-values. However the three g-values observed imply
lowering of the symmetry upon oxidation. Indeed, the trigonal
bipyramidal Rh(II) complex would be Jahn-Teller unstable and
may rearrange by widening the P-Rh-P angle20,21 which would
NMR and EPR Spectroscopy. The [RhII(H)(CO)(PPh3)3]+
cation also was electrogenerated for proton NMR and EPR
measurements using the same applied potential in the OTTLE
experiments but with a platinum basket working electrode. The
electrogeneration was carried out at both low (ca. 210 K) and
ambient temperature, and this had no effect the EPR spectrum
of the product. Coulometric monitoring of this oxidation process
is consistent with a one electron process, and comparison of
voltammograms before and after bulk electrolysis confirms the
(17) Dudley, C. W.; Read, G.; Walker, P. J. C. J. Chem. Soc., Dalton
Trans. 1974, 1926-1931.
(18) Dewhirst, K. C.; Keim, W.; Reilly, C. A. Inorg. Chem. 1968, 7,
546-551.
(19) La Placa, S. J.; Ibers, J. A. Acta Crystallogr. 1965, 18, 511-519.
(20) Jean, Y.; Eisenstein, O. Polyhedron 1988, 7, 405-407.
(21) Reihl, J.-F.; Jean, Y.; Eisenstein, O.; Pe´lissier, M. Organometallics
1992, 11, 729-737.
(16) Zotti, G.; Zecchin, S.; Pilloni, G. J. Organomet. Chem. 1983, 246,
61-71.

Properties of [RhII(H)(CO)(PPh3)3]+
J. Am. Chem. Soc., Vol. 120, No. 9, 1998 2089
and results confirm the assignment of parameters associated with
the 1H nucleus. The simulation of the deuteride analogue was
undertaken with the same g-values as for the hydride and the
following hyperfine coupling constants: A(Pap)1 ) 180.0 G
(177.77 × 10-4 cm-1); A(Pap)2 ) 182.0 G (175.79 × 10-4
cm-1), A(Pap)3 ) 235.0 G (219.53 × 10-4 cm-1) (one nucleus);
A(Peq)1 ) 6.0 G (5.93 × 10-4 cm-1), A(Peq)2 ) 16.0 G (15.45
× 10-4 cm-1), A(Peq)3 ) 12.0 G (11.21 × 10-4 cm-1) (two
nuclei); A(D)1 ) 0.6 G (0.59 × 10-4 cm-1); A(D)2 ) 1.96 G
(1.89 × 10-4 cm-1), A(D)3 ) 1.38 G (1.29 × 10-4 cm-1) (one
nucleus); A(Rh)1 ) 8.0 G (7.90 × 10-4 cm-1); A(Rh)2 ) 3.0 G
(2.90 × 10-4 cm-1); A(Rh)3 ) 4.0 G (3.74 × 10-4 cm-1) (one
nucleus); line widths for 1, 2, and 3 components are 9.0, 8.0,
and 6.0 G, respectively. Differences between the rhodium
hyperfine coupling constants in the protonated and deuterated
species are not regarded as being statistically reliable.
Figure 4. (A) Experimental EPR spectrum (Varian Instrument) of the
electrogenerated [RhII(D)(CO)(PPh3)3]+ complex at 77 K and (B)
simulated spectrum.
Finally, it can be noted that the g-values and A(Rh) are
comparable with those published for other compounds22,24,25
which further supports the assignment of the EPR parameters
for the [RhII(H)(CO)(PPh3)3]+ complex.
result in a square pyramidal structure (Cs symmetry, see Figure
3). This rearrangement may occur virtually without an activa-
tion barrier, and, therefore, the proposal of a structural rear-
rangement is not inconsistent with the fast homogeneous and
heterogeneous electron-transfer rates suggested by voltammetric
and NMR data, respectively. Crystallographic studies also
provide examples of pentacoordinate Rh(II) complexes having
near square pyramidal geometry.22,23
A square pyramidal structure with a phosphorus atom at the
apex would account for the anisotropy of the g-value. The
splitting between the two sets of lines (A, B, C and D, E, F)
appears to be due to the hyperfine coupling with the apical
phosphorus, which is amplified by the mixing of the singly
occupied dz2 rhodium orbital and the apical phosphorus s-orbital.
The hyperfine structure of the components C and F must be
due to the equatorial phosphorus nuclei, the proton and the
central rhodium atom, all of which have a spin of 1/2.
Conclusions
Electrochemical oxidation of the widely used 18 electron
rhodium(I) catalyst RhI(H)(CO)(PPh3)3 leads to an unusually
stable monomeric 17 electron rhodium(II) complex [RhII(H)-
(CO)(PPh3)3]+. The oxidation is so facile that it may need to
be considered in processes catalyzed by RhI(H)(CO)(PPh3)3.
Voltammetric and UV-visible spectral monitoring of the
oxidation process in the presence and absence of PPh3 ligand
confirm that the five-coordinate geometry is thermodynamically
favored in both oxidation states at low temperatures in dichlo-
romethane. The EPR study of the electrochemically generated
cation reveals an uncommonly rich spectrum and, in concert
with other techniques, indicates that a rapid rearrangement from
trigonal bipyramidal to square pyramidal structure occurs upon
oxidation. Commonly, monomeric rhodium redox chemistry
is dominated by the Rh(I) and Rh(III) oxidation states and
reductive elimination and oxidative addition reactions. Many
of the stable complexes of formally divalent rhodium are in
fact metal-metal bonded, dimeric, diamagnetic compounds.26
The identification of a stable 17 electron monomeric Rh(II)
paramagnetic compound therefore establishes another novel
feature of this well-known catalytic system where even number
diamagnetic 18 or 16 electron species are known to be important.
Further oxidation of [RhII(H)(CO)(PPh3)3]+ leads to the transient
formation of [RhIII(H)(CO)(PPh3)3]2+ which undergoes a reduc-
tive elimination reaction to form the well-known [RhI(CO)-
(PPh3)3]+ complex.
Computer simulation of the EPR spectrum using the Bruker
“Simfonia” program was undertaken in order to confirm both
the above interpretation of the spectrum and to quantify the
values of the EPR parameters. Figure 3b presents the result of
the simulation with the following parameters: g1 ) 2.1155, g2
) 2.0690, g3 ) 2.0010; A(Pap)1 ) 175.0 G (172.83 × 10-4
cm-1); A(Pap)2 ) 176.0 G (170.00 × 10-4 cm-1), A(Pap)3 )
230.0 G (214.85 × 10-4 cm-1) (one nucleus); A(eq)1 ) 4.0 G
(3.96 × 10-4 cm-1), A(eq)2 ) 13.0 G (12.57 × 10-4 cm-1),
A(eq)3 ) 9.4 G (8.79 × 10-4 cm-1) (three nuclei: two P and
one H); A(Rh)1 ) 12.0 G (11.87 × 10-4 cm-1); A(Rh)2 ) 4.0
G (3.87 × 10-4 cm-1); A(Rh)3 ) 16.5 G (15.44 × 10-4 cm-1
)
(one nucleus); line widths for 1, 2, and 3 components are 11.0,
9.0, and 4.0 G, respectively. A Gaussian line shape was used
in the simulation, and the comparison with experiment is
excellent.
Acknowledgment. The authors wish to thank Dr. Graham
Heath, Research School of Chemistry, The Australian National
University, Canberra for the use of UV/vis Spectroelectrochemi-
cal facilities. This paper is dedicated to the memory of Dr. Scott
A. Olsen, who died (aged 29) on April 29, 1996.
An EPR spectrum of the species prepared chemically by
ferricenium oxidation represents the superposition of the above
spectrum and that of the residual ferricenium cation.
The experimental and simulated spectra of the deuterated Rh-
(II) compound, [RhII(D)(CO)(PPh3)3]+, are shown in Figure 4
JA973164X
(24) Anderson, J. E.; Gregory, T. P. Inorg. Chem. 1989, 28, 3905-3909.
(25) Rotov, A. V.; Zhilyaev, A. N.; Baranovskii, I. B.; Larin, G. M.
Russ. J. Inorg. Chem. 1989, 34, 1079-1081.
(26) See, for example: Cotton, F. A., Walton, R. A. Multiple Bonds
between Metal Atoms, 2nd ed.; Clarendon Press: Oxford, 1993.
(22) Connelly, N. G.; Emslie, D. J. H.; Metz, B.; Orpen, A. G.; Quayle,
M. J. J. Chem. Soc., Chem. Commun. 1996, 2289-2290.
(23) Peng, S.-M.; Peters, K.; Peters, E.-M.; Simon, A. Inorg. Chim. Acta
1985, 101, L35-L36.