The metal-carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine-amide ligand

Article abstract of DOI:10.1016/S0022-328X(01)01097-X

Interactions of trans-[Rh(bpb)(CH₃)(H₂O)] (1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the respective adducts trans-[Rh(bpb)(CH₃)(L)], where L=4-substituted pyridine 4-Xpy (X=H (2), ^tBu (3), NMe₂ (4) CN (5)), PMe₂Ph (6) or benzimidazole (7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh-C distance in complex 6 (2.095(6) ?) is longer than that in complex 3 (2.02(1) ?), indicating that PMe₂Ph has a stronger trans influence than 4-^tBupy. The metal-carbon stretching frequencies for trans-[Rh(bpb)(CH₃)(L)] and [M(TTP)(CH₃)] [TTP=5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman spectroscopy. Complex 1 exhibits ν(Rh-C) at 562 cm^-1, which downshifts to 532 cm^-1 upon deuteriation of the axial methyl group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in ν(Rh-C). The ν(Rh-C) for trans-[Rh(bpb)(CH₃)(L)] was found to decrease in the order L: PMe₂Ph?4-Xpy~BzIm>H₂O, consistent the order of trans influence of L. For [M(TTP)(CH₃)] (M=Co, Rh, Ir, Ga, or In) the M-C force constant was found to decrease in the orders Ir>Rh>Co and Ga>In, consistent with the trends of metal-carbon bond strength for these metals. For trans-[Rh(bpb)(CH₃)(4-Xpy)] and trans-[Rh(TTP)(CH₃)(4-Xpy)], the ν(Rh-C) were found to be not very sensitive to the nature of X, suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh-C bonds for these rhodium alkyl complexes.

Full text of DOI:10.1016/S0022-328X(01)01097-X

Journal of Organometallic Chemistry 634 (2001) 61–68
www.elsevier.com/locate/jorganchem
The metal–carbon stretching frequencies in methyl complexes of
Rh, Ir, Ga and In with porphyrins and a tetradentate
pyridine–amide ligand
Willetta Lai a, Man-Kit Lau a, Victor Chong a, Wing-Tak Wong b, Wa-Hung Leung a,*,
Nai-Teng Yu a,*
a Department of Chemistry, The Hong Kong Uni6ersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
b Department of Chemistry, The Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong
Received 4 May 2001; accepted 27 June 2001
Abstract
Interactions of trans-[Rh(bpb)(CH3)(H2O)] (1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the
t
respective adducts trans-[Rh(bpb)(CH3)(L)], where L=4-substituted pyridine 4-Xpy (X=H (2), Bu (3), NMe2 (4) CN (5)),
PMe2Ph (6) or benzimidazole (7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh–C
,
,
distance in complex 6 (2.095(6) A) is longer than that in complex 3 (2.02(1) A), indicating that PMe2Ph has a stronger trans
influence than 4-tBupy. The metal–carbon stretching frequencies for trans-[Rh(bpb)(CH3)(L)] and [M(TTP)(CH3)] [TTP=
5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman
spectroscopy. Complex 1 exhibits w(Rh–C) at 562 cm−1, which downshifts to 532 cm−1 upon deuteriation of the axial methyl
group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in w(Rh–C). The
w(Rh–C) for trans-[Rh(bpb)(CH3)(L)] was found to decrease in the order L: PMe2Phꢀ4-XpyꢁBzIm\H2O, consistent the order
of trans influence of L. For [M(TTP)(CH3)] (M=Co, Rh, Ir, Ga, or In) the M–C force constant was found to decrease in the
orders Ir\Rh\Co and Ga\In, consistent with the trends of metal–carbon bond strength for these metals. For trans-
[Rh(bpb)(CH3)(4-Xpy)] and trans-[Rh(TTP)(CH3)(4-Xpy)], the w(Rh–C) were found to be not very sensitive to the nature of X,
suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh–C bonds for these
rhodium alkyl complexes. © 2001 Published by Elsevier Science B.V.
Keywords: Metal methyl compounds; FT-Raman spectroscopy; Metal–carbon stretching frequency; Trans ligand effect
better to understand the unique roles of certain metals
in catalysis. Of special interest are cobalt(III) alkyls
1. Introduction
that are found in the active sites of two types of
cobalamin, namely methylcobalamin and 5%-de-
Transition metal s-alkyl compounds play central
roles in organometallic [1] and bioinorganic [2] cataly-
oxyadenosylcobalamin, which mediate methyl group
sis. A knowledge of factors affecting metal-carbon
transfer and carbon skeletal rearrangement, respectively
bond strength of transition metal alkyls may help eluci-
[3,4]. Since the breaking of the cobalt–alkyl bond is
date mechanisms of organometallic reactions, and the
believed to be a key step in the catalytic cycles for these
two types of cobalamin, efforts have been made to
Abbre6iations: Por, porphyrin dianion; TTP, 5,10,15,20-tetrakis(4-
elucidate the factors affecting cobalt–alkyl bond
methylphenyl)porphyrin dianion; OEP, 2,3,7,8,12,13,17,18-octaethyl-
strength for organocobalt(III) compounds [3,5]. In par-
ticular, Raman spectroscopy has proved to be a conve-
nient technique to assess the ground-state properties of
B12 and model compounds due to the correlation be-
tween the metal–carbon stretching frequency and
porphyrin dianion; bpb, 1,2-bis(2-pyridinecarboxamido)benzene
dianion; 4-Xpy, 4-X-substituted pyridine; BzIm, benzimidazole.
* Corresponding authors. Tel.: +852-2358-7360; fax: +852-2358-
1594.
E-mail addresses: chleung@ust.hk (W.-H. Leung), chyu@ust.hk
(N.-T. Yu).
0022-328X/01/$ - see front matter © 2001 Published by Elsevier Science B.V.
PII: S0022-328X(01)01097-X

62
W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
metal–carbon bond dissociation energy. Using a near-
infrared laser source for Raman excitation, which elim-
inates the problem of photolysis of light-sensitive cobalt
alkyl compounds, the Co–C vibrational modes for
methylcobalamins and model compounds have been
unambiguously identified to be in the range of 500–515
cm−1 [6]. A similar stretching frequency was found for
methylcobalamin by resonance Raman spectroscopy
[7]. One issue that has attracted much attention is the
role of the axial ligand in the Co–C bond activation for
2. Experimental
2.1. General Information
Solvents were purified and distilled and degassed. All
synthetic manipulations, unless otherwise stated, were
carried out in air. NMR spectra were recorded on a
Bruker ARX 300 spectrometer operating at 300 MHz,
chemical shifts (l/ ppm) were reported with reference
to SiMe4 (1H) and H3PO4 (31P).
B
12 coenzyme. The displacement of the axial benzimida-
The methyl complexes trans-[Rh(bpb)R(H2O)] (R=
CH3 (1) or CD3) were prepared according to the Che’s
procedure [13] by oxidative addition of Na[Rh(bpb)]
with CH3I or CD3I, respectively, and purified by
column chromatography (neutral alumina). The metal-
loporphyrins [Rh(por)(CH3)] (por=TTP, OEP) [14],
and [M(TTP)(CH3)] (M=Ir [15], Ga [16], In [17]) were
prepared according to the literature methods. The
methyl-d3 derivatives trans-[Rh(bpb)(CD3)(H2O)] and
[Rh(por)(CD3)] were prepared as for the corresponding
methyl analogues using CD3I instead of CH3I, and
[M(ttp)(CD3)] prepared by methylation of [M(TTP)Cl]
zole group from the cobalt centre by a nitrogenous
ligand was indicated by recent X-ray diffraction [8] and
EPR [9] studies on methylmalonyl–CoA mutase. Al-
though the axial base is known to accelerate the Co–C
bond cleavage in B12 coenzyme [10], trans ligands were
found to have little or no electronic effects on w(Co–C)
for both alkylcobaloximes [11] and alkylcobalamins
[7,12]. In an effort to elucidate the factors governing
metal–alkyl bond strength, and to address the question
as to why cobalt is chosen as the metal centre for B12
chemistry, we set out to determine metal–carbon
stretching frequencies for a series of metal methyl com-
plexes by FT-Raman spectroscopy. Previously, we have
reported on the stereoelectronic factors affecting Co–C
stretching frequencies for cobalt(III) alkyls with a tetra-
aza chelate 1,2-bis(2-pyridinecarboxamido)benzene
(H2bpb) and porphyrins [6]. In this paper, we will
extend our study to methyl complexes of other transi-
tion metals. The effects of metal substitution and axial
base coordination on the metal–carbon stretching fre-
quencies for trans-[Rh(bpb)(CH3)(L)] (L=H2O, 4-sub-
1
with CD3MgI. The H-NMR spectra for the methyl-d3
complexes were found to be identical with those for the
methyl analogues except that the axial methyl resonant
1
signals were absent. H-NMR spectral data for trans-
[Rh(bpb)(CH3)(L)] and trans-[M(bpb)(CH3)(L)] (M=
Rh, Ir) are collected in Section 5.
2.2. Preparations of trans-[Rh(bpb)(CH3)(L)]
t
(L=4-Xpy where X=H (2), Bu (3), NMe2 (4), CN
(5), PMe2Ph (6), or BzIm (7))
stituted
pyridine,
BzIm
or
PMe2Ph)
and
[M(TTP)(CH3)(L)] (M=Rh or Ir, L=4-substituted
pyridine) (Scheme 1) were examined. The crystal struc-
tures of trans-[Rh(bpb)(CH3)(4-tBupy)] and trans-
[Rh(bpb)(CH3)(PMe2Ph)] have been determined.
Typically, to a solution of 1 (50 mg, 0.08 mmol) in
MeOH (10 ml) an excess of L (ca. two equivalents) was
added, and the mixture was stirred at room tempera-
ture (r.t.) for 30 min. Concentration (to ca. 5 cm3) and
Scheme 1.

W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
63
The spectra were recorded via 180o scattering. The laser
excitation used was 1.064 mm provided by a CW diode
laser-pumped Nd: YAG laser. The number of scans
taken was 50–200. The laser power employed was
50–100 mW (defocused) and the spectral resolution
was 4.0 cm−1. The samples were exposed to the laser
beam in the solid form in an aluminum sample holder,
which has a 2-mm hole driller at the centre.
Table 1
Crystallographic data and experimental details for trans-
[Rh(bpb)(CH3)(4-Butpy)]·1/2Et2O
(3·1/2Et2O)
and
trans-
[Rh(bpb)(CH3)(PMe2Ph)]·1/2H2O (6·1/2H2O)
3·1/2Et2O
6·1/2H2O
Empirical formula
Formula weight
Crystal color, habit
Crystal system
C30H33N5O2.5Rh C27H27N4O2.5PRh
606.53
581.41
Orange, block
Triclinic
Orange, block
Monoclinic
Unit cell dimensions
2.5. X-ray crystallography
,
a (A)
13.589(5)
18.223(6)
13.290(6)
110.85(3)
107.93(3)
73.30(3)
2866(2)
15.987(3)
16.481(4)
19.418(2)
,
b (A)
A summary of crystallographic data and experimen-
tal details for 3·1/2Et2O and 6·1/2H2O is shown in
Table 1. All data were collected on a Rigaku AFC7R
diffractometer using graphite-monochromated Mo–Ka
,
c (A)
h (°)
i (°)
g (°)
98.97(1)
3
,
V (A )
5053(1)
P21/a (no. 14)
8
1.528
2376
7.72
298.2
0.71073
ꢀ–2q
,
radiation (u=0.71073 A) at 25 °C. The structures were
(
Space group
Z
P1 (no. 2)
4
solved by direct methods. All data were corrected for
Lorentz and polarization and absorption effects. The
structures were refined on F by full matrix least-squares
analyses. Hydrogen atoms were placed at the idealised
Dcalc (g cm−3
F(000)
)
1.405
1252
6.31
v (Mo–Ka) (cm−1
T (K)
)
298.2
0.71073
ꢀ–2q
7883
,
positions (C–H=0.95 A). All calculations were per-
,
u (A)
formed using the TEXSAN [19] crystallographic software
package.
Scan type
Total no. of reflections
Weighting scheme
9589
1/[|2(Fo)
1/[|2(Fo)
+0.025F2o/4]
4379
+0.012F2o/4]
5967
3. Results
No. of observations
(I\1.50|(I))
R(F)
0.075
0.077
1.71
0.047
0.044
1.37
3.1. Syntheses of metal methyl complexes
Rw
Goodness-of-fit (F)
The aquo ligands in trans-[Rh(bpb)(CH3)(H2O)] (1)
was found to be labile and could be displaced readily
by Lewis bases. Thus, the interaction of complex 1 with
Lewis bases L afforded the corresponding adducts
trans-[Rh(bpb)(CH3)(L)] (L=4-Xpy where X=H (2),
tBu (3), NMe2 (4) and CN (5); PMe2Ph (6) or BzIm (7))
that were characterized by 1H-NMR spectroscopy.
These adducts are stable in both the solid state and
solution. Similarly, the six-coordinated pyridine ad-
ducts trans-[M(TTP)(CH3)(4-Xpy)] (M=Rh, X=H
(8), Bu (9), NMe2 (10), CN (11); Ir, X=H (12), Bu
(13), NMe2 (14), CN (15)) were prepared from
[M(TTP)(CH3)] and 4-Xpy. The FT-Raman spectra of
solid samples of complexes 8–15 showed that the M–C
stretching modes were shifted with respect to those for
[M(TTP)(CH3)] (see later section), indicating that the
4-Xpy ligands are coordinated in these complexes.
addition of a large amount of Et2O afforded an orange
solid, which was collected, washed with MeOH, recrys-
tallized from CH2Cl2–Et2O, and characterized by H-
1
NMR spectroscopy [13] (yield 50–70%).
2.3. trans-[M(TTP)(CH3)(4-Xpy)] (M=Rh, X=H (8),
t
tBu (9), NMe2 (10), CN (11); M=Ir, X=H (12), Bu
t
t
(13), NMe2 (14), CN (15))
Typically, to a solution of [M(TTP)(CH3)] (50 mg,
0.08 mmol) in CH2Cl2 (10 cm3) an excess of 4-Xpy (ca.
two equivalents) was added. The mixture was stirred at
r.t. for 30 min and was concentrated to ca. 5 cm3.
Addition of hexane (ca. 20 cm3) and cooling to
−10 °C afforded a purple solid, which was collected,
washed with cold MeOH, and characterized by 1H-
NMR spectroscopy in CDCl3 solution in the presence
of 4X-py [18] (yield 60–70%).
3.2. Crystal structures of trans-[Rh(bpb)(CH3)(L)]
(L=4-Butpy and PMe2Ph)
The solid-state structures of 3 and 6 have been
established by X-ray crystallography. For 3, two inde-
pendent molecules were found in the asymmetric unit.
The structure of one of these molecules containing
Rh(1) is shown in Fig. 1; selected bond lengths and
angles are listed in Table 2. The geometry around Rh in
2.4. FT-Raman measurements
FT-Raman data were acquired on a Bruker IFS
66/FRA FT-Raman spectrophotometer equipped with
a highly sensitive Ge detector cooled by liquid nitrogen.

64
W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
Fig. 1. Molecular structure of trans-[Rh(bpb)(CH3)(4-tBupy)] (3).
Table 2
3 is pseudo octahedral with the methyl group opposite
to the 4-tBupy. The average Rh–N(bpb amide) (1.967
,
Selected bond lengths (A) and bond angles (°) for trans-
[Rh(bpb)(CH3)(4-tBupy)]·1/2Et2O (3·1/2Et2O)
,
,
A) and Rh–N(bpb pyridyl) (2.084 A) are comparable
to those for Na[Rh(bpb)Cl2] [13]. The Rh–C bond
Bond lengths
Rh(1)–N(1)
Rh(1)–N(3)
Rh(1)–N(5)
Rh(2)–N(6)
Rh(2)–N(8)
Rh(2)–N(10)
2.082(9)
1.965(9)
2.203(9)
2.10(1)
1.977(10)
2.22(1)
Rh(1)–N(2)
Rh(1)–N(4)
Rh(1)–C(19)
Rh(2)–N(7)
Rh(2)–N(9)
Rh(2)–C(47)
1.97(1)
2.09(1)
2.02(1)
1.97(1)
2.08(1)
2.04(1)
,
distance (2.02(1) A) is typical of octahedral rhodiu-
,
m(III) methyl complexes, e.g. 2.032(4)
A
for
[{Rh(TTP)(CH3)}2(m-4-CNpy)] [20]. The Rh–N(4-
Bupy) bond (2.203(10) A) that is trans to the methyl
t
,
group is considerably longer than the Rh–N(bpb
pyridyl) bonds due to the trans influence of the methyl
group. For complex 6, the asymmetric unit also consists
of two independent molecules. The structure of one of
these molecules containing Rh(2) is shown in Fig. 2;
selected bond lengths and angles are listed in Table 3.
The geometry around Rh in complex 6 is pseudo
octahedral with the methyl group opposite to the phos-
phine ligand. The Rh–C, Rh–P, average Rh–N(bpb
amide) and average Rh–N(bpb pyridyl) distances are
Bond angles
N(1)–Rh(1)–N(2)
N(1)–Rh(1)–N(4)
N(1)–Rh(1)–C(19)
N(2)–Rh(1)–N(4)
N(2)–Rh(1)–C(19)
N(3)–Rh(1)–N(5)
N(4)–Rh(1)–N(5)
80.8(4)
114.8(4)
89.4(5)
164.1(4)
88.3(6)
92.0(4)
90.7(4)
N(1)–Rh(1)–N(3)
N(1)–Rh(1)–N(5)
N(2)–Rh(1)–N(3)
N(2)–Rh(1)–N(5)
N(3)–Rh(1)–N(4)
N(3)–Rh(1)–C(19)
N(4)–Rh(1)–C(19)
N(6)–Rh(2)–N(7)
N(6)–Rh(2)–N(9)
N(6)–Rh(2)–C(47)
N(7)–Rh(2)–N(9)
N(7)–Rh(2)–C(47)
N(8)–Rh(2)–N(10)
N(9)–Rh(2)–N(10)
164.1(5)
90.0(4)
83.4(5)
92.5(4)
80.9(4)
88.8(4)
88.8(6)
80.0(4)
115.3(4)
88.6(5)
164.0(4)
88.7(5)
89.5(4)
90.3(4)
N(5)–Rh(1)–C(19) 179.0(5)
N(6)–Rh(2)–N(8)
N(6)–Rh(2)–N(10)
N(7)–Rh(2)–N(8)
N(7)–Rh(2)–N(10)
N(8)–Rh(2)–N(9)
N(8)–Rh(2)–C(47)
N(9)–Rh(2)–C(47)
164.0(5)
93.5(4)
84.1(4)
93.3(4)
80.4(4)
88.9(5)
87.3(5)
,
2.095(6), 2.407(2), 1.980 and 2.096 A, respectively. De-
spite the observed standard deviations, it is apparent
that the average Rh–C bond distance in 6 is longer
than that in 3, indicating that the phosphine has a
stronger trans influence than pyridine.
N(10)–Rh(2)–C(47) 177.3(6)

W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
65
Fig. 2. Molecular structure of trans-[Rh(bpb)(CH3)(PMe2Ph)] (6).
3.4. w(M–C) for [M(TTP)(CH3)(L)] (M=Rh, Ir, Ga
or In)
3.3. Rh–C stretching frequencies for
trans-[Rh(bpb)(CH3)(L)]
The solid-state Raman w(M–C) for metal methyl
porphyrin complexes are listed in Table 5.
[Rh(TTP)(CH3)] exhibits a Raman-active band at 562
cm−1, which shifts to 524 cm−1 upon deuteriation of
the axial methyl group, is assigned as w(Rh–C). Like
The Rh–C stretching frequencies for trans-
[Rh(bpb)(CH3)(L)] (L=H2O, 4-Xpy, BzIm, PMe2Ph)
in the solid state have been determined by FT-Raman
spectroscopy and the results are summarized in Table 4.
The Raman spectrum of compound 1 (Fig. 3a) shows a
band at 562 cm−1, which downshifts to 532 cm−1 upon
deuteriation of the axial methyl group (Fig. 3b), is
confidently assigned to w(Rh–C). The observed isotope
shift of 30 cm−1 is smaller than the theoretical value of
43 cm−1 probably due to coupling of w(Rh–C) with
some other vibrational modes of bpb. The w(Rh–C) for
complex 1 is higher than that for the cobalt congener
Table 3
,
Selected bond lengths (A) and bond angles (°) for trans-
[Rh(bpb)(CH3)(PMe2Ph)]·1/2H2O (6·1/2H2O)
Bond lengths
Rh(1)–P(1)
Rh(1)–N(2)
Rh(1)–N(4)
Rh(2)–P(2)
Rh(2)–N(6)
Rh(2)–N(8)
2.413(2)
1.974(5)
2.074(5)
2.407(2)
1.982(5)
2.096(5)
Rh(1)–N(1)
Rh(1)–N(3)
Rh(1)–C(19)
Rh(2)–N(5)
Rh(2)–N(7)
Rh(2)–C(46)
1.980(4)
2.097(4)
2.089(6)
1.977(4)
2.095(4)
2.095(7)
trans-[Co(bpb)(CH3)(H2O)] (515 cm−1
) [6d]. Like
trans-[Co(bpb)(CH3)(H2O)], replacement of the aquo
ligand in complex 1 with 4-Xpy or BzIm resulted in
lowering of w(Rh–C). Although it appears that the
electron-rich pyridine adducts (X=tBu or NMe2) have
higher w(Rh–C) than the less electron-rich ones, the
electronic influence of the pyridine ligand on w(Rh–C)
is not big. The Dw(Rh–C) for trans-[Rh(bpb)(CH3)(4-
Xpy)], which is defined as the difference between w(Rh–
C) for the py adduct and that for the 4-Xpy adduct, is
in the range of +4 to −6 cm−1. A similar result
was found for [Co(DH)2(CH3)(4-Xpy)] (DH=
dimethylglyoxime anion) which shows a Dw(Co–C) of
ca. 10 cm−1 [6c]. The w(Rh–C) for the PMe2Ph adduct
6 is considerably lower than those for 1 and the pyri-
dine adducts consistent with the strong trans influence
of phosphine.
Bond angles
P(1)–Rh(1)–N(1)
P(1)–Rh(1)–N(3)
91.4(1)
89.5(1)
P(1)–Rh(1)–N(2)
P(1)–Rh(1)–N(4)
N(1)–Rh(1)–N(2)
N(1)–Rh(1)–N(4)
N(2)–Rh(1)–N(3)
N(2)–Rh(1)–C(19)
N(3)–Rh(1)–C(19)
P(2)–Rh(2)–N(5)
P(2)–Rh(2)–N(7)
P(2)–Rh(2)–C(46)
N(5)–Rh(2)–N(7)
N(5)–Rh(2)–C(46)
N(6)–Rh(2)–N(8)
N(7)–Rh(2)–N(8)
N(8)–Rh(2)–C(46)
90.6(1)
93.3(1)
84.6(2)
80.5(2)
80.5(2)
88.9(2)
89.5(2)
93.9(1)
88.8(1)
177.3(2)
164.3(2)
88.2(2)
163.4(2)
115.7(2)
87.1(2)
P(1)–Rh(1)–C(19) 178.9(2)
N(1)–Rh(1)–N(3) 165.1(2)
N(1)–Rh(1)–C(19) 89.5(2)
N(2)–Rh(1)–N(4) 164.6(2)
N(3)–Rh(1)–N(4) 114.4(2)
N(4)–Rh(1)–C(19) 87.4(2)
P(2)–Rh(2)–N(6)
P(2)–Rh(2)–N(8)
N(5)–Rh(2)–N(6)
N(5)–Rh(2)–N(8)
N(6)–Rh(2)–N(7)
93.8(1)
91.5(1)
84.3(2)
79.7(2)
80.1(2)
N(6)–Rh(2)–C(46) 88.1(2)
N(7)–Rh(2)–C(46) 89.7(2)

66
W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
the theoretical value (40 cm−1). [In(TTP)(CH3)] shows
w(In–C) at 501 cm−1 with an isotope shift of 44 cm−1
Table 4
Solid-state Rh–C stretching frequencies for trans-[Rh(bpb)(CH3)(L)]
,
which is also larger than the theoretical value. Thus, in
contrast to the methyl complexes of cobalt triad, the
metal–carbon stretching frequency for the Group 13
metal methyl complexes decreases down a group.
a
L
w(Rh–C) (cm−1
)
Dw(Rh–C) (cm−1
)
H2O
H2O
py
562
532 b
556
552
556
562
555
514
0
4
0
4-tBupy
4-Me2Npy
4-CNpy
BzIm
4. Discussion
−6
PMe2Ph
4.1. Trends of M–C force constants for metal methyl
complexes
a Dw(Rh–C)=w(Rh–C) for [Rh(bpb)(CH3)(py)]–w(Rh–C) for
[Rh(bpb)(CH3)(4-Xpy)].
b Methyl-d3 complexes.
In earlier studies, it was demonstrated that the
cobalt–carbon force constant for organocobalt B12
model compounds correlates well with the Co–C bond
strength [6]. The correlation of force constant with
bond strength for cobalt alkyls is derived from the
assumption that the Co–CH3 moiety conforms to the
Morse potential relationship, i.e. U(r)=De{1−exp[−
a(r−re)]}2 where U(r)=potential energy, De=bond
dissociation energy, r=bond length, re=equilibrium
bond length, and a=constant. Under such a circum-
stance, the harmonic force constant (s) of the Co–C
bond is equal to the second derivative of U(r) at r=re,
i.e. s=2Dea2. Thus s/s%=(De/D%).
e
If one assumes that the iso-structural metal methyl
complexes [M(TTP)(CH3)] have similar Morse potential
characteristics, an analogous correlation of s(M–C)
with De(M–C) may also exist. Our Raman data for
Fig. 3. FT-Raman spectra of trans-[Rh(bpb)R(H2O)] in the solid
state: R=CH3 (a), CD3 (b).
Table 5
Solid-state M–C stretching frequencies for trans-[M(por)(CH3)(L)]
complex 1, the observed isotope shift of 38 cm−1 for
[Rh(TTP)(CH3)] is smaller than the theoretical value. A
slightly lower wavenumber (558 cm−1) was found for
the more basic OEP analogue [Rh(OEP)(CH3)]. As
might be expected, coordination of 4-Xpy to
[Rh(ttp)(CH3)] led to a downshift in w(Rh–C), indicat-
ing that the Rh–C bonds in these octahedral adducts
are weaker than that in square pyramidal
[Rh(TTP)(CH3)]. Like the Rh(bpb) system, the w(Rh–
C) for [Rh(TTP)(CH3)(4-Xpy)] is not very sensitive to
the electronic factors of 4-Xpy [Dw(Rh–C)511 cm−1].
[Ir(TTP)(CH3)] exhibits w(Ir–C) at 578 cm−1, which is
higher than those for [Co(TTP)(CH3)] (503 cm−1) and
[Rh(TTP)(CH3)]. Thus, for [M(TTP)(CH3)], the M–C
stretching frequency increases in the order CoBRhB
Ir. Again binding of 4-Xpy to [Ir(TTP)(CH3)] results in
a downshift in w(Ir–C) with Dw(Ir–C)B10 cm−1. For
a
M
por
L
w(M–C) (cm−1
)
Dw(Rh–C) (cm−1
)
Co TTP
Rh TTP
Rh TTP
Rh OEP
Rh OEP
503 b
562
524 c
558
526 c
560
Rh TTP py
0
11
6
Rh TTP 4-tBupy
549
Rh TTP 4-NMe2py 554
Rh TTP 4-CNpy
Rh TTP BzIm
Ir
Ir
Ir
Ir
Ir
Ga TTP
Ga TTP
In TTP
In TTP
563
563
578
577
568
−3
TTP
TTP py
0
9
6
TTP 4-tBupy
TTP 4-NMe2py 571
TTP 4-CNpy
567
559
510c
501
457c
10
[Ga(TTP)(CH3)], the Raman-active band at 559 cm−1
,
which shifts to 510 cm−1 upon deuteriation of the axial
methyl group, is assigned to w(Ga–C). By contrast to
the rhodium and iridium analogues, the observed iso-
tope shift of 49 cm−1 for [Ga(TTP)(CH3)] is larger than
a Dw(M–C)=w(M–C)
[M(TTP)(CH3)(4-Xpy)].
b Ref. [6d].
for
[M(TTP)(CH3)(py)]–w(M–C)
for
c Methyl-d3 complexes.

W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
67
[M(TTP)(CH3)] are indeed in line with such as correla-
tion. For a diatomic oscillator M–CH3, w8(s/v)1/2 or
s8w2v (where v is the reduced mass of M and CH3).
Thus, for [M(TTP)(CH3)] (M=Co, Rh, Ir), s(Co–C):
s(Rh–C): s(Ir–C) is calculated to be (503)2×11.96:
(563)2×13.11: (577)2×13.91=1: 1.37: 1.53 (v for Co–
CH3, Rh–CH3 and Ir–CH3 are 11.96, 13.11, and 13.91,
respectively). The increasing order of M–C force con-
stant is consistent with the trend that the M–C bond
strength increases down a group for transition elements
[21]. On the other hand, for [M(TTP)(CH3)] (M=Ga,
In), s(Ga–C): s(In–C) is calculated to be (559)2×
12.37: (501)2×13.29=1: 0.863 (v for Ga–CH3 and
In–CH3 equal 12.37 and 13.29, respectively), which
again is consistent with the trend of metal–carbon
bond strength for main group elements [21].
values of w(M–C) for the phosphine adducts of these
alkyls. X-ray crystallography revealed that the Rh–C
bond in the phosphine adduct 6 is significantly longer
than that in the pyridine adduct 3. The strong trans
influence of phosphine is attributed to the covalency of
M–P bond and the ability of phosphines form p bond-
ing with soft transition metals such as cobalt and
rhodium.
4.3. Summary
In summary, the metal–carbon stretching modes for
trans-[M(bpb)(CH3)(L)] and trans-[M(bpb)(CH3)(L)]
have been determined by FT-Raman spectroscopy and
assigned by deuterium-labeling experiments. Although
a more sophisticated model of the metal–carbon vibra-
tional mode, which includes the coupling of the metal–
carbon stretches with other modes, is required in order
to have a better agreement between the observed and
theoretical isotope shifts, the conclusion of this paper
does not rest on this. This work not only describes
trends of metal–alkyl bond strengths but also gives
some numerical values that allows a qualitative/semi-
quantitative correlation between the metal–carbon
stretching frequencies and the trans ligand and metal
center effects. The conclusion is consistent with the
X-ray data, and the well-known trans influence of
ligands and trends of element-carbon bond strength:
namely, for [M(TTP)(CH3)], the M–C force constant
decreases in the orders Ir\Rh\Co and Ga\In. For
4.2. Trans ligand effect on w(M–C)
The w(M–C) for trans-[Rh(bpb)(CH3)(L)] (L=4-
Xpy and BzIm) are lower than those for 1, suggesting
that the N-donor ligands exhibit stronger trans influ-
ence than aquo. This finding is in contrast to the results
for the methylcobalamin, for which the w(Co–C) is
independent of the displacement of the benzimidazole
group [7]. Binding of 4-Xpy to [M(ttp)(CH3)] (M=Rh,
Ir) results in lowering in w(M–C), indicating that the
M–C bond is weakened by ligation of axial bases. It
may be noted that the axial Co–C bond in
[Co(OEP)(CH3)(4-NMe2py)] is longer than that in
,
[Co(OEP)(CH3)] by ca. 0.045 A [22]. The w(M–C) for
trans-[Rh(bpb)(CH3)(4-Xpy)] and [M(TTP)(CH3)(4-
Xpy)] are, however, not very sensitive to the nature of
X. Therefore, like the previously studied alkylcoba-
loxime system [6], the electronic factors of 4-Xpy do
not exhibit a significant influence on the ground-state
properties of the M–C bond in both trans-
[Rh(bpb)(CH3)(4-Xpy)] and [M(TTP)(CH3)(4-Xpy)]. It
appears that the insensitivity of M–C force constant to
nitrogen base coordination is quite common for alkyl
complexes of transition metals, at least for the cobalt
triads, and is apparently due to strong transition metal-
to-alkyl covalent bonds. For B12 and model com-
pounds, the strong covalent Co–C bond renders the
opposite Co–N bond relatively weak regardless of the
basicity of trans nitrogen ligands. Thus, coordination or
displacement of axial nitrogen ligands to cobalt alkyls
does not cause a significant change to the Co–C bond
strength. Although for trans-[M(TTP)(CH3)(4-Xpy)]
(M=Rh, Ir) the M–N(py) bond is expected to be
stronger than the Co–N counterpart, no electronic
influence of 4-Xpy on M–C bond is observed because
of the high M–C bond strength, as indicated by the
large value of s(M–C). On the other hand, phosphines
such as PMe2Ph exhibit strong trans influence to both
rhodium and cobalt alkyls, as indicated by the low
trans-[Rh(bpb)(CH3)(4-Xpy)]
and
trans-[M(TTP)-
(CH3)(4-Xpy)] (M=Rh, Ir), the metal–carbon stretch-
ing frequencies are not sensitive to the electronic factors
of pyridine ligand. It appears that the metal–carbon
force constant for transition metal alkyls is rather in-
sensitive to coordination of nitrogen ligands because of
the strong trans influence of the alkyl group. Thus, the
activation of B12 Co–C bond by enzyme is not due to
weakening of the Co–C bond by the axial nitrogen
bases. By contrast, phosphine ligands show a more
pronounced trans influence on transition metal–alkyl
bonds.
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 156615 for 3·1/2Et2O and
156616 for 6·1/2H2O. Copies of this information may
be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-
1233-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).

68
W. Lai et al. / Journal of Organometallic Chemistry 634 (2001) 61–68
[9] R. Padmakumar, S. Taoka, R. Padmakumar, R. Banerjee, J.
Acknowledgements
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Grants Council of Hong Kong, China (project no.
HKUST 6152/97P). We thank Tin-Ming Kwok for
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