Equilibrium, solid state behavior and reactions of four and five co-ordinate carbonyl stibine complexes of rhodium. Crystal Structures of trans-[Rh(Cl)(CO)(SbPh3)2], trans-[Rh(Cl)(CO)(SbPh3)3] and trans-[Rh(I)<s

Article abstract of DOI:10.1016/S0020-1693(01)00807-6

The crystal structures of the four-coordinate trans-[Rh(Cl)(CO)(SbPh₃)₂] (1) and the five-coordinate trans-[Rh(Cl)(CO)(SbPh₃)₃] (2) are reported, as well as the unexpected oxidative addition product, trans-[Rh(I

Full text of DOI:10.1016/S0020-1693(01)00807-6

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Inorganica Chimica Acta 331 (2002) 199–207
Equilibrium, solid state behavior and reactions of four and five
co-ordinate carbonyl stibine complexes of rhodium. Crystal
Structures of trans-[Rh(Cl)(CO)(SbPh3)2],
trans-[Rh(Cl)(CO)(SbPh3)3] and trans-[Rh(I)2(CH3)(CO)(SbPh3)2]
Stefanus Otto, Andreas Roodt *
Department of Chemistry, Uni6ersity of the Free State, Bloemfontein 9300, South Africa
Received 27 July 2001; accepted 26 November 2001
Dedicated to Professor A.G. Sykes
Abstract
The crystal structures of the four-coordinate trans-[Rh(Cl)(CO)(SbPh3)2] (1) and the five-coordinate trans-[Rh(Cl)(CO)(SbPh3)3]
(2) are reported, as well as the unexpected oxidative addition product, trans-[Rh(I)2(CH3)(CO)(SbPh3)2] (3), obtained from the
reaction of 2 with CH3I. The formation constants of the five-coordinate complex were determined in dichloromethane, benzene,
diethyl ether, acetone and ethyl acetate as 16398, 363910, 744934, 1043995 and 1261996 M−1, respectively. While
coordinating solvents facilitate the formation of the five-coordinate complex, the four-coordinate complex could be obtained from
diethyl ether due to the favorable low crystallization energy. The tendency of stibine ligands to form five-coordinate rhodium(I)
complexes is attributed mainly to electron deficient metal centers in these systems, with smaller contributions by the steric effects.
The average effective cone angle for the SbPh3 ligand in the three crystallographic studies was determined as 139° with individual
values ranging from 133 to 145°. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Rhodium stibine complexes; Vaska complexes; Oxidative addition; Five-coordination; Structures
cantly more resistant towards oxidative addition than
their iridium counterparts. This is believed to be linked
directly to the larger steric crowding around the
rhodium metal center, especially when employing large
phosphine ligands, thus hindering the nucleophilic at-
tack of the metal to a substrate molecule. In this regard
we found it interesting to investigate the solution and
solid state structural effect of arsine and stibine ligands
on the oxidative addition reactions of their respective
rhodium Vaska complexes [5].
Initial work on the preparation of these complexes [3]
describes the preparation of rhodium complexes using
three different stibine ligands. The color of the com-
plexes is reported to range from orange red to purple
red, whereas the analogous phosphine and arsine con-
taining complexes are yellow. Molecular mass determi-
nations and elemental analysis suggested the stibine
complexes to be the four-coordinate analogues of the
rhodium phosphine complexes that were already known
1. Introduction
Certainly one of the best known organometallic com-
plexes is trans-[Ir(Cl)(CO)(PPh3)2] which was first pre-
pared by Angoletta [1], but was later correctly
formulated by Vaska [2]. Interestingly enough, at that
time the Rh analogue was already known [3] and
investigated [4] to some extent. Today, complexes with
the general formula trans-[M(X)(CO)(L)2] (M=metal,
X=halogen or pseudo-halogen, L=neutral ligand) are
considered analogues to Vaska’s complex. These com-
plexes were soon recognized as important model sys-
tems for studies in homogeneous catalysis and undergo
oxidative addition reactions with a variety of sub-
strates. The rhodium complexes, however, are signifi-
* Corresponding author. Tel.: +27-51-401-2923; fax: +27-51-430-
7805.
E-mail address: roodta@sci.uovs.ac.za (A. Roodt).
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00807-6

200
S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
at that time. Two more accounts by other authors
describing the rhodium stibine complexes as having red
colors also appeared during that time [9,10]. A brief
correction given in a footnote was the first report of
these complexes to be five-coordinate and containing
three stibine ligands [6]. Confirmation of the presence
of three stibine ligands by elemental analysis and the
existence of solution equilibria between the four- and
five-coordinate rhodium complexes appeared shortly
afterwards [7]. Practical problems concerning the ele-
mental analysis were mentioned and the presence of an
additional benzene molecule in all the synthesized com-
plexes raised some questions concerning these results.
The first reference to the preparation of the yellow
four-coordinate complex is only sited as a personal
communication [7,8].
As part of our research directed towards the manipu-
lation of the reactivity of platinum group metal com-
plexes, with specific emphasis on rhodium, we
undertook this study to investigate the rhodium
triphenylstibine analogue of Vaska’s complex. To date,
stibine complexes have only been investigated to a very
limited extent as phosphine-containing complexes are in
general favored on the basis of the NMR activity of
phosphorous. Furthermore, large variations of phos-
phine ligands are commercially available, enabling
broad studies without time-consuming ligand synthesis
involving extremely hazardous chemicals. Thus, differ-
ent attempts to prepare and characterize the rhodium
stibine analogue to Vaska’s complex are reported in the
literature [3,9,10], of which several are incorrect.
Work by Wilkinson explored several aspects regard-
ing the steric and electronic properties of the Rh(I)
analogues [11], but definite crystal structural confirma-
tion of the reaction products was not achieved. Thus,
since not many crystallographic studies on stibine sys-
tems have been reported to date [12], in this paper,
special emphasis is placed on the characterization of the
complexes, specifically by detailed crystallographic in-
vestigations, as well as the solution equilibria involved
in their preparation and isolation.
NaCl solution cell in the range 2200–1600 cm−1 cor-
recting for the solvent background. NMR spectra were
collected on a Bruker 300 MHz instrument. The affix
‘a’ indicates the complexes in solution.
2.1.1. trans-[Rh(Cl)(CO)(SbPh3)2] (1a)
[Rh(m-Cl)(CO)2]2 (25 mg, 0.064 mmol) was dissolved
in ether (15 cm3) at 0 °C and an ether solution (10 cm3)
of SbPh3 (86 mg, 0.23 mmol) was added drop wise. At
low temperature the yellow product precipitates from
solution. The product was collected in fractions by
decanting the reaction solution from time to time to
ensure the minimum contamination of the unreacted
rhodium starting complex. At slightly elevated tempera-
tures (10 °C) X-ray quality crystals of 1 were obtained
by slow evaporation of solvent (yield 49 mg, 88%). IR
(w(CO)/KBr): 1952 cm−1; (w(CO)/CH2Cl2): 1970 cm−1
.
1H NMR (CDCl3): l(ppm) 7.36–7.46 (m, 18H); 7.62–
7.70 (m, 12H). UV–Vis (C6H6) [umax, nm (log m, M−1
cm−1)]: 365 (3.76), 410 (3.25).
2.1.2. trans-[Rh(Cl)(CO)(SbPh3)3] (2a)
[Rh(m-Cl)(CO)2]2 (25 mg; 0.064 mmol) was dissolved
in acetone (5 cm3) and solid SbPh3 (170 mg; 0.48 mmol)
was added in portions. The solution became increas-
ingly red with each addition of the ligand. Dark red
crystals of 2, suitable for X-ray analysis, formed within
an hour and were collected by filtration (yield 150 mg;
95%). IR (w(CO)/KBr): 1967 cm−1, (w(CO)/CH2Cl2):
1
1971 cm−1. H NMR (CDCl3): l(ppm) 7.25–7.38 (m,
27H); 7.48–7.56 (m, 18H). UV–Vis (C6H6) [umax, nm
(log m, M−1 cm−1)]: 424 (3.93), 470 (3.72).
2.1.3. trans-[Rh(I)2(CH3)(CO)(SbPh3)2] (3)
trans-[Rh(Cl)(CO)(SbPh3)3] (20 mg; 0.016 mmol) was
dissolved in acetone (4 cm3), an excess CH3I (1 cm3, 16
mmol) was added and the solution was covered with a
watch glass. Within five hours poor quality brownish
crystals of the a isomer (4) [13] (yield 5 mg; B30%)
was obtained, while a second batch of good quality
dark red–brown crystals were collected after 24 h,
corresponding to the b isomer (3) (yield 8 mg, B45%).
a: IR (w(CO)/KBr): 2050 cm−1, (w(CO)/CH2Cl2): 2048
2. Experimental
cm−1
.
b: IR (w(CO)/KBr): 2034 cm−1
,
(w(CO)/
1
CH2Cl2): 2048 cm−1. H NMR (CDCl3): l(ppm) 7.2–
2.1. Preparation of complexes
7.7 (m, 30H; 3xPh); 1.1 (m, 3H, CH3).
All solvents and chemicals used for the preparation
of the complexes were of reagent grade and were used
without further purification. Triphenylstibine was pur-
chased from Merck Chemicals. All preparations were
carried out under aerobic conditions and no special
precautions were taken to exclude atmospheric mois-
ture. Infrared measurements were done on a Hitachi-
270-50 spectrometer using either KBr discs in the range
4000–250 cm−1 or in dichloromethane solution in a
2.1.4. trans-[Rh(Cl)(I)(CH3)(CO)(SbPh3)2] (5a)
trans-[Rh(Cl)(CO)(SbPh3)2] (14 mg; 0.016 mmol) was
dissolved in acetone (4 cm3), an excess CH3I (2 cm3, 32
mmol) was added and the solution was covered with a
watch glass. The oxidative addition (accessible tempera-
tures) is much slower on 1a than on the corresponding
iodo (both bis and tris SbPh3 complexes) as well as on
2a (up to orders-of-magnitude [13]) and significant sol-
vent interaction was observed. Thus, this complex

S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
201
could not be isolated in pure form and the spectro-
biguously characterized by means of X-ray crystallogra-
phy. In general, IR spectroscopy is used extensively to
identify this kind of complexes, but in this case it is not
sufficient as both the solid- and solution state spectra of
1 and 2 exhibit carbonyl stretching frequencies within
the expected range for these complexes containing two
phosphine or arsine ligands in cis positions with respect
the OC–Rh–Cl core. As a dynamic equilibrium is
established between complexes 1a and 2a in solutions of
2, only a single carbonyl stretching frequency is ob-
served in solutions of the latter. The quite similar values
of 1970 and 1969 cm−1, respectively obtained for 1a
and 2a, are furthermore probably indicative of exten-
sive dissociation of the solvent interaction with 2, even
scopic data were obtained in situ. IR (w(CO)/CH2Cl2):
2050 cm−1 1H NMR (CDCl3): l(ppm) 7.3–7.7 (m,
.
30H; 3×Ph); 1.1 (m, 3H, CH3).
2.2. X-ray measurements and structure determinations
The densities of the crystals were determined by
flotation in aqueous NaI. All data collections were
performed at 293(2) K on a Siemens Smart diffractome-
,
ter using Mo Ka (0.71073 A) radiation. The structures
(except 1, since the Rh-atom lies on an inversion center)
were solved by direct methods (SHELXS-97) and refined
through full-matrix least-squares calculations using
SHELXL-97, with S(ꢀFoꢀ−ꢀFcꢀ)2 being minimized [14].
All non-hydrogen atoms were refined anisotropically,
while the hydrogen atom positions were calculated in
idealized positions as riding on the adjacent carbon
in
a
fairly non-coordinating solvent such as
dichloromethane.
In general, the Vaska type complexes of rhodium
containing phosphine- and arsine ligands can be conve-
niently prepared by reacting the rhodium carbonyl
chloride dimer, [Rh(m-Cl)(CO)2]2, with at least four
equivalents of the appropriate ligand in acetone
medium. In some cases if very soluble complexes are
being formed (typically when using ligands containing
alkyl substituents) the reaction medium should be ad-
justed accordingly to ensure easy isolation of the de-
sired product without contamination by excess ligand.
In the experimental section we give detailed procedures
for the selective preparation of both the red five-coordi-
nate and the yellow four-coordinate complexes. It is
clear that five-coordinate complexes of rhodium are
easily formed when utilizing stibine ligands and knowl-
edge of the solution behavior is imperative to manipu-
late the equilibria in such a way as to favor the
formation of the four-coordinate complexes. Addition
of SbPh3 (either as the solid or in solution) to an
acetone solution of [Rh(m-Cl)(CO)2]2 immediately re-
sults in a dark red color of the solution, indicative of
the formation of 2a. Complex 2 can thus be prepared
and isolated in high yield using many common organic
solvents.
It is clear from the results reported here that the
isolation of complex 1 is favored when diluted solutions
of non-polar solvents are used, and care should also be
taken to ensure that a non-excess of stibine is used.
Addition of a hexane solution of SbPh3 to an ice-cooled
solution of [Rh(m-Cl)(CO)2]2 in the same solvent results
in the formation of 1 as a fluffy yellowish precipitate.
Utilizing diethyl ether as solvent and performing the
preparation at a temperature of approximately 10 °C
without stirring, small, well formed yellow crystals of 1
can be obtained.
,
,
atom (aromatic C–H=0.93 A; methyl C–H=0.96 A).
The graphics were done with DIAMOND [15].
2.3. Cone angle calculations
Calculations of the cone angles were done based on
the Tolman model [16], but using the actual Rh–Sb
bond distances determined in the crystallographic stud-
ies, i.e. the effective cone angles, qE, in this case [17].
,
Bond distances of 0.93 A were assumed for the aro-
matic C–H hydrogen and a van der Waals radius of 1.2
,
A for hydrogen was used in all the calculations.
2.4. Solution studies
UV–Vis spectra were recorded at 298 K using a
Varian Cary 50 conc spectrophotometer between 350
and 600 nm. Solvents were distilled and purified prior
to use. The low stability of 1a, observed in solution,
necessitated the use of solvents wherein the decomposi-
tion rate was sufficiently slow to allow accurate deter-
mination of the formation constant of 2a. In all cases a
range of solutions were prepared containing 0.25 mM
of 1 (diethyl ether [Rh]f=0.18 mM) with varying
amounts of SbPh3. Control experiments were per-
formed under inert conditions and no effect was ob-
served. Absorbance values at fixed wavelengths were
fitted to the appropriate equations [18] using the SCIEN-
TIST least-squares program [19].
3. Results and discussion
3.1. Preparation of complexes
The behavior of 2 was investigated by reacting with
CH3I in acetone medium. Two different kinds of crys-
tals were isolated from the reaction and were character-
ized as two different polymorphs of [Rh(I)2(CH3)(CO)-
(SbPh3)2]. The isolation of this complex containing two
The complexes were prepared as described in the
experimental section and although the CO stretching
frequencies are reported the compounds were unam-

202
S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
expected for a complex containing a d8 metal center
with the SbPh3 ligands trans to each other, see Fig. 1.
The angles within the coordination polyhedron are
close to that expected for an ideal square planar geome-
try with C(1)–Rh–Sb=89.8(7)° and Cl–Rh–Sb=
89.35(12)°, while a Rh–C–O angle of 178(2)°, linear
within standard deviation, was encountered for the CO
moiety. All the bonds in the coordination polyhedron,
i.e. Rh–Sb=2.5655(2), Rh–C=1.797(13) and Rh–
Scheme 1. Stoichiometric mechanism of reaction with iodomethane (a
indicates complexes in solution).
,
iodo ligands can be rationalized as illustrated in Scheme
1.
Cl=2.315(3) A, are shorter when compared to those
observed in 2 and 3, respectively.
Upon dissolution of 2 in acetone, a mixture of 1a, 2a
and liberated SbPh3 is formed. Addition of CH3I to
such a solution results in CH3I oxidative addition, also
to the liberated SbPh3, thus forming [SbMePh3]I, in
conjunction with the oxidative addition to 1a and 2a.
The [SbMePh3]I thus produced acts as a source of I−
ions to substitute the Cl− from the rhodium metal
centres, resulting in 3 formed as the final product. The
exact sequence of reactions, as well as the reactivity of
the different species toward iodomethane oxidative ad-
dition in this complicated mechanism are not resolved
at this stage, but are under further investigation.
Relative small C–Sb–C tetrahedral angles of almost
equal size were found and ranges from 99.62(12) to
101.04(12)°. In conjunction with this observation are
the long Sb–C bond lengths, deviating very little from
,
each other and ranging from 2.128(3) to 2.132(3) A.
This suggests that the steric crowding of the phenyl
rings is significantly less pronounced than what is en-
countered for analogous phosphine and arsine ligands.
This distortion from normal tetrahedral geometry prob-
ably has an effect on the lone pair character of the
stibine ligand and may be due to draining of the
electron density away from the Sb ligand by the elec-
tron deficient metal center.
3.2. Crystallography
The effective (qE) cone angles for the SbPh3 ligand in
this structure were calculated as 137.4°.
Practical problems and the resulting uncertainty as-
sociated with elemental analysis reported previously
were encountered. Furthermore, by NMR spec-
troscopy, only poorly resolved proton signals (anti-
mony has two NMR active isotopes, I=5/2 (57%) and
7/2, respectively [20], introducing quadrupolar effects)
are observed. Moreover, since in this rhodium system
the stibine ligands are associated with exchange dynam-
ics, and thus NMR could only be used to a limited
extent, we relied heavily on X-ray crystallography for
the unambiguous characterization of the different com-
plexes. These crystal structure determinations represent
some of the few rhodium stibine complexes structurally
characterized to date. Crystals suitable for crystallo-
graphic purposes were obtained as described in the
experimental section. Crystal-, data collection- and
refinement details for the crystal structures of 1, 2 and
3 are reported in Table 1, while the numbering schemes
and molecular geometries are shown in Figs. 1–3,
respectively. A selection of the most important geomet-
rical parameters is given in Tables 2–4.
3.2.2. Crystal structure of trans-[Rh(Cl)(CO)(SbPh3)3]
(2)
The compound crystallizes in the monoclinic space
group P21/n with two crystallographically independent,
but chemically equivalent, molecules in the asymmetric
unit.
The
solid
state
structure
of
trans-
[Rh(Cl)(CO)(SbPh3)3] represents another example of a
five-coordinate Rh(I) metal center, see Fig. 2. The
molecules exhibit a trigonal bipyramidal geometry with
the CO and Cl ligands trans to each other in the apical
positions with the three SbPh3 ligands describing the
equatorial plane. Angles close to the expected 90° from
the apical positions to the equatorial plane, and close to
120° between the stibine ligands within the equatorial
plane were observed, see Table 3.
An interesting observation again made, as in 1, is the
quite small C–Sb–C tetrahedral angles ranging be-
tween 95.5(2) and 102.2(2)° with an average of 99.1(2)°.
This is most likely due to the large Sb–C bond lengths,
,
ranging from 2.127(6) to 2.146(6) A with an average of
,
2.138(6) A, making it possible for the phenyl rings to be
3.2.1. Crystal structure of trans-[Rh(Cl)(CO)(SbPh3)2]
(1)
The compound crystallizes in the triclinic space
compressed a little more than what is encountered for
analogous phosphine and arsine ligands. It is clear from
the appropriate torsion angles that the two Sb ligands
with the shortest Rh–Sb bond length on each molecule
(Sb(2), Sb(3), Sb(5) and Sb(6)) have torsion angles so
that one of the phenyl rings is directed almost directly
away from the chloro ligand. This may give an indica-
tion to an energy minimum conformation present in
(
group P1 with one molecule in the unit cell implicating
that the Rh-atom lies on an inversion center and suffers
a 50% statistical disorder in the CO and Cl positions.
The molecular structure of trans-[Rh(Cl)(CO)-
(SbPh3)2] exhibits the normal square planar geometry,

S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
203
Table 1
Crystallographic and refinement data for 1, 2 and 3
a
1
2 b
3 c
Formula
C37H30ClOSb2Rh
872.47
triclinic
C110H90Cl2O2Sb6Rh2
2451.04
monoclinic
P21/n
C38H33I2OSb2Rh
1105.85
monoclinic
P21/c
Formula weight
Crystal system
Space group
(
P1
Unit cell dimensions
,
a (A)
9.5488(7)
9.8783(7)
10.0947(8)
104.355(1)
95.463(1)
111.753(1)
838.05(11)
1
18.8147(6)
13.7789(5)
38.4810(12)
90
96.101(1)
90
9919.5(6)
4
1.641
14.1532(7)
14.5888(7)
18.9031(9)
90
107.600(1)
90
3720.4(3)
4
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (g cm−3
)
1.729
2.194
1.974
3.567
v (mm−1
)
2.034
T
max/Tmin
0.880/0.694
424
0.854/0.545
4784
0.763/0.377
2088
F(000)
Crystal size (mm3)
Index ranges
0.18×0.14×0.06
−125h512, −135k512,
−135l513
5.65–28.35
9463
0.34×0.18×0.08
−245h524, −185k515,
−485l543
1.16–28.30
59944
0.34×0.20×0.08
−125h518, −195k519,
−255l520
1.51–28.22
23286
q range (°)
Collected data
Unique data (Rint
Observed data
[I\4|(I)]
)
4104 (0.0307)
3027
22476 (0.0325)
18526
8477 (0.0276)
7507
Data/restraints/paramete 4104/0/206
22476/0/1100
8477/0/399
rs
S
0.948
0.0292
0.0597
1.279
0.0567
0.0878
1.169
0.0362
0.0757
R1 [I\4|(I)] d
e
wR2
R (all data)
0.0512
0.0752
0.0440
wR2
0.0656
0.391; −0.468
0.0933
0.463; −0.772
0.0792
1.313; −0.813
−3
,
Dzmax; Dzmin (e A
)
a trans-[Rh(Cl)(CO)(SbPh3)2].
b trans-[Rh(Cl)(CO)(SbPh3)3].
c trans-[Rh(I)2(Me)(CO)(SbPh3)2].
d R1=[(S DF)/(S Fo)].
e wR2=S[w(Fo2−Fc2)2]/S[w(Fo2)2]1/2
.
these crowded systems. The less accurate and shorter
than expected C–O bond distance (also the case for 3
described below) is indicative of the presence of many
heavy atoms (Sb and Rh) in the structure, see Tables 3
and 4.
The effective cone angles for the SbPh3 ligands were
determined as qE(1)=133.3°, qE(2)=144.5° and qE(3)
=
139.7° for molecule 1 and qE(4)=138.4°, qE(5)=143.5°
and qE(6)=134.5° for molecule 2, respectively, averag-
ing at 139°. Interestingly enough, the largest effective
cone angle corresponds to the shortest Rh–Sb bond
Fig. 1. Molecular diagram of complex 1 showing the numbering
scheme and displacement ellipsoids (30% probability) in trans-
[Rh(Cl)(CO)(SbPh3)2]. The first digit of the carbon atoms in the
phenyl rings refers to the ring number and the second digit to the
number of the C atom in the ring. Hydrogen atoms were omitted for
clarity.
,
(Sb(2): 144.5°, 2.5620(5) A) while the smallest effective
cone angle corresponding to the longest Rh–Sb bond
,
(Sb(1): 133.3°, 2.6263(5) A). Furthermore, both these
ligands are coordinated to the same metal center
(Rh(1)).

204
S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
Table 2
,
Selected bond lengths (A), bond and steric angles (°) for 1
Bond lengths
Rh–Sb
2.5655(2)
1.797(13)
2.128(3)
2.129(3)
Rh–Cl
O(1)–C(1)
Sb–C(31)
2.315(3)
1.175(13)
2.132(3)
Rh–C(1)
Sb–C(11)
Sb–C(21)
Bond angles
C(1)–Rh–Cl
178.2(9)
89.8(7)
90.2(8)
115.60(8)
114.73(8)
121.60(8)
O(1)–C(1)–Rh
Cl–Rh–Sb
Clc1–Rh–Sb
C(11)–Sb–C(21)
C(11)–Sb–C(31)
C(21)–Sb–C(31)
178(2)
C(1)–Rh–Sb
C(1)c1–Rh–Sb
C(11)–Sb–Rh
C(21)–Sb–Rh
C(31)–Sb–Rh
89.35(12)
90.65(12)
99.62(12)
101.04(12)
100.94(12)
Steric angles
qE(1)
a
137.4
Fig. 2. Molecular diagram of molecule 1 of complex 2 showing the
numbering scheme and displacement ellipsoids (30% probability) in
trans-[Rh(Cl)(CO)(SbPh3)3], molecule 2 is numbered accordingly con-
taining Sb(4); Sb(5) and Sb(6). The first digit in the phenyl rings
refers to the number of the Sb ligand (1–6), the second digit to the
number of the ring on the Sb ligand (1–3) and the third digit to the
number of the C atom in the ring (1–6). Hydrogen atoms were
omitted for clarity.
a Effective cone angle, see text.
,
Rh–I bond distances of 2.7736(5) and 2.7272(5) A
opposite of these, respectively, with the longer corre-
sponding to the bond trans to the CH3 ligand. The
C–Rh–C angle of 83.3(2)° between the carbonyl and
methyl ligands is indicative of the steric repulsion in-
duced by the bulky iodo ligands which exhibits an
I–Rh–I angle of 97.721(15)°. The average tetrahedral
angles around the Sb donor atoms are very similar with
C–Sb(1)–C and C–Sb(2)–C at 101.7(2) and
101.76(18)° and C–Sb(1)–Rh and C–Sb(2)–Rh at
116.40(13) and 116.35(13)°, respectively. The effective
cone angles for Sb(1) and Sb(2) were calculated as
140.1 and 141.8°.
3.3. Solution studies
Equilibrium constants for the formation of 2 (Eq. (1)
in Scheme 1) in different solvents from the reaction of
1a with SbPh3 are presented in Table 5 while Fig. 4
shows the least squares fits of the absorbance data
changes in dichloromethane, ether and acetone. The
solutions turned from the initial yellow color of 1a into
the dark red color characteristic of 2a with an increase
in the SbPh3 concentration. The progressive increase of
the stability constant of 2a in the five solvents is clear.
The stability constants determined in different sol-
vents according to Eq. (1) in Scheme 1 indicate that the
formation of 1 is more favoured in less polar/non-coor-
dinating solvents. In fact, there is almost an order of
magnitude difference in the K(dcm) value versus those
obtained in acetone and ethyl acetate (Table 5). It is,
however, not certain whether the inherent solvent char-
acteristics (donicity, polarity) are necessarily the only
reason for isolation 1. Lower crystallization energy of 1
versus that of 2 in solvents such as diethyl ether and
hexane added some driving force for obtaining the
four-coordinate complex. The solubility of 1 in diethyl
ether is B0.2 mM, thus the success of the synthetic
Fig. 3. Molecular diagram of 3 showing the numbering scheme and
displacement ellipsoids (30% probability) in [Rh(I)2(CH3)(CO)-
(SbPh3)2]. The first digit in the phenyl rings refers to the number of
the Sb ligand, the second digit to the number of the ring on the Sb
ligand (1–3) and the third digit to the number of the C atom in the
ring (1–6). Hydrogen atoms were omitted for clarity.
3.2.3. Crystal structure of
trans–[Rh(I)2(CH3)(CO)(SbPh3)2] (3)
The complex crystallizes in the monoclinic space
group P21/c with Z=4, with distorted octahedral ge-
ometry as shown in Fig. 3, as expected for a six-coordi-
nate rhodium(III) complex. The title complex contains
two stibine ligands in a trans orientation, two iodo
ligands cis to each other as well as a CO and CH3
group cis to one another, respectively. A significant
difference was observed between the Rh–C bond dis-
tances to the CH3 and CO ligands of 2.272(7) and
,
1.933(7) A, respectively; the difference being attributed
to the p back-bonding component in the coordination
mode of the CO ligand. The difference in the trans-infl-
uences of the CH3 and CO ligands is evident from the

S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
205
Table 3
Selected bond lengths (A), bond and steric angles (°) for 2
Table 4
,
,
Selected bond lengths (A), bond and steric angles (°) for 3
Molecule 1
Molecule 2
Bond lengths
Rh(1)–C(1)
Rh(1)–Sb(1)
Rh(1)–I(1)
[Sb(1)–C]avg
O(1)–C(1)
1.933(7)
2.5845(5)
2.7272(5)
2.131(6)
0.932(6)
Rh(1)–C(2)
Rh(1)–Sb(2)
Rh(1)–I(2)
[Sb(2)–C]avg
2.272(7)
2.5950(5)
2.7736(5)
2.131(5)
Bond lengths
Rh(1)–C(1)
Rh(1)–Cl(1)
Rh(1)–Sb(1)
Rh(1)–Sb(2)
Rh(1)–Sb(3)
[Sb(1)–C]avg
[Sb(2)–C]avg
[Sb(3)–C]avg
C(1)–O(1)
1.833(6)
2.4147(15)
2.6263(5)
2.5620(5)
2.5914(5)
2.141(6)
2.136(5)
2.137(6)
1.098(6)
Rh(2)–C(2)
Rh(2)–Cl(2)
Rh(2)–Sb(4)
Rh(2)–Sb(5)
Rh(2)–Sb(6)
[Sb(4)–C]avg
[Sb(5)–C]avg
[Sb(6)–C]avg
C(2)–O(2)
1.916(7)
2.4041(18)
2.6220(5)
2.5806(5)
2.6061(5)
2.141(6)
2.141(5)
2.134(6)
0.972(6)
Bond angles
C(1)–Rh(1)–C(2)
Sb(1)–Rh(1)–Sb(2)
C(1)–Rh(1)–Sb(1)
C(1)–Rh(1)–Sb(2)
C(1)–Rh(1)–I(1)
Sb(1)–Rh(1)–I(1)
C(1)–Rh(1)–I(2)
Sb(1)–Rh(1)–I(2)
83.3(2)
I(1)–Rh(1)–I(2)
97.721(15)
174.1(6)
89.32(11)
94.26(11)
89.60(13)
88.183(14)
172.20(13)
88.702(15)
174.212(16) O(1)–C(1)–Rh(1)
93.96(15) C(2)–Rh(1)–Sb(1)
90.99(15) C(2)–Rh(1)–Sb(2)
172.76(17) C(2)–Rh(1)–I(1)
87.288(14) Sb(2)–Rh(1)–I(1)
89.45(17) C(2)–Rh(1)–I(2)
88.338(15) Sb(2)–Rh(1)–I(2)
Bond angles
C(1)–Rh(1)–Cl(1)
C(1)–Rh(1)–Sb(1)
C(1)–Rh(1)–Sb(2)
C(1)–Rh(1)–Sb(3)
Cl(1)–Rh(1)–Sb(1)
Cl(1)–Rh(1)–Sb(2)
Cl(1)–Rh(1)–Sb(3)
176.71(17)
87.99(18)
91.50(17)
94.22(18)
94.55(4)
C(2)–Rh(2)–Cl(2)
C(2)–Rh(2)–Sb(4)
C(2)–Rh(2)–Sb(5)
C(2)–Rh(2)–Sb(6)
Cl(2)–Rh(2)–Sb(4)
Cl(2)–Rh(2)–Sb(5)
Cl(2)–Rh(2)–Sb(6)
177.11(16)
85.87(16)
90.68(16)
93.44(17)
95.87(5)
C(111)–Sb(1)–C(121) 102.4(2)
C(211)–Sb(2)–C(221) 100.92(18)
C(111)–Sb(1)–C(131) 102.18(18) C(211)–Sb(2)–C(231) 102.81(18)
C(121)–Sb(1)–C(131) 100.49(18) C(221)–Sb(2)–C(231) 101.55(18)
C(111)–Sb(1)–Rh(1) 111.85(13) C(211)–Sb(2)–Rh(1) 115.12(13)
C(121)–Sb(1)–Rh(1) 115.21(13) C(221)–Sb(2)–Rh(1) 114.81(12)
C(131)–Sb(1)–Rh(1) 122.13(13) C(231)–Sb(2)–Rh(1) 119.11(14)
85.43(4)
86.56(5)
86.44(5)
87.97(5)
Sb(1)–Rh(1)–Sb(2) 119.552(19) Sb(4)–Rh(2)–Sb(5) 120.72(2)
Sb(1)–Rh(1)–Sb(3) 115.912(18) Sb(4)–Rh(2)–Sb(6) 114.161(19)
Steric angles
qE(1)
a
a
140.1
qE(2)
141.8
Sb(2)–Rh(1)–Sb(3) 124.383(19) Sb(5)–Rh(2)–Sb(6) 125.115(19)
a Effective cone angle, see text.
[C–Sb(1)–C]avg
[C–Sb(2)–C]avg
[C–Sb(3)–C]avg
[C–Sb(1)–Rh(1)]avg 119.03(16)
[C–Sb(2)–Rh(1)]avg 117.27(14)
[C–Sb(3)–Rh(1)]avg 118.21(19)
98.0(2)
100.7(2)
99.3(3)
[C–Sb(4)–C]avg
[C–Sb(5)–C]avg
[C–Sb(6)–C]avg
[C–Sb(4)–Rh(2)]avg 118.26(16)
[C–Sb(5)–Rh(2)]avg 118.30(15)
[C–Sb(6)–Rh(2)]avg 119.16(16)
99.3(3)
99.3(2)
98.2(2)
Table 5
Stability constants for trans-[Rh(Cl)(CO)(SbPh3)3] in different sol-
vents, 25 °C
a
b
c
d
Solvent
DN
DS
m
K
Steric angles
(M−1
)
a
a
qE(1)
qE(2)
qE(3)
133.3
144.5
139.7
qE(4)
qE(5)
qE(6)
138.4
143.5
134.5
a
a
a
a
Dichloromethane
Benzene
Nitromethane
Diethylether
Acetone
4 e
0.1
2.7
19
17
17
6
9
9
12
15
14
8.9
2.3
35.8
4.3
20.7
6.0
16398
36397
\200 f
a Effective cone angle, see text.
744934
1043995
1261996
Ethyl Acetate
method is driven by the favourably low energy of
crystallization, even though the stability constant in this
solvent is quite large (Table 5).
a Ref. [21], DN=donor number.
b Ref. [22], DS=donor strength.
c Ref. [23], m=dielectric constant.
d Fig. 4, Eqs. in Ref. [18].
3.4. Structural correlation with other Vaska type
complexes
e Estimated from chloroform.
f Estimated value; significant decomposition.
In Table 6 comparative crystallographic data for
selected Rh Vaska complexes are presented. An in-
crease in the Rh–L (L=P, As and Sb) bond lengths is
observed when going from P to As to Sb as the donor
atom on the ligands with average values ranging from
centers in that order. The crystal structures of the two
Rh(I) complexes, 1 and 2, are informative in terms of
the nature of rhodium stibine complexes in general. It is
evident that the bond distances in the coordination
polyhedron of 1 are significantly shorter than those
encountered for 2. This is particularly true for the
Rh–Sb and Rh–Cl bond distance of 2.5981(5) and
,
2.322(1) to 2.4226(4) to 2.5655(2) A for the four-coordi-
nate complexes.
Furthermore, it was found that the Rh–Cl bond
distance decreased from 2.382(1) to 2.3538(14) to
,
2.4094(18) A for 2 compared to the 2.5655(2) and
,
,
2.315(3) A in the same order. This observation is
2.315(3) A for 1. The bond lengthening in 2 can hardly
indicative of a decrease in the cis effect of these ligands
when descending in Group 15 and adds further evim-
dence to the formation of more electrophilic metal
be solely attributed to steric effects since the average
effective cone angles calculated for the SbPh3 ligands
from the crystallographic data are identical to those

206
S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
experimental conditions. These complexes were further-
more unambiguously characterized by single crystal
X-ray crystallography. Complex 2a undergoes oxidative
addition with CH3I to form a Rh(III) complex contain-
ing only two SbPh3 ligands coordinated to the metal
center in the final product. This is in contrast to a
recent study [27] where the reaction of 2a with
HCꢀCCH2Cl gave [Rh(Cl)(h2-C(ꢁO)CHꢁCClCH2)-
(SbPh3)3] as the final product. In this case the dissoci-
ated SbPh3 ligand re-coordinated to the Rh(III) oxida-
tive addition product, after cycloaddition and Cl−
migration occurred, to retain a saturated coordination
sphere. In the current study CH3I oxidative addition to
the dissociated SbPh3, resulting in the formation of
[SbMePh3]+I−, effectively removes the SbPh3 as poten-
tial ligand from the reaction medium. The originally
liberated SbPh3 ligand is however simultaneously con-
verted into a source of I−, resulting in the diiodo
Rh(III) complex (3) being obtained as final product.
In order to understand the coordination chemistry of
stibine ligands it is imperative to obtain as much infor-
mation as possible concerning both the electronic- and
steric characteristics thereof. Important information ob-
tained from the three crystallographic studies in terms
of the steric demand and geometry of the stibine lig-
ands is that an average effective cone angle of 139° was
obtained for the nine SbPh3 ligands with individual
values ranging between 133.3 and 144.5°. These results
indicate a small degree of freedom in the flexibility of
the SbPh3 ligands, but that external packing forces, as
opposed to intermolecular strain, mainly dominate the
individual values. This conclusion is apparent from the
average values obtained for the four-, five- and six-co-
ordinate complexes of 137.4, 139.0 and 141.0°, which
are very similar.
Fig. 4. Least-squares fits (lines) of absorbance change vs. [SbPh3] for
the formation of trans-[Rh(Cl)(CO)(SbPh3)3] (Scheme 1) in acetone
(ac, 450 nm), diethyl ether (eth, 420 nm), dichloromethane (dcm, 450
nm), at 25 °C.
found in 1 and 3, as mentioned above. These short
bond distances obtained for 1 are assumed to be a
result of an electron deficient metal center, drawing its
ligands in as close as possible. Given the appropriate
conditions and if a suitable ligand is available, it will
even add a fifth ligand in order to compensate for this
lack of electron density.
4. Conclusions
Our results indeed indicate the dark red complex, 2,
of which variations have been previously reported but
not structurally characterized [3,6,7,9,10], to be five-co-
ordinate in the solid state with a trigonal bipyramidal
geometry containing three stibine ligand in the equato-
rial plane with carbonyl and chloro ligands in a trans
orientation in the apical positions. The yellow complex,
1, is the ‘normal’ four-coordinate square-planar ana-
logue to Vaska’s complex and contains only two stibine
ligands with a trans configuration, established for these
types of complexes.
In conclusion it may be noted that several Rh(I) and
Ir(I) bis arsine complexes have been structurally charac-
terized [12] having coordination numbers higher than
four. All of these complexes contain the normal Vaska
composition with the fifth ligand adding to the metal
via a C–C double bond, thus also a ligand capable of
accommodating electron density away from the central
metal atom via p back-bonding.
Both the four- and five-coordinate rhodium stibine
analogues of Vaska’s complex could be prepared in
satisfactory yields by appropriate adjustment of the
Table 6
Comparison of structural data for trans-[Rh(Cl)(CO)(L)n] (n=2 or 3) complexes
,
,
,
,
Complex/(L)n
Rh–L (A)
Rh–Cl (A)
Rh–CO (A)
C–O (A)
L–Pt–L (°)
Ref.
(PPh3)2
2.322(1)
2.4226(4)
2.5981(5)
2.5655(2)
2.568(2)
2.382(1)
1.77(1)
1.140(2)
0.915(25)
1.035(6)
1.175(13)
1.121(25)
180.0(1)
175.97(6)
119.97(2)
180
[24]
[25]
(AsPh3)2
(SbPh3)3
(SbPh3)2
2.3538(14)
2.4094(18)
2.315(3)
2.017(7)
1.875(7)
1.797(13)
1.911(20)
TW a
TW a
[26]
b
(SbPh3)3
120.0(1)
a TW=this work.
b trans-[Rh(CO)(COCH3)(SbPh3)3].

S. Otto, A. Roodt / Inorganica Chimica Acta 331 (2002) 199–207
207
[7] R. Ugo, F. Bonati, S. Cenini, Inorg. Chim. Acta 3 (1969) 220.
[8] L. Vallarino, Personal communication.
5. Supplementary material
[9] W. Hieber, H. Heusinger, O. Vohler, Chem. Ber. 90 (1957) 2425.
[10] M.A. Jennings, A. Wojcicki, Inorg. Chem. 6 (1967) 1854.
[11] (a) M.C. Baird, G. Wilkinson, J. Chem. Soc. Chem. Commun.
(1966) 267;
(b) M.J. Mays, G. Wilkinson, J. Chem. Soc. (1965) 6629;
(c) I.C. Douek, G. Wilkinson, J. Chem. Soc. (A) (1969) 2604.
[12] (a) F.H. Allen, O. Kennard, Chem. Des. Autom. News 8 (1993)
31;
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center under nos. CCDC 177091, 177092 and
177093 for structures 1, 2 and 3, respectively. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44-1223-336033; e-mail: deposit@
uccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk].
UV–Vis spectra and tables of absorbance changes at
selected wavelengths in the different solvents for the
determination of K, with appropriate figures of the
spectral changes, are available from the authors upon
request.
(b) Cambridge Structural Database, Cambridge Crystallographic
Data Centre, Cambridge, UK, October 2000.
[13] T. Hill, S. Otto, A. Roodt, Unpublished results, polymorph of 3.
[14] (a) G.M. Sheldrick, SHELXS-97: Program for Structure Determi-
nation, University of Go¨ttingen, Germany, 1997;
(b) G.M. Sheldrick, SHELXL-97: Program for Structure Refine-
ment, University of Go¨ttingen, Germany, 1997.
[15] K. Brandenburg, M. Bernt, DIAMOND: Program for Crystal
Structure Visualization, ver. 2.1, Crystal Impact, GmbH, Bonn,
Germany, 1999.
[16] A.C. Tolman, Chem. Rev. 77 (1977) 313.
Acknowledgements
[17] S. Otto, A. Roodt, J. Smith, Inorg. Chim. Acta 303 (2000) 295.
[18] S. Otto, A. Roodt, Inorg. Chem. Commun. 4 (2001) 49.
[19] SCIENTIST for Windows: Program for Least-squares Parameter
Estimation, ver. 4.00, Micromath Inc. Utah, 1990.
[20] J.W. Akitt, B.E. Mann, NMR and Chemistry, 4th ed., Stanley
Thornes, Cheltenham, UK, 2000, p. 4.
Financial assistance from the South African NRF
and the research fund of the University of the Free
State is gratefully acknowledged.
[21] V. Gutmann, Coord. Chem. Rev. 18 (1976) 225.
[22] M. Sandstrom, I. Persson, P. Persson, Acta Chim. Scand. 44
(1990) 653.
[23] J. Rydberg, in: J. Rydberg, C. Musikas, G. Choppin (Eds.),
Principles and Practices of Solvent Extraction, Marcel Dekker,
New York, 1992, p. 23.
[24] K.R. Dunbar, S.C. Haefner, Inorg.Chem. 31 (1992) 3676.
[25] S. Otto, PhD Thesis, University of the Free State, Bloemfontein,
South Africa, 1999.
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