A kinetic study of the oxidative addition of methyl iodide to [Rh((C4H3S)COCHCOCF3)(CO)(PPh3)] utilizing UV/vis and IR spectrophotometry and 1H, 19F and 31P NMR spectroscopy. Synthesis of [Rh((C4H3S)COCHCOCF3)(CO)(PPh3)(CH3)(I)]

Article abstract of DOI:10.1016/j.ica.2007.07.010

IR and UV/vis spectrophotometric and ¹H, ¹⁹F and ³¹P NMR spectroscopic techniques have been utilized to study the kinetics of oxidative addition of CH₃I to [Rh(tta)(CO)(PPh₃)] with Htta = (C₄H₃S)COCH₂COCF₃ = 2-thenoyltrifluoroacetone. Two definite reactions steps involving isomers of at least two distinctly different classes of Rh^III-alkyl and a Rh^III-acyl species were observed. NMR spectroscopy revealed that each reaction product exists in solution of two observable isomers in equilibrium with each other. The observed rate of oxidative addition of iodomethane to the different [Rh(tta)(CO)(PPh₃)] isomers was the same, but the rate of formation of the two isomers of the final Rh^III-alkyl2 reaction product, [Rh(tta)(CO)(CH₃)(PPh₃)(I)], differs. Results provided the following reaction mechanism. {A figure is presented}. The equilibrium K₂ was fast enough to be maintained during the [Rh(tta)(CO)(PPh₃)] depletion in the first reaction step and during the Rh^III-alkyl2 formation in the second reaction step. The molecular formulae of all the Rh^III-alkyl and Rh^III-acyl species are [Rh(tta)(CH₃)(CO)(PPh₃)(I)] and [Rh(tta)(COCH₃)(PPh₃)(I)], respectively, but the geometries are different due to different co-ordination positions of the ligands. The final Rh^III-alkyl2 reaction product is isolated and characterized.

Full text of DOI:10.1016/j.ica.2007.07.010

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Inorganica Chimica Acta 361 (2008) 208–218
www.elsevier.com/locate/ica
A kinetic study of the oxidative addition of methyl iodide
to [Rh((C4H3S)COCHCOCF3)(CO)(PPh3)] utilizing UV/vis and
1
19
31
IR spectrophotometry and H, F and P NMR
spectroscopy. Synthesis of
[Rh((C4H3S)COCHCOCF3)(CO)(PPh3)(CH3)(I)]
*
Marrigje M. Conradie, Jeanet Conradie
Department of Chemistry, University of the Free State, Bloemfontein 9301, South Africa
Received 5 May 2007; received in revised form 8 July 2007; accepted 9 July 2007
Available online 19 July 2007
Abstract
1
IR and UV/vis spectrophotometric and H, 19F and 31P NMR spectroscopic techniques have been utilized to study the kinetics of
oxidative addition of CH3I to [Rh(tta)(CO)(PPh3)] with Htta = (C4H3S)COCH2COCF3 = 2-thenoyltrifluoroacetone. Two definite reac-
tions steps involving isomers of at least two distinctly different classes of RhIII-alkyl and a RhIII-acyl species were observed. NMR spec-
troscopy revealed that each reaction product exists in solution of two observable isomers in equilibrium with each other. The observed
rate of oxidative addition of iodomethane to the different [Rh(tta)(CO)(PPh3)] isomers was the same, but the rate of formation of the two
isomers of the final RhIII-alkyl2 reaction product, [Rh(tta)(CO)(CH3)(PPh3)(I)], differs. Results provided the following reaction
mechanism.
RhI-A
Kc1
RhI-B
[RhIII-alkyl1A]
[RhIII-acyl1A]
[RhIII-alkyl2A]
k3A
K2, k2
K1, k1
k-3A
k3B
Kc2
Kc3
Kc4
[RhIII-alkyl2B]
+ CH3I
slow
k-1
k-2
[RhIII-alkyl1B]
[RhIII-acyl1B]
k-3B
The equilibrium K2 was fast enough to be maintained during the [Rh(tta)(CO)(PPh3)] depletion in the first reaction step and during
the RhIII-alkyl2 formation in the second reaction step. The molecular formulae of all the RhIII-alkyl and RhIII-acyl species are
[Rh(tta)(CH3)(CO)(PPh3)(I)] and [Rh(tta)(COCH3)(PPh3)(I)], respectively, but the geometries are different due to different co-ordination
positions of the ligands. The final RhIII-alkyl2 reaction product is isolated and characterized.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: b-Diketonato; Thienyl; Rhodium(I); Rhodium(III); Oxidative addition; NMR; Carbonyl; Phosphine
1. Introduction
Rhodium complex compounds are one of the most
widely spread industrial homogeneous catalysts for organic
raw material processing. As classic examples of efficacious
catalyst system, one may name the reactions of
*
methanol carbonylation to acetic acid in the presence of
Corresponding author. Tel.: +27 51 4012194; fax: +27 51 4446384.
E-mail address: conradj.sci@ufs.ac.za (J. Conradie).
[Rh(CO)2I2]ꢀ complex, alkene hydroformylation on
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.07.010

M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
209
RhHCO(PPh3)2 catalyst and hydrogenation of olefins and
2. Experimental
acetylenes with the help of RhCl(PPh3)3 [1]. The oxidative
addition of methyl iodide to rhodium(I) carbonyl com-
pounds has significant implications in catalysis, especially
when followed by methyl migration to give the acyl deriv-
ative [2,3]. The high catalytic reactivity of rhodium com-
plexes is in many respects due to the nature of ligand
surroundings [4,5]. There upon, in order to achieve a better
understanding of the ways of purposeful alteration of the
reactivity of such systems, a study of the effects of the ancil-
lary ligands on the rate of oxidative addition and subse-
quent carbonyl insertion and deinsertion steps, a detailed
kinetic study of such systems is of great importance.
Reported studies on the oxidative addition of methyl
iodide to [RhI(b-diketonato)(CO)(PPh3)] complexes, pro-
posed an initial ionic intermediate followed by two consec-
utive equilibrium steps to give an alkyl [RhIII(b-
diketonato)(CO)(PPh3)(CH3)(I)] as final product [6,7]:
2.1. Materials and apparatus
Solid reagents used in preparations (Merck, Aldrich and
Fluka) were used without further purification. Liquid
reactants and solvents were distilled prior to use; water
was doubly distilled. Organic solvents were dried according
to published methods [16]. Melting points (M.p.) were
determined with an Olympus BX51 system microscope
assembled on top of a Linkam THMS600 stage and
connected to a Linkam TMS94 temperature programmer.
2.2. Synthesis
2.2.1. [Rh(tta)(CO)2] complex
[Rh2Cl2(CO)4] was prepared in situ by refluxing
RhCl3 Æ 3H2O (0.1001 g, 0.38 mmol) in DMF (3 ml) until
K1, k1
k3
k-3
k4
[Rh(β−diketonato)(CO)(PPh3)] + CH3I
[Rh(β−diketonato)(CO)(PPh3)(CH3)]+I-
[Rh(β−diketonato)(COCH3)(PPh3)(I)]
[Rh(β−diketonato)(CO)(PPh3)(CH3)(I)]
ð1Þ
k-1
k-4
Rh(III)-alkyl 1
Rh(III)-acyl
Rh(III)-alkyl 2
with b-diketonato = acacꢀ (acetylacetonato), tfdmaaꢀ (tri-
fluoro-dimethylacetylacetonato), tfaaꢀ (trifluoroacetyl-
acetonato) and hfaaꢀ (hexafluoroacetylacetonato). The
second alkyl species is not observed for the oxidative addi-
the colour changed from red to yellow (ca. 30 min)
[17,18]. The dimer-containing solution was allowed to cool
on ice and an equivalent amount of solid b-diketone Htta
(0.0844 g, 0.38 mmol) was slowly added while stirring.
After 30 min of stirring at room temperature, the crude
[Rh(tta)(CO)2] complex was precipitated with an excess
of water, filtered, dried in air and recrystallized from hex-
ane. Yield: 0.1294 g, 89.6%. M.p. 143.3–134.7 °C. IR
(cmꢀ1) = 2008, 2026, 2708 and 2098. 1H NMR (d/ppm,
CDCl3) 6.58 (1H, s, CH), 7.18 (1H, dd, 3J = 4.9 Hz,
tion reactions of methyl iodide to
a
series of
[RhI(L,L0)(CO)(PPh3)] complexes with L,L0 = monoanion-
ic bidentate ligand with donor atoms L and L0 other than a
b-diketonato ligand. Studies on different O,O [8], O,N [9],
O,S [9,10] and N,S [9,11] bidentate ligand systems revealed
that all the oxidative addition reactions of methyl iodide to
the different [RhI(L,L0)(CO)(PPh3)] complexes are special
cases of the following general reaction mechanism [9]:
3
4
3J = 4.0 Hz, CH), 7.72 (1H, dd, J = 4.9 Hz, J = 1.0 Hz,
3
4
CH), 7.80 (1H, dd, J = 4.0 Hz, J = 1.0 Hz, CH).
K1, k1
[Rh(L,L')(CO)(PPh3)] + CH3I
[Rh(L,L')(CO)(PPh3)(CH3)(I)]
k-1
Rh(III)-alkyl 1
2.2.2. [Rh(tta)(CO)(PPh3)] complex
k3
k-3
solvent
pathway
+S
S
-S
ð2Þ
-S
To a solution of [Rh(tta)(CO)2] (0.0760 g, 0.20 mmol) in
warm n-hexane (3 ml) was added an equivalent amount of
solution of PPh3 (0.0535 g, 0.20 mmol) in warm n-hexane
(3 ml). After 1 min of stirring the product [Rh(tta)-
(CO)(PPh3)] precipitated. The solvent was removed by
decantation, resulting in spectroscopically pure [Rh(tta)-
(CO)(PPh3)] as a yellow powder. Yield: 0.0493 g, 40.1%.
M.p. 181.9–182.4 °C. IR (cmꢀ1) = 1981. d1H NMR (d/
ppm, CDCl3) 6.42 (1H, s, isomer B CH), 6.43 (1H, s, iso-
mer A CH), 6.92 (1H, dd, 3J = 4.9 Hz, 3J = 3.8 Hz, isomer
+S
L
CO
[Rh(L,L')(COCH3)(PPh3)(I)]
MeI
+
Rh
Rh(III)-acyl
L'
PPh3
The solvent pathway was often not observed.
Although it is known that two isomers of the rhodium(I)
complexes [RhI(b-diketonato)(CO)(PPh3)] [12,13] and
[RhI(L,L0)(CO)(PPh3)] [14,15] may exist in solution for
unsymmetrical b-diketonato and L,L0 ligands, the above
studies utilizing UV/vis and IR techniques, did not distin-
guish between the reaction rates of the different isomers.
NMR spectroscopic techniques was used in this study to
obtain a cleared insight into the overall process that takes
place during the oxidative addition reaction of methyl
iodide to the two [Rh(tta)(CO)(PPh3)] isomers with
Htta = (C4H3S)COCHCOCF3 = thenoyltrifluoroacetone.
3
3
A CH), 7.13 (1H, dd, J = 4.9 Hz, J = 3.8 Hz, isomer B
CH), 7.18 (1H, dd, 3J = 3.8 Hz, 4J = 1.1 Hz, isomer A
CH), 7.37 (1H, dd, 3J = 4.9 Hz, 4J = 1.1 Hz, isomer A
CH), 7.40–7.45 (12H, m, CH), 7.45–7.50 (6H, m, CH),
7.63 (1H, dd, 3J = 4.9 Hz, 4J = 1.1 Hz, isomer B CH),

210
M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
7.65–7.73 (12H, m, CH), 7.76 (1H, dd, 3J = 3.8 Hz,
4J = 1.1 Hz, isomer B CH).
infrared spectrophotometer utilizing a He–Ne laser at
632.6 nm. UV/vis spectra were recorded on a Cary 50
Probe UV/vis spectrophotometer.
2.2.3. [Rh(tta)(CO)(PPh3)(CH3)(I)] complex
Methyliodide (60 ll, 0.96 mmol, a 20 fold excess) was
added to a solution of [Rh(tta)(CO)(PPh3)] (0.0300 g,
0.049 mmol) in chloroform (1.0 ml) at room temperature.
The reaction vessel was sealed and left at room tempera-
ture in the dark. After 1 day the solvent and excess
methyliodide were removed by evaporation, resulting
in spectroscopically pure [Rh(tta)(CO)(PPh3)(CH3)(I)].
Yield: 0.0329 g, 89%. M.p. 107.7–109.2 °C. IR (cmꢀ1) =
2064. 1H NMR (d/ppm, CDCl3) 1.89 (3H, dd,
2.4. Calculation of thermodynamic quantities
The variation of the equilibrium constant Kc with tem-
perature for the equilibrium shown in Scheme 1 may be
mathematically quantified by
ꢀ
ꢁ
DrH
R
1
1
ln KcðiiÞ ¼ ln KcðiÞ ꢀ
ꢀ
T
T
ðiiÞ
ðiÞ
where lnKc(ii) and lnKc(i) are the equilibrium constants at
temperatures T(ii) and T(i), R = 8.314 J Kꢀ1 molꢀ1 and
DrH the reaction enthalpy as defined elsewhere [19,20].
The thermodynamic quantity ‘‘Gibbs free energy’’, DrG,
and reaction entropy, DrS, may be calculated from the equa-
tions DrG = ꢀ RTlnKc and DrG = DrH ꢀ TDrS [20]. The
above relationship also implies that a graph of lnKc versus
1/T should be linear [21] with a slope equal to ꢀDrH/R.
3
2JH–C–Rh = 1.9 Hz, JH–C–Rh–P = 2.6 Hz, isomer B CH3),
2
3
1.90 (3H, dd, JH–C–Rh = 1.9 Hz, JH–C–Rh–P = 2.6 Hz, iso-
mer A CH3), 5.72 (1H, s, isomer A CH), 5.80 (1H, s, iso-
3
3
mer B CH), 7.04 (1H, dd, J = 4.9 Hz, J = 3.8 Hz, isomer
3
3
B CH), 7.06 (1H, dd, J = 4.9 Hz, J = 3.8 Hz, isomer A
CH), 7.59–7.66 (34H, m, CH); dC (600 MHz, CDCl3):
167.5 (q, JC–F = 33.0 Hz, C–CF3), 170.2 (q, JC–F
2
2
=
33.0 Hz, C–CF3), 179.1 (s, C–C4H3S), 180.2 (s, C–
1
2
C4H3S), 184.3 (dd, JC–Rh = 11.5 Hz, JC–P = 5.9 Hz,
2.5. Kinetic measurements
1
2
Rh–CO), 184.7 (dd, JC–Rh = 11.0 Hz, JC–P = 5.5 Hz,
Rh–CO). Elemental Anal. Calc. for RhC28PH22SO3IF3:
C, 44.5; H, 2.9. Found: C, 44.4; H, 2.8%.
Oxidative addition reactions were monitored on the IR
(by monitoring formation and disappearance of the car-
bonyl peaks at 1991, 2079, 2064 and 1728 cmꢀ1) and UV/
vis (by monitoring the change in absorbance at the wave-
length 370 and 400 nm) spectrophotometers in solution of
chloroform and on the NMR (by monitoring the change
in integration units of the specified signals) spectrometer
in solution of deuterated chloroform (CDCl3). All kinetic
measurements were monitored under pseudo-first-order
conditions with [CH3I] 10–5000 times the concentration
of the [RhI(b-diketonato)(CO)(PPh3)] complex in the
specified solution. The concentration [RhI(b-diketona-
to)(CO)(PPh3)] = 0.00005 mol dmꢀ3 for UV/vis measure-
2.3. Spectroscopy and spectrophotometry
NMR measurements at 298 K were recorded on a Bru-
ker Advance DPX 300 NMR spectrometer [1H
(300.130 MHz)] and a Bruker Advance II 600 NMR spec-
trometer [1H (600.130 MHz), 13C (150.903 MHz), 19F
(564.686 MHz) and 31P (242.937 MHz)]. The chemical
shifts were reported relative to SiMe4 (0.00 ppm) for the
1H spectra, relative to CFCl3 (0.00 ppm) for the 19F spec-
tra, relative to 85% H3PO4 (0 ppm) for the 31P spectra
and to CDCl3 (77.04 ppm) as internal marker for the 13C
spectra. Positive values indicate downfield shift. IR spectra
were recorded from neat samples on a Digilab FTS 2000
ments,
=0.008 mol dmꢀ3
for
IR
measurements
and = 0.014 mol dmꢀ3 for NMR measurements. Kinetic
measurements under pseudo-first-order conditions for dif-
O
O
F3C
S
2
O
O
CF3
CO
CO
2
Rh
DMF
OC
OC
Cl
Cl
CO
CO
2 RhCl3.H2O
Rh
Rh
S
- 2 CO
2 PPh3
F3C
enantiomers not displayed
F3C
F3C
F3C
I
I
O
O
O
O
O
O
O
O
CH3
CO
CO
CO
PPh3
CO
Kc4
Kc1
2 CH3I
Rh
Rh
Rh
Rh
CH3
PPh3
PPh3
PPh3
S
S
isomer A
isomer B
S
S
isomer 2A
isomer 2B
Scheme 1. Synthetic route for the synthesis of the [Rh(tta)(CO)2] and [Rh(tta)(CO)(PPh3)] from RhCl3 Æ 3H2O. Oxidative addition of CH3I to
[Rh(tta)(CO)(PPh3)] to yield [Rh(tta)(CO)(PPh3)(CH3)(I)] was shown by 1H NMR to be a slow equilibrium reaction.

M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
211
ferent concentrations of a [RhI(b-diketonato)(CO)(PPh3)]
complex, for a constant [CH3I] confirmed that the concen-
tration [RhI(b-diketonato)(CO)(PPh3)] did not influence
the value of the observed kinetic rate constant. The
observed first-order rate constants were obtained from
least-squares fits of absorbance (IR and UV/vis) or integra-
tion units (NMR) versus time data [21].
It is known that for complexes of the type [RhI (b-dike-
tonato)(CO)(PPh3)] with an unsymmetrical b-diketonato
ligand, two isomers, as shown in Scheme 1, exist in solution
[12,24]. The equilibrium constant, defined as Kc1 = [isomer
B]/[isomer A] for the [Rh(tta)(CO)(PPh3)] isomers applica-
ble to the equilibrium shown in Scheme 1, may be deter-
mined by calculating the ratio of peak integral values of
non-overlapping corresponding signals of each isomer
and averaging all answers. For [Rh(tta)(CO)(PPh3)] for
example, Kc1 may be calculated from the peak integrals
of the methine proton of the b-diketonato ligand tta of
the two isomers at 6.43 and 6.42 ppm, respectively, giving
Kc1 = 1.05/1.00 = 1.05 at 25 °C, see Fig. 1. 31P and 19F
NMR also differentiate between isomers 1 and 2 resulting
in the same equilibrium constant. The time required for
two [Rh(tta)(CO)(PPh3)] isomers to reach the solution
equilibrium position according to Scheme 1 was shorter
than the time required to dissolve the complex and to
record data for a NMR spectrum. The two [Rh(tta)(-
CO)(PPh3)] isomers co-exist in dynamic equilibrium in
The [Rh(tta)(CO)(PPh3)] complex was tested for stabil-
ity in chloroform by means of overlay IR and UV spectra
1
for at least 24 h. A H NMR spectrum of the title com-
pound after 48 h in CDCl3 confirmed stability in solution.
A linear relationship between UV absorbance (A) and con-
centration (c) for the complex [Rh(tta)(CO)(PPh3)] con-
firms the validity of the Beer Lambert law (A = ecl with
l = path length = 1 cm) for the [Rh(tta)(CO)(PPh3)] com-
plex. The extinction coefficient (e) in chloroform is
31(1) dm3 molꢀ1 cmꢀ1 at 390 nm.
2.6. Calculations
´
Pseudo-first-order rate constants, kobs, were calculated
solution. Although Trzeciak and Ziolkowski [25] reported
by fitting [21] kinetic data to the first-order equation [22]
that changes in temperature have little influence on the
spectra, it was possible to quantify the effect of temperature
on the isomer ratio. Fig. 1 illustrates the linear relationship
between lnKc1 and 1/T for [Rh(tta)(CO)(PPh3)] in CDCl3,
resulting in DrH = 12.1 kJ molꢀ1, DrG = ꢀ 111.2 J molꢀ1
(at 25 °C) and DrS = 41.1 J molꢀ1 Kꢀ1. The enthalpy effect
of the interconversion of the [Rh(tta)(CO)(PPh3)] isomers,
is of the same order as was found for [RhI(b-diketona-
to)(CO)(PPh3)] complexes with b-diketones containing a
ferrocene group [12]. The isomer ratio is solvent-dependent
with Kc1 = [B]/[A] = 1.05 in chloroform, 0.70 in acetone
and 1.50 in benzene. In acetonitrile, only isomer A was
observed.
½Aꢁt ¼ ½Aꢁ0eðꢀkobs tÞ
with [A]t and [A]0 the absorbance of the indicated species at
time t and at t = 0 (UV/vis or IR experiments) or integral
values for the specified peaks on NMR spectra. The exper-
imentally determined pseudo-first-order rate constants
were converted to second-order rate constants, k1, by deter-
mining the slope of the linear plots of kobs against the con-
centration of the incoming CH3I ligand. Non-zero
intercepts implied that kobs = k1 [CH3I] + k
and that
ꢀ1
the k step in the proposed reaction sequence exists.
ꢀ1
The activation parameters DH# (activation enthalpy)
and DS# (activation entropy), for the oxidative addition
reactions were determined from least-squares fits [21] of
the reaction rate constants versus temperature data
according to the Eyring relationship [23], in the linear
form
Regarding the two possible isomers, we chose to label
the isomer with CO trans to the oxygen atom nearest to
the thienyl group of the b-diketonato chelate ring as isomer
A, while the other isomer is labelled isomer B (Scheme 1).
Focussing on the positions of the signals of the thienyl pro-
k
T
DH# DS#
RT
kB
h
ln ¼ ꢀ
þ
þ ln
R
with k = rate constant, kB = Boltzmann’s constant, T =
temperature, h = Planck’s constant and R = universal gas
constant. The activation free energy DG# = DH# ꢀ TDS#.
All kinetic mathematical fits were done utilizing the fit-
ting program MINSQ [21]. The error of all the data is pre-
sented according to crystallographic conventions. For
example kobs = 0.0236(1) sꢀ1 implies kobs = (0.0236
0.0001) sꢀ1
.
3. Results and discussion
3.1. Synthesis and identification of complexes
Fig. 1. Temperature dependence of Kc1 for the equilibrium between the
two isomers of [Rh(tta)(CO)(PPh3)] (Scheme 1) as determined from the
ratio of the peak integral values of the 1H NMR methine signals
representing the two isomers, see inset.
The route utilized to obtain the rhodium(I) and the new
rhodium(III) complex is given in Scheme 1.

212
M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
tons of the 1H NMR spectrum of [Rh(tta)(CO)(PPh3)], the
signals of isomer B (7.13, 7.63 and 7.76 ppm) lie in the same
low field region as the position of the thienyl protons of the
free ligand Htta (7.23, 7.78 and 7.86 ppm) and of the dicar-
bonyl complex [Rh(tta)(CO)2] (7.18, 7.72 and 7.80 ppm).
The resonances of isomer B (6.92, 7.18 and 7.37 ppm),
however, is shifted 0.20–0.45 ppm to high field relative to
isomer B due to the shielding effect through space of the
PPh3 group. The position of the thienyl resonances of the
two [Rh(tta)(CO)(PPh3)] isomers was confirmed by
1H–1H TOCSY.
1.87 ppm (inset in Fig. 2), but they overlap in CDCl3 at
1.89 ppm (high resolution inset in Fig. 2) with
2J(1H–13C–103Rh) = 1.9 Hz and 3J(1H–13C–103Rh–31P) =
2.6 Hz.
In general, the multinuclear NMR spectra of the
solution, containing two isomeric [RhI(b-diketona-
to)(CO)(PPh3)] or [RhIII(b-diketonato)(CO)(PPh3)(CH3)-
(I)] complexes, are as follow: (i) 31P, two doublets with
1
similar values of d31P and J(P–Rh) corresponding to the
isomers A and B, (ii)13C, two doublets of doublets repre-
senting the Rh–CO resonance, with similar values of
d13C, 1J(C–Rh), and 2J(C–Rh–P) corresponding to the iso-
mers A and B and (iii) 19F, if the b-diketonato ligand con-
tains a CF3 group, two singlets with similar values of d19F,
representing the CF3 resonance (C and H decoupled) of the
b-diketonato ligand of isomers A and B, see Fig. 2 for
[Rh(tta)(CO)(PPh3)(CH3)(I)]. NMR spectral parameters
for [Rh(tta)(CO)(PPh3)] and [Rh(tta)(CO)(PPh3)(Me)(I)]-
alkyl2 are summarized in Tables 1 and 2. Characteristic
spectral data of the reaction intermediates during the oxi-
dative addition reaction of methyl iodide to [Rh(tta)-
(CO)(PPh3)], are also included in Table 1. The tabulated
data are in agreement with the observed resonance signals
of related complexes [27,15], see Table 2 for 13C NMR data
of selected rhodium(I) and rhodium(III) complexes. Plot-
ting the data of chemical shifts against m(CO) gives an
approximate correlation: on increasing m(CO) the d13C
tends to decrease (Fig. 3). The data also indicate that car-
bonyl shielding is sensitive to the composition of the com-
plex and to the oxidation state of the central atom. An
upfield shift of 4–6 ppm is observed for the RhIII-alkyl
complexes relative to rhodium(I). 1J(103Rh–13C) of the rho-
dium(I) complexes varies from 73.6 to 79.3 Hz when the
CO group is situated trans to O, 63.8–66.9 Hz for CO trans
to N and is 72.1 for CO trans to S.
The rhodium(I) complex [Rh(tta)(CO)(PPh3)] undergoes
oxidative addition with CH3I to give [Rh(tta)(CO)-
(CH3)(I)(PPh3)] as a final reaction product. [Rh(tta)(CO)-
(CH3)(I)(PPh3)], that will be referred to as the alkyl2
complex, was isolated and stable in the solid state for
months on end as well as in CDCl3 solution for at least
1
three weeks. The H NMR spectrum of alkyl2, presented
in Fig. 2, shows two sets of resonances representing two
alkyl2 isomers. The two low field signals at 5.80 and
5.72 ppm present the resonance of the methine protons of
the thenoyltrifluoroacetonato ligand of the [Rh(tta)(CO)-
(CH3)(I)(PPh3)] isomers. Changes in the temperature
(ꢀ30 to 55 °C in CDCl3 and ꢀ70 to 45 °C in acetone-d)
have little influence on the equilibrium constant, Kc4, of
the two [Rh(tta)(CO)(PPh3)(CH3)(I)]-alkyl2 isomers which
is 1.00 in chloroform. The observed equilibrium constants
were concentration independent over a concentration range
of ꢂ0.05–3 mmol dmꢀ3. After 2 weeks in CDCl3 at 25 °C,
the equilibrium constant, Kc4, was still 1.0. The equilibrium
constant between the two alkyl2 isomers is solvent-depen-
dent with Kc4 = 0.92 in acetonitrile, 1.05 in acetone and
1.10 in benzene.
In acetone-d6 at 25 °C, the doublet of doublets of each
of the methyl groups of the two isomers of [Rh(tta)-
(CO)(PPh3)(Me)(I)]-alkyl2 due to spin–spin coupling to
Rh and P [26], can clearly be distinguished at 1.83 and
3.2. Kinetics
3.2.1. UV/vis study
Two different reactions were observed when monitoring
the reaction between CH3I and [Rh(tta)(CO)(PPh3)] on a
UV/vis spectrophotometer. The first reaction is best fol-
lowed at 370 nm and the second reaction at 400 nm, as illus-
trated by the absorbance versus time data at these two
wavelengths in Fig. 4. The first reaction (at 25 °C:
k1 = 0.00171(4) dm3 molꢀ1 sꢀ1, kꢀ1 = 0.000023(6) sꢀ1, see
Fig. 5) showed a first-order dependence on CH3I, while
the second slower reaction with k3 = 0.0005(1) sꢀ1 is inde-
pendent of CH3I concentration.
Results of the UV/vis study of the reaction between
CH3I and [Rh(tta)(CO)(PPh3)] are interpreted to imply
that the first reaction represents an oxidative addition
whereby the RhI nucleus converts to an unknown RhIII
-
Fig. 2. NMR spectra of the two [Rh(tta)(CO)(PPh3)(CH3)(I)]-alkyl2
isomers in CDCl3: 1H NMR (bottom, inset bottom middle the methyl
region in acetone-d and bottom right a high resolution spectrum of the
methyl region in CDCl3), 31P NMR (top left), 19F NMR (top middle) and
the Rh–CO region of the 13C NMR spectrum (top right).
product. The product of this oxidative addition step may
be either a RhIII-alkyl or RhIII-acyl species, [Rh(tta)-
(CH3)(CO)(PPh3)(I)] or [Rh(tta)(COCH3)(PPh3)(I)] (or
both), but in the absence of further experimental evidence,

M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
213
the nature of the oxidative addition product can only be
speculated.
Since the second reaction is independent of [CH3I] one
may deduce that this reaction either involves solvent
exchange with any of the ligands of the newly formed rho-
dium(III) complex or it may be isomerization from the
RhIII-product to form the final reaction product which is
identified as the two RhIII-alkyl2 isomers. Based on the
UV/vis study as well as the fact that the final reaction prod-
uct is identified as RhIII-alkyl2 isomers, one may write a
preliminary reaction sequence consistent with the reaction
pathway proposed by Basson et al. [6,7] for a series of
[RhI(b-diketonato)(CO)(PPh3)] complexes:
RhI-A
Kc1
RhI-B
[RhIII-alkyl2A]
K1, k1
isomerization or
solvent exchange
[RhIII-oxidative
Kc4
+
CH3I
ð3Þ
{
}
addition product X]
k-1
[RhIII-alkyl2B]
From Fig. 5 it is clear that the relationship between kobs
and [CH3I] in chloroform does not pass through the origin,
implying that the k step in the above reaction sequence
ꢀ1
exists. The large value of K1 = k1/kꢀ1 = 74 dm3 molꢀ1 at
25 °C, implies that the formation of the intermediate
RhIII-product(s) is thermodynamically favoured. Rate
constants and activation parameters is determined from
the linear relationship between ln(k1/T) and (1/T) (inset
in Fig. 5). A relatively small DH# value (31.1(5) kJ molꢀ1
is compensated by
)
a
highly negative DS# value
(ꢀ194(2) J molꢀ1 Kꢀ1), both of which may be considered
as characteristic of an associative mechanism [28]. The
large negative DS# value for the oxidative addition step
indicates an increase in the coordination number during
the formation of the transition state.
3.2.2. Infrared study
To obtain more insight into the nature of the intermedi-
ate RhIII-product(s) X, an infrared-based study of the reac-
tion was conducted as this technique is ideal to distinguish
between CO bonds in metal–CO complexes that resonate at
ꢂ1980–2000 cmꢀ1 for RhI–carbonyl complexes, at ꢂ2050–
2100 cmꢀ1 for RhIII–carbonyl complexes, and CO bonds in
metal–COCH3 complexes that resonates at ꢂ1700–
1750 cmꢀ1 for RhIII-acyl species [12]. Fig. 6 illustrates the
observed reaction progress in the range 1700–2200 cmꢀ1
as monitored by IR spectrophotometry in chloroform at
25 °C. Consistent with the UV/vis results, two reaction
steps were identified.
The first step involves the disappearance of [Rh(tta)-
(CO)(PPh3)] (0.00188(1) molꢀ1 dm3 sꢀ1) as (monitored by
the disappearance of the CO peak at 1991 cmꢀ1) with the
simultaneous formation of
a
RhIII-alkyl1 species
[Rh(tta)(CH3)(CO)(PPh3)(I)] (0.0022(2) sꢀ1, monitored at
2079 cmꢀ1) and a RhIII-acyl1 species [Rh(tta)(COCH3)-
(PPh3)(I)] (0.0021(2) sꢀ1, monitored at 1728 cmꢀ1) at the
same rate. Since it is highly unlikely that the RhIII-acyl1
species can be formed from an undetected RhIII-alkyl1 spe-
cies with a total different ligand coordination pattern than

214
M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
Table 2
13C NMR spectral parameters of the Rh-CO region of the selected [RhI(L,L0)(CO)(PPh3)] and [RhIII(L,L0)(CO)(PPh3)(CH3)(I)] complexes
Ligandb
L,L0
mCO (cmꢀ1
)
d13C (ppm)
1J(C–Rh) (Hz)
2J(C–Rh–P) (Hz)
CO trans to
Rhodium(I)a
fctfa A
fctfa B
tta A
O,O
O,O
O,O
O,O
O,O
O,O
O,O
N,O
S,O
1986
1986
1981
1981
1983
1983
1983
1960
1978
1944
1960
1965
189.00
189.07
188.27
188.12
188.24
188.18
189.10
191.20
190.83
190.02
190.30
192.22
79.3
79.3
78.5
77.8
77.6
77.4
75.7
73.8
73.6
72.1
66.9
63.8
24.1
24.1
24.4
24.6
24.6
24.8
24.8
22.8
21.1
21.8
20.9
20.5
O
O
O
O
O
O
O
O
O
S
tta B
tfaa A
tfaa B
acac
dmavk
sacac
cacsm
dmavk
hacsm
N,S
N,O
N,S
N
N
Rhodium(III)a
acac-alkyl1
acac-alkyl2
tta-alkyl2Ac
tta-alkyl2B
a
O,O
O,O
O,O
O,O
2072
2062
2064
2064
185.30
185.60
184.31
184.73
64.0
62.5
11.5
11.0
18.1
11.0
5.9
5.5
Data from Ref. [12] (fctfa), [15] (tfaa, dmavk, sacac, cacsm, hacsm), [27] (acac), and this study (tta).
b
Hfctfa = ferrocenoyltrifluoroacetone, Hcacsm = methyl (2-cyclohexylamino-1-cyclopentene-1-dithiocarboxylate), Hhacsm = methyl (2-amino-1-
cyclopentene-1-dithiocarboxylate), Hdmavk = dimethylaminovinylketone, Hsacac = thioacetylacetone, Hhacsm = methyl(2-amino-1-cyclopentene-1-
dithiocarboxylate).
c
The tta-alkyl2A isomer does not specifically correspond to tta-alkyl2A isomer in Table 2.
194
0.6
hacsm
370 nm
RhI(N,S or O)
192
dmavk A
RhI(S,O)
1st reaction
0.5
dmavk B
sacac
acac
190
188
186
184
182
180
cacsm
tta A
RhI(O,O)
fctfa
tfaa A
tfaa B
tta B
0.4
0.3
0.2
0.1
400 nm
acac-alkyl2
acac-alkyl1
tta-alkyl2A
tta-alkyl2B
RhIII(O,O)
2050
2nd reaction
1900
1950
2000
2100
0
500
1000
Time / min
1500
2000
(CO) / cm-1
ν
Fig. 3. Plot of d13C chemical shifts for carbonyl carbon resonances in
[RhI(L,L0)(CO)(PPh3)] and [RhIII(L,L0)(CO)(PPh3)(CH3)(I)] complexes
vs. stretching frequencies. The indicated L,L0 monoanionic bidentate
ligand correspond to complexes in Table 2.
Fig. 4. Relative absorbance vs. time data at 25 °C for the UV/vis
monitored oxidative addition of CH3I to [Rh(tta)(CO)(PPh3)] in chloro-
form at the indicated wavelengths for the first and second reaction step
(starting at after approximately 6 h), [Rh(tta)(CO)(PPh3)] =
0.00006 mol dmꢀ3, [CH3I] = 0.154 mol dmꢀ3. Rate constants for the first
reaction as obtained at different wavelengths were mutually consistent. For
the first reaction k1obs was determined as 0.0196 sꢀ1 (370 nm) and for the
second [CH3I] independent reaction k3 = 0.0006(1) sꢀ1 (400 nm).
the detected RhIII-alkyl1 species at the same rate as the
detected RhIII-alkyl1 species, this result is considered to
imply that an equilibrium exists between the RhIII-alkyl1
and RhIII-acyl1 species that is fast enough to be maintained
on the timescale of the [Rh(tta)(CO)(PPh3)] disappearance.
are unknown. It is presumed, though, that the RhIII-alkyl1
species has the CH3 and CO ligands in a cis configuration
to allow carbonyl insertion during the generation of the
RhIII-acyl1 species.
This first reaction may thus be presented by the reaction
sequence:
1
The same conclusion could be drawn using H NMR data
and will be discussed in the next paragraph. The structure
of [Rh(tta)(CO)(PPh3)] is reported elsewhere [29], but the
structures of the RhIII-alkyl1 and the RhIII-acyl1 species

M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
215
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
existence of the fast equilibrium between the RhIII-alkyl1
and a RhIII-acyl1 species and the existence of two isomers
of the final reaction product.
-11.4
-12.2
-13.0
35˚C
25˚C
1
3.2.3. H, 19F and 31P NMR study
32
34
36
104 1/T / K-1
NMR spectroscopy can be used as an analytical tool in
which the strength of the signal is a measurement of the
concentration of a particular complex. The advantage of
NMR spectroscopy is that the different isomers in reaction
can be monitored separately. By carefully comparing the
positions and integrals of the different signals, the spectral
parameters of the different isomers during the oxidative
addition reaction and subsequent carbonyl insertion and
deinsertion reaction reactions could be identified, Table 1.
The same reaction sequence as observed on IR (Fig. 6)
15˚C
0.00
0.05
0.10
0.15
0.20
0.25
0.30
1
[CH3I] / mol dm-3
and UV/vis (Fig. 4) was observed on H NMR (Fig. 7),
31P NMR (Fig. 8) and 19F NMR. During the first reaction,
the two major RhI isomers (referred to as RhI-A and RhI-
B, the choice of the labels are arbitrary and has no signif-
icance) reacted with CH3I to form at least two isomers of
the RhIII-alkyl1 product (referred to as RhIII-alkyl1A and
RhIII-alkyl1B) which underwent a CO insertion reaction
to form at least two isomers of the RhIII-acyl1 product
(referred to as RhIII-acyl1A and RhIII-acyl1B). As before,
RhIII-alkyl1 and RhIII-acyl1 formed at the same rate, indi-
cating again that these two products are in equilibrium
with each other. During the second reaction, the acyl ligand
of the two isomers of the RhIII-acyl1 product underwent
decarbonylation to form two isomers of the RhIII-alkyl2
complex (referred to as RhIII-alkyl2A and RhIII-alkyl2B)
at different rates.
Fig. 5. Temperature and CH3I concentration dependence of the oxidative
addition reaction of CH3I to [Rh(tta)(CO)(PPh3)], as monitored on the
UV/vis spectrophotometer at 370 nm in chloroform. Inset: linear depen-
dence between ln(k1/T) and 1/T as predicted by the Eyring equation.
RhI-A
K1, k1
K2, k2
{
[RhIII-alkyl1]
[RhIII-acyl1]
Kc1
RhI-B
+
ð4Þ
CH3I
{
k-1
k-2
where ki represents the forward rate constants and k the
ꢀi
reverse rate constants, i = 1 or 2.
A slower second reaction (t1/2 ꢃ 15 min), that is CH3I
independent, is observed by the simultaneous disappear-
ance of the RhIII-alkyl1 species at 2079 cmꢀ1 and the
RhIII-acyl1 species at 1728 cmꢀ1 (Fig. 6), both same rate
(0.00057(4) sꢀ1 and 0.00057(4) sꢀ1, respectively), as the for-
mation of a second alkyl species, the RhIII-alkyl2 species
(0.00054(2) sꢀ1) at 2064 cmꢀ1. Again, this is regarded as
proof that the equilibrium K2 between the RhIII-alkyl1
The new feature introduced by the NMR study is the
existence two observable isomers for each intermediate,
making it possible to calculate the rate of formation or
depletion of each isomer separately, see Table 3.
It was already shown that the RhI-A and RhI-B isomers
exist in a fast equilibrium with each other. NMR kinetic
results indicate that the RhIII-alkyl1A and the RhIII-
alkyl1B, as well as the RhIII-acyl1A and RhIII-acyl1B iso-
mers, also exist in a fast equilibrium with each other since
the ratio RhIII-alkyl1A/RhIII-alkyl1B = 0.7, as well as the
ratio RhIII-acyl1A/RhIII-acyl1B = 0.5, stayed constant
throughout the reaction. This, however, was not true for
RhIII-alkyl2A and RhIII-alkyl2B. The RhIII-alkyl2A iso-
mer, the kinetic favored product presumably with a lower
activation energy, formed at a much faster rate than the
RhIII-alkyl2B isomer up to a 5:1 ratio. After ca. 10 000 s,
when practically all the intermediate alkyl1A, alkyl1B,
acyl1A and acyl1B isomers have been converted to the
alkyl2 isomers, the RhIII-alkyl2A isomer slowly isomerizes
over 2–3 days (kobs = 0.000030(3) sꢀ1) to the thermody-
namically more stable RhIII-alkyl2B isomer until a 1:1
equilibrium sets in. Once the equilibrium between the
RhIII-alkyl2A and RhIII-alkyl2B isomers is established,
Kc4 stays constant for months on end in both the solid state
and in CDCl3 solution at room temperature. At 55 °C in
CDCl3, however, isomerization of the alkyl2 isomers to
and RhIII-acyl1 species is fast. The kinetick data of the
3
second reaction are consistent with an A ꢀ B type of
k
ꢀ3
reaction with kꢀ3nk3 as shown in
[RhIII-alkyl2A]
K2, k2
k3
{
[RhIII-alkyl1]
[RhIII-acyl1]
Kc4
{
ð5Þ
k-2
k-3
[RhIII-alkyl2B]
Rate constants obtained by the IR spectrophotometer for
both reactions, are consistent with the results from the
UV/vis spectrophotometer, see Table 3.
The IR results made it possible to identify the interme-
diate reaction product as a RhIII-alkyl1 and a RhIII-acyl1
species in a fast equilibrium with each other. The overall
reaction sequence for this oxidative addition reaction can
therefore be represented as
RhI-A
[RhIII-alkyl2A]
K1, k1
K2, k2
k3
{
[RhIII-alkyl1]
[RhIII-acyl1]
Kc1
+
Kc4
CH3I
{
ð6Þ
k-1
k-2
k-3
RhI-B
[RhIII-alkyl2B]
The only difference between the above reaction sequence
and the mechanism proposed by Basson et al. [6,7] is the

216
M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
1.0
0.15
0.8
0.6
0.4
0.2
0.0
2ndreaction
2nd reaction
0.10
2
4
0.8
3
0.05
0.6
0.4
0.2
0.0
1st reaction
0.00
0
5000
0
10000
Time / s
Time / s
1
1st reaction
4000 5000
0
1000
2000
3000
Time / s
Fig. 6. Illustration of the oxidative addition reaction and alkyl and acyl interconversions for the reaction between 0.814 mol dmꢀ3 CH3I and
0.0081 mol dmꢀ3 [Rh(tta)(CO)(PPh3)] as monitored on the IR spectrophotometer between 1700 and 2200 cmꢀ1 in chloroform at 25 °C. Left selected IR
spectra and right the absorbance-time data. The first reaction is indicated by the disappearance of RhI at 1991 cmꢀ1 (1 in IR bottom left) and the
simultaneous appearance of RhIII-alkyl1 at 2079 cmꢀ1 and RhIII-acyl1 at 1728 cmꢀ1 (2 and 3, respectively, in IR bottom left). The second reaction is
indicated by the simultaneous disappearance of the RhIII-alkyl1 and RhIII-acyl1 intermediates (2 and 3, respectively, in IR top left) with the formation of a
RhIII-alkyl2 species at 2064 cmꢀ1 (4 in IR top left). Right the absorbance time data of RhI (1 on main graph), RhIII-alkyl1 (2 in inset left) and RhIII-acyl1
(3 in inset left) and RhIII-alkyl2 (4 in inset right).
Table 3
The kinetic rate constants of the oxidative addition reaction between CH3I and [Rh(tta)(CO)(PPh3)] as obtained by various spectrophotometric and
spectroscopic methods in chloroform at 25 °C
Method
RhI disappearance
RhIII-alkyl1 appearance
kobs (sꢀ1
RhIII-acyl1 appearance
kobs (sꢀ1
k1 (dm3 molꢀ1 sꢀ1
)
)
)
1st reaction
UVa
0.00171(4)
0.00172(1)
0.0015(3)
0.0017(2)
0.00144(2)
0.00159(1)
0.0015(2)
0.0015(2)
IRb
0.00184(2)
0.003(1)
0.004(5)
0.0018(1)
0.0015(2)
0.0024(2)
1H NMRc
19F NMRc
31P NMRc
0.0018(1)c
0.0015(1)c
0.00225(5)
0.00118(9)
0.0018(3)c
0.0007(5)
0.00086(1)
RhIII-alkyl1 disappearance
RhIII-acyl1 disappearance
RhIII-alkyl2 appearancee
k3 (sꢀ1 kisomerization (sꢀ1
)
k3 (sꢀ1
)
k3 (sꢀ1
)
)
2nd reaction
UV
IR
1H NMRc
0.0005(1)
0.00042(2)
0.0007(2)
0.00031(8)
0.00019(1)d
0.00018(1)d
0.0011(4)
0.0004(1)
0.00066(2)
0.00025(5)
0.00027(6)
0.00039(2)
0.00039(6)
0.00022(3)
0.00024(3)
0.000038(7)
0.000031(4)
0.000036(4)
0.0000266(8)
0.000037(3)
0.000019(3)
19F NMRc
31P NMRc
0.00022(2)d
0.0004(2)
0.0011(4)
k1 and k3 are the rate constants associated with the first and the second steps of the oxidative addition reaction between CH3I and [Rh(tta)(CO)(PPh3)].
a
k
ꢀ1 = 0.000023(6) sꢀ1, DH# = 31.1(5) kJ molꢀ1, DS# = ꢀ 194(2) J molꢀ1 Kꢀ1, DG# = 89(1) kJ molꢀ1 (at 25 °C).
b
c
Average values for [CH3I] = 81, 205, 452, 630 and 814 mol dmꢀ3
Two isomers observed for each product.
.
d
e
Not able to distinguish between RhIII-alkyl1 and RhIII-acyl1 peaks, fourth peak under the signal of RhI.
One isomer increases and then decreases to point of equilibrium, as indicated in Fig. 8.
RhI, RhIII-alkyl1 and RhIII-acyl1 was observed, implying
The NMR kinetic results also reveal that the observed
rate of disappearance of RhI-A and RhI-B was the same
within experimental error, due to the fast equilibrium
between the isomers. Similarly the RhIII-alkyl1A, RhIII-
that the k
step exists, which were found to be
ꢀ3
0.000067(2) sꢀ1 and 0.000065(1) sꢀ1 at 55° for the two
alkyl2 isomers.

M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
217
Fig. 7. Fragments of the methine (left) and methyl (right) region of the 1H NMR spectra in CDCl3 illustrating the reaction sequence during the oxidative
addition and the following carbonyl insertion and deinsertion reactions of 0.0271 mol dmꢀ3 [Rh(tta)(CO)(PPh3)] reacting with 0.267 mol dmꢀ3 CH3I in
CDCl3 (T = 25 °C) at the indicated time intervals. Two isomers for each specie can be observed. The first reaction is observed by the decrease in the signal
of the methine proton of the b-diketonato ligand of the RhI-A and RhI-B isomers (1) at 6.43 and 6.42 ppm with the simultaneous formation of the signals
of the methine proton of the b-diketonato ligand of the RhIII-alkyl1A and RhIII-alkyl1B isomers (2) (6.44 and 6.49 ppm), as well as the RhIII-acyl1A and
RhIII-acyl1B isomers (3) at 6.43 and 6.45 ppm illustrated on left. The reaction sequence is also illustrated by the CH3-group of the RhIII-acyl1A and RhIII
-
acyl1B isomers (3) at 3.07 and 2.99 ppm and the RhIII-alkyl1A and RhIII-alkyl1B isomers (2) at 1.55 and 1.54 ppm illustrated on right. The second reaction
is illustrated by the decrease of the various signals representing RhIII-alkyl1A and RhIII-alkyl1B and RhIII-acyl1A and RhIII-acyl1B with the simultaneous
increase in the signals of the methine proton of the b-diketonato ligand of the RhIII-alkyl2A and RhIII-alkyl2B isomers (4) at 5.72 and 5.80 ppm (see left),
as well as the increase in the signals of the CH3-group of the RhIII-alkyl2A and RhIII-alkyl2B isomers (4) at 1.90 and 1.89 ppm (see right), at different rates.
Note that the one RhIII-alkyl2 isomer increases to a maximum (kobs = 0.00039(6) sꢀ1) and then decreases to an equilibrium position (isomerization) with
kobs = 0.000031(4) sꢀ1. For the other RhIII-alkyl2 isomer, kobs = 0.000038(7) sꢀ1. The multiplet of the CH3 group of the RhIII-alkyl1 and RhIII-alkyl2
isomers is due to coupling with Rh (spin 1/2) and P (spin 1/2).
1
alkyl1B, RhIII-acyl1A and RhIII-acyl1B isomers all appear
at the same rate during the first reaction step (average at
0.0018 sꢀ1) and disappear at the same rate during the sec-
ond reaction step (average at 0.00048 sꢀ1). Good agree-
ment exists between the kinetic rate constants obtained
by utilizing UV/vis and IR spectrophotometric and H,
19F and 31P NMR spectroscopic methods (Table 3). It
was not possible to distinguish between the different rates
of formation of the RhIII-alkyl2A and RhIII-alkyl2B iso-
mers on UV/vis or IR.
60
9
6
3
0
90
60
30
0
2nd reaction
isomerization
50
40
30
20
10
0
3
7
8
5
4
6
1st reaction
2nd reaction
0
10000
0
300000
Time / s
Time / s
1
2
1st reaction
0
2000
4000
6000
8000
10000
Time / s
Fig. 8. Selected 31P NMR spectra illustrating doublet 31P peaks of the indicated reactants and products during the two reaction steps for the oxidative
addition 0.267 mol dmꢀ3 CH3I and 0.0271 mol dmꢀ3 [Rh(tta)(CO)(PPh3)] in CDCl3 at 25° (left) with the absorbance-time data (right). The first reaction
involves the disappearance of the two RhI isomers (observed by the doublets at 47.61 and 47.66 ppm) and the simultaneous appearance of two RhIII-alkyl1
isomers (at 33.50 and 34.42 ppm) and two RhIII-acyl1 isomers (at 37.51 and 37.93 ppm), all at the same rate. The second reaction is indicated by the
simultaneous disappearance at the same rate of the two RhIII-alkyl1 and the two RhIII-acyl1 isomers and the formation of two RhIII-alkyl2 isomers (at
27.07 and 28.74 ppm), but at different rates. After 2 h isomerization slowly set in between the two alkyl2 isomers. Right the absorbance time data of RhI
(main graphs 1 and 2), RhIII-alkyl1 (3 and 4 in inset left) and RhIII-acyl1 (5 and 6 in inset left) and RhIII-alkyl2 (7 and 8 in inset right) isomers.

218
M.M. Conradie, J. Conradie / Inorganica Chimica Acta 361 (2008) 208–218
Taking into account that there exists two isomers of
References
each reactant, intermediate species and product, the com-
plete reaction sequence for the oxidative addition reaction
of methyl iodide to [Rh(tta)(CO)(PPh3)] can be presented
as follows:
[1] C. Masters, Homogeneous Transition–Metal Catalysis: A Gentle Art,
Chapman & Hall, London, 1981.
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RhI-A
Kc1
RhI-B
[RhIII-alkyl1A]
[RhIII-acyl1A]
[RhIII-alkyl2A]
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k3A
K2, k2
K1, k1
k-3A
k3B
Kc2
Kc3
Kc4
[RhIII-alkyl2B]
+ CH3I
slow
ð7Þ
k-2
k-1
[RhIII-alkyl1B]
[RhIII-acyl1B]
k-3B
The equilibrium constants for the equilibrium between the
isomers, defined as Kci = [Rh-B-species]/[Rh-A-species],
i = 1, 2 or 3, were determined from the NMR data at
25 °C in chloroform: Kc1 = 1.1, Kc2 = 0.7, Kc3 = 0.5 and
Kc4 = 1.0.
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iodomethane on grounds of IR or UV/vis techniques alone
[6,7] could not identify the existence of two observable iso-
mers of each species in the reaction scheme or distinguish
between the different formation rates of the isomers of
the final reaction product, the alkyl2A and alkyl2B
isomers.
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4. Conclusions
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Utilizing NMR to explore the oxidative addition kinet-
ics reaction of CH3I to [Rh(tta)(CO)(PPh3)], it was for the
first time possible to distinguish between the reaction rates
of the different isomers in solution. Results provided the
reaction sequence given in reaction (7). The equilibrium
involving Kc1, Kc2, Kc3 and K2 is fast enough for all the
intermediate reaction products RhIII-alkyl1A, RhIII-
alkyl1B, RhIII-acyl1A and RhIII-acyl1B to appear and dis-
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appear at the same rate. The same is not found for RhIII
-
alkyl2A and RhIII-alkyl2B which appeared at different
rates. After all the intermediate species were converted to
RhIII-alkyl2A or RhIII-alkyl2B, a 1:1 equilibrium between
RhIII-alkyl2A and RhIII-alkyl2B slowly sets in over 2–3
days. This study is the first to observe different rates of for-
mation of the two isomers of the final alkyl reaction
product.
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Rhodium Express 14 (1995) 10.
´
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Acknowledgements
Financial assistance by the South African National Re-
search Foundation under Grant number 2067416 and the
Central Research Fund of the University of the Free State
is gratefully acknowledged.
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