Synthesis and Characterization of a Stable trans-Dioxo Tungsten(IV) Complex and Series of Mono-Oxo Molybdenum(IV) and Tungsten(IV) Complexes. Structural and Electronic Effects of π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems

Article abstract of DOI:10.1021/ic980342u

trans-[W(O)(F)(dppe)²](BF₄) (dppe = Ph₂PCH₂CH₂PPh₂, K2-bis(diphenylphosphino)ethane) (10) has been synthesized. From this, the neutral trans-[W(O)₂(dppe)₂]·2CH₃OH (11) was prepared by a hydrolysis and deprotonation reaction with Et₄NOH in methanol/water. Together with the analogous molybdenum compound (2) this compound afforded by substitution under acid conditions salts of trans-[M(O)(X)(dppe)₂]⁺ ions (M = Mo, W; X = OH, OCH₃, Cl. Br, I, NCS) and trans-[Mo(O)(N₃)(dppe)₂](CF₃SO₃) (9). All the compounds have low-spin d² electronic configuration. The compounds were characterized by ³¹P{¹H} NMR, positive FAB-ionization mass spectrometry, UV-vis spectroscopy, and vibrational spectroscopy. X-ray crystallographic studies were carried out on compounds 10, 11, trans-[W(O)(OH)(dppe)₂](ClO₄) (12), and trans-[W(O)(NCS)(dppe)₂](BPh₄) (16). The electronic spectra of the mono-oxo species are consistent with the lowest energy transitions being of d_xy → {d_zx, d_yz} character. No ligand-field transitions are observed for the dioxo tungsten complex. Identical μ-spectrochemical series were found for the two metals. The shielding of the phosphorus nuclei determined by ³¹P{¹H} NMR is significantly influenced by the nature of the axial ligands.

Full text of DOI:10.1021/ic980342u

5992
Inorg. Chem. 1998, 37, 5992-6001
Synthesis and Characterization of a Stable trans-Dioxo Tungsten(IV) Complex and Series of
Mono-Oxo Molybdenum(IV) and Tungsten(IV) Complexes. Structural and Electronic
Effects of π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Jesper Bendix*,† and Anders Bøgevig‡
Department of Inorganic Chemistry and Centre for Crystallographic Studies, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen, Denmark
ReceiVed March 25, 1998
trans-[W(O)(F)(dppe)2](BF4) (dppe ) Ph2PCH2CH2PPh2, 1,2-bis(diphenylphosphino)ethane) (10) has been
synthesized. From this, the neutral trans-[W(O)2(dppe)2]‚2CH3OH (11) was prepared by a hydrolysis and
deprotonation reaction with Et4NOH in methanol/water. Together with the analogous molybdenum compound
(2) this compound afforded by substitution under acid conditions salts of trans-[M(O)(X)(dppe)2]+ ions (M )
Mo, W; X ) OH, OCH3, Cl, Br, I, NCS) and trans-[Mo(O)(N3)(dppe)2](CF3SO3) (9). All the compounds have
low-spin d2 electronic configuration. The compounds were characterized by 31P{1H} NMR, positive FAB-ionization
mass spectrometry, UV-vis spectroscopy, and vibrational spectroscopy. X-ray crystallographic studies were
carried out on compounds 10, 11, trans-[W(O)(OH)(dppe)2](ClO4) (12), and trans-[W(O)(NCS)(dppe)2](BPh4)
(16). The electronic spectra of the mono-oxo species are consistent with the lowest energy transitions being of
dxy f {dzx, dyz} character. No ligand-field transitions are observed for the dioxo tungsten complex. Identical
π-spectrochemical series were found for the two metals. The shielding of the phosphorus nuclei determined by
31P{1H} NMR is significantly influenced by the nature of the axial ligands.
Introduction
complexes have been described. These are the trans-[M(O)2-
(CN)4]4- ions8 and the matrix-isolated trans-[M(O)2(CO)4] (M
) Mo, W) complexes.9
Multiple bonding between transition metals and ligands has
received much interest recently, fueled by its importance for
atom-transfer reactions in catalysis and biochemistry.1,2 Among
the systems with multiple-bonding between metals and ligands
a rich subclass of strongly tetragonally compressed trans-dioxo
complexes of metal centers d2 electronic configurations has
developed. This class encompasses complexes of techne-
tium(V),3 rhenium(V),4 ruthenium(VI),5 and osmium(VI).6 The
photochemical and photophysical properties of these compounds
have been the focus of much interest.7 Contrary to these four
metals, only very few molybdenum(IV) and tungsten(IV) dioxo
Recently, Cotton et al.10,11 and we12 have augmented these
with compounds of the type trans-[Mo(O)2(PP)2]‚2(solvate)
where PP represents one of the two bidentate phosphine ligands,
dppe (Ph2PCH2CH2PPh2, 1,2-bis(diphenylphosphino)ethane) or
dppee (Ph2PCHdCHPPh2, cis-1,2-bis(diphenylphosphino)eth-
ylene). Analogous tungsten(IV) complexes have, however, not
been described despite the facts that trans-[Re(O)2(dppe)2]-
(ReO4) has been known for 30 years13 and that similar
complexes of the heavier calcogenides (trans-[W(Q)2(P(CH3)3)4],
Q ) S, Se, Te) were described in a pioneering work by
Rabinovich and Parkin.14
† Department of Inorganic Chemistry.
‡ Centre for Crystallographic Studies.
Some examples of related cationic complexes of the type
trans-[M(O)(X)(dppe)2]+ (M ) Mo, W) have been reported:
trans-[Mo(O)(Cl)(dppe)2]+,15 trans-[W(O)(Cl)(dppe)2]+,16 trans-
(1) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley:
New York, 1988.
(2) Holm, R. H. Chem. ReV. 1987, 87, 1401.
(3) (a) Trop, H. S.; Jones, A. G.; Davison, A. Inorg. Chem. 1980, 19,
1993. (b) Kastner, M. E.; Lindsay, M. J.; Clarke, M. J. Inorg. Chem.
1982, 21, 2037. (c) Fackler, P. H.; Lindsay, M. J.; Clarke, M. J. Inorg.
Chem. Acta 1985, 109, 39. (d) Vanderheyden, J.-L.; Ketring, A. R.;
Libson, K.; Heeg, M. J.; Roecker, L.; Motz, P.; Whittle, R.; Elder, R.
C.; Deutsch, E. Inorg. Chem. 1984, 23, 3184.
(7) (a) Che, C. M.; Yam, V. W. W.; Cho, K. C.; Gray, H. B. J. Chem.
Soc., Chem. Commun. 1987. 948. (b) Yam, V. W. W.; Che, C. M.
Coord. Chem. ReV. 1990, 97, 93. (c) Lam, H. W.; Chin, K. F.; Che,
C. M.; Wang, R. J.; Mak, T. C. W. Inorg. Chim. Acta 1993, 204, 133.
(d) Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346.
(8) (a) Day, V. W.; Hoard, J. L. J. Am. Chem. Soc. 1968, 90, 3374. (b)
Lippard, S. J.; Russ, B. J. Inorg. Chem. 1967, 6, 1943. (c) Samotus,
A.; Dudek, M.; Kanas, A. J. Inorg. Nucl. Chem. 1975, 37, 943.
(9) Crayston, J. A.; Almond, M. J.; Downs, A. J.; Poliakoff, M.; Turner,
J. J. Inorg. Chem. 1984, 23, 3051.
(10) Cotton, F. A.; Feng, X. Inorg. Chem. 1996, 35, 4921.
(11) Cotton, F. A.; Schmid, G. Inorg. Chem. 1997, 36, 2267.
(12) Bendix, J.; Birkedal, H.; Bøgevig, A. Inorg. Chem. 1997, 36, 2702.
(13) Freni, M.; Giusto, D.; Romiti, P. Gazz. Chim. Ital. 1967, 97, 833.
(14) Rabinowich, D.; Parkin, G. Inorg. Chem. 1995, 34, 6341.
(15) (a) Bishop, M. W.; Chatt, J.; Dilworth, J. R.; Hursthouse, M. B.;
Motevalli, M. J. Chem. Soc., Dalton Trans. 1979, 1603. (b) Adam,
V. C.; Gregory. U. A.; Kilbourn, B. T. J. Chem. Soc., Chem. Commun.
1970, 1400.
(4) (a) Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346. (b) Winkler,
J. R.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 1373. (c) Pipes, D.
W.; Meyer, T. J. Inorg. Chem. 1986, 25, 3256. (d) Yam, V. W-W.;
Tam, K.-K.; Cheng, M.-C.; Peng, S.-M.; Wang, Y. J. Chem. Soc.,
Dalton Trans. 1992, 1727.
(5) (a) Griffith, W. P.; Pawson, D. J. Chem. Soc., Dalton Trans. 1973,
1315. (b) Tokita, Y.; Yamaguchi, K.; Watanabe, Y.; Morishima, I.
Inorg. Chem. 1993, 32, 329. (c) Hepworth, M. A.; Robinson, P. L.
Inorg. Nucl. Chem. 1957, 4, 24. (d) Mak, T. C. W.; Che, C.-M.; Wong,
K.-Y. J. Chem. Soc., Chem. Commun. 1985, 986.
(6) (a) Che, C.-M.; Yam, V. W-W.; Cho, K.-C.; Gray, H. B. J. Chem.
Soc., Chem. Commun. 1987, 948. (b) Malin, J. M.; Schlemper, E. O.;
Murmann, R. K. Inorg. Chem. 1977, 16, 615. (c) Phillips, F. L.;
Skarpski, A. Acta Crystallogr. 1975, B31, 1814. (d) Kruse, F. H. Acta
Crystallogr. 1961, 14, 1035.
10.1021/ic980342u CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/24/1998

π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Inorganic Chemistry, Vol. 37, No. 23, 1998 5993
FAB-MS (glycerol matrix): m/z 929 (M+, rel intensity 63%), 531 ((M
- dppe)+, rel intensity 100%).
[Mo(O)(F)(dppe)2]+,17 and trans-[Mo(O)(OCH3)(dppe)2]+.18
However, only the first of these complexes was a result of an
intended synthesis with a reported yield. In addition to these
four complexes also trans-[Mo(O)(OH)(dppe)2]+ 19 has been
claimed, but this result has been shown12 to be a misinterpreta-
tion and the complex actually described to be trans-[Mo(O)-
(F)(dppe)2]+.
Here we describe the synthesis of trans-[W(O)(F)(dppe)2]-
(BF4) (10) and therefrom trans-[W(O)2(dppe)2]‚2CH3OH (11).
We further describe the rational synthesis of the two series of
complexes: trans-[M(O)(X)(dppe)2]+ (M ) W; OH (12), Cl
(13), Br (14), I (15), NCS (16), OCH3 (17), and M ) Mo; X )
OH (3), Cl (4), Br (5), I (6), NCS (7), OCH3 (8), and N3 (9))
utilizing the respective trans-dioxo complexes as versatile
starting materials. The spectral and structural variations within
and between these series are discussed.
Synthesis of trans-[Mo(O)2(dppe)2]‚2CH3OH (2). 1 (200 mg, 0.20
mmol) was added to a solution of 15 mL of acetone and 10 mL of
methanol. When the pink trans-[Mo(O)(F)(dppe)2](BF)4 had com-
pletely dissolved, the solution was filtered and tetraethylammonium
hydroxide (5 drops of 40% aqueous solution, 0.7 mmol) was layered
with the solution. The reaction mixture was cooled to 5 °C. During
several hours yellow crystals grew. The compound does not survive
for more than 1 day under these conditions. If good quality crystals
are not needed, addition of 3-5 times larger amount of base and
immediate mixing will produce a yellow microcrystalline powder. Yield
170 mg (87%). Anal. Calcd for MoO4P4C54H56: C, 65.59; H, 5.71.
Found: C, 64.71; H, 5.79. Mp 131-133 °C (dec). FAB-MS (m-NBA
matrix): m/z 927 ((M + H)+, rel intensity 17%), 529 ((M + H -
dppe)+, rel intensity 100%). Adducts with water, ethanol, or 2-propanol
can be made in similar yields by substituting these solvents for the
methanol above.
Synthesis of trans-[Mo(O)(OH)(dppe)2](ClO4) (3). To a suspen-
sion of 2 (200 mg) in methanol (30 mL) was slowly added 1 M HClO4
(30 mL). After addition of the first 5 drops a clear orange solution
resulted. Upon addition of the remaining HClO4 the compound
crystallized as yellow-orange plates. The yield was quantitative (200
mg). Anal. Calcd for MoO6P4ClC52H49: C, 60.92; H, 4.82; Cl, 3.46.
Found: C, 59.94; H, 4.65; Cl, 3.53. Mp 170 °C (expl.). FAB-MS
(m-NBA matrix): m/z 927 (M+, rel intensity 91%), 529 ((M - dppe)+,
rel intensity 67%).
Experimental Section
General Procedures. dppe and triphenylphosphine were purchased
(Strem) and used without further purification. Aqueous solutions of
HBF4 (Fluka), Pb(BF4)2 (Riedel-de Hae¨n), and Et4NOH (Fluka) were
purchased and used as received. CF3SO3H was purchased (3M-
Company) and purified by distillation of the mono hydrate. Trimethyl-
silyl azide was purchased (Fluka) and used as received. trans-W(PPh3)2-
Cl4 and Na3[Mo(HCOO)6]21 were prepared by literature procedures.
20
Synthesis of trans-[Mo(O)(Cl)(dppe)2]Cl‚2H2O (4). To a suspen-
sion of 2 (100 mg, 0.101 mmol) in methanol (15 mL) was added 0.2
mL of 1 M aqueous HCl, the solution was filtered and added 1 M HCl
(10 mL). The blue solution deposits 90 mg (88%) of blue flaky crystals.
Anal. Calcd for MoO2Cl2P4C52H50: C, 61.49; H, 5.16; Cl, 6.98.
Found: C, 59.07; H, 5.06; Cl, 6.81. Mp 147-151 °C (dec). FAB-
MS (m-NBA matrix): m/z 945 (M+, rel intensity 100%), 547 ((M -
dppe)+, rel intensity 70%).
Synthesis of trans-[Mo(O)(Br)(dppe)2]Br‚2H2O (5). This com-
pound was synthesized in the same way as the chloro-complex,
substituting HBr for HCl. Yield 82%. Anal. Calcd for MoO2-
Br2P4C52H50: C, 56.54; H, 4.75; Br, 14.47. Found: C, 55.14; H, 4.69;
Br, 14.31. Mp ca. 131 °C (dec). FAB-MS (m-NBA matrix): m/z 989
(M+, rel intensity 100%), 591 ((M - dppe)+, rel intensity 72%).
Synthesis of trans-[Mo(O)(I)(dppe)2](BPh4) (6). 2 (150 mg, 0.152
mmol), NH4I (1.35 g, 13.1 mmol), and 0.75 mL of aqueous CF3SO3H
was refluxed in 2-propanol (15 mL) for 1 h. The green solution was
cooled to 20 °C and diluted with methanol (45 mL). Slow addition of
NaBPh4 (400 mg, 1.17 mmol) in methanol (30 mL) afforded green
crystals which were washed with methanol (4×) and diethyl ether (2×).
Yield 128 mg (62%). Anal. Calcd for MoOIBP4C76H68: C, 67.37; H,
5.06; I, 9.37. Found: C, 67.04; H, 5.00; I, 9.87. Mp 200-208 °C
(dec). FAB-MS (m-NBA matrix): m/z 1036 (M+, rel intensity 22%),
638 ((M - dppe)+, rel intensity 15%).
Synthesis of trans-[Mo(O)(NCS)(dppe)2](BPh4) (7). 2 (160 mg,
0.162 mmol) and NH4NCS (1.00 g, 13.1 mmol) was refluxed in
2-propanol (12 mL) for 0.5 h. The green solution was cooled to 20
°C and diluted with methanol (25 mL). Slow addition of NaBPh4 (480
mg, 1.40 mmol) in methanol (45 mL) afforded blue solid. The raw
product was dissolved in acetone (12 mL). To the filtered solution
was slowly added 0.50 g NaBPh4 in methanol (40 mL). Yield 110 mg
(53%) of blue-violet needles. Anal. Calcd for MoONSBP4C77H68: C,
71.91; H, 5.33; N, 1.09. Found: C, 71.62; H, 5.17; N, 1.05. Mp 200-
206 °C (dec). FAB-MS (m-NBA matrix): m/z 968 (M+, rel intensity
24%).
Synthesis of trans-[Mo(O)(OCH3)(dppe)2](CF3SO3) (8). To a
suspension of 2 (150 mg, 0.152 mmol) in methanol (15 mL) was added
neat CF3SO3H (0.25 mL). The solution was refluxed for 2 h, cooled
to 0 °C, and 10 mL of 2 M aqueous CF3SO3H was added. The product
was washed with cold methanol (1×) and diethyl ether (2×). Yield
125 mg (76%) of yellow orange crystals. Anal. Calcd for MoO5P4-
C54H51SF3: C, 59.57; H, 4.72. Found: C, 57.93; H, 4.67. Mp ca. 190
°C (dec). FAB-MS (m-NBA matrix): m/z 941 (M+, rel intensity 100%,
543 (M - dppe)+, rel intensity 95%).
The syntheses of trans-[Mo(O)(F)(dppe)2](BF4) (1), trans-[Mo(O)2-
(dppe)2]‚2CH3OH (2), and trans-[Mo(O)(OH)(dppe)2](ClO4) (3) have
been outlined in a previous communication;12 the details are given here.
Fast atom bombardment (FAB) mass spectra (Xe ions, accelerated by
6 kV) were recorded on a Jeol JMS-HX/HX110A tandem mass
spectrometer (positive ion detection. Matrix: m-nitrobenzyl alcohol
(m-NBA) or glycerol). In all cases only peaks which could be
rationalized as fragments of the molecular ions were observed. Good
agreement between calculated and observed isotope patterns was found
for all important peaks. UV-vis spectra were recorded on a Perkin-
Elmer, Lambda 17 spectrometer. 31P and 19F NMR spectra were
recorded on a Varian UNITY 400 spectrometer operating at 161.90
and 376.28 MHz, respectively. All spectra were recorded in CDCl3.
31P NMR spectra were recorded with H3PO4 as external reference and
19F NMR spectra with CF3C6H5 (δ ) -63.75 ppm vs CFCl3) as external
reference. The quoted 19F chemical shifts are relative to CFCl3.
Infrared and Raman (λ ) 1064 nm) spectra were recorded on solid
samples (KBr disks) on a Perkin-Elmer 2000 FT-IR/FT-NIR Raman
spectrometer. Melting points were measured on a Bu¨chi melting point
apparatus. Elemental analyses were made at the Microanalytical
Laboratory at the Department of Chemistry, University of Copenhagen.
Safety Note. Azide complexes and perchlorate salts of metal
complexes are potentially explosiVe. Only in small amounts of material
should be prepared and these should be handled with great caution.
The described perchlorates explode when heated in the solid state.
Synthesis of trans-[Mo(O)(F)(dppe)2](BF4) (1). Na3[Mo(HCOO)6]
(100 mg, 0.23 mmol) and dppe (180 mg, 0.45 mmol) were refluxed
for 3 h in a mixture of 96% ethanol (18 mL), 40% aqueous HBF4 (1
mL, 8 mmol), and water (1 mL). From the resulting dark red solution
pink crystals precipitated upon cooling in ice. The precipitate was
washed with a 1:1 mixture of methanol/diethyl ether followed by diethyl
ether. Yield 170 mg (74%). Anal. Calcd for MoOP4F5BC52H48: C,
61.56; H, 4.77. Found: C, 61.62; H, 4.82. Mp 275-285 °C (dec).
(16) Cotton, F. A.; Mandal, S. K. Eur. J. Solid State Inorg. Chem. 1991,
28, 775.
(17) Morris, R. H.; Sawyer, J. F.; Schweitzer, C. T.; Sella, A. Organo-
metallics 1989, 8, 2099.
(18) Adachi, T.; Hughes, D. L.; Ibrahim, S. K.; Okamoto, S.; Pickett, C.
J.; Yabanouchi, N.; Youshida., T. J. Chem. Soc., Chem. Commun.
1995, 1081.
(19) Churchill, M. R.; Rotella, F. J. Inorg. Chem. 1978, 17, 668.
(20) Butcher, A. V.; Chatt, J.; Leigh, G. J.; Richards, R. L. J. Chem. Soc.,
Dalton Trans. 1972, 1064.
(21) Brorson, M.; Scha¨ffer, C. E. Acta Chem. Scand. 1986, A 40, 358.

5994 Inorganic Chemistry, Vol. 37, No. 23, 1998
Bendix and Bøgevig
Table 1. Crystallographic Data of Complexes
10
11
12
16
empirical formula
fw (g/mol)
temp (K)
wavelength (Å)
space group
unit cell dimensions
a (Å)
C52H48BF5OP4W
1102.44
122(2)
0.710 73
Cc (No. 9)
C54H56O4P4W
1076.72
122(2)
0.710 73
P1h (No. 2)
C52H49ClO6P4W
1113.09
122(2)
0.710 73
P21/n (No. 14)
C77H68BNOP4SW
1373.92
122(2)
0.710 73
P1h (No. 2)
9.814(5)
22.472(3)
21.1029(17)
-
92.522(17)
-
4649(2)
4
1.575
26.81
0.0306
0.0620
9.464(2)
11.429(2)
13.126(3)
110.86(2)
99.05(2)
110.24(2)
1179.4(5)
1
1.516
26.31
0.0173
0.0433
16.864(3)
16.639(4)
17.114(4)
-
102.256(18)
-
4692.5(18)
4
1.576
27.06
0.0278
0.0587
13.8122(3)
19.0123(4)
24.9475(5)
92.5320(10)
98.6970(10)
93.7920(10)
6452.0(2)
4
1.414
19.69
0.0698
0.1638
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calc (g/cm3)
µ (cm-1
)
R1 [I > 2σ(I)]a
wR2 [I > 2σ(I)]a
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑{w(|Fo| - |Fc| )2}/∑{w(|Fo| )2}]1/2
.
Synthesis of trans-[Mo(O)(N3)(dppe)2](CF3SO3) (9). 2 (140 mg,
0.142 mmol) and (CH3)3SiN3 (1 mL) were added to a mixture of 3 mL
of methanol, 3 mL of acetone, and 2 mL of 2 M aqueous CF3SO3H.
The clear solution was left in an open beaker for 24 h whereupon the
deposited large blue needle-shaped crystals were collected and washed
with methanol/diethyl ether (1:5) (3×) and diethyl ether (2×). Yield
120 mg (77%). Anal. Calcd for MoO4N3P4C53H48F3S: C, 57.88; H,
4.40; N, 3.82. Found: C, 56.68; H, 4.40; N, 3.50. Mp 212-215 °C
(dec). FAB-MS (m-NBA matrix): m/z 952 (M+, rel intensity 100%),
526 ((M - dppe - 2N)+, rel intensity 15%), 924 ((M - 2N)+, rel
intensity 10%).
(expl.). FAB-MS (m-NBA matrix): m/z 1032 (M+, rel intensity 100%),
634 ((M - dppe)+, rel intensity 68%).
Synthesis of trans-[W(O)(Br)(dppe)2](BPh4) (14). To a suspension
of 10 (150 mg, 0.139 mmol) in 10 mL of acetone was added 0.8 mL
of 48% aqueous HBr. The dioxo compound dissolves giving an orange
solution which upon heating to 50 °C (5 min) turns violet. The solution
was cooled to 0 °C and methanol (10 mL) added, followed by slow
addition of NaBPh4 (0.45 g, 1.32 mmol). The precipitate (80 mg, 41%)
of blue-violet flaky crystals was filtered of and washed with methanol
(3×). Anal. Calcd for WOBrBP4C76H68: C, 65.40; H, 4.91; Br, 5.72.
Found: C, 64.90; H, 4.90; Br, 5.70. Mp ca. 197 °C (dec). FAB-MS
(m-NBA matrix): m/z 1078 (M+, rel intensity 32%), 680 ((M - dppe)+,
rel intensity 9%).
Synthesis of trans-[W(O)(F)(dppe)2](BF4) (10). trans-W(PPh3)2-
Cl4 (2.55 g, 3.00 mmol) and dppe (3.10 g, 7.78 mmol) were suspended
in a degassed mixture of 175 mL of 2-propanol, 25 mL of 50% aqueous
HBF4, and 12 mL of 50% aqueous Pb(BF4)2. The mixture was refluxed
under N2 for 16 h. After cooling the solids were filtered off, washed
with water and diethyl ether, and then extracted with 200 mL of hot
acetone. To the cooled extract was slowly added 200 mL of 0.4 M
aqueous HBF4. Yield 1.76 g (53%) of flesh-colored needles. Anal.
Calcd for WOP4F5BC52H48: C, 56.65; H, 4.39. Found: C, 56.69; H,
4.44. Mp 285-290 °C dec. FAB-MS (glycerol matrix): m/z 1015
(M+, rel intensity 100%), 617 ((M - dppe)+, rel intensity 31%).
Synthesis of trans-[W(O)2(dppe)2]‚2CH3OH (11). 10 (1.10 g, 1.00
mmol) was dissolved in a mixture of 110 mL of acetone and 70 mL of
methanol. To the filtered solution was added 12 mL of 40% aqueous
NEt4OH solution, and the reaction mixture was heated to 55 °C for 2
h. After cooling to room temperature, 0.99 g (92%) of bright yellow
microcrystals was collected by filtration and washed with diethyl ether.
Anal. Calcd for WO4P4C54H56: C, 60.23; H, 5.24. Found: C, 59.64;
H, 5.15. Mp 121-122 °C (dec). FAB-MS (m-NBA matrix): m/z 1013
((M + H)+, rel intensity 90%), 615 ((M + H - dppe)+, rel intensity
39%).
Synthesis of trans-[W(O)(I)(dppe)2](BPh4) (15). 10 (250 mg, 0.232
mmol) and 3.2 g NaI was dissolved in 10 mL of neat HCOOH by
heating to close to the boiling point for 5 min. The reaction mixture
was cooled to 0 °C, and 10 mL of methanol was added to give a clear
solution, followed by slow addition of a solution of 0.25 g NaBPh4 in
10 mL of methanol. The precipitate was allowed to settle for 30 min
and then filtered off and washed with methanol (3×). Yield 210 mg
(63%). Anal. Calcd for WOIBP4C76H68: C, 63.27; H, 4.75; I, 8.80.
Found: C, 63.19; H, 4.70; I, 8.54. Mp ca. 201 °C (dec). FAB-MS
(m-NBA matrix): m/z 1123 (M+, rel intensity 12%), 725 ((M - dppe)+,
rel intensity 5%).
Synthesis of trans-[W(O)(NCS)(dppe)2](BPh4) (16). A solution
of 10 (100 mg, 0.093 mmol) and NH4SCN (0.62 g, 8.1 mmol) in 15
mL of methanol was refluxed for 2 h. To the resulting green solution
was slowly added NaBPh4 (0.30 g, 0.88 mmol) in methanol (10 mL).
A purple crystalline precipitate was collected by filtration and washed
with methanol (3×) and diethyl ether (2×). Anal. Calcd for
WONSBP4C77H68: C, 67.31; H, 4.99; N, 1.02. Found: C, 66.56; H,
5.01; N, 1.19. Mp 212-217 °C (dec). FAB-MS (m-NBA matrix):
m/z 1054 (M+, rel intensity 42%), 656 ((M - dppe)+, rel intensity 13%).
Synthesis of trans-[W(O)(OCH3)(dppe)2]I (17). 10 (150 mg, 0.139
mmol) was dissolved in 5 mL of CH3I. To the solution was added 5
mL of methanol, and the mixture was refluxed for 2 h, followed by
stripping off the solvent on a rotatory evaporator. The yellow product
was dissolved in 5 mL of methanol by heating. Cooling to 0 °C
followed by slow addition of 3.5 mL of H2O precipitated 123 mg (76%)
of yellow-orange flakes. Anal. Calcd for WO2P4IC53H51: C, 55.13;
H, 4.45; I, 10.99. Found: C, 54.39; H, 4.40; I, 11.38. Mp 224-226
°C (dec). FAB-MS (m-NBA matrix): m/z 1028 (M+, rel intensity
13%), 630 ((M - dppe)+, rel intensity 7%).
Synthesis of trans-[W(O)(OH)(dppe)2](ClO4) (12). To a suspen-
sion of 10 (150 mg, 0.139 mmol) in 10 mL of methanol cooled to 0
°C was slowly added 2.0 mL of 1 M HClO4. The orange-yellow
crystalline product was filtered off and washed with methanol (3×)
and diethyl ether (2×). Yield 144 mg (93%). Anal. Calcd for WO6P4-
ClC52H49: C, 56.11; H, 4.44; Cl, 3.19. Found: C, 55.55; H, 4.38; Cl,
3.05. Mp 215 °C (expl.). FAB-MS (m-NBA matrix): m/z 1014 (M+,
rel intensity 100%), 616 ((M - dppe)+, rel intensity 35%).
Synthesis of trans-[W(O)(Cl)(dppe)2](ClO4) (13). To a suspension
of 10 (155 mg, 0.144 mmol) in 5.0 mL of acetone was added 2.0 mL
of 12 M HCl. Immediately the dioxo compound dissolves and the
solution turns red-violet. Addition of 1.5 mL of 1 M HClO4, heating
to give a clear solution, filtering, and cooling to 0 °C afforded purple
crystals which were washed with ethanol (3×) and diethyl ether (2×).
Yield 110 mg (68%). Anal. Calcd for WO5P4Cl2C52H48: C, 55.13;
H, 4.28; Cl, 6.27. Found: C, 54.81; H, 4.23; Cl, 6.31. Mp 198 °C
X-ray Crystallography. Crystal structure and refinement data for
10, 11, 12, and 16 are summarized in Table 1. Details of the structure
determinations are available in the Supporting Information. The
disorder in 11 was modeled in a similar way as done for trans-[Mo(O)-

π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Inorganic Chemistry, Vol. 37, No. 23, 1998 5995
Scheme 1. Schematic Representation of the Synthetic
Routes to the Molybdenum Complexes and Labeling Scheme
for These
Figure 1. Coordinate system employed for the trans-[M(O)(X)-
(dppe)2]0/+ complexes. The orbital splitting pattern together with the
orbital population in the resulting low-spin d2 system are depicted on
the right.
(F)(Ph2PCHdCHPPh2)2](BF4) by Cotton et al.22 The O, W, and F
atoms were described by two fragments with opposite orientations:
F-W-O and O-W-F. No restraints were made on the geometries
or multiplicities of the two fragments, but common thermal parameters
were imposed on the two fragments.
For 16 all of the non-hydrogen atoms were refined with anisotropic
temperature factors, but all of the phenyl carbon atoms were restrained
to be similar. The distance between neighboring carbon atoms in the
phenyl groups was constrained to 1.39 Å. The distance from tungsten
atoms to coordinating O and N atoms were restrained to be the same
in both independent cations. The molecular structure diagrams were
made with the ORTEP-II program.23
Results and Discussion
The rational synthesis and the characterization, by means of
UV-vis, vibrational, and 31P{1H} NMR spectroscopies as well
as by single-crystal X-ray diffraction of the two parallel series
trans-[W(O)(X)(dppe)2]+, provides an unprecedented possibility
for studying the variation of π-antibonding effects in similar
systems.
outlined in Schemes 1 and 2, respectively. The generality of
the approach of replacing one oxo ligand in the neutral trans-
dioxo species with singly charged anions by substitution under
acidic conditions is clearly demonstrated by the range of
examples provided here. Yields between 40% and 90% from
the easily accessible dioxo complexes make these routes the
first general synthetically useful approach to this type of
complexes. Compared with other solvents, dilute methanol
solutions were found to give superior crystallinity and purity
of the product in the cases where tetraphenylborate salts of the
cations were isolated. However, for the iodo complexes heating
with methanol resulted in contamination with the corresponding
methoxy complexes. For these systems 2-propanol or formic
acid were suitable reaction media in the syntheses.
For strongly tetragonally compressed octahedral complexes,
like the ones in question, the orbital splitting scheme in Figure
1 is applicable. This orbital scheme was originally invoked by
Jørgensen24 and by Ballhausen and Gray25 to explain the
electronic structure of the vanadyl ion.
The low-spin d2 electronic configuration of the trans-[M(O)-
1
(X)(dppe)2]+ was confirmed by the narrow-line unshifted H,
13C, and 31P NMR spectra recorded on the complexes. Fur-
thermore, the magnetic susceptibilities of the fluoro complexes
1 and 10 were measured and gave, when corrected for
diamagnetism, a temperature independent (60-290 K) magnetic
susceptibility (TIP) of around 110 × 10-6 cgsu, which corre-
sponds to a room-temperature magnetic moment of 0.50 µB.
This value is in the range expected for low-spin d2 systems.
Although several studies26 report metal centers with this
electronic configuration to be diamagnetic, complete configu-
ration ligand-field calculations predict, irrespective of the
magnitude of the spin-orbit coupling parameter, room-tem-
perature magnetic moments of around 0.5 µB.27
By letting the known high-spin tungsten(IV) compound trans-
WCl4(PPh3)2 react with dppe and HBF4/Pb(BF4)2 in an acetone
water mixture, flesh-colored 10 was obtained. Addition of a
source of Pb2+ ions was found to be necessary to avoid
contamination of the product with large amounts of trans-
-
[W(O)(Cl)(dppe)2](BF4). The possibility of BF4 acting as a
fluoride source was also observed in the molybdenum chem-
istry.12 The synthetic pathway to 11 by a substitution-
deprotonation reaction of trans-[W(O)(F)(dppe)2]+ is similar to
the route previously described for the molybdenum analogue.12
This yellow compound is nearly insoluble in most solvents but
like its molybdenum analogue it reacts smoothly with halogen-
ated methanes.
The acid-catalyzed substitution reactions are likely to occur
via the diprotonated trans-oxo-aqua complexes. We have not
investigated this point quantitatively, but noted that the substitu-
tion rate increases with the concentration and strength of the
added acid. A dissociative mechanism, with the trans-oxo aqua
Syntheses. The synthetic pathways to the molybdenum and
tungsten series of complexes trans-[M(O)(X)(dppe)2]+ are
(22) Cotton, F. A.; Eglin, J. L.; Wiesinger, K. J. Inorg. Chim. Acta 1992,
195, 11.
(23) Johnson, C. K. ORTEP-II; Report ORNL-5138; Oak Ridge National
Laboratory, TN, 1976.
(24) Jørgensen, C. K. Acta Chem. Scand. 1957, 11, 73.
(25) (a) Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962, 1, 111. (b)
Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346.
(26) (a) Novotny, M.; Lippard, S. J. Inorg. Chem. 1974, 13, 828. (b) Fackler,
P. H.; Lindsay, M. J.; Clarke, M. J.; Kastner, M. E. Inorg. Chim. Acta
1985, 109, 39.
(27) Bendix, J. To be published.

5996 Inorganic Chemistry, Vol. 37, No. 23, 1998
Bendix and Bøgevig
Scheme 2. Schematic Representation of the Synthetic
Routes to the Tungsten Complexes and Labeling Scheme for
These
Figure 2. Electronic absorption spectra of trans-[M(O)2(dppe)2] and
trans-[M(O)(X)(dppe)2]+ (X ) OH, F, I; M ) Mo (top), W (bottom)).
All spectra were recorded in CHCl3; cf. Table 2 for spectral charac-
teristics.
complex as the only reactive species, has been established for
the substitution reactions of trans-dioxo tetracyano complexes
of Mo(IV), W(IV), Re(V), and Tc(V)28 and of trans-[Os(N)-
(H2O)(CN)4]-.29
On the other hand is the difference in reactivity between the
dioxo and the hydroxo-oxo complexes notable. The former
species thus react with neat (CH3)3SiCl within seconds at room
temperature to give the trans-[M(O)(Cl)(dppe)2]Cl (M ) Mo,
W) compounds in quantitative yield, whereas 2 h at room
temperature in neat (CH3)3SiCl leave the hydroxo complexes
unchanged.
equatorial phosphine ligands. These transitions should thus be
independent of the nature of the axial ligands.
In Figure 2 the UV-vis spectra of the trans-[M(O)(X)-
(dppe)2]0/+ species for X ) O, OH, F, and I are shown for
comparison. The remaining spectral data are summarized in
Table 2. As for the molybdenum analogue,12 we observe no
bands in the spectrum of 11 with wavelengths above 400 nm.
The spectrum of this complex was recorded immediately after
dissolving the compound. This is necessary since the compound
reacts fast with halogenated organic solvents to produce
solutions with distinctly different spectra. The featureless
spectrum of 11 contains no information regarding the energetic
ordering of the metal d2 states. For the mono-oxo species the
pronounced dependence of the position of the lowest energy
band on the nature of the axial ligands proves this transition to
be of dxy f {dyz, dzx} character.
UV-Vis Spectroscopy. For systems with d2 electronic
configuration, Figure 1 lead us to expect transitions with the
following symmetries, orbital character, and energetic order-
3
2
1
ing: 1A1(C4V) f E(C4V) (dxy f dxy, {dyz, dzx}), A1(C4V) f
2
1
3
2
1E(C4V) (dxy f dxy, {dyz, dzx}), A1(C4V) f A2(C4V) (dxy
f
1
1
2
2
2
2
2
dxy, dx -y ), and A1(C4V) f A2(C4V) (dxy f dxy, dx -y ). The
most significant difference between the transitions to states with
E-symmetry to states with A2 symmetry, respectively, is their
dependence on the ligation sphere. The transition to 1,3E states
promotes an electron from the nonbonding dxy orbital to the
degenerate set {dyz, dzx} which has π-symmetry with respect to
the axial ligands. This transition is therefore expected to depend
on the nature of the ligand trans to the oxo ligand directly as
well as through its influence on the oxo-ligand. The transitions
to the 1,3A2 states on the other hand involve shifting an electron
Importantly, the intensity of the band or band system with
lowest energy is very similar for the two metals. If the
transitions were spin forbidden, their intensity should be
proportional to the square of the spin-orbit coupling parameter.
Since the spin-orbit coupling parameters for gaseous Mo(IV)
and W(IV) are 920 and 3100 cm-1, respectively,30 it can be
concluded that the observed bands of lowest energy with molar
extinction coefficients in the range 50-150 dm3 mol-1 cm-1
2
2
to the dx -y orbital, which σ- and π-interacts only with the
1
are spin allowed (1A1 f E).
(28) (a) Roodt, A.; Leipoldt, J. G.; Helm, L.; Merbach, A. E. Inorg. Chem.
1994, 33, 140. (b) Leipoldt, J. G.; van Eldik, R.; Basson, S. S.; Roodt,
A. Inorg. Chem. 1986, 25, 4639. (c) Leipoldt, J. G.; Basson, S. S.;
Roodt, A.; Purcell, W. Transition Met. Chem. 1987, 12, 209.
(29) Van Der Westhuizen, H. J.; Basson, S. S.; Leipoldt, J. G.; Purcell,
W. Polyhedron 1994, 13, 717.
The data in Table 2 encompasses seven pairs of analogous
1
1
tungsten and molybdenum where the A1 f E transition has
(30) Bendix, J.; Brorson, M.; Scha¨ffer, C. E. Inorg. Chem. 1993, 32, 2838.

π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Inorganic Chemistry, Vol. 37, No. 23, 1998 5997
Table 2. Spectroscopic Data for Complexes
UV-vis
λ/nm (ꢀ/mol-1
IR/Raman
NMR
complex
dm3 cm-1
)
ν
MdO/cm-1
δ(31P)/ppm
1
509 (83.7)
362 (262)
-
944 (IR)
39.8 (d, 2JP-F ) 40.3 Hz)
2
3
808 (Raman) 34.9
897 (IR)
461 (82.7)
37.2
371 (300) sh
566 (64.9)
573 (64.2)
587 (83.8)
589 (112)
463 (107)
377 (324) sh
572 (104)
457 (119)
4
5
6
7
8
943 (IR)
942 (IR)
945 (IR)
945 (IR)
889 (IR)
40.3
37.4
32.8
44.6
37.6
9
10
933 (IR)
945 (IR)
42.0
31.7 (1JP-W ) 320 Hz)
(2JP-F ) 45.1 Hz)
11
12
13
14
15
16
17
-
840 (Raman) 22.5 (s,d, 1JP-W ) 328 Hz)
411 (151)
502 (81.3)
505 (90.3)
517 (84.5)
536 (73.3)
416 (181)
902 (IR)
952 (IR)
954 (IR)
952 (IR)
951 (IR)
892 (IR)
27.0 (s,d, 1JP-W ) 318 Hz)
27.6 (s,d, 1JP-W ) 314 Hz)
23.8 (s,d, 1JP-W ) 314 Hz)
17.2 (s,d, 1JP-W ) 314 Hz)
32.5 (s,d, 1JP-W ) 312 Hz)
28.2 (s,d, 1JP-W ) 318 Hz)
Figure 3. Plot showing the correspondence between the energy of
the first band maximum in the absorption spectrum and the chemical
shift of the phosphorus nuclei. The complexes have been abbreviated
by the metal and the trans-to-oxygen ligand.
of this absorption is quite similar for the two systems suggesting,
in accordance with the scheme in Figure 1, the transition to
have the orbital character dxy f dxy, dx -y . The intensity of
been observed. The ratios between the energy of the lowest
absorption band maximum in these pairs of tungsten and
molybdenum complexes fall within the narrow range 1.10-
1.14. For octahedral systems the ratio between the spectro-
chemical parameters for 5d and 4d metals is generally higher:
1.2-1.3, but with much variation.31 Our results are though in
good agreement with the trends observed for the B2 f E
transition in tetragonal d1 oxo-halide systems. This transition
energy is determined only by π-bonding effects and varies little
across and down the periodic table. The energy of this transition
increases by a factor of 1.10-1.15 [sic] upon going from one
period to the next.1
2
2
2
this absorption band when compared to that of the spin-allowed
1
bands assigned above makes us assign it to the transition A1
1
f A2.
By dissolving salts of the trans-[M(O)(OH)(dppe)2]+ ions in
acetone and adding excess of neat CF3SO3H, blue solutions with
red shifted spectra compared to the hydroxo species were
obtained (Mo, 587 nm, 85.0 mol-1 dm3 cm-1; W, 521 nm, 79.1
mol-1 dm3 cm-1). The blue species produced are believed to
be the trans-[M(O)(H2O)(dppe)2]2+ ions. The oxo-aqua for-
mulation is in line with the known34 behavior of the cyanide
complexes. However, the tautomeric di-hydroxo formulation
cannot be ruled out, as shown by the fact that the isoelectronic
trans-[Mo(NH)(OH)(dppe)2]+ upon protonation crystallizes as
a salt of the trans-[Mo(NH2)(OH)(dppe)2]2+ ion.35 The shifting
of the absorption spectrum is in accordance with a the quoted
assignment and a decreasing π-donor strength along the series
O2- > OH- > H2O.
2
2
The spectra of the tungsten species all show a first absorption
band with distinct structure. Since the 1E state cannot be split
by spin-orbit coupling, we conclude that the structure is due
1
3
to spin-orbit coupling mixing of the E and E states. The
energy difference between these states is expected to be similar
for the molybdenum and tungsten species since interelectronic
repulsion parameters generally are very similar for correspond-
ing 4dq and 5dq systems.32 This together with the much higher
value of the spin-orbit coupling parameter increases the
31P{1H} NMR Spectroscopy. 31P NMR shifts of the
complexes have been summarized in Table 2. In the spectra
of the molybdenum species only a single resonance is observed
except for the fluoro complex (1) where it is split into a doublet
due to coupling to the fluorine nucleus (2JP-F ) 40.3 Hz). In
the tungsten complexes singlets with sidebands due to coupling
1
3
intermixing of the E and E states in the tungsten complexes.
The low-energy, lower intensity components of these composite
bands are calculated to be predominantly 3E and the component
1
with highest energy to be predominantly E. It is noteworthy
that in the spectrum of trans-[Mo(O)(I)(dppe)2]+, but not in the
spectra of any of the other molybdenum complexes, there is a
clear asymmetry of the absorption band. This supports the
suggestion that spin-orbit coupling is responsible for the shape
of the tungsten spectra. Other examples are known where
transition metal iodo-complexes, such as [Cr(NH3)5I]2+, show
large spin-orbit coupling effects in formally metal centered
states.33
1
1
to the 183W nuclei (I ) /2, 14.27%) are observed. The JP-W
values range from 312 to 328 Hz, which is in the range reported
for similar systems.36 In the spectrum of 10 these three lines
are further split into doublets by a smaller coupling (45.1 Hz)
to the fluorine nucleus.
It is a noteworthy common feature of the hitherto described
trans-dioxo molybdenum and tungsten complexes that they in
addition to the strong axial π-donors have typical π-acceptors
as auxiliary equatorial ligands. The coexistence of ligands,
In the spectra of [Mo(O)(F)(dppe)2]+ and [Mo(O)(OH)-
(dppe)2]+ a second absorption band is observed. The energy
(34) (a) Robinson, P. R.; Schlemper, E. O.; Murmann, R. K. Inorg. Chem.
1975, 14, 2035. (b) Smit, J. P.; Purcell, W.; Roodt, A.; Leipoldt, J. G.
Polyhedron 1993, 12, 2271.
(35) Adachi, T.; Hughes, D. L.; Ibrahim, S. K.; Okamoyo, S.; Pickett, J.;
Yabanouchi, N.; Yoshida, T. J. Chem. Soc., Chem. Commun. 1995,
1081.
(31) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Am-
sterdam, 1984.
(32) For Rh(III) and Ir(III), see: Jørgensen, C. K. Modern Aspects of Ligand
Field Theory; North-Holland: Amsterdam, 1971. For the d2 systems
Nb(III) and Ta(III), see: Brorson, M.; Scha¨ffer, C. E. Inorg. Chem.
1988, 27, 2522.
(36) Brevard, C.; Granger, P. Handbook of High-Resolution Multinuclear
NMR; John Wiley: New York, 1981.
(33) Pedersen, E.; Toftlund, H. Inorg. Chem. 1974, 13, 1603.

5998 Inorganic Chemistry, Vol. 37, No. 23, 1998
Bendix and Bøgevig
Table 3. Calculated and Measured Intensities in the 19F NMR
Spectrum of trans-[W(O)(F)(dppe)2]+
which normally are associated with different oxidation states,
is facilitated by the geometry in these tetragonal systems where
the equatorial π-acceptors are optimally arranged to relieve the
π-electron density donated by the axial ligands. This combina-
tion of strong π-donors and π-acceptors continues to exist for
group 7 and group 8 d2 systems exemplified by systems such
as trans-[Re(O)2(dppe)2]+ 37 and trans-[Os(O)2(2,2′-bipyri-
relative intensity
calcd
exptl
100.0
68.8
20.0
1.25
100.0
68.6/69.5
20.2/20.6
1.53/1.57
dine)2]2+ 38
However, for these centers with their higher
.
oxidation states +5 and +6, the prevalence of back-bonding
equatorial ligands ceases to be the rule and π-donating equatorial
ligands, like OH- and Cl-, are also found.
have stretching frequencies between 889 and 902 cm-1, which
is distinctly lower than those in the halide and pseudo-halide
complexes (933-952 cm-1). This grouping is different from
the UV-vis spectra where fluoride was found to be different
from the other halides and intermediate between these and the
hydroxide and methoxide. The data compares well with the
literature data on similar complexes, e.g., Na3[Mo(O)(OH)-
(CN)4]‚4H2O (937 cm-1),39 [Mo(O)(Cl)(dppe)2](BPh4) (940
cm-1),40 and trans-(NMe4)3[W(O)(NCS)(CN)4]‚NaNCS (954
cm-1).41 The symmetric Raman active stretch in the trans-dioxo
complexes is found at 811 cm-1 (2) for molybdenum and 840
cm-1 (11) for tungsten. This is between those reported for
It would be interesting to examine if this coexistence of
ligands which normally stabilize widely different oxidation states
can be ascribed to inter ligand π-bonding synergy. To try to
illuminate this question we have plotted in Figure 3 the chemical
shifts of the coordinated phosphines versus the energy of the
first maximum in the absorption spectra. Representing the data
in that way exhibits a high resemblance between the distribution
patterns of the Mo and the W complexes.
A π-bonding synergy between the axial ligands and the
equatorial phosphines would be expected to result in negative
1
1
correlation between δ(31P) and the energy of the A1 f E
transition. Clearly the relation is more complicated than so.
The data for each metal fall into two groups: On one hand
those systems where the ligating atoms are second row elements,
and on the other hand the heavier (Cl, Br, I) halide complexes.
The former group exhibits a negative correlation whereas the
chloro, bromo, and iodo complexes deviate increasingly, show-
ing larger NMR shielding of the phosphorus nuclei than
expected from the spectrally determined donor strength of the
axial ligands. The grouping of thiocyanate with the second-
row donor ligands suggests it to be N-bound in these complexes.
This assessment is verified by the X-ray structural analysis of
the tungsten complex (vide infra).
NaK3[M(O)2(CN)4]‚6H2O (Mo, 783 cm-1; W, 795 cm-1 42
) and
M(O)2(CO)4 (Mo, 820 cm-1; W, 850 cm-1).9 The ordering with
higher νOdMdO symmetry for tungsten than for molybdenum is
thus found for the neutral phosphine and the carbonyl complexes
as well as for the tetraanionic cyano complexes indicating
slightly stronger tungsten-oxo than molybdenum-oxo bonds.
X-ray Crystallography. The molecular structures of the
tungsten complexes 10, 11, 12, and 16 have been determined
from single-crystal X-ray diffraction. The structures are shown
in Figures 4-7. Selected bond lengths and angles are listed in
Tables 4-7. Complete structural data are available in the
Supporting Information. The structures are similar, based on
octahedral coordination of the tungsten, with the phosphine
ligators coordinating in the equatorial plane and the oxo and
auxiliary ligand situated trans to each other. The preference
for a trans arrangement of two π-donor ligands in systems with
d2 electronic configuration has been thoroughly discussed.43
trans-[W(O)2(dppe)2]‚2CH3OH (11) is the first structurally
characterized dioxo-W(IV) complex. It is isomorphous with
the analogous molybdenum complex (2)12 crystallizing in the
triclinic space group P1h with the tungsten atom situated on a
center of inversion. The tungsten-oxygen bond length of
1.8298(11) Å is a little, but significantly, longer than the
1.8183(8) Å found for the Mo-O bond length in trans-[Mo(O)2-
(dppe)2]‚2CH3OH. Conversely, the metal-phosphorus bond
lengths are 0.010(1) Å shorter in the tungsten complex than in
the molybdenum analogue. In isoelectronic trans-[Re(O)2-
(dppe)2]+ 44 the rhenium-oxo bond length is 1.781(6) Å and the
average Re-P bond length is 2.491(3) Å almost identical to
that in 11.
From the data it is clear that axial ligands from different rows
of The Periodic Table causes very different shieldings of the
phosphorus nuclei. This result is surprising in view of the data
presented in a recent paper dealing with trans-biscalcogenide
complexes of molybdenum(IV): trans-[Mo(Q)2(PP)2] where
PP is a bidentate phosphine and Q ) O, S, Se, Te.11 For these
closely related systems it was found that there was essentially
no dependence of the δ(31P) on the axial ligands.
19F NMR Spectroscopy. The fluoro complexes show 19F
NMR spectra which consist of 5 lines at -119.8 ppm (Mo)
respectively seven lines at -136.1 ppm (W). From the 31P
NMR values for the coupling constants 1JP-W ) 320 Hz (183W,
I ) 1/2, 14.27%) and 2JP-F ) 45.1 Hz can be determined. The
19F NMR spectrum of trans-[W(O)(F)(dppe)2](BF4) consists of
a seven-line pattern with apparently one coupling constant
identical to the 45 Hz determined from the 31P NMR spectrum.
1
The seven-line spectrum is due to the ratio JF-W/2JP-F
accidentally being very close to 2. This assessment can be
verified by comparing the relative intensities of the lines in the
seven-line pattern. Calculated and experimentally determined
relative intensities are collected in Table 3. The measured and
calculated relative intensities agree well and rule out the
The tungsten-oxo bond lengths show the expected large
dependence on the trans ligand: 1.8298(11) Å (11), 1.7610(14)
(39) Dudek, M.; Kanas, A.; Samotus, A. J. Inorg. Nucl. Chem. 1979, 41,
1135.
(40) Levason, W.; McAuliffe, C. A.; Sayle, B. J. J. Chem. Soc., Dalton
Trans. 1976, 1177.
(41) Roodt, A.; Leipoldt, J. G.; Basson, S. S.; Potgeiter, I. M. Transition
Met. Chem. 1990, 15, 439.
(42) Dudek, M.; Kanas, A.; Samotus, A. J. Inorg. Nucl. Chem. 1980, 42,
1701.
1
alternative possibility that the JF-W is a higher multiplum of
2
the JP-F and some small lines are unobserved.
Vibrational Spectroscopy. The MdO stretching frequencies
are collected in Table 2. The hydroxo and methoxo complexes
(43) (a) Mingos, D. M. P. J. Organomet. Chem. 1979, 179, C29. (b)
Demachy, I.; Jean, Y. Inorg. Chem. 1997, 36, 5956 and references
therein.
(44) Meyer, K. E.; Root, D. R.; Fanwick, P. E.; Walton, R. A. Inorg. Chem.
1992, 31, 3067.
(37) Meyer, K. E.; Root, D. R.; Fanwick, P. E.; Walton, R. R. Inorg. Chem.
1992, 31, 3067.
(38) Dobson, J. C.; Takeuchi, K. J.; Pipes, D. W.; Geselowitz, D. A.; Meyer,
T. J. Inorg. Chem. 1986, 25, 2357.

π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Inorganic Chemistry, Vol. 37, No. 23, 1998 5999
Figure 4. ORTEP-II drawing and atom-numbering scheme of the
cation in 10. Displacement ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are shown as spheres of arbitrary size. Both of
the two almost equally populated independent O-W-F fragments are
shown.
Figure 6. ORTEP-II drawing and atom-numbering scheme of 12.
Displacement ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are shown as spheres of arbitrary size.
Figure 7. ORTEP-II drawing and atom-numbering scheme of the
cation in 16. Displacement ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are shown as spheres of arbitrary size.
Figure 3. The W-N bond length is 2.104(5) Å. This is
significantly shorter than the value (2.23(2) Å) found in (NMe4)3-
[W(O)(NCS)(CN)4]‚NaNCS,41 but in the range (2.090-2.205
Figure 5. ORTEP-II drawing and labeling scheme of the molecular
structure of 11. Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are shown as spheres of arbitrary
size.
Å) found in [W3S4(NCS)9]5- 45
.
The variations in the tungsten phosphorus bond lengths, albeit
small, are in accordance with the notion of inter ligand
π-bonding synergy. The average W-P bond length thus
increases along the series 11 (2.500 Å), 12 (2.521 Å), 10 (2.53
Å), and 16 (2.53 Å) contrary to what is expected from charge
considerations. The same ordering is found for the molybdenum
compounds 2 (2.510 Å),12 3 (2.536 Å),46 and 1 (2.54 Å).47
Å (12), 1.721(5) Å (16), and 1.718(9) Å (10). As discussed by
Cotton et al.11 the maximal bond order in the centrosymmetric
dioxo complex is two for symmetry reasons. With decreasing
trans-influence exerted upon the oxo-ligand the bond order
approaches three in the fluoride and isothiocyanate complexes.
An even shorter WIV-O bond length of 1.687(7) Å has been
found in trans-[W(O)(Cl)(dppe)2]+.16
(45) Shibahara, T.; Kohda, K.; Ohtsuji, A.; Yasuda, K.; Kuroya, H. J. Am.
Chem. Soc. 1986, 108, 2757.
(46) Bendix, J.; Bøgevig, A. Acta Crystallogr. (C) 1998, 54, 206.
(47) Bendix, J.; Bøgevig, A. Unpublished results.
The structure of 16 confirms that the thiocyanate is N-bonded.
A conclusion also reached from the data point distribution in

6000 Inorganic Chemistry, Vol. 37, No. 23, 1998
Bendix and Bøgevig
Table 7. Selected Bond Lengths [Å] and Angles [deg] for
Table 4. Selected Bond Lengths [Å] and Angles [deg] for
trans-[W(O)(F)(dppe)2](BF4) (10)a
trans-[W(O)(NCS)(dppe)2](BPh4) (16)
W(1)-O(1)
W(1)-F(1)
W(1)-P(3)
W(1)-P(4)
W(1)-P(2)
W(1)-P(1)
1.718(9)
2.001(7)
2.512(2)
2.5195(17)
2.522(2)
2.5582(17)
W(2)-O(2)
W(2)-F(2)
W(2)-P(2)
W(2)-P(1)
W(2)-P(3)
W(2)-P(4)
1.718(9)
2.001(7)
2.508(2)
2.5186(18)
2.544(2)
2.5586(18)
W(1)-O(1)
W(1)-N(1)
W(1)-P(2)
W(1)-P(3)
1.721(5)
2.104(5)
2.527(2)
2.536(2)
W(1)-P(1)
W(1)-P(4)
N(1)-C(9)
C(9)-S(1)
2.536(2)
2.541(2)
1.184(13)
1.593(13)
O(1)-W(1)-N(1)
O(1)-W(1)-P(2)
N(1)-W(1)-P(2)
O(1)-W(1)-P(3)
N(1)-W(1)-P(3)
P(2)-W(1)-P(3)
O(1)-W(1)-P(1)
N(1)-W(1)-P(1)
P(2)-W(1)-P(1)
177.1(3)
102.4(2)
80.5(2)
91.6(2)
85.5(2)
166.00(7)
94.9(2)
85.5(2)
78.19(8)
P(3)-W(1)-P(1)
O(1)-W(1)-P(4)
N(1)-W(1)-P(4)
P(2)-W(1)-P(4)
P(3)-W(1)-P(4)
P(1)-W(1)-P(4)
C(9)-N(1)-W(1)
N(1)-C(9)-S(1)
100.58(8)
89.1(2)
90.5(2)
100.47(8)
79.78(8)
175.95(8)
178.8(8)
175.8(11)
O(1)-W(1)-F(1)
O(1)-W(1)-P(3)
F(1)-W(1)-P(3)
O(1)-W(1)-P(4)
F(1)-W(1)-P(4)
P(3)-W(1)-P(4)
O(1)-W(1)-P(2)
F(1)-W(1)-P(2)
P(3)-W(1)-P(2)
P(4)-W(1)-P(2)
O(1)-W(1)-P(1)
F(1)-W(1)-P(1)
P(3)-W(1)-P(1)
P(4)-W(1)-P(1)
P(2)-W(1)-P(1)
178.1(4)
82.6(5)
95.6(3)
87.5(4)
91.9(4)
79.76(5)
100.6(5)
81.3(3)
176.35(9)
98.44(7)
99.2(4)
O(2)-W(2)-F(2)
O(2)-W(2)-P(2)
F(2)-W(2)-P(2)
O(2)-W(2)-P(1)
F(2)-W(2)-P(1)
P(2)-W(2)-P(1)
O(2)-W(2)-P(3)
F(2)-W(2)-P(3)
P(2)-W(2)-P(3)
P(1)-W(2)-P(3)
O(2)-W(2)-P(4)
F(2)-W(2)-P(4)
P(2)-W(2)-P(4)
P(1)-W(2)-P(4)
P(3)-W(2)-P(4)
178.3(6)
92.1(5)
89.5(3)
91.1(5)
89.1(3)
79.63(5)
97.6(5)
80.7(3)
169.83(9)
103.04(7)
95.3(5)
81.5(4)
84.6(3)
102.82(7)
173.05(10)
78.61(5)
97.80(7)
173.24(9)
78.43(5)
a O(2), W(2), and F(2) belong to the second oppositely oriented
fragment introduced to describe the disorder of these atoms.
Table 5. Selected Bond Lengths [Å] and Angles [deg] for
trans-[W(O)2(dppe)2]‚2CH3OH (11)a
W-O(1)
W-P(2)
W-P(1)
1.8298(11)
2.4955(7)
2.5065(12)
O(1)-W-O(1)i
O(1)-W-P(2)
O(1)i-W-P(2)
O(1)-W-P(1)
O(1)i-W-P(1)
P(2)-W-P(1)
P(2)i-W-P(1)
180.0
84.10(4)
95.90(4)
84.40(4)
95.60(4)
79.49(3)
100.51(3)
a Symmetry transformations used to generate equivalent atoms: (i)
-x, -y, -z.
Figure 8. Top view of trans-[W(O)2(dppe)2]‚2CH3OH (11) showing
the interactions between the terminal oxo ligand and hydrogen atoms
from three phenyl groups and the hydrogen bond to the methanol adduct
molecule. The molecule lies on a center of inversion. Only one-half of
the molecule is shown.
Table 6. Selected Bond Lengths [Å] and Angles [deg] for
trans-[W(O)(OH)(dppe)2](ClO4) (12)
W(1)-O(1)
W(1)-O(2)
W(1)-P(3)
1.7610(14)
1.9157(14)
2.5137(8)
W(1)-P(1)
W(1)-P(2)
W(1)-P(4)
2.5182(6)
2.5212(8)
2.5292(7)
O2) is a little longer (2.769(3) Å) than that found in the dioxo
complex, but like the latter it is nearly linear (169(5)°).
O(1)-W(1)-O(2) 179.29(6)
P(3)-W(1)-P(2) 173.220(16)
O(1)-W(1)-P(3)
O(2)-W(1)-P(3)
O(1)-W(1)-P(1)
O(2)-W(1)-P(1)
P(3)-W(1)-P(1)
O(1)-W(1)-P(2)
O(2)-W(1)-P(2)
87.39(5)
93.08(5)
96.57(5)
83.89(5)
P(1)-W(1)-P(2)
O(1)-W(1)-P(4)
O(2)-W(1)-P(4)
P(3)-W(1)-P(4)
79.077(19)
86.58(5)
92.98(5)
Except for the trans-dioxo compound where the tungsten atom
lies on an inversion center, the tungsten atom is raised out of
the plane spanned by the four equatorial phosphorus ligators
toward the oxo-ligand. This type of distortion is common in
octahedral systems with one strong π-donor. For six-coordinate
systems the out-of-plane distance is generally in the range 0.05-
0.30 Å.1 In five-coordinate complexes the metal out-of-plane
displacement is generally larger with the 0.82 Å in [(AsPh4)-
Mo(O)(SPh)4] representing the high end.49 This pattern of
parallel variation of the out-of-plane displacement and the
asymmetry in the donor strength of the axial ligators is also
found in the complexes in this study. The out-of-plane
displacements are 0.1069(3) Å (12), 0.107(2)/0.178(2) Å (10),
and 0.198(4) Å (16), respectively, for the three mono cations.
Often this type of deviation from orthoaxiality is discussed in
terms of the angle between the multiple-donor and the cis-
ligands. However, in the present context this measure loses it
importance due to fairly large tilt of the metal-oxo axis with
79.509(19)
98.535(19) P(1)-W(1)-P(4) 176.240(15)
99.16(5)
80.39(5)
P(2)-W(1)-P(4) 102.502(19)
In 11 a methanol molecule is hydrogen bonded to each of
the terminal oxo-ligands (Figure 5). The bridging hydrogen
atom was located in the electron density map and its position
refined. The distance between the donor (OME) and the
acceptor (O1) is 2.710 Å, rendering it a medium strength
hydrogen bond on distance-based criteria.48 In 12, a hydrogen
bond between the hydroxo ligand in the cation and the
perchlorate counterion (Figure 6) prevents the type of disorder
found in 10. The arrangement is similar to that found in the
analogous Mo compound.46 The hydrogen bond (O6‚‚‚H1-
(48) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University
Press: New York, 1997.
(49) Bradbury, J. R.; Mackay, M. F.; Wedd, A. G. Aust. J. Chem. 1978,
31, 2423.
(50) Levason, W.; Champness, N. R.; Webster, N. Acta Crystallogr. (C)
1993, 49, 1884.

π-Bonding in trans-[M(O)(X)(dppe)2]+/0 Systems
Inorganic Chemistry, Vol. 37, No. 23, 1998 6001
such as in trans-[Mo(O)(S)(dppee)2] (89.08°/88.51°).11 We have
found that the complete body of structural data can be
rationalized by a repulsive interaction between the donated
electron density from the phosphine (arsine) ligands and the
electron density in the bonds to the axial ligands. A schematic
representation of this model is given in Figure 9, which shows
how the backbone conformation of the phosphine ligands is
crucial in determining the tilt. A full account these results will
be presented elsewhere. In summary, the contacts between the
hydrogen atoms from the phenyl groups and the axial ligands
are facilitated by the tilting of the axial ligands with respect to
the P4 plane, but not the cause of it.
Conclusions
The synthesis and acid-catalyzed substitution reactions of
trans-[W(O)2(dppe)2]‚2CH3OH has shown tungsten to have a
chemistry that closely parallels that of molybdenum in com-
pounds of this type. These neutral complexes are versatile
starting materials for mono-oxo complexes. Their full potential
in this respect is not exhausted by the compounds described
here. Both 31P NMR and UV-vis spectrochemical parameters
show completely identical dependence on the auxiliary ligand
for molybdenum and tungsten. The dependence of the 31P
chemical shifts on the nature of the axial ligands is significant
and can be taken as evidence for π-bonding synergy between
π-donor and -acceptor ligands. The data for the metal-
phosphorus bond lengths corroborate the existence of such a
synergy. From the electronic spectra identical π-spectrochemi-
cal series for the two metals could be deduced: O2- > OH-
≈ CH3O- > F- > Cl- ≈ Br- > I- > NCS-. The magnitude
of energy splittings determined mainly by π-bonding effects
increases by a factor of 1.10-1.14 upon going from Mo(IV) to
W(IV).
Figure 9. Illustration of the influence of the phosphine backbone
orientations on the angle between the P4 plane and the π-donor axis.
respect to the plane spanned by the four equatorial phosphorus
donors. The angle between MdO axis and the P4 plane is
82.51(5)° (11), 82.87(5)° (12), 83.3(4)° (average) (10), and
84.5(2)° (average) (16), respectively. In trans-[Re(O)2(dppe)2]-
(ReO4) a similar angle (83.4°) between the RedO axis and the
P4 plane is found.48
When Figures 4-7 are examined, it is seen that there are
close contacts between hydrogen atoms from four to six of the
phenyl groups and the electronegative axial ligands (cf. Figure
8). The contacts are in the range 2.25-2.8 Å, which corre-
sponds to accepted values for C-H‚‚‚O hydrogen bonds.48 It
is however peculiar how the hydrogen bond acceptor is bent
away to accommodate this quite weak interaction. An examina-
tion of all structurally characterized trans-bis complexes of
bidentate phosphine and arsine complexes revealed the tilt
between the axial ligands and the P4/As4 plane to be a quite
general property. It is not only found for axial ligands which
are good hydrogen bond acceptors, but also in complexes such
as trans-[OsCl2(dppe)2] (82.10°)50 and trans-[Mo(N2)2(dppe)2]
(84.55°).51 On the other hand the tilt is absent in some
complexes with good hydrogen bond accepting axial ligands
Acknowledgment. We thank Prof. C. E. Scha¨ffer and Prof.
S. Larsen for valuable discussions. Mr. F. Hansen and Mrs. H.K.
Andersen are thanked for valuable technical assistance. We
thank Mr. A. Goebbls for measuring the magnetic susceptibili-
ties.
Supporting Information Available: Complete X-ray crystal-
lographic files in CIF format are available on the Internet only. FAB+
mass spectra of all new complexes, magnetic susceptibility data for
trans-[M(O)(F)(dppe)2](BF4) (M ) Mo, W), and details of the structure
determinations (9 pages). Ordering information is given on any current
masthead page.
(51) Uchida, T.; Uchida, Y.; Hidai, M.; Kodama, T. Acta Crystallogr. (B)
1975, 31, 1997.
IC980342U