Synthesis and characterization of cis-Mo(CO)4(L-L′) and cis-Mo(CO)2(L-L′)2 complexes of N(1)-methyl-2-(arylazo)imidazoles (L-L′). Correlations of spectroscopic data with substituent effects

Article abstract of DOI:10.1016/S0022-328X(03)00793-9

N(1)-Methyl-2-(p-X-phenylazo)imidazoles (L-L′; X=CH₃O, CH₃, H, Br, CF₃, NO₂) react with cis -(norbornadiene)Mo(CO)₄ and Mo(CO)₃ (CH₃CN)₃ to provide the complexes cis-M

Full text of DOI:10.1016/S0022-328X(03)00793-9

Journal of Organometallic Chemistry 682 (2003) 248ꢀ254
/
www.elsevier.com/locate/jorganchem
Synthesis and characterization of cis-Mo(CO)4(Lꢀ
Mo(CO)2(LꢀL?)2 complexes of N(1)-methyl-2-(arylazo)imidazoles
(LꢀL?). Correlations of spectroscopic data with substituent effects
/
L?) and cis-
/
/
Martin N. Ackermann *, Marisa P. Robinson, Ian A. Maher, Eric B. LeBlanc,
Richard V. Raz
Department of Chemistry, Oberlin College, 119 Woodland Street, Oberlin, OH 44074, USA
Received 27 June 2003; received in revised form 30 July 2003; accepted 30 July 2003
Abstract
N(1)-Methyl-2-(p-X-phenylazo)imidazoles (Lꢀ
/
L?; Xꢁ/CH3O, CH3, H, Br, CF3, NO2) react with cis-(norbornadiene)Mo(CO)4
and Mo(CO)3(CH3CN)3 to provide the complexes cis-Mo(CO)4(Lꢀ
/
L?) (XꢁCH3O, CH3, H, Br, CF3, NO2) and cis-Mo(CO)2(Lꢀ
/
/
L?)2 (Xꢁ
CH3, H, Br, CF3), respectively. The complexes were characterized by visible and infrared spectroscopy, by H-, 13C- and
/
1
95Mo-NMR spectroscopy, and by cyclic voltammetry. Correlations among the data and correlations of the data with the Hammett
sigma parameter within each set of complexes were investigated and are generally very good. These data, as well as the inability to
synthesize complexes of the type Mo(LꢀL?)3, indicate that 2-(arylazo)imidazoles are less effective ligands at stabilizing complexes of
/
zerovalent metals than are 2-(arylazo)pyridines.
# 2003 Elsevier B.V. All rights reserved.
Keywords: 2-(Arylazo)imidazole complexes; Molybdenum(0) complexes; Nuclear magnetic resonance; Azoimine complexes
1. Introduction
The strong p-acceptor ability of azoimines (ꢀ
NÄNꢀ) makes them unusual among chelating nitrogen
/
NÄ
/
Cꢀ
/
/
/
ligands in their ability to stabilize metals in their lower
oxidation states. This was first recognized in the
complexes of 2-(arylazo)pyridines with various metal
ions [1ꢀ
copper(I) [5ꢀ
14], and rhenium(II) [15ꢀ
/
4]. Subsequent studies of azoimine complexes of
7], osmium(II) [8ꢀ10], ruthenium(II) [11ꢀ
18] have demonstrated that the
/
/
/
/
extent of this stabilization depends on the N-hetero-
cyclic ring to which the arylazo linkage is attached.
Based on electrochemical data the ability of an azoimine
ligand to stabilize lower metal oxidation states increases
Our own work with azoimines has focused on
zerovalent complexes of iron [19] and the Group 6
metals [20ꢀ22] and shows that azoimines also can have a
/
significant effect on stabilizing complexes in which the
metal is in a formally zero oxidation state. Thus,
reaction of 2-(phenylazo)pyridine (2-PAP) with
in the order 2-(arylazo)imidazoleB
/2-(arylazo)pyrid-
ineB2-(arylazo)pyrimidine.
/
M(CO)6 (MꢁCr, Mo, W) yielded the series of com-
/
plexes cis-M(CO)4(2-PAP), cis-M(CO)2(2-PAP)2, and
M(2-PAP)3 [20]. The stability of the cis-M(CO)2(2-
PAP)2, and M(2-PAP)3 type complexes is unusual for
complexes of chelating nitrogen ligands as is the ability
* Corresponding author. Tel.: ꢂ
6682.
E-mail
Ackermann).
/
1-440-775-8066; fax: ꢂ
/
1-440-775-
address:
martin.ackermann@oberlin.edu (M.N.
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00793-9

M.N. Ackermann et al. / Journal of Organometallic Chemistry 682 (2003) 248ꢀ
/254
249
to replace all six carbonyls by direct reaction of 2-PAP
with M(CO)6. Studies using molybdenum as the metal
have demonstrated that the properties of these three
types of complexes vary with the substituents on the
pyridyl ring [21,22].
zo)imidazoles [23] as described in the literature [5]. The
only major departure from the literature procedure was
that the crude methylation product was not chromato-
graphed. Instead, If was crystallized from an ethanolꢀ
/
water mixture and the others were crystallized from an
acetone/hexane mixture.
N(1)-Methyl-2-(p-trifluoromethylphenylazo)imida-
zole (Ie) is a new compound and was prepared in a
manner similar to the other ligands. The intermediate 2-
(p-trifluoromethylphenylazo)imidazole, previously un-
Given these results, we were interested in what effect
replacing the pyridyl group with a different heterocycle
would have on the types and properties of complexes
obtained with zerovalent metals. Herein we report on
our studies using N(1)-methyl-2-(arylazo)imidazoles (I)
as the ligands. Since the studies with metal ions indicate
that 2-(arylazo)imidazoles are the least effective at
stabilizing lower metal oxidation states, it was expected
that these azoimines might show a marked difference
from the results using 2-(arylazo)pyridines. Our use of
the N(1) methylated ligand follows the lead of others in
order to avoid possible reactivity at that nitrogen
[5,6,10,12,14,15,18]. We also continue to use molybde-
num as the metal because of the ability to probe directly
the metal center in the complexes through 95Mo-NMR.
reported, was prepared from p-trifluoromethylaniline in
1
56% yield on a 30 mmol scale. M.p. 204ꢀ
/
205 8C. H-
NMR (CDCl3, d): 7.20 (s, 1H), 7.47 (s, 1H), 7.76 d, 2H),
8.02 (d, 2H). Treatment of 2-(p-trifluoromethylpheny-
lazo)imidazole with methyl iodide in THF and recrys-
tallization of the crude product from an acetone/hexane
mixture gave Ie in 57% yield on a 33 mmole scale. For
Ie: m. p. 119ꢀ
121 8C. 1H-NMR (CDCl3, d): 4.06 (s, 3H,
/
CH3), 7.19 (s, 1H, H5), 7.31 (s, 1H, H4), 7.74 (d, 2H, H8
and H10), 8.05 (d, 2H, H7 and H11). Anal. Found: C,
51.97; H, 3.67; N, 21.85. Calc. for C11H9F3N4: C, 51.97;
H, 3.57; N, 22.04%.
2.2.2. cis-Mo(CO)4(Lꢀ
A mixture of 1.0 mmol of cis-(C7H8)Mo(CO)4
(C7H8ꢁnorbornadiene) [24] and 1.0 mmol of I were
stirred in 50 ml of hexane (Ia
/
L?) (II) (Lꢀ
/
L?ꢁ
/
I)
/
/d) or CH2Cl2 (Ie, If) at
ꢀ
room temperature (r.t.) until the infrared spectrum in
the carbonyl stretching region showed the reaction was
complete (1ꢀ2 days). The solvent was removed under
/
reduced pressure. The residue was dissolved in CH2Cl2
and filtered through a column of Celite. Hexane was
added to the filtrate and the volume of the solution was
reduced under vacuum, causing crystals of II to form.
The remaining liquid was removed when it was judged
that crystal formation was complete. The crystals were
washed with hexane and dried under vacuum. The yield,
melting point, and elemental analysis of the complexes
are summarized in Table 1.
2. Experimental
2.1. General procedures
The general procedures for carrying out reactions, for
handling air-sensitive materials, for purifying solvents,
and for obtaining infrared, NMR, and cyclic voltam-
metry data have been described [20,21]. In addition,
some of the NMR spectra were obtained on a 600 MHz
system-95Mo at 39.07 MHz, 13C at 151 MHz, and
HETCORR. Electronic spectra were recorded on a
Cary 5E spectrophotometer. Melting points were taken
in open capillaries and are uncorrected. Elemental
analyses were performed by Atlantic Microlab Inc.,
Norcross, GA.
2.2.3. cis-Mo(CO)2(Lꢀ
/
L?)2 (III) (Lꢀ
/
L?ꢁI)
/
Molybdenum hexacarbonyl (1.0 mmol) was refluxed
in 25 ml of CH3CN until the infrared spectrum in the
carbonyl stretching region showed complete conversion
to Mo(CO)3(CH3CN)3 [25]. The solvent was removed
under vacuum and 30 ml of hexane and 2.0 mmole of I
was added. The mixtures were refluxed for 16 h and then
the solvent was removed under vacuum. The solid
residue was crystallized as described for II. The yields,
melting points, and elemental analyses of the complexes
are summarized in Table 1.
2.2. Syntheses
2.2.1. Synthesis of N(1)-methyl-2-(arylazo)imidazoles
(I)
The N(1)-methyl-2-(arylazo)imidazole ligands were
prepared by methylation of the corresponding 2-(aryla-

250
M.N. Ackermann et al. / Journal of Organometallic Chemistry 682 (2003) 248ꢀ254
/
Table 1
Yields, melting points, and elemental analyses for complexes cis-Mo(CO)4(Lꢀ
/
L’) (II) and cis-Mo(CO)2(Lꢀ
/
L’)2 (III)
Compound
Yield (%)
M.p. a (8C)
Elemental analysis (Found (Calc.) (%))
C
H
N
IIa
IIb
IIc
79
78
61
59
63
75
43
65
60
63
165ꢀ
160ꢀ
119ꢀ
159ꢀ
137ꢀ
/
166
162
121
160
139
270
180
165
205
145
42.36 (42.47)
43.97 (44.13)
42.62 (42.66)
35.53 (35.54)
38.40 (38.98)
38.29 (37.94)
51.53 (52.18)
50.39 (50.16)
38.74 (38.74)
43.40 (43.65)
2.83 (2.85)
3.00 (2.96)
2.60 (2.56)
1.97 (1.92)
2.01 (1.96)
2.07 (2.14)
4.36 (4.38)
3.84 (3.87)
2.72 (2.66)
2.92 (2.75)
13.28 (13.21)
13.62 (13.72)
14.26 (14.21)
11.86 (11.84)
12.14 (12.12)
15.95(15.78)
20.14 (20.28)
21.37 (21.25)
16.25 (16.42)
16.79 (16.97)
/
/
IId
IIe
/
/
IIf
ꢀ
/
IIIb
IIIc
IIId
IIIe
Â
/
Â
/
Â
/
Â/
a
A single temperature indicates the compound undergoes decomposition that depends on the rate of heating. The temperature given is where
melting/decomposition occurs immediately when the sample is placed in a preheated bath.
3. Results and discussion
In both II and III the carbonyl frequencies increase with
the electron-withdrawing ability of the substituent on
the phenyl ring of the ligand as was observed with the
analogous 2-PAP complexes [21,22]. Both II and III
show a strong correlation between the sum of the CO-
stretching frequencies and the Hammett sigma para-
meter, a measure of the electron-withdrawing or -
donating ability of the phenyl substituent [27]. For II,
3.1. Syntheses
The tetracarbonyl complexes cis-(CO)4Mo(LꢀL?) (II)
/
were readily obtained for all ligands (I) from the
reaction of the ligand with cis-(C7H8)Mo(CO)4. How-
ever, reaction of I with Mo(CO)3(CH3CN)3 only yielded
rꢁ
/
0.992 (nꢁ
/
6); for III rꢁ
/
0.978 (nꢁ4). The data for II
/
complexes of the type cis-(CO)2Mo(LꢀL?)2 (III) for Ib-
/
e. Complex IIIa was observed but was too unstable to
isolate in pure form, while no product could be detected
for If. Attempts to obtain complexes of type III, cis-
are plotted in Fig. 1. The sum of the frequencies is used
since all of the COs are affected by the non-carbonyl
ligand(s) [28]. The strong fit for the tetracarbonyl
frequencies occurs even though only the three observed
bands are used for the sum, presumably because the
fourth band changes in the same direction and to the
same extent as the others.
(CO)2Mo(Lꢀ
/
L?)2, and possibly even M(LꢀL?)3 by
/
refluxing I and Mo(CO)6 at a variety of temperatures,
a successful approach using 2-(phenylazo)pyridine as the
ligand [20], gave only intractable solid. In no case have
we seen any evidence for a complex of the type M(Lꢀ
/
L?)3 with the arylazoimidazoles (I).
The complexes II and III are slightly soluble in
nonpolar solvents but reasonably soluble in polar
solvents. Solutions of II in CH2Cl2 are blue or blueꢀ
/
green, while those of III are violet. For d6 octahedral
metal complexes the lowest energy electronic transition
normally involves excitation of an electron from a filled
metal d orbital to the lowest energy p* orbital of the
ligand [26]. The intensity of the bands (o Â
104) is
/
consistent with such a MLCT transition. Within each
type of complex the wavelength of the visible absorp-
tion(s) generally increases as the electron-withdrawing
ability of the phenyl substituent increases.
3.2. Infrared spectra
The tetracarbonyl complexes (II) show three bands in
the infrared spectra in the carbonyl-stretching region.
The expected fourth band appears only as a slight
shoulder on the low frequency side of the second band.
Fig. 1. Plot of the Hammett sigma parameter vs. the sum of the
carbonyl stretching frequencies for type II complexes (rꢁ0.992).
/

M.N. Ackermann et al. / Journal of Organometallic Chemistry 682 (2003) 248ꢀ
/254
251
3.3. Electrochemistry
meter is excellent with rꢁ
/
0.948 (nꢁ
/
6) for II and rꢁ
/
0.980 (nꢁ4) for III.
/
The electrochemistry of complexes II and III in
CH2Cl2 shows one irreversible oxidation and two
reversible or quasi-reversible reductions. As with the 2-
PAP complexes, we take the oxidation in these d6
complexes to be from an orbital that is metal-centered
and the reductions to be to a p* ligand orbital [26]. This
assignment is consistent with the effect of the phenyl
substituent on the potentials. For both II and III the
potentials for oxidation and reduction increase as the
electron-withdrawing ability of the substituent increases.
Attempts to draw correlations using the data for
oxidation is of limited value since only Ea and not E1/2
3.4. NMR spectra
Proton and carbon NMR data for II and III are
summarized in Tables 3 and 4. The carbon assignments
were guided by the effect of the substituents on the
phenyl ring, by HETCORR on some of the ligands, and
by model compounds [29]. We attribute the absence of
carbonyl carbon signals for the tetracarbonyl complexes
(II) to fluxional behavior as occurs with the analogous
2-PAP complexes [21]. In the dicarbonyl complexes (III)
the CO resonance shifts upfield as the electron-with-
drawing ability of the phenyl substituent increases. This
is consistent with the proposal that increasing p-
acceptor ability of the non-carbonyl ligand should result
in shielding of the CO carbon [30,31]. There is a good
values are available. The correlation is poor for II (rꢁ
/
0.771, nꢁ6) but good for III (rꢁ0.960, nꢁ4). How-
/
/
/
ever, the correlation between the first reduction, for
which E1/2 is available, and the Hammett sigma para-
Table 2
Infrared and 95Mo-NMR data for complexes cis-Mo(CO)4(Lꢀ
/
L?) (II) and cis-Mo(CO)2(Lꢀ
/
L?)2 (III) taken in CH2Cl2
a
d
Compound
n(CO) (cmꢃ1
)
lmax (nm)
d
95Mo (Dn1/2
(ppm) (Hz)
)
E1/2 (V) b
sp
ꢂ
/
1/0 c
0/ꢃ
/
1
ꢃ
/
1/ꢃ2
/
IIa
IIb
IIc
2017m, 1925s, 1852m
2018m, 1927s, 1856m
2019m, 1929s, 1859m
2020m, 1931s, 1860m
2022m, 1935s, 1865m
2022m, 1939s, 1869m
1929s, 1858m
596.3
595.5
ꢃ
/
1119 (45)
1103 (27)
1090 (22)
1074 (27)
1049 (24)
1017 (21)
324 (95)
0.79
0.87
0.86
0.93
0.89
0.92
0.26
0.25
0.33
0.37
ꢃ
/
0.87
0.89
0.82
0.78
0.72
0.53
1.09
1.09
1.01
0.93
ꢃ
/
1.82 e
1.73
1.62
1.57
1.58 e
0.87
1.6
ꢃ
ꢃ/
/
0.28
0.14
0
ꢃ
/
ꢃ
/
ꢃ
/
594.4
605.1
ꢃ
/
ꢃ
/
ꢃ
/
IId
IIe
ꢃ
/
ꢃ
/
ꢃ
/
0.26
0.53
0.81
0.14
0
606.8
623.1
ꢃ
/
ꢃ
/
ꢃ
/
IIf
ꢃ/
ꢃ
/
ꢃ
/
IIIb
IIIc
IIId
IIIe
471.5, 561.4
471.3, 558.7
475.4, 566.0
478.8, 564.8
ꢃ
/
Â
/
ꢃ
/
ꢃ/
1933s, 1862m
1934s, 1864m
1939s, 1869m
331 (107)
354 (130)
392 (210)
ꢃ
/
ꢃ
/
1.66 e
1.50
1.34
ꢃ
/
ꢃ
/
0.26
0.53
ꢃ/
ꢃ/
a
Uncertainty 91 ppm.
/
b
c
Values vs. SCE in CH2Cl2 with 0.1 M tetrabutylammonium tetrafluoroborate and a scan rate of 100 mV sꢃ1
Ea values.
From Ref. [27].
.
d
Table 3
1H-NMR assignment for complexes cis-Mo(CO)4(Lꢀ
/
L?) (II) and cis-Mo(CO)2(Lꢀ
L?)2 (III) a
/
b
Compound
H4
H5 b
NCH3
H8,H10 b
H7,H11 b
Other
IIa
IIb
7.47
7.50
7.48
7.53
7.56
7.62
6.78
6.80
6.82
6.86
7.26
7.29
7.31
7.34
7.38
7.48
6.57
6.60
6.56
6.60
4.06
4.08
4.07
4.07
4.09
4.08
3.84
3.83
3.85
3.87
6.98
7.28
7.48
7.61
7.75
8.35
7.19
8.15
7.96
8.02
7.91
8.07
8.13
7.47
3.90 (OCH3)
2.44 (CH3)
7.48 (H9)
IIc
IId
IIe
IIf c
IIIb
IIIc
IIId
IIIe
2.38 (CH3)
7.3ꢀ
7.5m
7.66s
/7.6m
7.3ꢀ
7.5m
7.66s
/
7.6m
7.3ꢀ7.6 (H9)
/
a
Recorded in CDCl3. Chemical shifts are in ppm relative to tetramethylsilane.
Doublet unless indicated otherwise.
Recorded in CD2Cl2.
b
c

252
M.N. Ackermann et al. / Journal of Organometallic Chemistry 682 (2003) 248ꢀ254
/
Table 4
13C-NMR assignment for ligands (I) and complexes cis-Mo(CO)4(Lꢀ
/
L?) (II) and cis-Mo(CO)2(Lꢀ
L?)2 (III) a
/
Compound
C2
C4
C5
C6
C7,C11
C8,C10
C9
NCH3
Other
Ia
Ib
Ic
Id
Ie
152.7
152.5
152.5 b
152.5
152.4
129.9
130.1
130.3
132.2
131.0
123.1
123.5
123.8
124.2
124.8
147.4
151.1
152.9 b
151.6
154.6
125.0
123.0
123.0
124.4
123.0
114.1
129.6
162.4
142.0
131.3
125.7
132.1qt
(32.5) c
148.8
162.6
141.8
130.8
125.6
131.7qt
(32.9) c
149.0
137.1
32.5
32.6
32.6
32.7
32.7
55.4 OCH3
21.4 CCH3
128.9
130.7
126.1qt
(3.9) c
124.8 d
113.8
123.7qt CF3
(272.2) c
If
152.8
157.0
157.2
157.3 b
157.5
157.8
131.9
129.8
129.7
129.8
129.7
129.7
125.7
125.0
125.2
125.5
125.0
126.1
156.4
150.9
155.0
156.9 b
155.8
159.1
123.7 d
124.7
122.8
122.9
124.3
123.3
33.1
33.8
33.9
34.0
31.5
34.0
IIa
IIb
IIc
IId
IIe
55.6 OCH3
21.3 CCH3
129.3
128.7
131.8
126.0qt
(3.7) c
124.8 d
128.8
123.8qt CF3
(272.2) c
IIf e
IIIb
158.8
157.0
130.5
125.7
127.6
120.0
161.3
156.4
124.4 d
122.9
34.7
33.1
224.7 CO
21.0 CCH3
224.4 CO
224.3 CO
224.2 CO
124.4qt CF3
(271.8) c
IIIc
IIId
IIIe
157.0
157.0 b
157.0
125.8
125.9
126.1
120.2
120.5
120.8
158.5
157.2 b
160.5
123.2
124.7
123.6
128.3
131.3
127.3
120.9
33.1
33.1
33.2
125.6qt
(3.8) c
129.0qt
(32.6) c
a
Recorded in CDCl3.
Assignments of C2 and C6 may be reversed.
b
c
Number in parenthesis is JCF (Hz) for the quartet.
Assignments of C7,C11 and C8,C10 may be reversed.
Recorded in CD2Cl2.
d
e
correlation of d(13CO) with the Hammett sigma para-
meter of the phenyl substituents (rꢁ0.908, nꢁ4) and
with the sum of the CO stretching frequencies (rꢁ0.942,
nꢁ4).
Although the cis-Mo(CO)2(Lꢀ
Ramsey equation (sꢁ
is separated into diamagnetic (sd) and paramagnetic
/
sdꢂ
/
sp) where the total shielding
/
/
(sp) terms [34ꢀ37]. The paramagnetic term is dominant
/
/
in heavy nuclei such as 95Mo and is represented by the
equation
/
/
L?)2 complexes (III)
can exist in three isomeric forms (N and N? refer to the
imidazole and azo nitrogen atoms, respectively), the
data and bonding considerations strongly support the
cis/trans isomer. This structure is consistent with the
equivalence of the two azoimidazole ligands indicated
by the proton and carbon spectra and also optimizes p
bonding between the metal and carbonyls by having the
COs trans to an imidazole group rather than to the
better p-accepting azo group. Additionally, the data and
analysis parallel that for the analogous 2-PAP com-
plexes, for which an X-ray crystal structure confirmed
the cis/trans isomer [22].
sp ꢁꢃK DEꢃ1Žrꢃ34dk2
for comparison of complexes with the same symmetry.
Here K is a constant and DE, the spectrochemical
parameter, is the average electronic energy approxi-
mated by the HOMOꢀLUMO gap of the Mo d orbitals.
/
The radial term, Žrꢃ34d, which is the average distance
of the 4d electrons from the nucleus, and k2, which
describes their angular distribution, together comprise
the nephelauxetic effect, which is associated with the
metalꢀligand bond covalency [32]. The chemical shift
/
generally moves downfield with decreased s bonding
and/or p bonding or increased steric hindrance.
The patterns of the 95Mo chemical shifts for II and III
are similar to those of the 2-PAP complexes [21,22]. The
linewidths of III are larger than those of II due to the
larger size of III. The increase of d(95Mo) from II to III
is consistent with the ligands (I) being both poorer s
donors and poorer p acceptors than CO. The type II
complexes show a very good correlation between
The Mo-95 NMR data for II and III are presented in
Table 2. Molybdenum NMR has become a useful probe
in the study of molybdenum compounds [32,33]. Inter-
pretation of the chemical shifts is usually based on the
d(95Mo) and lmax (8
Such a correlation can arise in a series of complexes if
/
1/DE) with rꢁ
/
0.932 (nꢁ6).
/

M.N. Ackermann et al. / Journal of Organometallic Chemistry 682 (2003) 248ꢀ
/254
253
the nephelauxetic terms are constant and DE is taken as
the lowest energy absorption [38,39]. However, in II the
visible band involved is due to a MLCT transition and
not a ligand field transition, which is the most important
excitation in the Ramsey equation [40]. Hence, the
observed correlation is either fortuitous or a ligand field
band is coincident with the MLCT band.
Both types of data show that the molybdenum center is
more electron rich in the N(1)-methyl-2-(phenylazo)
imidazole complexes.
Acknowledgements
The correlation of d(95Mo) with the Hammett sigma
The authors are grateful for support from the
National Science Foundation (CHE-0079470) toward
the acquisition of a 600 MHz NMR spectrometer at
Oberlin College. We thank Manish Mehta for assistance
with obtaining some of the spectra.
parameter is excellent for both II and III with rꢁ0.996
/
in each case. Within each set of complexes d(95Mo)
shifts downfield as the electron-withdrawing ability of
the phenyl substituent increases. Since S n(CO) corre-
lates well with the Hammett sigma parameter, it is not
surprising that d(95Mo) and Sn(CO) show a strong
correlation. For II rꢁ
/
0.995 (nꢁ
/
6) and for III rꢁ0.950
/
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