Synthesis, spectroscopy, and structures of the seven-coordinate complexes (CH3)2AsC(CF3) to C double bond (CF3)As(CH3)2 W(CO)2 I2P(OC6H5)3 and [(CH3)2AsC(CF3) to C double bond (CF3)As(CH3)2]2W(CO)

Article abstract of DOI:10.1139/v98-008

(L-L)W(CO)₃I₂(L-L = (CH₃)₂AsC(CF₃) to C double bond (CF₃)As(CH₃)₂) reacts with the monodentate phosphite P(OC₆H₅)₃ and (L-L)W(CO)₃Br₂ reacts with L-L to form new seven-coordinate complexes (L-L)W(CO)₂I₂P(OC₆H₅)₃ and (L-L)₂W(CO)Br₂. Low-temperature X-ray diffraction analyses show the tungsten atom to be seven coordinate in both complexes, with the geometry most closely approximated by a monocapped octahedral environment, the capping group being a carbonyl in the dicarbonyl complex; the geometry is most closely approximated by a pentagonal bipyramidal environment in the monocarbonyl complex. The ¹H, ¹³C, and ¹⁹F NMR data indicate that the dicarbonyl complex is stereochemically nonrigid at 298 K and rigid at lower temperatures, while the monocarbonyl is nonrigid both at 298 K and at lower temperatures. ΔG^≠ values calculated at coalescence temperatures are consistent with an intramolecular rearrangement process for both complexes. The ¹³C chemical shifts and ²J(¹³C-³¹P) values provide important structural considerations in the assignment of a seven-coordinate geometry. Spectroscopic properties for the related seven-coordinate dicarbonyl complexes (L-L)W(CO)₂PX₂ (P = P(OC₆H₅)₃; X = Br; P = P(OCH₃)₃, P(C₆H₅)₃; X = Br, I) and monocarbonyl complexes (L-L)₂W(CO)I₂ and (L-L)W(CO)X₂[P(OCH₃)₃]₂ (X = Br, I) are presented and compared to those of the two title complexes.

Full text of DOI:10.1139/v98-008

245
Synthesis, spectroscopy, and structures of the
seven-coordinate complexes (CH3)2AsC(CF3)==
C(CF3)As(CH3)2W(CO)2I2P(OC6H5)3 and
[(CH3)2AsC(CF3)==C(CF3)As(CH3)2]2W(CO)Br2 and
spectroscopy of related seven-coordinate
complexes
Richard J. Barton, Sushil K. Manocha, Beverly E. Robertson, and Lynn
M. Mihichuk
Abstract: (L-L)W(CO)3I2 (L-L = (CH3)2AsC(CF3)==C(CF3)As(CH3)2) reacts with the monodentate phosphite P(OC6H5)3 and
(L-L)W(CO)3Br2 reacts with L-L to form new seven-coordinate complexes (L-L)W(CO)2I2P(OC6H5)3 and (L-L)2W(CO)Br2.
Low-temperature X-ray diffraction analyses show the tungsten atom to be seven coordinate in both complexes, with the
geometry most closely approximated by a monocapped octahedral environment, the capping group being a carbonyl in the
dicarbonyl complex; the geometry is most closely approximated by a pentagonal bipyramidal environment in the
monocarbonyl complex. The 1H, 13C, and 19F NMR data indicate that the dicarbonyl complex is stereochemically nonrigid at
298 K and rigid at lower temperatures, while the monocarbonyl is nonrigid both at 298 K and at lower temperatures. ∆G≠
values calculated at coalescence temperatures are consistent with an intramolecular rearrangement process for both
complexes. The 13C chemical shifts and 2J(13C-31P) values provide important structural considerations in the assignment of a
seven-coordinate geometry. Spectroscopic properties for the related seven-coordinate dicarbonyl complexes (L-L)W(CO)2PX2
(P = P(OC6H5)3; X = Br; P = P(OCH3)3, P(C6H5)3; X = Br, I) and monocarbonyl complexes (L-L)2W(CO)I2 and
(L-L)W(CO)X2[P(OCH3)3]2 (X = Br, I) are presented and compared to those of the two title complexes.
Key words: seven-coordination, X-ray, NMR analysis.
Résumé : Le (L-L)W(CO)3I2 (L-L = (CH3)2AsC(CF3)==C(CF3)As(CH3)2) réagit avec le phosphite monodentate P(OC6H5)3
alors que (L-L)W(CO3)Br2 réagit avec L-L pour former respectivement les nouveaux complexes heptacoordinés
(L-L)W(CO)2I2P(OC6H5)3 et (L-L)2W(CO)Br2. Des analyses de diffraction des rayons X à basse température ont permis de
montrer que l’atome de tungstène est heptacoordiné dans les deux complexes: dans le complexe dicarbonyle, sa géométrie
peut être décrite approximativement par un environnement octaédrique à cap unique qui est le dicarbonyle; dans le complexe
monocarbonyle, la géométrie peut être décrite approximativement par un environnement bipyramidal pentagonal. Les données
de RMN du 1H, du 13C et du 19F indiquent que le complexe dicarbonyle est stéréochimiquement non rigide à 298 K mais qu’il
est rigide à des températures inférieures; par ailleurs, le complexe monocarbonyle n’est pas rigide ni à 298 K ni à des
températures inférieures. Pour chacun de ces complexes, les valeurs de ∆G≠ calculées à partir des températures de coalescence
sont en accord avec un processus de réarrangement intramoléculaire. Les valeurs des déplacements chimiques en 13C ainsi que
celles des 2J(13C-31P) fournissent des considérations structurales importantes dans l’attribution de la géométrie heptacoordinée.
On rapporte les propriétés spectroscopiques des complexes dicarbonyles heptacoordinés apparentés (L-L)W(CO)2PX2 (P =
P(OC6H5)3; X = Br; P = P(OCH3)3 ou P(C6H5)3; X = Br, I) et des complexes monocarbonyles (L-L)2W(CO)I2 et
(L-L)W(CO)X2[P(OCH3)3]2 (X = Br, I) et on les compare à celles des deux complexes mentionnés dans le titre.
Mots clés : heptacoordination, rayons X, analyse par RMN.
[Traduit par la rédaction]
Introduction
Received August 12, 1997.
In the area of seven coordination the relevant polytopal poly-
hedra are the pentagonal bipyramid, the monocapped octahe-
dron, and the quadrilaterally monocapped trigonal prism.
X-ray crystallographic studies of seven-coordinate transition
metal carbonyl complexes have shown the latter two types of
geometry to predominate (1–12). However, the actual struc-
tures seldom match the ideal polyhedral arrangements, and
R.J. Barton, S.K. Manocha, B.E. Robertson, and
L.M. Mihichuk.1 Department of Chemistry, University of
Regina, Regina, SK S4S 0A2, Canada.
1
Author to whom correspondence may be addressed.
Telephone: (306) 585-4793. Fax: (306) 585-4894. E-mail:
mihichuk@meena.cc.uregina.ca
Can. J. Chem. 76: 245–253 (1998)
© 1998 NRC Canada

246
Can. J. Chem. Vol. 76, 1998
Table 1. Crystallographic data for (L-L)W(CO)2I2P(OC6H5)3 and
(L-L)2W(CO)Br2.
various criteria have been proposed to aid in the assignment of
the most appropriate description to an experimentally deter-
mined polyhedron (13–15).
(L-L)W(CO)2I2P(OC6H5)3 (L-L)2W(CO)Br2
We have been interested in the seven-coordinate chemistry
of group six metal (Mo, W) carbonyl complexes from the point
of view of structure and dynamic behaviour (10–12, 16). Struc-
tural studies have been restricted to the tungsten analogues
because the corresponding molybdenum complexes decom-
pose in the X-ray beam during X-ray analysis, preventing the
collection of useful structural data. In addition the 14.4% spin
one-half 183W atom provides important coupling parameters
not present in the molybdenum series. The molybdenum and
tungsten complexes nevertheless show similar properties and
are assumed to behave in a similar manner with regard to flux-
ionality and reactivity. Our present study, which includes a
detailed spectroscopic investigation of tungsten complexes
containing one bidentate ligand and extends into a study of
corresponding complexes incorporating two bidentate ligands,
allows for a greater understanding of structural features in
terms of a predominant geometry and fluxional behaviour.
Seven-coordinate carbonyl complexes also exhibit fluxion-
ality in solution, resulting in room-temperature NMR spectra
often not being consistent with the X-ray crystal structure. The
reason for this behaviour is thought to be due to the small
energy barriers between stereochemical isomers. To explain
the mechanisms of these dynamic processes both intramolecu-
lar and (or) intermolecular pathways are possible (14, 16–18),
and no typical mechanism is responsible for these processes
although most are described as intramolecular rearrangements.
We are compiling structural and solution NMR data in order
to better understand the dynamic behaviour and elucidate the
mechanism of the ligand motion.
Mol. formula
Mol. wt.
Cryst. syst.
Space group
a, D
C28H27As2F6I2O5PW
1175.91
Monoclinic
C2/c
C17H24As4Br2F12OW
1115.58
Monoclinic
P21/c
24.444(2)
15.704(1)
19.038(1)
97.12(1)
7252
14.073(2)
14.523(2)
29.434(4)
105.08(2)
5808
b, D
c, D
β, deg
V, D3
Z
8
8
D
D
calcd, g cm–3
measd, g cm–3
2.15
2.55
2.20
2.48
F(0,0,0)
4400
4144
µ(Mo Kα), cm–1 69.06
Ra
wRb
120.25
0.047
0.073
0.063
0.100
a R = Σ||Fo| – |Fc||/Σ|Fo|.
b wR = [Σw(|Fo| – |Fc|)2/Σw|Fo|2]1/2
.
recorded on a Bruker AC 200 QNP NMR spectrometer (1H,
200.13 MHz; 31P, 81.02 MHz; 19F, 188.31 MHz; 13C, 50.3
MHz). Chemical shifts are expressed in parts per million,
downfield from Si(CH3)4 (1H, 13C), downfield (positive) or
upfield (negative) from 85% H3PO4(31P) and upfield (nega-
tive) from CFCl3(19F).
Infrared spectra were recorded on a Perkin–Elmer 1600 FT
instrument in dichloromethane and are reported in cm–1. Ele-
mental analyses were measured using a Perkin–Elmer 2400
CHN elemental analyzer.
In this paper we describe the synthesis, X-ray structure,
spectroscopic properties, and fluxional behaviour of the seven-
coordinate complexes (CH3)2AsC(CF3)== C(CF3)As(CH3)2-
W(CO)2I2P(OC6H5)3 and [(CH3)2AsC(CF3)==C(CF3)As(CH3)2]2-
W(CO)Br2 and spectroscopic properties of related seven-coordinate
complexes.
Preparation of (L-L)W(CO)2I2P(OC6H5)3
The (L-L)W(CO)3I2 (0.25 g, 0.28 mmol) and P(OC6H5)3 (0.12 g,
0.32 mmol) were refluxed in 50 mL of benzene for 17 h. The
infrared spectra indicated complete conversion of the tricar-
bonyl complex to the dicarbonyl complex. The solvent was
removed in vacuo, and the resulting yellow solid was washed
with n-hexane to remove any unreacted P(OC6H5)3 and was
then recrystallized from a dichloromethane hexane mixture at
–20°C to give 0.25 g (75%) of (L-L)W(CO)2I2P(OC6H5)3. 1H
NMR (CDCl3): 2.01 (s, 12H, As(CH3)2), 7.27 (m, 15H,
P(OC6H5)3); 31P-{1H}NMR (CDCl3): 90.7 (s, W-P, J(183W-
31P) = 403 Hz); 19F-{1H}NMR (CDCl3): –53.1 (s, CF3). Anal.
calcd. for C28H27As2F6I2O5PW: C 28.57, H 2.30, I 21.60;
found: C 28.90, H 2.29, I 21.77.
Experimental section
General considerations
All reactions were carried out under an argon atmosphere us-
ing standard Schlenk tube techniques. Solvents were dried and
purified by known procedures and distilled under nitrogen
prior to use. The ligand cis-2,3-bis(dimethylarsino)-
1,1,1,4,4,4-hexafluorobut-2-ene(L-L) (19, 20) and the starting
complexes (L-L)W(CO)3I2 and (L-L)W(CO)3Br2 were pre-
pared as previously described (16).
Preparation of (L-L)2W(CO)Br2
All seven-coordinate dicarbonyl complexes for which spec-
troscopic data are presented were prepared in a similar manner
to that of (L-L)W(CO)2I2P(OC6H5)3, discussed below. Spe-
cifically, the preparation of (L-L)W(CO)2I2P(OCH3)3 (11) and
(L-L)W(CO)2Br2P(C6H5)3 (12) have been previously described.
The synthesis of the seven-coordinate monocarbonyl complex
(L-L)W(CO)Br2[P(OCH3)3]2 has been previously described
(10), and the iodo analogue was prepared in a similar manner
to that of (L-L)2W(CO)Br2, discussed below, as was
(L-L)2W(CO)I2.
The (L-L)W(CO)3Br2 (0.25 g, 0.31 mmol) and (L-L) (0.25 g,
0.67 mmol) were refluxed in 50 mL of toluene for 22 h. The
colour of the reaction mixture changed from yellow to orange
to dark brown. Infrared spectroscopy indicated complete con-
version of the tricarbonyl complex to the monocarbonyl com-
plex. The solvent was removed in vacuo, and the resulting
brown–black solid was recrystallized from a toluene–hexane
mixture at –20°C to give 0.23 g (65%) of (L-L)2W(CO)Br2. 1H
NMR (CD2Cl2): 2.05 (s, As(CH3)2); 19F-{1H}NMR (CDCl3):
–50.6 (s, CF3). Anal. calcd. for C17H24As4Br2F12OW: C 18.29,
H 2.15, Br 14.34; found: C 17.93, H 2.08, Br 14.81.
The 1H, 31P-{1H}, 19F-{1H}, and 13C NMR data were
© 1998 NRC Canada

247
Fig. 1. ORTEP diagram for (L-L)W(CO)2I2P(OC6H5)3 showing the
atomic labelling scheme with 50% probability thermal ellipsoids.
Barton et al.
Table 2. Selected bond distances (D) and angles (deg) for
(L-L)W(CO)2I2P(OC6H5)3.
F(1)
Bond
Distance (D)
Bond
W—C(1)
Distance (D)
F(5)
F(3)
F(2)
C(6)
W—As(1)
W—As(2)
W—I(1)
W—I(2)
W—P
2.5603(7)
2.6004(7)
2.8684(6)
2.8698(7)
2.457(2)
1.942(7)
1.957(8)
1.16(1)
1.17(1)
F(4)
W—C(2)
C(1)—O(1)
C(2)—O(2)
C(5)
F(6)
C(8)
C(3)
C(7)
Bond
Angle (deg)
Bond
Angle (deg)
C(4)
As(2)
As(1)-W-As(2)
As(1)-W-I(1)
As(1)-W-I(2)
As(1)-W-P
76.02(2)
159.74(2)
76.88(4)
110.88(4)
114.8(2)
71.9(2)
P-W-C(1)
110.7(2)
77.2(2)
C(10)
P-W-C(2)
As(1)
C(9)
C(1)-W-C(2)
71.2(3)
W-C(1)-O(1)
W-C(2)-O(2)
W-As(1)-C(8)
W-As(1)-C(9)
W-As(1)-C(10)
C(8)-As(1)-C(9)
C(8)-As(1)-C(10)
C(9)-As(1)-C(10)
W-As(2)-C(3)
W-As(2)-C(4)
W-As(2)-C(5)
C(3)-As(2)-C(4)
C(3)-As(2)-C(5)
C(4)-As(2)-C(5)
179.1(7)
176.6(6)
109.7(2)
114.9(3)
120.6(3)
110.7(4)
98.9(3)
C(1)
O(1)
I(1)
I(2)
W
As(1)-W-C(1)
As(1)-W-C(2)
As(2)-W-I(1)
As(2)-W-I(2)
As(2)-W-P
C(2)
O(2)
89.51(2)
83.24(2)
161.67(4)
79.6(2)
O(4)
P
C(18)
As(2)-W-C(1)
As(2)-W-C(2)
I(1)-W-I(2)
I(1)-W-P
C(17)
C(12)
C(14)
O(5)
C(23)
C(11)
121.0(2)
87.54(2)
79.09(4)
75.4(2)
100.6(4)
118.7(3)
115.4(3)
110.3(2)
104.5(5)
101.8(4)
104.3(4)
O(3)
C(19)
C(13)
C(28)
C(16)
C(15)
I(1)-W-C(1)
I(1)-W-C(2)
I(2)-W-P
C(24)
C(22)
128.2(2)
82.00(4)
155.8(2)
132.7(2)
C(20)
C(21)
C(27)
I(2)-W-C(1)
I(2)-W-C(2)
C(25)
C(26)
full-matrix least-squares method. Weights were assigned ac-
cording to the REGWT program with w = ( σ2c + σm2 )−1, where
σ2c is the variance and σ2m is the term calculated by REGWT
using iterative methods to remove the systematic trends with
respect to F and sin θ/λ in the residuals of the structure factors
(21). Several cycles of least-squares refinement were per-
formed to achieve a convergence with a conventional R value
of 0.047 and a weighted R value of 0.063. The maximum and
average shift/esd of the least-squares parameters were 1.8 ×
10–3 and 1.1 × 10–4, respectively. In the final Fourier difference
map a peak corresponding to 1.71 e D–3 was located approxi-
mately 1.07 D from the center of the W atom. The residual
density near the heavy transition metal is not unexpected in
light of the approximations used in the absorption corrections.
All calculations were carried out using the XTAL 2.4 system
of programs (22). Important bond lengths and bond angles are
given in Table 2.2 An ORTEP drawing showing the atomic
labelling scheme is given in Fig. 1, and a schematic diagram
showing the torsional angles in the five-membered ring of the
chelate ligand for (L-L)W(CO)2I2P(OC6H5)3 and (L-
L)W(CO)2I2P(OCH3)3 is given in Fig. 2.
Crystallographic characterization of
(L-L)W(CO)2I2P(OC6H5)3
A yellow–orange crystal with approximate dimensions 0.41 ×
0.44 × 0.45 mm was selected and coated with collodion. Pre-
liminary investigation of the precession photographs showed
symmetry and systematic extinction conditions (hkl: h + k =
2n; h01: h, l = 2n; and 0k0: k = 2n) indicative of the space
group C2/c or Cc. The accurate unit cell parameters were ob-
tained from the least-squares refinement of 24 reflections and
their Friedel pairs in the 2θ range 35–45°, as observed on a
modified Picker FACS-1 four-circle diffractometer. The inten-
sity data were collected at 218 ± 4 K using an Enraf–Nonius
Universal low-temperature device. Three standard reflections
were measured after every 47 reflections. No significant decay
was observed during data collection.
The intensities of 8291 independent reflections were meas-
ured for 2θ values between 3° and 55° of which 6663 were
considered to be observed with I ≥ 2σ(I). Absorption correc-
tions were applied using the Gaussian integration method. The
crystal data are given in Table 1. The intensity statistics indi-
cated the centrosymmetric space group C2/c. The structure
was solved by combined direct methods and Fourier syntheses.
All non-hydrogen atoms were refined anisotropically by the
Crystallographic characterization of (L-L)2W(CO)Br2
Brown–black plates were crystallized from a toluene–hexane
2
The supplementary material includes tables of atomic coordinates for the non-hydrogen atoms, additional bond distances and angles,
anisotropic and isotropic thermal parameters, free energy values, and coalescence temperatures. The complete set of data (11 pages) may be
purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Council of Canada, Ottawa, Canada
K1A 0S2. Tables of atomic coordinates for the non-hydrogen atoms and additional bond distances and angles have also been deposited with
the Cambridge Crystallographic Data Centre, and can be obtained on request from The Director, Cambridge Crystallographic Data Centre,
University Chemical Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K. Two tables of observed and calculated structure factors (94
pages) can be obtained from the corresponding author.
© 1998 NRC Canada

248
Can. J. Chem. Vol. 76, 1998
Table 3. Selected bond distances (D) and angles (deg) for
(L-L)2W(CO)Br2.
Fig. 2. Schematic diagram showing the torsional angles in the
five-membered ring of the chelate ligand for
(L-L)W(CO)2I2P(OC6H5)3 (top numbers) and the related complex
(L-L)W(CO)2I2P(OCH3)3 (bottom numbers).
Bond
W—As(1)
Distance (D)
Bond
Distance (D)
2.573(2) W′—As(1)′
2.626(3) W′—As(2)′
2.578(3) W′—As(3)′
2.585(2) W′—As(4)′
2.624(3) W′—Br(1)′
2.647(3) W—Br(2)′
2.575(2)
2.620(2)
2.577(2)
2.584(2)
2.624(3)
2.664(3)
1.78(2)
W—As(2)
W—As(3)
W—As(4)
W—Br(1)
W—Br(2)
W—C(1)
W
19.7(2)
12.2(3)
-23.6(3)
-14.3(3)
As
28.3(8)
As
-10.6(7)
-7.3(9)
16.3(9)
1.78(2)
W′—C(1)′
C(1)′—O(1)′
C
C
-11.0(9)
-5.5(9)
C(1)—O(1)
1.22(3)
1.19(3)
Bond
Angle (deg)
Bond
Angle (deg)
As(1)-W-As(2)
As(3)-W-As(4)
As(1)-W-Br(1)
As(1)-W-Br(2)
As(1)-W-As(3)
As(1)-W-As(4)
As(1)-W-C(1)
As(2)-W-C(1)
As(2)-W-Br(1)
As(2)-W-Br(2)
As(2)-W-As(3)
As(2)-W-As(4)
As(3)-W-Br(1)
As(3)-W-Br(2)
As(3)-W-C(1)
As(4)-W-C(1)
As(4)-W-Br(1)
As(4)-W-Br(2)
80.30(8) W-As(1)-C(2)
75.63(8) W-As(1)-C(3)
76.51(9) W-As(1)-C(4)
121.0(7)
116.1(8)
110.6(7)
100(1)
mixture at –20°C under an inert atmosphere. A hexagonal plate
with approximate dimensions 0.39 × 0.22 × 0.19 mm was se-
lected and coated with collodion. Precession photographs
showed systematic extinction conditions (h0l: l = 2n; 0k0: k = 2n)
and symmetry indicative of the space group P21/c. The accu-
rate unit cell parameters were obtained from the least-squares
refinement of 36 reflections in the 2θ range 25–45°, observed
as for the previous compound. The data were collected at
203 ± 4 K. The intensities of three standard reflections, meas-
ured after every 47 data points, showed an average decay of
20% over the period of the data collection.
71.9(9)
C(2)-As(1)-C(3)
137.36(8) C(2)-As(1)-C(4)
145.91(9) C(3)-As(1)-C(4)
100(1)
106(1)
84.5(7)
W-As(2)-C(5)
W-As(2)-C(8)
106.1(7)
121.8(8)
118.9(7)
98.7(9)
104(1)
164.8(7)
82.30(10) W-As(2)-C(9)
76.51(10) C(5)-As(2)-C(8)
106.17(9) C(5)-As(2)-C(9)
100.57(8) C(8)-As(2)-C(9)
145.51(9) W-As(3)-C(10)
69.00(9) W-As(3)-C(11)
103(1)
The intensities of 7611 reflections were measured (hkl,
_
124.1(7)
110.8(8)
113.9(7)
103(1)
hkl for 2θ values in the range 3–45°) of which 5364 were
considered to be observed with I ≥ 2σ(I). The data were cor-
rected for the decay in the standard reflections, and absorption
corrections were applied as for the previous compound. Faces
of the crystal were not well defined and were approximated.
The crystal data are given in Table 1. The structure has two
independent molecules in the unit cell. The structure was
solved by the heavy-atom method, using direct methods to
confirm the positions of the two tungsten atoms. The remain-
ing non-hydrogen atoms were found from successive Fourier
maps. The positions of the hydrogen atoms were not apparent
on the difference maps. The structure was refined by full-ma-
trix least squares, treating the tungsten, arsenic, bromine, and
fluorine atoms anisotropically and the carbon and oxygen at-
oms isotropically. Weights were assigned as w = σ−c2, where
σ2c is the variance in F as calculated from counting statistics
and the instability factor.
85.9(7)
91.2(7)
W-As(3)-C(12)
C(10)-As(3)-C(11)
69.93(8) C(10)-As(3)-C(12)
141.77(10) C(11)-As(3)-C(12)
99(1)
102(1)
Br(1)-W-C(1)
92.9(7)
W-As(4)-C(13)
W-As(4)-C(16)
114.4(6)
120.5(7)
115.6(7)
103(1)
Br(2)-W-C(1)
100.1(7)
Br(1)-W-Br(2)
144.39(10) W-As(4)-C(17)
75.63(8) C(13)-As(4)-C(16)
80.38(8) C(13)-As(4)-C(17)
75.75(8) C(16)-As(4)-C(17)
77.03(8) W-C(1)-O(1)
As(3)-W-As(4)
As(1)′-W′-As(2)′
As(3)′-W′-As(4)′
As(1)′-W′-Br(1)′
As(1)′-W′-Br(2)′
As(1)′-W′-As(3)′
As(1)′-W′-As(4)′
As(1)′-W′-C(1)′
As(2)′-W′-C(1)′
As(2)′-W′-Br(1)′
As(2)′-W′-Br(2)′
As(2)′-W′-As(3)′
As(2)′-W′-As(4)′
As(3)′-W′-Br(1)′
As(3)′-W′-Br(2)′
As(3)′-W′-C(1)′
As(4)′-W′-C(1)′
As(4)′-W′-Br(1)′
As(4)′-W′-Br(2)′
Br(1)′-W′-C(1)′
Br(2)′-W′-C(1)′
Br(1)′-W′-Br(2)′
As(3)′-W′-As(4)′
W′-As(4)′-C(16)′
W′-As(4)′-C(17)′
97(1)
102(1)
176(1)
71.72(9) W′-As(1)′-C(2)′
136.89(8) W′-As(1)′-C(3)′
146.41(9) W′-As(1)′-C(4)′
122.2(6)
116.1(7)
110.0(7)
99(1)
85.3(7)
C(2)′-As(1)′-C(3)′
C(2)′-As(1)′-C(4)′
165.7(7)
100(1)
The refinement converged to a conventional R value of
0.073 and a weighted R value of 0.10. These high values reflect
the decay of the sample during data collection and the poor
quality of the sample as manifested by a large number of weak
reflections. The maximum and average shift/esd were 1.2 ×
10–3 and 3.1 × 10–5, respectively, at the end of refinement. The
coordinates of the two independent molecules were approxi-
mately related by the operator 1/2 + x, 1/2 – y, z. If this relation
were exact, reflections h0l would be absent. The data showed
that these reflections were weak but not absent. Further exami-
nation showed that they were not due to multiple scattering.
Therefore, the apparent symmetry is only approximate. Pairs
of residual electron-density peaks, with a maximum height of
3.6 e D–3, were observed near each of the tungsten, bromine,
and arsenic atoms, reflecting the limitations in the approximate
absorption corrections.
82.50(10) C(3)′-As(1)′-C(4)′
77.20(9) W′-As(2)′-C(5)′
105.95(9) W′-As(2)′-C(8)′
100.07(8) W′-As(2)′-C(9)′
145.50(9) C(5)′-As(2)′-C(8)′
68.53(8) C(5)′-As(2)′-C(9)′
106(1)
106.9(7)
121.3(7)
119.1(7)
99(1)
103(1)
84.9(7)
C(8)′-As(2)′-C(9)′
W′-As(3)′-C(10)′
103(1)
91.5(6)
123.9(7)
110.8(7)
113.7(7)
69.81(8) W′-As(3)′-C(11)′
141.55(10) W′-As(3)′-C(12)′
93.8(7)
C(10)′-As(3)′-C(11)′ 104(1)
98.9(6)
C(10)′-As(3)′-C(12)′
99(1)
145.00(10) C(11)′-As(3)′-C(12)′ 102(1)
75.75(8) W′-As(4)′-C(13)′
114.9(7)
120.0(7)
C(13)′-As(4)′-C(17)′
99(1)
114.4(7)
C(16)′-As(4)′-C(17)′ 102(1)
W′-C(1)′-O(1)′ 175(1)
All calculations were carried out using the XTAL 2.4 sys-
tem of programs (22). Important bond lengths and angles are
given in Table 3. The labelling for the two molecules are
C(13)′-As(4)′-C(16)′ 103(1)
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Barton et al.
Fig. 3. ORTEP diagram for (L-L)2W(CO)Br2 showing the atomic
labelling scheme for the unprimed molecule with 50% probability
thermal ellipsoids.
ligands on a sphere of unit radius and with the same angles
subtended at the center as in the real coordination polyhedron)
less than 1.11 would be expected to exhibit pentago-
nal–bipyramidal geometry, and those with a normalized bite
greater than 1.11 would be expected to exhibit either mono-
capped octahedral or monocapped trigonal prismatic geome-
try. The observed geometry of the tungsten complex with a
normalized bite of 1.24 for the L-L ligand is in accord with
Kepert’s (24) conclusions. A similar geometry is observed for
the related complex (L-L)W(CO)2I2P(OCH3)3 with an L-L
normalized bite of 1.23 (11). Both complexes also exhibit
other features associated with a monocapped octahedral ge-
ometry. The mean Cc-W-Ac and Cc-W-Au angles (Cc is a cap-
ping atom, Ac is an atom in the capped face, and Au is an atom
in the uncapped face) are 73.4° and 127.3° for the triphenyl-
phosphite compound and 72.7° and 127.8° for the trimethyl
phosphite complex (11), respectively, with corresponding an-
gles for an ideal monocapped octahedron being 74.1° and
125.5° (25). The other feature is that the unique capping atom
has very close contacts with the three atoms in the capped face.
Thus in the triphenylphosphite complex the capping carbon is
2.27, 2.69, and 2.77 D from the C, As, and P atoms, respec-
tively, in the capped face and in the related trimethyl phosphite
complex 2.28, 2.71, and 2.69 D from these atoms (11).
The five-membered ring of the chelate ligand shows a mod-
erate degree of puckering in both the triphenylphosphite and
trimethyl phosphite complexes. The torsional angles for both
complexes are shown in Fig. 2. The ring-puckering parame-
ters, according to the method of Cremer and Pople (26) are q
= 0.42 and φ = 14° for (L-L)W(CO)2I2P(OC6H5)3 and q = 0.26
and φ = 10.1° for (L-L)W(CO)2I2P(OCH3)3 (11), indicating a
moderate degree of puckering, intermediate between a twist
and envelope conformation. The greater degree of puckering
in the triphenylphosphite complex is primarily due to the pres-
ence of the bulky phenyl rings compared to the methyl groups.
The tetrahedral coordination of the arsenic atoms is also dis-
torted by the contacts.
The two carbon monoxide ligands are in a cis arrangement
to each other (71.2°), the arrangement required in order to
minimize competition for dπ electron density from the d4 tung-
sten atom. The lengths of the two W—As bonds are signifi-
cantly different from each other at 2.5603(7) and 2.6004(7) D,
which can be explained by a consideration of their respective
trans ligands. The I(1) atom trans to As(1) is not as strong an
acceptor as the P atom trans to As(2), resulting in greater back
donation to As(1). This results in a shortening of the W—As(1)
bond compared to the W—As(2) bond, the difference being
0.04(1) D. The average W—I bond length of 2.87 D in this
complex is close to the average of 2.85 D reported for similar
complexes (27, 28). The W—P bond length of 2.457(2) D is
similar to that in (L-L)W(CO)2I2P(OCH3)2 (2.466(3) D) (11)
but considerably shorter as compared to (L-
L)W(CO)2Br2P(C6H5)3 (2.548(2) D) (12). This difference may
reflect both the steric restrictions of the greater bulk of the
P(C6H5)3 groups and the presence of more double-bond char-
acter through back donation from the tungsten in (L-
L)W(CO)2I2P(OC6H5)3. These results are in accord with
differences in the π-acceptor ability of the phosphorus ligands.
F(2)
F(1)
F(3)
F(4)
F(5)
C(6)
C(4)
F(6)
C(3)
C(7)
C(5)
C(2)
As(1)
C(8)
Br(2)
As(2)
Br(1)
W
C(9)
C(1)
O(1)
C(11)
As(3)
C(10)
As(4)
C(17)
C(16)
C(12)
C(14)
C(13)
C(15)
F(8)
F(11)
F(10)
F(7)
F(12)
F(9)
identical except that the chemical symbols for one are primed.
An ORTEP drawing showing the labelling scheme for the un-
primed molecule is given in Fig. 3.
Results and discussion
Synthesis of the seven-coordinate complexes
The dicarbonyl complexes containing
a monodentate
phosphine or phosphite ligand were readily synthesized from
the corresponding tricarbonyl complexes in refluxing benzene.
More vigorous conditions (refluxing toluene) were necessary
to form the monocarbonyl complexes; attempts yielded only
(L-L)2W(CO)X2 and (L-L)W(CO)X2[P(OCH3)3]2 (X = Br, I).
Presumably there is congestion around the tungsten atom with
ligands such as P(C6H5)3, P(OC6H5)3, P(OC2H5)3, and
P(C4H9)3.
Crystal structure of (L-L)W(CO)2I2P(OC6H5)3
The tungsten atom is seven-coordinate with the coordination
sphere containing the two arsenic atoms of the L-L ligand, two
iodine atoms, two carbon atoms of the carbonyl groups, and the
phosphorus atom of the triphenylphosphite group. To deter-
mine the stereochemistry of the title compound we have used
the δ parameters of Porai–Koshits and Aslanov (23), as calcu-
lated by Muetterties and Guggenberger (13). The δ parameter
values (21°, 17°, 6°), when compared with the idealized mono-
capped octahedron (24.2°, 24.2°, 24.2°) and the idealized
monocapped trigonal prism (0°, 0°, 41.5°), indicate the geome-
try to be closer to a monocapped octahedron. For this geometry
the capping atom is the C(2) carbonyl group with the capped
face containing the As(1), C(1), and P atoms. Kepert (24) has
shown that seven-coordinate complexes containing one biden-
tate ligand with a normalized bite (defined as the distance be-
tween points representing the donor atoms of the bidentate
Crystal structure of (L-L)2W(CO)Br2
The structure contains two crystallographically independent
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Can. J. Chem. Vol. 76, 1998
Fig. 4. Schematic diagram showing the torsional angles in the two
five-membered rings of the chelate ligands for (L-L)2W(CO)Br2.
As(1) and As(2) occupy equatorial and axial positions, respec-
tively; As(3) and As(4), equatorial positions. The top numbers refer
to each ring of the unprimed molecule and the bottom numbers to
each corresponding ring of the primed molecule.
Table 4. The ν(CO) and 13C NMR data (carbonyl region) for
seven-coordinate complexes.
ν(CO)
(cm–1)a
δ
2J(13C-31P)
(Hz)
Complex
(ppm)b
(L-L)W(CO)2I2P(OC6H5)3
1959 (vs) 212.4(d)
1875 (s) 240.4(d)
1956 (vs) 217.0(d)
1872 (s) 245.8(d)
1948 (vs) 215.0(d)
1870 (s) 240.4(d)
1950 (vs) 220.0(d)
1868 (s) 246.1(d)
1937 (vs) 216.7(d)
1855 (s) 246.5(d)
1940 (vs) 222.2(d)
9.3
39.9
8.3
7.2(8)
W
W
-8.2(8)
9.3(9)
-8.8(8)
8.4(9)
12.2(8)
(L-L)W(CO)2Br2P(OC6H5)3
(L-L)W(CO)2I2P(OCH3)3
(L-L)W(CO)2Br2P(OCH3)3
(L-L)W(CO)2I2P(C6H5)3
(L-L)W(CO)2Br2P(C6H5)3
(L-L)W(CO)I2[P(OCH3)3]2
-9.9(8)
-11.8(9)
37.0
9.7
(1)
(2)
(3)
As
-4(2)
As
(4)
As
-16(2)
As
3(2)
9(2)
42.1
5.8
16(2)
9(2)
-8(2)
-2(2)
(4)
(12)
C
(5)
(13)
C
C
C
-4(3)
-1(3)
8(2)
-9(2)
39.6
5.1
29.0
5.0
molecules that are related as near enantiomers. They are
chemically equivalent; equivalent metal—ligand bond dis-
tances differ by less than 0.02 D, equivalent interligand dis-
tances differ by an average of 0.04 D, and intramolecular bond
angles agree within 2.5°. In both molecules the two L-L li-
gands chelate the metal to form a seven-coordinate tungsten
(II) complex with the two bromine atoms and the carbonyl
group. The tungsten atom has a slightly distorted pentagonal
bipyramidal geometry. The δ parameters of Porai–Koshits and
Aslanov (23) are 37° and 52°. In the pentagonal bipyramid the
third δ parameter represents an interatomic vector internal to
the polyhedron. The δ values for an ideal pentagonal
bipyramid with identical ligands are both 54.4°. The equatorial
plane of the pyramid is occupied by an L-L ligand, the two
bromine atoms, and one arsenic atom of the other L-L ligand,
leaving the second arsenic atom and the carbonyl group to fill
the apical positions. The tungsten atom is displaced from the
average plane of the pentagonal ring by 0.0144(2) D. Thus this
less symmetric pentagonal bipyramid has fewer steric con-
straints than the more symmetric one with both L-L ligands in
the pentagonal plane, with repulsions between the methyl
groups on adjacent arsenic atoms.
1854 (s)
1833
250.8(d)
27.3
254.8(dd) 37.0, 39.6
259.0(dd) 34.8, 36.0
241.5(s)
(L-L)W(CO)Br2[P(OCH3)3]2 1829
(L-L)2W(CO)I2
1811
1809
(L-L)2W(CO)Br2
260.5(s)
a Spectra were recorded in CH2Cl2; vs = very strong, s = strong.
b Spectra were recorded in CD2Cl2 at 200 K in the presence of a trace
amount of [Cr(acac)3]; s = singlet, d = doublet, dd = doublet of doublets.
(0.20 for each) and puckering phase angles different by ap-
proximately 180° (200° in one molecule and 22° in the other
molecule). These phase angles correspond to the twist confor-
mation of a five-membered ring. The equatorial–axial chelates
are less similar, with puckering amplitudes of 0.15 and 0.18,
and the respective puckering phase angles of 168° and 1°.
These phase angles correspond to an intermediate twist-boat
conformation and a boat conformation, respectively. In this
case, four of the torsional angles show opposite signs, with the
fifth small and negative in both cases.
This type of geometry is rare (29, 30) for seven-coordinate
carbonyl complexes of the type MX2(CO)nL5–n (M = Mo, W;
n = 1–3). The tungsten—carbon bond is unique, being ex-
tremely short (1.78(2) D) compared to a related monocarbonyl
complex (10) containing two monodentate P(OCH3)3 ligands
(1.86(3) D) and the related dicarbonyl complexes (11, 12) (av-
erage 1.94 D). The relative shortness of the tungsten—carbon
bond here represents considerable double-bond character aris-
ing from back donation of π electrons from tungsten.
The four five-membered rings of the chelate ligands in the
two independent molecules of (L-L)2W(CO)Br2 show a vari-
ation in conformation. Their torsional angles are shown in
Fig. 4. One of the two chelates in each independent molecule
has a bite that spans from the equatorial plane of the pentagonal
bipyramid to one of the axial vertices (equatorial–axial), in-
volving the atoms As(1) and As(2). The other chelate in each
molecule has a bite that forms an edge of the equatorial plane
(equatorial–equatorial), involving the atoms As(3) and As(4).
Spectroscopic properties of (L-L)W(CO)2I2P(OC6H5)3
and related seven-coordinate dicarbonyl complexes
Table 4 lists carbonyl IR stretching frequencies and 13C NMR
data, and Table 5 lists 19F-{1H}and 31P-{1H}NMR data for all
the seven-coordinate complexes under study. The observation
of a single As–CH3 resonance in the 1H NMR spectrum (297
K) and a single CF3 resonance in the 19F-{1H}NMR spectrum
of (L-L)W(CO)2I2P(OC6H5)3 are not consistent with the mole-
cule having a static solution structure equivalent to that deter-
mined from the X-ray determination. The static structure
consistent with the solid-state structure is frozen out at 200
K. Thus, As–CH3 resonances are found at 1.50, 1.84, 2.51, two
CF3 quartets at –52.2 and –53.8, and two 13C doublets (car-
bonyl region) at 212.4 and 240.4. It is assumed that accidental
degeneracy may have caused the 1:2:1 As–CH3 triplet, since
all other related complexes exhibited four equal-area As–CH3
resonances. The dicarbonyl complexes are related as shown by
the similarity of carbonyl stretching frequencies, chemical
Because of the approximate enantiomorphic relation be-
tween the two independent molecules, the torsional angles of
their equatorial–equatorial five-membered rings have similar
torsional angle magnitudes but with opposite signs. This is
further illustrated by the ring-puckering parameters of Cremer
and Pople (26), which show similar puckering amplitudes
1
shifts, coupling constants, and variable temperature H and
19F-{1H}NMR data. As the temperature is increased, four
As–CH3 peaks (three for the title compound) indicative of the
low-temperature slow-exchange limiting spectra coalesce into
two peaks. The line broadening continues until an averaged
© 1998 NRC Canada

251
Barton et al.
Table 5. The 19F-{1H} and 31P-{1H} NMR data for seven-coordinate complexes.
δ(19F) (ppm)a
Complex
297 K
210 K
J(19F-19F) (Hz)
δ(31P) (ppm)b
J(31P-183W) (Hz)
(L-L)W(CO)2I2P(OC6H5)3
–53.1 (s)
–52.2 (q, 3F)
–53.8 (q, 3F)
–50.7 (q, 3F)
–52.3 (q, 3F)
–51.5 (m, 6F)
–51.4 (m, 6F)
–52.3 (m, 6F)
–51.4 (m, 6F)
–53.0 (q, 3F)
–54.1 (q, 3F)
14.3
90.7
402.8
(L-L)W(CO)2Br2P(OC6H5)3
–51.4 (s)
14.1
101.1
406.1
(L-L)W(CO)2I2P(OCH3)3
(L-L)W(CO)2Br2P(OCH3)3
(L-L)W(CO)2I2P(C6H5)3
(L-L)W(CO)2Br2P(C6H5)3
(L-L)W(CO)I2[P(OCH3)3]2
–51.5 (s)
–51.3 (s)
–52.2 (s)
–51.1 (s)
–52.7 (s)
—
—
104.7
364.7
377.1
216.1
222.3
411.9
389.5
112.3
—
–7.1
—
4.8
14.3
104.5
130.2
2J(31P-31P) = 31.9 Hz
113.5
(L-L)W(CO)Br2[P(OCH3)3]2
–51.7 (s)
–51.1 (q, 3F)
–52.3 (q, 3F)
14.3
418.1
384.5
138.3
2J(31P-31P) = 30.8 Hz
—
(L-L)2W(CO)I2
–50.7 (s)
–50.6 (s)
–51.0 (m)
–51.0 (m)
—
—
—
—
(L-L)2W(CO)Br2
—
a Spectra were recorded in CD2Cl2; s = singlet, q = quartet, m = A3B3 multiplet.
b Each resonance (297 K) appeared as a singlet with 183W satellites except for (L-L)W(CO)X2[P(OCH3)3]2 (X = Br, I) for which each resonance
(210 K) appeared as a doublet with 183W satellites.
single As–CH3 resonance is observed, indicative of the high-
temperature fast-exchange limiting spectra.
involving dissociative processes and intramolecular rearrange-
ments. Dissociative processes such as phosphite or phosphine
dissociation or carbonyl dissociation for the dicarbonyl com-
plexes can be eliminated by the fact that the 31P-{1H}fast-ex-
change limiting NMR spectra exhibited 31P-183W coupling,
and 13C fast-exchange limiting NMR spectra exhibited both
13C-183W and 13C-31P coupling. Dissociation of one end of the
chelated ligand would probably require a much higher barrier
than that which is observed (∆G≠ values in the range 45.1–54.0
kJ/mol).
The activation energy barriers ∆G≠ for methyl group ex-
change (calculated from the Eyring equation (35) at the coa-
lescence temperature) show an increase, with an increase in
the size of the halide and the bulk of the phosphorus ligand,
consistent with an intramolecular process (36) in which the
tungsten—halide and tungsten—phosphorus bonds are re-
tained throughout the dynamic process.
To explain the dynamic behaviour of the title compound an
exchange process can be envisaged, which itself varies over a
narrow temperature range. The process might begin with ran-
dom scrambling of the atoms I(1), C(1), and P, leaving the
atom I(2) rigidly held in the “pocket” of the chelate ring. This
would equilibrate the atoms C(4) and C(9) as one pair, and the
atoms C(3) and C(10) as the other pair. As the temperature is
raised further the atom I(2) is gradually added to the random
scrambling motion, causing coalescence to a single peak. The
∆G≠ value associated with the lower temperature part of the
process is 51.7(2) kJ/mol and with the higher temperature part
of the process is 54.0(2) kJ/mol. The similarity of these two
values supports the description of the phenomenon responsible
for the coalescence as being one process.
The 13C chemical shifts (carbonyl region), carbonyl stretch-
ing frequencies, and J(31P-183W) values are consistent with the
electronic differences of the three phosphorus monodentate
ligands in which P(C6H5)3 is a better net donor: P(OC6H5)3,
P(OCH3)3, P(C6H5)3. An examination of the low-temperature
13C chemical shifts in Table 4 shows two unique resonances
for the dicarbonyl complexes, one that is approximately 30
ppm downfield from the other. For the title dicarbonyl com-
plex the downfield resonance is assigned to the unique capping
carbonyl group consistent with the work of Colton and Kevek-
ordes (31), where they assigned the low field carbonyl chemi-
cal shift absorption to the capping carbon in a structure best
described as a monocapped octahedron. Another unique fea-
ture of the 13C NMR data is the large differences in 2J(13C-31P)
values that are consistent with cis coupling. Previous work has
2
shown an average J(13C-31P) value of 32 Hz for angles sub-
tended at tungsten by the phosphorus and carbonyl carbon in
2
the range of 72–78° and a J(13C-31P) value of 6 Hz for a
corresponding angle of 95° (25, 32–34). On this basis the
larger 2J(13C-31P) value could be assigned to the carbonyl with
a P-W-C angle in the range 70–80° and the lower 2J(13C-31P)
value to the carbonyl with a P-W-C angle above 100°. For the
title dicarbonyl complex, the capping carbonyl (<P-W-C = 77.2°)
is assigned the resonance with 2J(13C-31P) = 39.9 Hz, and the
noncapping carbonyl (<P-W-C = 110.7°) the resonance with
2J(13C-31P) = 9.3 Hz. These assignments are compatible with
the earlier assignments made for the carbonyl group chemical
shift on the basis of the unique capping position.
In view of the difficulty in assigning a configuration for
seven-coordinate complexes based on multinuclear NMR data,
we believe 13C chemical shifts and 2J(13C-31P) values provide
important insights in elucidating structural considerations.
There have been many possible pathways suggested to ex-
plain the fluxional behaviour in seven-coordinate compounds
Spectroscopic properties of (L-L)2W(CO)Br2 and related
seven-coordinate monocarbonyl complexes
As with the previous complex, the observations of a single
1
As–CH3 resonance in the H NMR spectrum (297 K) and a
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Can. J. Chem. Vol. 76, 1998
Fig. 5. Schematic diagram illustrating a pathway for the dynamic behaviour for (L-L)2W(CO)Br2.
Fig. 6. Schematic diagram illustrating a pathway for the dynamic behaviour for (L-L)W(CO)Br2[P(OCH3)3]2.
single CF3 resonance in the 19F-{1H}NMR spectrum of (L-
L)2W(CO)Br2 are not consistent with the X-ray crystal struc-
ture, suggesting dynamic behaviour. Lowering the
temperature to 180 K produced two As–CH3 resonances at
1.76 and 2.15 and a complex CF3 multiplet centred at –51.0.
Further lowering of the temperature to 170 K (lowest attain-
able due to solvent limitations) caused the low field As–CH3
resonance to broaden. Thus it was not possible to obtain a
low-temperature slow-exchange limiting spectrum indicative
of the solid-state X-ray crystal structure. The ∆G≠ value of 43.5
kJ/mol is comparatively low for a complex containing two
bidentate ligands relative to the dicarbonyl complexes; per-
haps the small bite of the two L-L ligands provides extra space
around the tungsten centre for other ligands, thus enhancing
the possibility for rearrangements.
A nondissociative mechanism consistent with the NMR
data could involve a concerted rearrangement in which the
roles of the two bidentate L-L ligands are reversed, and the
environments of their methyl groups are also equilibrated. An
intermediate polyhedron between the two pentagonal
bipyramidal geometries is a monocapped trigonal prism, with
the carbonyl ligand capping the quadrilateral face formed by
the four arsenic atoms as illustrated in Fig. 5. Then the poly-
hedron found in the crystal can be generated from it by moving
any one of the four As atoms through the Br- -Br edge of that
polyhedron to become the apical polyhedron of the pentagonal
bipyramid, leaving its ligand mate near the pentagonal plane
between the two Br atoms.
back to the intermediate polyhedron may take place with equal
probability for the apical As to arrive back in the monocapped
trigonal pyramid sharing an edge with Br(1) or with Br(2).
Then all eight As–CH3 resonances would be equilibrated.
Attempts to produce other related monocarbonyl com-
plexes have only yielded (L-L)W(CO)X2[P(OCH3)3]2 (X = Br,
I). We have previously published the X-ray crystal structure
of (L-L)W(CO)Br2[P(OCH3)3]2 (10) and have included exten-
sive spectroscopic data in the present paper. Low-temperature
NMR studies of the trimethylphosphite monocarbonyl com-
plex suggest a static structure consistent with the X-ray crystal
structure (best described as a monocapped trigonal prism with
a Br in the capping position and two As atoms, a P, and a Br
atom in the quadrilateral capped face; four 1H As-CH3 singlets;
and two 31P-{1H}doublets). As the temperature is increased
the four As–CH3 resonances coalesce into two and then into
an averaged As–CH3 resonance. Similarly, the two 31P-
{1H}doublets coalesce into a single resonance with 183W sat-
ellites. The ∆G≠ values (51.3 and 69.7 kJ/mol) are considerably
higher than the previously discussed monocarbonyl containing
the two bidentate L-L ligands, reflecting a greater steric hin-
drance exerted by the bulkier P(OCH3)3 monodentate ligands.
The equilibration of four As–CH3 resonances into two and two
31P resonances into one can be rationalized by the dynamic
behaviour illustrated in Fig. 6, producing a pentagonal
bipyramidal intermediate with As1As2Br2P1P2 in the equato-
rial plane. This would equilibrate the As–CH3 groups above
the pentagonal plane as one set and As–CH3 groups below the
pentagonal plane as the other set as well as equilibrating the
two 31P environments of the trimethylphosphite ligands. The
equilibration of two As–CH3 resonances into one at a coales-
cence temperature of 348 K occurs with a higher activation
energy than that for the first process (69.7 vs. 51.3 kJ/mol),
suggesting either a dissociative process or a high-energy ran-
dom scrambling process. The possibility of phosphorus disso-
ciation can be eliminated on the basis of the retention of
J(31P-183W) coupling in the high-temperature fast-exchange
limiting spectra.
If the pentagonal symmetry is not fully formed, as is seen
in the crystal structure, then there will be a tendency for re-
laxation back to the monocapped trigonal prism intermediate
polyhedron to proceed along the same route as that by which
the pentagonal bipyramid was formed. This would then leave
two broad peaks, corresponding to the As–CH3 resonances for
the CH3 on the two sides of the plane of the intermediate. The
deviations from the full symmetry of the pentagonal bipyramid
are associated with the puckering of the plane of the bidentate
ligand. However, if the full symmetry of the pentagonal
bipyramid is formed at higher temperature, then relaxation
Finally, the low-temperature 13C NMR data (carbonyl
© 1998 NRC Canada

253
Barton et al.
region) are consistent with cis coupling on the basis of 2J(13C-
31P) values (Table 4), and the values of 34.8 and 36.0 Hz for
the bromomonocarbonyl are consistent with the angles sub-
tended at tungsten by the phosphorus and carbonyl carbon at-
oms (71° and 76°) (10), a correlation previously discussed for
the dicarbonyl complexes.
12. S.K. Manocha, L.M. Mihichuk, R.J. Barton, and B.E. Robert-
son. Acta Crystallogr. Sect. C, C47, 722 (1991).
13. E.L. Muetterties and L.J. Guggenberger. J. Am. Chem. Soc. 96,
1748 (1974).
14. M.G.B. Drew. Prog. Inorg. Chem. 23, 67 (1977).
15. M.G.B. Drew and C.J. Rix. J. Organomet. Chem. 102, 467
(1975).
To determine the influence of the bidentate ligand and to
provide additional 31P NMR parameters that were lacking in
this study further work is continuing in the area of seven co-
ordination incorporating the ligands (C6H5)2PCH2P(C6H5)2,
(C6H5)2PCH2CH2P(C6H5)2, and cis-(C6H5)2PCH== CHP(C6H5)2.
This work is aimed at the study of monocarbonyl complexes
containing two identical bidentate ligands and two different
bidentate ligands. In addition the potential use of 183W NMR
is being investigated.
16. W.R. Cullen and L.M. Mihichuk. Can. J. Chem. 54, 2548
(1976).
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Acknowledgment
We thank the Faculty of Science, University of Regina, for
financial support and Quinn Major for computing assistance.
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© 1998 NRC Canada