Four-coordinate Mo(II) as (silox)2Mo(PMe3) 2 and its W(IV) congener (silox)2HW(η2- CH2PMe2)(PMe3) (silox = tBu 3SiO)

Article abstract of DOI:10.1021/ic8011958

The reduction of [(^tBu₃SiO)₂MoCl] ₂ (2₂) provided the cyclometalated derivative, (silox)₂HMoMo(κ-O,C-OSi^tBu₂CMe ₂CH₂)(silox) (3), and alkylation of 2₂ with MeMgBr afforded [(^tBu₃SiO)₂MoCH ₃]₂ (4₂). The hydrogenation of 4₂ was ineffective, but the reduction of 2₂ under H₂ generated [(^tBu₃SiO)₂MoH]₂ (5 ₂), and the addition of 2-butyne to 3 gave [(silox) ₂Mo]₂(μ:η²η²-C ₂Me₂) (6), thereby implicating the existence of [(silox)₂Mo]₂ (1₂). The addition of (silox)H to Mo(NMe₂)₄ led to (silox)₂Mo(NMe ₂)₂ (7), but further elaboration of the core proved ineffective. The silanolysis of MoCl₅ afforded (silox) ₂MoCl₄ (8) and (silox)₃MoCl₃ (9) as a mixture from which pure 8 could be isolated, and the addition of THF or PMe₃ resulted in derivatives of 9 as (silox)₂Cl ₃MoL (L = THF, 10; PMe₃, 11). Reductions of 11 and (silox)₂WCl₄ (15) in the presence of excess PMe ₃ provided (silox)₂Cl₂MPMe₃ (M = Mo, 12; W, 16) or (silox)₂HW(η²-CH₂PMe ₂)PMe₃ (14). While (silox)₂W(PMe ₃)₂ was unstable with respect to W(IV) as 14, a reduction of 12 led to the stable Mo(II) diphosphine, (silox) ₂Mo(PMe₃)₂ (17). X-ray crystal structures of 10 (pseudo-O_h), 12 (square pyramidal), and 14 and 17 (distorted T _d) are reported. Calculations address the diamagnetism of 12 and 16, and the distortion of 17 and its stability to cyclometalation in contrast to 14.

Full text of DOI:10.1021/ic8011958

Inorg. Chem. 2008, 47, 10542-10553
Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2 and Its W(IV) Congener
t
(silox)2HW(η2-CH2PMe2)(PMe3) (silox ) Bu3SiO)
David S. Kuiper,† Peter T. Wolczanski,*,† Emil B. Lobkovsky,† and Thomas R. Cundari*,‡
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853, and Department of Chemistry, Center for AdVanced Scientific Computing
and Modeling (CASCaM), Box 305070, UniVersity of North Texas, Denton, Texas 76203
Received June 27, 2008
The reduction of [(tBu3SiO)2MoCl]2 (22) provided the cyclometalated derivative, (silox)2HMoMo(κ-O,C-
OSitBu2CMe2CH2)(silox) (3), and alkylation of 22 with MeMgBr afforded [(tBu3SiO)2MoCH3]2 (42). The hydrogenation
of 42 was ineffective, but the reduction of 22 under H2 generated [(tBu3SiO)2MoH]2 (52), and the addition of 2-butyne
2
2
to 3 gave [(silox)2Mo]2(µ:η η -C2Me2) (6), thereby implicating the existence of [(silox)2Mo]2 (12). The addition of
(silox)H to Mo(NMe2)4 led to (silox)2Mo(NMe2)2 (7), but further elaboration of the core proved ineffective. The
silanolysis of MoCl5 afforded (silox)2MoCl4 (8) and (silox)3MoCl3 (9) as a mixture from which pure 8 could be
isolated, and the addition of THF or PMe3 resulted in derivatives of 9 as (silox)2Cl3MoL (L ) THF, 10; PMe3, 11).
Reductions of 11 and (silox)2WCl4 (15) in the presence of excess PMe3 provided (silox)2Cl2MPMe3 (M ) Mo, 12;
2
W, 16) or (silox)2HW(η -CH2PMe2)PMe3 (14). While “(silox)2W(PMe3)2” was unstable with respect to W(IV) as 14,
a reduction of 12 led to the stable Mo(II) diphosphine, (silox)2Mo(PMe3)2 (17). X-ray crystal structures of 10 (pseudo-
Oh), 12 (square pyramidal), and 14 and 17 (distorted Td) are reported. Calculations address the diamagnetism of
12 and 16, and the distortion of 17 and its stability to cyclometalation in contrast to 14.
Introduction
electronic features prevented generation of the typical
metal-metal triply bonded compounds, whose chemistry
comprises a significant fraction of the coordination chemistry
of this oxidation state in group 6.7,8
Similar to molybdenum and tungsten(III), low-coordinate,
mononuclear Mo(II) and W(II) compounds are difficult to
prepare due to their propensity to form metal-metal bonds.
Quadruple-bonded complexes readily form, and these occupy
a historically important niche in inorganic coordination
chemistry due to the presence of δ-bonding.7,9 While the
use of the silox (tBu3SiO) ligand has enabled the synthesis
Recent investigations of group 5 niobium and tantalum
species,1,2 and group 6 molybdenum and tungsten com-
pounds,3,4 have focused on low-coordinate d2 and d3
5,6
2
complexes that reveal the influence of ndz /(n + 1)s mixing
on structure and reactivity. Key syntheses of trigonal
monoprismatic (silox)3MoCl and squashed tetrahedral
(silox)3WCl3 led to the preparation of (silox)3MPMe3 (M )
t
Mo, W; silox ) Bu3SiO), (silox)3Mo, and related C3V
derivatives including (silox)3MP (M ) Mo, W).4 The M(III)
complexes are particularly interesting because steric and
(5) (a) Firman, T. K.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 11728–
11742. (b) Landis, C. R.; Cleveland, T.; Firman, T. K. J. Am. Chem.
Soc. 1998, 120, 2641–2649. (c) Landis, C. R.; Firman, T. K.; Root,
D. M.; Cleveland, T. J. Am. Chem. Soc. 1998, 120, 1842–1854. (d)
Weinhold, F.; Landis, C. R. Valency and Bonding; Cambridge
University Press: Cambridge, U.K., 2005. (e) For a convenient plot
of orbital energies vs Z, see Figure 15.39 in: Oxtoby, D. W.; Gillis,
H. P.; Nachtrieb, N. H. Principles of Modern Chemistry, 5th ed.;
Thomson Brooks/Cole: Florence, KY, 2002.
(6) Pyykko¨, P. Chem. ReV. 1988, 88, 563–594.
(7) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms;
Oxford University Press: New York, 1993.
(8) (a) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419–425. (b) Chisholm,
M. H.; Cotton, F. A. Acc. Chem. Res. 1978, 11, 356–362.
(9) Cotton, F. A.; Nocera, D. G. Acc. Chem. Res. 2000, 33, 483–490.
* Authors to whom correspondence should be addressed. E-mail:
ptw2@cornell.edu (P.T.W.), tomc@unt.edu (T.R.C.).
†
Cornell University.
University of North Texas.
‡
(1) Veige, A. S.; Slaughter, L. M.; Lobkovsky, E. B.; Wolczanski, P. T.;
Matsunaga, N.; Decker, S. A.; Cundari, T. R. Inorg. Chem. 2003, 42,
6204–6224.
(2) Hirsekorn, K. F.; Hulley, E. B.; Wolczanski, P. T.; Cundari, T. R.
J. Am. Chem. Soc. 2008, 130, 1183–1196.
(3) Kuiper, D. S.; Douthwaite, R. E.; Mayol, A.-R.; Wolczanski, P. T.;
Lobkovsky, E. B.; Cundari, T. R.; Lam, O. P.; Meyer, K. Inorg. Chem.
2008, 47, 7139–7153.
(4) Kuiper, D. S.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari, T. R.
J. Am. Chem. Soc. 2008, 130, 12931-12943.
10542 Inorganic Chemistry, Vol. 47, No. 22, 2008
10.1021/ic8011958 CCC: $40.75  2008 American Chemical Society
Published on Web 10/21/2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
Scheme 1
of numerous low-coordinate, low-valent compounds,
“(silox)2M” (M ) Mo, W) is unlikely to be a stable entity,
and even the quadruple-bonded “[(silox)2M]2” dimers are
probably too reactive to isolate on the basis of previous W2
chemistry. Efforts at generating [(silox)2W]2 resulted in
cyclometalation to afford (silox)2HWW(κ-O,C-OSitBu2-
CMe2CH2)(silox),10 and the transient dimer is also the likely
reactive species that degrades ethylene to [(silox)2W]2(µ-
CH)2 and dihydrogen via [(silox)2W]2(µ-CH)(µ-CH2)(µ-H).11
The first-row group 6 analogue, “(silox)2Cr”, exists as the
siloxide-bridged dimer, [(tBu3SiO)Cr]2(µ-OSitBu3)2;12 hence,
there is modest precedent for stabilizing group 6 M(II)
centers in dimeric form.
During these studies, the use of PMe3 has been extremely
advantageous in trapping low-valent silox-containing inter-
mediates prior to degradation or metal-metal bond forma-
tion.1,4,13,14 Herein, efforts to generate low-coordinate, low-
valent group 6 M(II) compounds are described, including
efficient syntheses of critical starting materials, the successful
generation of (silox)2Mo(PMe3)2, and an analysis of its
electronics via computation.
unfortunately resulted in the cyclometalated derivative,
(silox)2HMoMo(κ-O,C-OSitBu2CMe2CH2)(silox) (3), which
was isolated as a green-brown solid in 90% yield. 1H NMR
spectra of 3 revealed three silox groups, two singlets for the
remaining tert-butyl groups, singlets for two Me groups,
diastereotopic methylene hydrogens, and a hydride at δ
14.30, as Table 1 indicates.
Crystallization of crude 3 proved unsuccessful; hence,
other methods of preparation were attempted. The addition
of 2.0 equiv of MeMgBr in Et2O to [(tBu3SiO)2MoCl]2 (22),
with subsequent filtration and recrystallization from pentane,
afforded the purple dimethyl compound, [(tBu3SiO)2MoCH3]2
(42), in 85% yield. Its gauche C2 symmetric structure3,10 was
substantiated by the presence of two inequivalent silox
1
resonances in the H NMR spectrum (δ 1.19, δ 1.28),
equivalent methyl resonances (δ 3.48), and related 13C{1H}
NMR spectral signals. The stoichiometric hydrogenation of
42 to (silox)2HMoMo(κ-O,C-OSitBu2CMe2CH2)(silox) (3)
and 2 equiv of MeH could not be affected, and 42 proved to
be unreactive to 1 atm of H2 at 23 °C. Upon extended
thermolysis under excess H2 at 70 °C, evidence of a dihydride
was present, but continued heating or higher temperatures
resulted in degradation.
Results
2. Trapping of [(silox)2Mo]2. The reduction of [(tBu3SiO)2-
MoCl]2 (22) with 2.0 equiv of Na/Hg in the presence of 1
atm of H2 in THF resulted in the formation of reddish-brown
Approaches to [(silox)2Mo]2. 1. Reduction and Methyl-
ation of [(silox)2MoCl]2. Although the putative D2d
[(silox)2W]2 transient dimer proved too reactive to isolate,10
the lowered proclivity of Mo(II) to oxidize via CH oxidative
addition was enough incentive to consider the synthesis of
[(silox)2Mo] (12). The synthesis of gauche C2 [(tBu3SiO)2-
MoCl]2 (22) from MoCl3 and Na(silox) has been previously
described,3 and this was deemed an attractive starting
material. As Scheme 1 illustrates, the reduction of 22
1
[(tBu3SiO)2MoH]2 (52). Two silox singlets in the H NMR
spectrum again revealed gauche C2 symmetry of the dihy-
dride,10 and an additional resonance at δ 16.85 characterized
the MoH units (Table 1). A strong ν(MoH) was observed at
1965 cm-1 in the infrared spectrum of 52. In contrast to the
tungsten analog,10 52 could not be isolated by recrystallization
from various hydrocarbon and ethereal solvents, and the
presence of ∼5% tBu3SiONa, ∼3% tBu3SiOH, and residual
THF were typically evident in its 1H NMR spectrum.
Unfortunately, the percentages of these impurities did not
change when the reaction times were varied (3-48 h).
In an NMR tube experiment, (silox)2HMoMo(κ-O,C-
OSitBu2CMe2CH2)(silox) (3) was explored as an alternative
means to generating the putative [(silox)2Mo]2 (12). With 1
equiv of 2-butyne as a trapping reagent, thermolysis at 100
(10) Miller, R. L.; Lawler, K. A.; Bennett, J. L.; Wolczanski, P. T. Inorg.
Chem. 1996, 35, 3242–3253.
(11) Chamberlin, R. L. M.; Rosenfeld, D. C.; Wolczanski, P. T.; Lobkovsky,
E. B. Organometallics 2002, 21, 2724–2735.
(12) Sydora, O. L.; Kuiper, D. S.; Wolczanski, P. T.; Lobkovsky, E. B.;
Dinescu, A.; Cundari, T. R. Inorg. Chem. 2006, 45, 2008–2021.
(13) Rosenfeld, D. C.; Wolczanski, P. T.; Barakat, K. A.; Buda, C.; Cundari,
T. R.; Schroeder, F. C.; Lobkovsky, E. B. Inorg. Chem. 2007, 46,
9715–9735.
(14) Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E. B. J. Chem. Soc.,
Chem. Comm. 2001, 2734–2735.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10543

Kuiper et al.
a
1
Table 1. H, 13C{1H}, and 31P NMR Spectral Assignments for Diamagnetic Group 6 Siloxide Complexes
1H NMR (δ (J(Hz)), assmt)
13C{1H} NMR (δ,(J(Hz) assmt))
compound
[(silox)2MoCl]2 (22)b
tBu
R/H etc.
SiC
SiC(CH3)3
R/H etc.
1.25
1.27
1.13
1.24
1.34
23.98
24.53
24.42
24.06
23.21
30.98
31.67
30.74
31.23
31.24
(silox)2HMoMo(silox)(κ-O,C-SitBuCMe2CH2) (3)
1.27, 1.37 (tBu)
23.94, 23.40 (SiC)
23.85 (CMe2)
29.89 (MoCH2)
31.66, 31.93 (C(CH3)3)
34.24, 34.43 (C(CH3)2)
48.27 (Me)
1.54, 1.56 (Me)
3.78 (d, 11, CHH)
3.86 (d, 11, CHH)
14.30 (MoH)
[(silox)2MoCH3]2 (42)
1.19
1.28
1.22
1.31
1.05
1.35
1.21
1.28
1.25
1.31
1.35
3.48 (Me)
24.18
24.66
23.20
23.97
31.38
31.41
30.79
31.40
[(silox)2MoH]2 (52)c
16.85 (H)
[(silox)2Mo]2(µ:η2,η2-C2Me2) (6)
3.17 (Me)
3.27 (NMe2)
(silox)2Mo(NMe2)2 (7)
22.85
26.11
24.99
24.00
24.24
30.15
30.69
30.95
31.46
31.57
60.56 (N(CH3)2)
(silox)2MoCl4 (8)
(silox)2MoCl2(PMe3) (12)d
(silox)2HW(η2-CH2PMe2)PMe3 (14)g
1.06 (d, 11,e PMe3)
18.31 (32,f PMe3)
19.63 (br, PMe)
25.35 (21,f PMe)
26.14 (32,f PMe3)
1.25 (d, 10,e PMe)
1.26 (d, 8,e PMe3)
1.34 (d, 10,e PMe)
2.35 (ddd, 7, 7,e 5,e CHH)
4.83 (dd, 87,e 27,e 53,h WH)
5.05 (ddd, 14,e 7,e 7, CHH)
(silox)2WCl4 (15)b
1.38
1.29
27.23
24.96
32.55
30.95
d
(silox)2WCl2(PMe3) (16)i
1.18 (d, 12,e PMe3)
31.06 (36,f PMe3)
31P NMR δ -6.35. e JPH f JPC
a Benzene-d6 unless otherwise noted; JHH unless otherwise noted. b Taken from ref 3. c IR (nujol), n(MoH) ) 1965 cm-1
. . .
31P{1H} NMR δ -3.01 (d with satellites, JPP ) 8, JWP ) 204), -33.16 (d with satellites, JPP ) 7, JWP ) 178). h From tungsten satellites, this is JWH 31P
.
g
i
NMR δ -68.13 (JPW ) 610).
°C for 48 h produced only 80% [(silox)2Mo]2(µ:η2η2-C2Me2)
(6), according to 1H NMR analysis (K ∼ 460 M-1; ∆G ° ∼
-4.5 kcal/mol). 2-Butyne-bridged 6 possesses a C2 twist that
is observed via two silox resonances in its 1H NMR spectrum,
just like its tungsten analogue.11 Along with the formation
of [(tBu3SiO)2MoH]2 (52) from a reduction of [(tBu3SiO)2-
MoCl]2 (22), these results strongly suggest that [(silox)2Mo]2
(12) is a viable species, but unstable with respect to its
cyclometalation product, 3. Further trapping reactions with
alkynes and olefins were not attempted with 3, nor were any
attempts made to isolate pure 6. Unlike the alkyne scission
that occurs in the thermolysis of [(silox)2W]2(µ:η2η2-C2Me2)
at 80 °C,11 6 remained thermally stable to 120 °C.
Precursors to the (silox)2Mo Core. 1. (silox)2Mo-
(NMe2)2. The aforementioned experiments suggested that
[(silox)2Mo]2 (12) was not stable, and that cleavage of
multiply bonded dimolybdenum complexes was unlikely to
be a successful approach toward Mo(II) species. As a
consequence, routes derived from mononuclear starting
materials were sought. Despite its potential, the Mo(IV)
tetraamide, Mo(NMe2)4, has not been routinely used as a
starting material because its original synthesissfrom MoCl5
and LiMe2sresults in ∼10% yield and requires separation
from [(Me2N)3Mo]2.15,16 The use of Mo(IV) starting materi-
als MoCl4(THF)2 and MoCl4(DME) gave modestly better
yields (∼10-15%), and the use of Zr(NMe2)4 and Sn(NMe2)4
as nonreductive amide sources led to mostly diamagnetic
product mixtures. The best procedure was determined to be
the direct treatment of MoCl4(OEt2)2 with a stoichiometric
amount of LiNMe2 in Et2O, which afforded 25% purple
Mo(NMe2)4 upon sublimation. The treatment of Mo(NMe2)4
with 2 equiv of (silox)H in pentane provided (silox)2Mo-
(NMe2)2 (7) as a maroon diamagnetic solid in 70% yield
(eq 1).
pentane
Mo(NMe2)4 + 2HOSitBu3
8
23 °C, 30 min
(silox)2Mo(NMe2)2 + HNMe2 (1)
7
Somewhat surprisingly, (silox)2Mo(NMe2)2 (7) was re-
markably unreactive toward NH3, H2PPh, 2,6-Ph2-C6H3NH2,
and RNH2 (R ) adamantly, SitBu3) at 150 °C in C6D6. The
diamide did react with MeI and TMSCl in C6D6, but only
TMS-NMe2 could be identified among many paramagnetic
products in each reaction. As a consequence, molybdenum
halides were revisited as potential starting materials.
2. bis-Silox Derivatives from MoCl5. As in Weiden-
bruch’s original report of silox-containing transition metal
complexes,17 the silanolysis of MoCl5 was explored. Treat-
ment of MoCl5 with (silox)H in refluxing CCl4 led to
(silox)2MoCl4 (8) and (silox)2MoCl3 (9) in proportions that
were highly dependent on the purity of the initial halide and
the length of the reaction. During a 2 h procedure with an
Ar purge, the ratio of 8/9 was roughly 1:4 on the basis of
1H NMR integration of a singlet corresponding to a
diamagnetic product at δ 1.28 in relation to a broad (ν1/2
∼
35 Hz) singlet at δ 1.36. Extended periods (>6 h) provided
almost exclusively the Mo(VI) tetrachloride 8 in preference
to the Mo(V) trichloride 9, as shown in Scheme 2. Presum-
ably, extended reaction times permitted a slower oxidation
of 9 by CCl4 to occur. Unfortunately, the greater reflux times
(15) Bradley, D. C.; Chisholm, M. H. J. Chem. Soc. A 1971, 2741–2744.
(16) Chisholm, M. H.; Cotton, F. A.; Extine, M. W. Inorg. Chem. 1978,
17, 1329–1332.
(17) Weidenbruch, M; Pierrard, C.; Pesel, H. Z. Naturforsch. 1978, 33B,
1468–1471.
10544 Inorganic Chemistry, Vol. 47, No. 22, 2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
Scheme 2
Table 2. Crystallographic Data for (silox)2MoCl3(THF) (10), (silox)2MoCl2(PMe3) (12), (silox)2HW(η2-CH2PMe2)PMe3 (14), and (silox)2Mo(PMe3)2
(17)
10a
12
14
17
formula
fw
space group
Z
C28H62O3Si2Cl3Mo
705.27
C2/c
C27H63O2Si2Cl2PMo
673.79
P21/n
C30H72O2Si2P2W
766.88
P1
C30H72O2Si2P2Mo
678.94
P21/n
j
4
4
2
4
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
8.7262(8)
15.4639(17)
27.145(2)
90
96.374(3)
90
3640.3(6)
1.287
0.672
8.5082(6)
15.5481(10)
26.5089(17)
90
97.070(3)
90
3480.1(4)
1.286
0.667
8.6908(9)
13.0276(16)
17.333(2)
99.970(6)
94.754(6)
101.066(6)
1883.2(4)
1.352
3.239
173(2)
0.71073
R1 ) 0.0438
wR2 ) 0.0957
R1 ) 0.0660
wR2 ) 0.1017
1.006
9.2725(4)
28.0138(13)
15.0447(7)
90
90.252(3)
90
3907.9(3)
1.152
0.501
rcalcd, g cm-3
µ, mm-1
temp, K
λ (Å)
173(2)
173(2)
0.71073
173(2)
0.71073
R1 ) 0.0340
wR2 ) 0.0841
R1 ) 0.0516
wR2 ) 0.0876
1.102
0.71073
R1 ) 0.0572
wR2 ) 0.1191
R1 ) 0.0684
wR2 ) 0.1251
1.008
R indices [I > 2σ(I)]b c
R1 ) 0.0246
wR2 ) 0.0647
R1 ) 0.0320
wR2 ) 0.0662
1.124
,
R indices (all data)b c
,
GOFd
a Half of a molecule of 10 constitutes an asymmetric unit. b R1 ) ∑|Fo| - |Fc|/Σ|Fo|. c wR2 ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2
.
d GOF (all data) ) [∑w(|Fo| -
2
|Fc|)2/(n - p)]1/2, n ) number of independent reflections, p ) number of parameters.
also increased the amount of an impurity, which was
spectroscopically identified as (silox)2Mo(O)2. It is not
angles are given in Table 3. Figure 1 shows that 10 is a
nearly perfect octahedron with one chloride ligand and the
THF opposite one another (180° by symmetry) and a set of
trans-silox groups (179.42(8)°). The remaining trans-
chlorides (172.86(2)°) are tipped slightly toward the THF
(86.43(2)°), and the corresponding Cl1-Mo-Cl2 angles are
93.57(2)°. All remaining angles are within 1.2° of 90°, and
the bond distances are normal for Mo(V), with a d(MoO) of
1.8348(12) Å and 2.3429(7) Å that describe the silox and
THF interactions, respectively. The chloride opposite THF
is slightly closer to Mo (2.3429(7) Å) than the others
(2.369(2) Å), as expected from trans-influence arguments.
18
certain whether the dioxo species is derived from impure
starting material or whether the copious HCl produced as a
byproduct helps dehydrate the glass reaction vessel, but it
does limit the isolated yield of 8 to ∼40%. The tungsten
analogue, (silox)2WCl4, was produced in similar fashion from
WCl6, and (silox)2WCl3 can be generated from it via
reduction.3
Attempts to separate (silox)3MoCl3 (9) from (silox)2MoCl4
(8) via fractional crystallization failed in most solvents, but in
THF, orange-red crystals of
a
THF adduct, mer-
(silox)2MoCl3(THF) (10), were produced in 40% yield. A more
efficient means of producing a Mo(V) starting material involved
treatment of the 8/9 mixture with PMe3 to precipitate the nearly
insoluble phosphine adduct, (silox)2MoCl3(PMe3) (11), from
benzene in ∼75% yield on the basis of MoCl5.
3. Structure of mer-(silox)2MoCl3(THF) (10). Table 2
provides the X-ray crystallographic data pertaining to mer-
(silox)2MoCl3(THF) (10), while pertinent bond distances and
4. Synthesis of (silox)2MoCl2(PMe3) (12). The phosphine
adduct, (silox)2MoCl3(PMe3) (11), proved to be a convenient
starting material. The reduction of 11 with Na/Hg in diethyl
ether with PMe3 present (∼3 equiv gave optimum yields)
afforded the diamagnetic blue Mo(IV) complex, (silox)2Mo-
1
Cl2(PMe3) (12), in 90% yield according to eq 2. In the H
NMR spectrum of 12, the singlet corresponding to the silox
was at δ 1.25, and a doublet attributed to the PMe3 was
observed at δ 1.06 (JPH ) 11 Hz). The 13C{1H} NMR
(18) Rosenfeld, D. C.; Kuiper, D. S.; Lobkovsky, E. B.; Wolczanski, P. T.
Polyhedron 2006, 25, 251–258.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10545

Kuiper et al.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
mer-(silox)2MoCl3(THF) (10), (silox)2MoCl2(PMe3) (12),
(silox)2HW(η2-CH2PMe2)PMe3 (14), and (silox)2Mo(PMe3)2 (17)
mer-(silox)2MoCl3(THF) (10). Taking the chlorides along the
x axis, this lengthening may manifest 2e- occupation of the
Cl f Mo dπ* bonding dxz orbital, rather than the π*
component of the stronger π-donor siloxides (dyz). The
phosphine has a slight lean away from Cl2 (P1-Mo-Cl2
) 93.08(2)°, P1-Mo-Cl1 ) 89.81(2)°), and the corre-
sponding asymmetry in the C-P1-Mo angles (110.85(5)°,
112.08(5)°, and 117.93(5)°) is consistent with slight σ/π
mixing to maximize Mo-P bonding.4,13 The d(MoO) aver-
age is 1.834(3) Å, essentially the same as in 10, and the
d(MoP) is normal at 2.3933(3).
10a
12b
14c
17d
M-O1
1.8348(12) 1.8317(7) 1.918(3)
1.8358(7) 1.914(3)
2.1676(17)
1.975(3)
1.943(3)
M-O2
Mo-O(THF)
Mo-Cl1
2.3689(5)
2.3429(7)
2.4277(3)
2.4424(3)
Mo-Cl2
M-P1
2.3933(3) 2.3537(13) 2.3374(13)
2.3713(13) 2.3330(13)
2.235(6)
M-P2
W-C1
(P-C)av
1.802(7)
1.7066(13) 1.691(3)
1.801(38)
1.638(6)
1.806(7)
1.616(5)
(O-Si)ave
O1-M-O1′/2
O1-M-O(THF)
O1-Mo-Cl1
O-Mo-Cl1′
O1-Mo-Cl2
179.42(8)
90.29(4)
91.18(4)
88.85(4)
89.71(4)
157.91(3) 104.87(13) 136.67(13)
Approaches to the (silox)2W Core. 1. Via (silox)3-
WCl (13). The W(III) PMe3 complex (silox)3WPMe3 was
prepared via Na/Hg (∼1 equiv) reduction of (silox)3WCl (13)
in moderate yield (∼40%),4 but 13 was discovered to be
prone to overreduction. The product of a controlled reduction
using 2 equiv of Na/Hg in the presence of 10 equiv of PMe3
was the red hydride (silox)2HW(η2-CH2PMe2)PMe3 (14; eq
3), which was isolated in only 15% yield via fractional
crystallization despite being present in ∼90% yield according
90.43(3)
89.28(3)
Cl1-Mo-O(THF) 86.43(2)
Cl2-Mo-O(THF) 180
O2-Mo-Cl1
90.48(3)
88.71(3)
O2-Mo-Cl2
Cl1-Mo-Cl1′
Cl1-Mo-Cl2
P1-M-O1
P1-M-O2
P2-M-O1
P2-M-O2
P1-M-P2
P1-Mo-Cl1
P1-Mo-Cl2
O1-W-C1
O2-W-C1
P1-W-C1
P2-W-C1
W-P1-C1
W-P1-C2
W-P1-C3
W-C1-P1
M-O1-Si1
M-O2-Si2
172.86(2)
93.57(2)
177.11(2)
100.57(3)
101.49(2) 132.79(9)
134.62(9)
96.84(9)
96.85(10)
111.74(10)
99.62(10)
1
to H NMR analysis. Its origin is likely to be the transient
96.83(10) 110.14(10)
96.74(5)
93.10(5)
(silox)2W(PMe3)2 (15) complex, which is expected to be
susceptible to CH bond oxidative addition to its W(II) center.
89.81(2)
93.08(2)
132.33(19)
91.04(19)
45.47(17)
85.56(18)
63.7(2)
136.4(3)
118.23(18)
70.8(2)
171.86(8)
163.57(5) 157.2(2)
158.85(5) 167.2(2)
152.2(2)
175.1(2)
a The apical chloride is Cl2. b The Mo-P1-C’s are 111.85(5)°,
112.08(5)°, and 117.93(5)°. c The W-P2-C angles are 114.1(2)°,
122.1(3)°, and 120.8(4)°; the C-P2-C angles are 90.1(4)°, 97.1(4)°,
and 105.7(5)°; the C-P1-C angles are 109.7(4)°, 111.4(3)°, and
104.5(4)°. d The Mo-P1-C angles are 114.0(2)°, 114.9(2)°, and 123.8(2)°,
and the Mo-P2-C angles are 114.8(2)°, 118.8(2)°, and 120.6(2)°.
spectrum was consistent with this formulation, and the 31P
NMR spectrum showed a single resonance at δ -6.35.
Figure 1. Molecular view of mer-(silox)2MoCl3(THF) (10).
(silox) MoCl PMe . Na ⁄ Hg, 3PMe , - NaCl8
3
(
3
)
3
2
Et2O, 23 °C, 4 h
11
(silox) MoCl PMe (2)
(
)
2
2
3
12
5. Structure of (silox)2MoCl2(PMe3) (12). Structural
details of (silox)2MoCl2(PMe3) (12) can be found in Table
2, and core bond distances and angles are provided in Table
3. As Figure 2 illustrates, 12 is a square-pyramidal molecule
with PMe3 in the apical position, and trans-silox and trans-
chlorides occupying the basal positions. There is a modest
distortionspresumably stericsfrom
a perfect square
pyramid as the O-Mo-P angles average 101.0(7)°, while
the Cl-Mo-P angles are 89.81(2)° and 93.08(2)°. All
of the O-Mo-Cl angles within the base average 89.7(9)°,
and the O-Mo-O and Cl-Mo-Cl angles are 157.91(3)°
and 177.11(2)°, respectively. The d(MoCl) averages 2.435(10)
Å, which is >0.06 Å longer than the d(MoCl) average in
Figure 2. Molecular structure of (silox)2MoCl2(PMe3) (12).
10546 Inorganic Chemistry, Vol. 47, No. 22, 2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
There is ample precedent for CH bond activation by W(II)
in this manner.19-22
2Na ⁄ Hg, 10PMe3,
(silox)3WCl. - NaCl, - Na(silox)8 (silox)2HW(η2-CH2PMe2)PMe3
Et2O, 24h, 23 °C
14
13
(3)
The composition of the cyclometalated product, (silox)2-
HW(η2-CH2PMe2)PMe3 (14), was initially determined by
NMR spectroscopy, with the aid of correlation spectroscopy
and heteronuclear multiple-quantum coherence experiments.
1
Inequivalent silox groups were observed in the H NMR
spectrum in addition to diastereotopic methylene and PMe
signals corresponding to the η2-CH2PMe2 ligand. The hydride
signal was located at δ 4.83 with 87 and 27 Hz coupling to
the phosphorus atoms and JWH ) 57 Hz, as revealed by
tungsten satellites (183W, I ) 1/2, 14.3%); a ν(WH) of 1820
cm-1 was observed in the IR spectrum of 14.
Figure 3. Molecular view of (silox)2HW(η2-CH2PMe2)PMe3 (14) with
hydride placed in most sterically probable location.
(CO){trans-(Ph2MeP)2}WP(2,4,6-tBu3-C6H2) by Cowley et
al.23 This may be an indication of the significant 5dz /6s
2
2. (silox)2WCl4 (15) and (silox)2WCl2(PMe3) (16). The
separation difficulties of the process described above, and
the low efficiency of removing a siloxide group, prompted
mixing common to third row transition elements, since a
hybrid orbital of this type would be utilized in bonding to
the apical position of the presumed square pyramid. While
(silox)2WCl2(PMe3) (16) can be used as a precursor to
(silox)2HW(η2-CH2PMe2)PMe3 (14), it is even more efficient
to reduce (silox)2WCl4 directly, as eq 5 indicates. With 4
equiv of Na/Hg in the presence of 3 equiv of PMe3 in Et2O,
14 was cleanly produced in 85% yield.
an effort toward
a
better route to (silox)2HW(η2-
CH2PMe2)PMe3 (14). As previously discovered, silanolysis
of WCl6 can be used to produce (silox)2WCl4 (15) in 90%
isolated yield, and it can be efficiently utilized as a starting
material.
3. Structure of (silox)2HW(η2-CH2PMe2)PMe3 (14).
Data collection information pertaining to (silox)2HW(η2-
CH2PMe2)PMe3 (14) can be found in Table 2, and its
distances and angles are listed in Table 3. As Figure 3
illustrates, 14 has a solid-state configuration consistent with
the aforementioned NMR spectrum, including inequivalent
siloxides that are adjacent (O1-W-O2 ) 104.87(13)°) to
one another, and a W(η2-CH2PMe2) moiety that is roughly
parallel to one WOSi unit (O2-W-P1 ) 132.79(9)°,
C1-W-O2 ) 91.04(19)°), and perpendicular to the other
(O1-W-P1 ) 96.84(9)°, C1-W-O1 ) 132.33(19)°) and
the PMe3 ligand (P1-W-P2 ) 96.74(5)°, C1-W-P2 )
85.56(18)°). Ignoring the hydride, which could not be
located, and counting the η2-CH2PMe2 ligand as a single
“site”, the structure adopts a pseudo-Td geometry with
O1-W-P2 opened to 134.62(9)° to accommodate the PMe2
fragment. With the η2-CH2PMe2 ligand (W-P1-C1 )
63.7(2)°, P1-C1-W ) 70.8(2)°, and C1-W-P1 )
45.47(17)°) taken as the apex, the hydride, when placed
according to “best sterics” criteria, resides roughly equidistant
from the PMe3 (O2-W-P2 ) 96.83(10)°) and the silox
ligands (O1-W-O2 ) 104.87(13)°). The distances in the
structure appear normal, with a d(WP) of 2.3713(13) Å for
the PMe3 group, slightly longer than the 2.3537(13) Å of
the η2-CH2PMe2 ligand (d(WC) ) 2.235(6) Å), and d(WO)’s
of 1.914(3) and 1.918 (3) Å.
(silox)2WCl4. 2Na ⁄ Hg, 3PMe , - 2NaCl8 (silox)2WCl2(PMe3)
3
Et2O, 23 °C, 4h
15
16
(4)
(silox)2WCl4. 4Na ⁄ Hg, 3PMe , - 4NaCl8
3
Et2O, 23 °C, 12h
15
(silox)2HW(η2-CH2PMe2)PMe3 (5)
14
The reduction of 15 with 2 equiv of Na/Hg in the presence
of 3 equiv of PMe3 in diethyl ether generated the purple
W(IV) (silox)2WCl2(PMe3) (16) complex in 72% yield, as
shown in eq 4. The structure of diamagnetic 16 is assumed
to be analogous to its Mo congener (12) and its spectral
parameters are listed in Table 1. One very unusual spectro-
scopic parameter was noted. The JWP, as evidenced via
satellite resonances (183W, I ) 1/2, 14.3%), is 610 Hz, a value
considerably higher than that of (silox)3WP (1JWP ) 162
Hz),4 and only slightly less than the 1JWP of 649 Hz reported
for the nearly linear (168.2(2)°) phosphinidene (cis-Cl2)
(19) (a) Green, M. L. H.; Parkin, G.; Mingquin, C.; Prout, K. J. Chem.
Soc., Chem. Commun. 1984, 1400–1402. (b) Green, M. L. H.; Parkin,
G.; Chen, M.; Prout, K. J. Chem. Soc., Dalton Trans. 1986, 2227–
2236. (c) Green, M. L. H.; Parkin, G.; O’Hare, D.; Wong, L. L.;
Derome, A. E. J. Organomet. Chem. 1986, 317, 61–68.
(20) (a) Buccella, D.; Tanski, J. M.; Parkin, G. Organometallics 2007, 26,
3275–3278. (b) Buccella, D.; Parkin, G. J. Am. Chem. Soc. 2006, 128,
16358–16364. (c) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.;
Parkin, G. J. Am. Chem. Soc. 2003, 125, 1403–1420.
(21) (a) Gibson, V. C.; Grebenik, P. D.; Green, M. L. H. J. Chem. Soc.,
Chem. Commun. 1983, 1101–1102. (b) Gibson, V. C.; Graimann, C. E.;
Hare, P. M.; Green, M. L. H.; Bandy, J. A.; Grebenik, P. D.; Prout,
K. J. Chem. Soc., Dalton Trans. 1985, 2025–2035.
The (silox)2Mo Core. 1. (silox)2Mo(PMe3)2 (17) via
Reduction. Although W(II) was not stabilized by a combi-
nation of silox and PMe3, precedent suggested that a related
(22) Hirsekorn, K. F.; Veige, A. S.; Wolczanski, P. T. J. Am. Chem. Soc.
2006, 128, 2192–2193.
(23) Cowley, A. H.; Pellerin, B.; Atwood, J. L.; Bott, S. G. J. Am. Chem.
Soc. 1990, 112, 6734–6735.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10547

Kuiper et al.
cantly greater (111.74(10)° and 110.14(10)°, respectively).
O1, P1, and P2 occupy fac positions of an octahedron, and
O2 is leaning away from the phosphines in a manner that
does not make sense on pure steric grounds.
Discussion
Synthetic Studies. It is not surprising that the stability of
the dimolybdenum triple bond hampered the elaboration of
Mo(III),7,8 but the inability to manipulate (silox)2Mo(NMe2)2
(7) was disappointing given the investment in improving the
yield of Mo(NMe2)4.15,16 Fortunately, a return to Weiden-
bruch’s silanolysis routes17 has enabled the ready preparation
of silox halides of molybdenum and tungsten. Aside from
some modest separation problems, the syntheses of (silox)2-
MCl4 (M ) Mo, 8; W, 15),3 (silox)2MoCl3 (9), and its
derivatives have solved the problem of gaining an entryway
into the chemistry of the second and third row transition
metals of group 6.
Figure 4. Molecular view of (tBu3SiO)2Mo(PMe3)2 (17).
attempt would be successful for Mo(II).19-22,24 The reduction
of (silox)2MoCl2(PMe3) (12) with Na/Hg in Et2O with 3
equiv of PMe3 present afforded red (tBu3SiO)2Mo(PMe3)2
(17) in 68% yield upon crystallization from pentane.
Compound 17 is paramagnetic, with a solution µeff of 2.6 at
(silox)2Mo(PMe3)2 (17) versus (silox)2HW(η2-CH2PMe2)-
PMe3 (14). Similar to the case of (silox)3M (M ) Mo, W),
where the molybdenum complex is isolable but the tungsten
is not,4 Mo(II) can be stabilized by the combination of silox
and PMe3,1,4,13,14 whereas “(silox)2W(PMe3)2” is prone to
the oxidative addition of an internal CH bond.19-22,24 These
observations follow the periodic trend that third row transi-
tion elements are more easily oxidized than their second row
congeners.20 However, it is surprising that Mo(II) is stabilized
in a four-coordinate environment. Now that Cotton and
Schmid have debunked the claims of “Cl2Mo(PMe3)2” (i.e.,
Cl2Zn(PMe3)2),29,30 (silox)2Mo(PMe3)2 (17) is likely the only
four-coordinate Mo(II) species known. Related use of 2,6-
Ph2-C6H3O by the groups of Parkin and Rothwell led to
discovery of the η6-Ph,η1-O coordination mode, yielding the
23 °C, but NMR spectra were revealing. Two resonances
1
were observed in the H NMR spectrum at δ 2.33 (υ1/2
)
75 Hz) and δ 11.44 (υ1/2 ) 120 Hz), while only one signal
was apparent in the 13C{1H} NMR spectrum at δ 91.40
(C(CH3)3). The data appeared promising for an S ) 1 Mo(II)
species, but an X-ray crystal structure was necessary to
confirm the composition.
(silox)2MoCl2(PMe3). 2Na ⁄ Hg, 3PMe , - 2NaCl8
3
Et2O, 23 °C, 24h
15
(tBu3SiO)2Mo(PMe3)2 (6)
17
2. Structure of (silox)2Mo(PMe3)2 (17). Information on
data collection regarding (tBu3SiO)2Mo(PMe3)2 (17) is avail-
able in Table 2, metric features are given in Table 3, and its
pseudo-Td geometry is evident in Figure 4. Elongated
d(MoO)’s of 1.943(3) and 1.975(3) Å provide testament to
the low oxidation state of the complex, whereas the shorter
d(MoP)’s of 2.3330(13) and 2.3374(13) Å give evidence of
the backbonding that occurs when PMe3 binds to Mo(II).
On the basis of a pure tetrahedral geometry, a diamagnetic
complex would not be surprising for a second row element,25
but instead, 17 manifests certain distortions that do not appear
to be steric in origin. The O-Mo-O angle is opened up to
136.67(13)°, whereas the P-Mo-P angle is closed down
to 93.10(5)°, despite the cone angle of PMe3 (118°)26,27 being
roughly the same as that of silox (∼120° (est)).28 What is
more curious is the asymmetry of the P-Mo-P plane to
the O-Mo-O plane. The O1-Mo-P1 and O1-Mo-P2
angles are 96.85(10)° and 99.62(10)°, respectively, but the
related O2-Mo-P1 and O2-Mo-P2 angles are signifi-
seven-coordinate [η1-(η6-2-Ph)(6-Ph)C6H3O]Mo(PR1R2 ) H
2 2
(R1 ) R2 ) Me;31 R1 ) Me, R2 ) Ph)32 derivatives. These
studies were preceded by the synthesis of octahedral tert-
butoxide complexes by Chisholm et al. as (tBuO)2Mo-
33
(py)2(CO)2 and (tBuO)2Mo(bipy)2.34 Thiolates have also
seen activity as ligands compatible with Mo(II) centers, but
35,36
37
t
these BuS-,
2,6-Ph6-C6H3S-, and 2,4,6-iPrC6H2S-
compounds all possess higher coordination numbers than
four.
(29) (a) Fromm, K.; Plaikner, M.; Hey-Hawkins, E. Z. Naturforsch., B:
Chem. Sci. 1995, 50, 894–898. (b) Fromm, K.; Plaikner, M.; Hey-
Hawkins, E. Z. Naturforsch., B: Chem. Sci. 1996, 51, 1669–1669.
(30) Cotton, F. A.; Schmid, G. Polyhedron 1996, 15, 4053–4059.
(31) Hascall, T.; Baik, M.-H.; Bridgewater Brian, M.; Shin Jun, H.;
Churchill David, G.; Friesner Richard, A.; Parkin, G. Chem Comm.
2002, 2644–2645.
(32) Kerschner, J. L.; Torres, E. M.; Fanwick, P. E.; Rothwell, I. P.;
Huffman, J. C. Organometallics 1989, 8, 1424–1431.
(33) Chisholm, M. H.; Huffman, J. C.; Kelly, R. L. J. Am. Chem. Soc.
1979, 101, 7615–7617.
(24) (a) Murphy, V. J.; Parkin, G. J. Am. Chem. Soc. 1995, 117, 3522–
3528. (b) Zhu, G.; Tanski, J. M.; Churchill, D. G.; Janak, K. E.; Parkin,
G. J. Am. Chem. Soc. 2002, 124, 13658–13659.
(25) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its
Applications; Wiley-VCH: New York, 2000.
(34) Chisholm, M. H.; Huffman, J. C.; Rothwell, I. P.; Bradley, P. G.;
Kress, N.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 4945–
4947.
(35) Kamata, M.; Yoshida, T.; Otsuka, S.; Hirotsu, K.; Higuchi, T. J. Am.
Chem. Soc. 1981, 103, 3572–3574.
(26) Tolman, C. A. Chem. ReV. 1977, 77, 313–348.
(27) Bunten, K. A.; Chen, L.; Fernandez, A. L.; Poe¨, A. J. Coord. Chem.
ReV. 2002, 233, 41–51.
(36) Kamata, M.; Yoshida, T.; Otsuka, S.; Hirotsu, K.; Higuchi, T.; Kido,
M.; Tatsumi, K.; Hoffmann, R. Organometallics 1982, 1, 227–230.
(37) Bishop, P. T.; Blower, P. J.; Dilworth, J. R.; Zubieta, J. A. Polyhedron
1986, 5, 363–367.
(28) Wolczanski, P. T. Polyhedron 1995, 14, 3335–3362.
10548 Inorganic Chemistry, Vol. 47, No. 22, 2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
Figure 5. Calculated geometries and d-orbital splitting diagrams of (silox)2MoCl2PMe3 (12′) and (silox)2WCl2PMe3 (16′). Distances (black) are in angstroms;
angles are in italicized red.
As for the cyclometalation of PMe3 to afford (silox)2HW(η2-
CH2PMe2)PMe3 (14), this CH bond activation was first
observed for tungsten by Green et al. in their attempt to
prepare “W(PMe3)6”.19,21 The cyclometalation product
(Me3P)4HW(η2-CH2PMe2) was isolated instead. Later efforts
by Parkin demonstrated that the Mo0 center of (Me3P)6Mo
is less prone to activating the CH bond of a PMe3 ligand
than its tungsten counterpart.24
Electronic Structures. 1. (silox)2MCl2PMe3 (M )
Mo, 12; W, 16). The X-ray crystal structure determination
of (silox)2MoCl2PMe3 (12) confirmed its expected square-
pyramidal arrangement, but the diamagnetism observed for
12 and its third row congener (silox)2WCl2PMe3 (16) was
not an obvious consequence. Figure 5 illustrates the d-orbital
splitting diagrams for the full models (12′ and 16′) as
determined from high-level quantum mechanics/molecular
mechanics calculations (QM/MM).38-43 The calculated
structure (12′) essentially reproduces the geometry deter-
mined by the crystal structure, except that the bond lengths
are ∼0.04-0.06 Å longer. The tungsten analogue (16′) has
a virually identical arrangement, with the exception of a
significantly shorter d(WP) of 2.41 Å (d(MoP) ) 2.44 Å),
which is in concert with its anomalously large JWP of 610
Hz.
The ordering of the orbitals portrayed in Figure 5 is
standard, and the spread of energies is greater for W (3.42
eV) relative to the second row element (3.16 eV), as
predicted on the basis of the slightly larger radial extent of
the 5d orbitals. The slightly better σ and π overlaps for the
third row metal are reflected in the generally greater gaps
between each orbital pair. The lowest orbital, dyz, is π* with
respect to the MCl bonds and πb to phosphorus. There is a
considerable gap (Mo, 0.77 eV; W, 0.91 eV) between dyz
and the next orbital, dxz, that explains the diamagnetism. This
is in part due to a modest amount of metal-oxygen σ*
character to the orbital, which is primarily metal-oxygen
π* and MP π-bonding. The cant of the siloxides relative to
PMe3 (e.g., P-Mo-O ∼ P-W-O ∼ 101°) turns on the σ
antibonding character and elevates the orbital considerably,
since it approaches the dxy π* orbital in energy (Mo, 0.30
eV; W, 0.44 eV). It is interesting to note that dxy has virtually
no MCl π* character but possesses a considerable metal-
(38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
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Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford CT, 2004.
2
oxygen π* interaction. The dz orbital for W is slightly
(40) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 612–630.
(41) Hirsekorn, K. F.; Veige, A. S.; Marshak, M. P.; Koldobskaya, Y.;
Wolczanski, P. T.; Cundari, T. R.; Lobkovsky, E. B. J. Am. Chem.
Soc. 2005, 127, 4809–4830.
(42) Vreven, T.; Morokuma, K. J. Comput. Chem. 2000, 21, 1419–1432.
(43) Rappe´, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, K. S., III;
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024–10035.
(39) Parr, R. G.; Yang, W. Density-functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, U.K., 1989.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10549

Kuiper et al.
perusal of the orbitals in Figure 6 reveals nothing obvious
that would cause the “twist” described above, so the
contention that subtle bonding interactions are at play appears
reasonable. The lowest orbital in the ligand field is best
2
2
construed as dx -y , and it is at -2.42 eV relative to the two
singly occupied dxz (-0.95 eV) and dyz (-0.76 eV) orbitals.
2
2
It is clear that dx -y has a significant Mo(dπ) f P(d/σ*)
backbonding component, while dxz and dyz carry the brunt
of the O(pπ) f Mo(dπ)* antibonding interaction of the silox
ligands, and dyz has some σ* character. At -0.44 eV and
2
-0.22 eV are dxy and “dz ”, and these are σ* towards the
phosphines and the silox oxygens, respectively, although the
low symmetry renders the latter orbital rather difficult to
identify. The orbital energies must be treated with caution,
as density functional theory (DFT) is prone to error in
comparing filled versus half-filled versus empty orbitals,44
but the expected “3 over 2” configuration is not apparent.
By opening the O-Mo-O angle, dyz becomes significantly
less σ* in character and is lower in energy, but the penalty
is greater π* character in dxz, which is higher in energy; the
proximity of the two leads to the S ) 1 ground state, and
their ∼0.19 eV separation is not enough to overcome a
pairing energy. Thus “3 over 2” becomes “2 over 2 over 1”,
Figure 6. Calculated geometry and d-orbital splitting diagram of
(silox)2Mo(PMe3)2 (17′). Distances (black) are in angstroms; angles are in
italicized red.
2
2
with dx -y greatly stabilized by the backbonding interaction,
highersrelative to dyzsthan in Mo, perhaps indicative of the
2
and through mixing with “dz ”, which takes any σ* character
3
2
2
greater 6s involvement in WP bonding. Note that the dx -y
away.
orbital has more MCl than metal-oxygen σ* character, as
expected on the basis of ∆EMX arguments.
Conclusions
The combination of tBu3SiO (silox) and PMe3 again proves
to make a potent duo in the synthesis of low-coordinate, low-
oxidation-state species via the stabilization of Mo(II) as
(silox)2Mo(PMe3)2 (17). This combination is unable to
overcome the propensity of W(II) to oxidation; hence, the
related “(silox)2W(PMe3)2” formula is found as the PMe3
cyclometalation product, (silox)2HW(η2-CH2PMe2)PMe3
(14).
2. (silox)2Mo(PMe3)2 (17). In a pseudo-Td geometry, a
“3 over 2” orbital arrangement would be expected to generate
a low-spin (S ) 0) d4 configuration for a second or third
row transition element.25 In an attempt to better understand
the observed S ) 1 electronic state of (silox)2Mo(PMe3)2
(17), high-level QM/MM calculations were conducted on the
full model (17′).38-43 While most of the major geometric
features of the structure were effectively modeled by 17′,
the tilt of the P-Mo-P plane with respect to the O-Mo-O
plane was not. Instead of tilting toward one silox ligand, the
P-Mo-P plane has a much greater twist with respect to
the O-Mo-O plane (O-Mo-P ) 90.7° and 103.1° vs
110.4° and 121.3°), as Figure 6 reveals, and the calculated
d(MoO) and d(MoP) are 0.03-0.04 Å and 0.07-0.10 Å
greater, respectively. The geometric distortions occurred
whether or not the crystal structure coordinates or a geometry
generated from a molecular-mechanics-based conformational
search was used to initiate the QM/MM-based geometry
optimization. There is no obvious rationale for the angular
discrepancies, and such subtleties are unlikely to alter the
electronics to a great extent. The calculated P-Mo-P angle
of 96.8° and O-Mo-O of 128.8° are also off by ∼3.7° and
∼ 7.8°, respectively, but the general features of the core are
similar to the X-ray crystal structure and may simply indicate
that 17 lies in a shallow minimum, whereby its angular
changes are sensitive to the chemical environment.
Experimental Section
General Considerations. All manipulations were performed
using either glovebox or high vacuum line techniques. Hydrocarbon
solvents containing 1-2 mL of added tetraglyme and ethereal
solvents were distilled under nitrogen from purple sodium ben-
zophenone ketyl and vacuum-transferred from the same prior to
use. Benzene-d6 was dried over activated 4 Å molecular sieves,
vacuum-transferred, and stored under N2. All glassware was oven-
dried, and NMR tubes for sealed tube experiments were additionally
flame-dried under a dynamic vacuum. MoCl5 was obtained from
Strem Chemical, and MoCl4(THF)2, MoCl3(THF)3,45 and
MoCl4(OEt2)2,46 were prepared according to literature procedures,
as were [(tBu3SiO)2MoCl]2 (22), (silox)3WCl (2-Cl), and (silox)2-
WCl4 (15).3
NMR spectra were obtained using Varian XL-400, INOVA-400,
and Unity-500 spectrometers, and chemical shifts are reported
relative to benzene-d6 (1H, δ 7.15; 13C{1H}, δ 128.39). Infrared
spectra were recorded on a Nicolet Impact 410 spectrophotometer
The calculations reproduce the S ) 1 ground state
implicated by the solution magnetism of (silox)2Mo(PMe3)2
(17), and corresponding singlet and quintet excited states are
2.3 and 13.8 kcal/mol above the triplet, respectively. A
(44) Zhang, G.; Musgrave, C. B. J. Phys. Chem. A 2007, 111, 1554–1561.
(45) Dilworth, J. R.; Zubieta, J. Inorg. Synth. 1986, 24, 193–194.
(46) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, 269,
9–2703.
10550 Inorganic Chemistry, Vol. 47, No. 22, 2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
interfaced to a Gateway PC, or a Nicolet Avatar 370 DTGS
spectrophotometer interfaced with an IBM ThinkPad. Solution
magnetic measurements were conducted via Evans’ method in C6D6
unless otherwise noted.47 Elemental analyses were performed by
Oneida Research Services, Whitesboro, New York, or Robertson
Microlit Laboratories, Madison, New Jersey.
(7.80 g) deposited on the condenser was collected, and the process
was repeated to give an additional 1.88 g for a combined yield of
25%. H NMR (C6D6, 400 MHz): δ 3.27.
1
6. (silox)2Mo(NMe2)2 (7). To a 100 mL flask containing 2.00 g
t
(7.34 mmol) of Mo(NMe2)4 and 3.17 g (14.7 mmol) of Bu3SiOH
was distilled 30 mL of pentane at -78 °C. The solution developed
a deep maroon color upon warming. After stirring for 30 min, the
solution was filtered and stripped of all volatiles. The solid was
redissolved in minimal pentane (∼5 mL), cooled to -78 °C, and
filtered to give 2.73 g (5.13 mmol, 70%) of 7 as maroon
Procedures. 1. (silox)2HMoMo(K-O,C-OSitBu2CMe2CH2)-
(silox) (3). To a 100 mL flask containing 1.00 g (1.78 mmol) of
[(tBu3SiO)2MoCl]2 (22) and 4.52 g of Na/Hg (0.95%, 1.87 mmol)
was distilled 50 mL of THF at -78 °C. The solution was allowed
to warm slowly from -78 to 23 °C. After the green-brown solution
was stirred for 4 h at 23 °C, all volatiles were removed in Vacuo.
The reaction mixture was triturated three times with 10 mL of
hexanes and filtered in hexanes. The removal of all volatiles yielded
0.843 g (0.80 mmol, 90%) of 3 as a green-brown solid. IR (nujol
mull, Na/Cl, cm-1): 1945 (υ, MoH). Anal. calcd for C50H108Cl2-
Mo2O4Si4: C, 54.61; H, 10.50. Found: C, 54.24; H, 10.65.
2. [(tBu3SiO)2MoCH3]2 (42). To a 100 mL flask containing
1.00 g (0.889 mmol) of 22 was distilled 20 mL of Et2O at -78
°C. CH3MgBr (0.6 mL, 3.0 M in Et2O) was added via syringe.
The solution was allowed to warm to 23 °C and was stirred for
3 h. The flask was evacuated and the reaction mixture stripped
of all volatiles. The resulting dusty-purple solid was dissolved
in 50 mL of pentane and filtered, and the salt cake was repeatedly
washed until no traces of product remained. The filtrates were
concentrated to ∼5 mL, cooled to -78 °C, and filtered to give
0.818 g (0.755 mmol, 85%) of 4 as dusty-purple microcrystals.
Anal. calcd for C50H108Cl2Mo2O4Si4: C, 55.42; H, 10.60. Found:
C, 54.89; H, 10.76.
3. [(tBu3SiO)2MoH]2 (52). To a 100 mL round-bottom flask
containing 2.00 g (1.78 mmol) of 22 and 9.04 g of Na/Hg (0.95%,
3.73 mmol) was distilled 50 mL of THF at -78 °C. The solution
was exposed to 1 atm of H2 and allowed to warm slowly from
-78 to 23 °C. After the red-brown solution was stirred for 4 h at
23 °C, all volatiles were removed in vacuo, and the reaction mixture
was filtered in hexanes. The removal of all volatiles yielded 1.78 g
(1.69 mmol, 95%) of 52 as a red-brown solid. IR (nujol mull, Na/
Cl, cm-1): 1965 (ν(MoH)). Anal. calcd for C50H108Cl2Mo2O4Si4:
C, 54.61; H, 10.50. Found: C, 54.24; H, 10.65.
1
t
microcrystals. H NMR (C6D6, 400 MHz): δ 1.21 (s, Bu, 27H),
3.27 (s, N(CH3)2, 6H). 13C{1H} NMR (C6D6, 400 MHz): δ 23.84
(C(CH3)), 23.99 (C(CH3)), 30.88 (C(CH3)), 31.42 (C(CH3)). Anal.
calcd for C29H66N2MoO2Si2: C, 65.59; H, 12.53; N, 5.28. Found:
C, 65.43; H, 12.63; N, 5.11.
7. (silox)2MoCl4 (8) and (silox)3MoCl3 (9) in 1:4 Ratio. To a
500 mL flask containing 10.0 g of tBu3SiOH (46.2 mmol) and 6.94 g
of MoCl5 (25.4 mmol) was vacuum-transferred 250 mL of CCl4 at
-78 °C. The dark purple suspension evolved copious amounts of
HCl gas as the reaction mixture warmed to 23 °C and changed to
a deep red color. The removal of HCl was further affected by the
periodic exposure of the reaction mixure to a liquid-nitrogen-cooled
trap under static vacuum for 1 h (∼15 cycles of HCl removal).
The reaction mixture was then heated in a 45 °C water bath under
a static vacuum with occasional evacuation of the vacuum manifold
(∼15 min intervals). The reaction mixture was then stripped of all
volatiles and assayed by 1H NMR to reveal a 1:4 mixture of
(tBu3SiO)2MoCl4 (8) and (tBu3SiO)2MoCl3 (9), respectively.
8. (silox)2MoCl4 (8). To a 250 mL flask containing 5.00 g of
tBu3SiOH (23.1 mmol) and 6.31 g of MoCl5 (23.1 mmol) was
vacuum-transferred 100 mL of CCl4 at -78 °C. The purple
suspension was warmed to 23 °C under a heavy argon purge and
allowed to stir for an hour. The deep red solution containing a dark
precipitate was then heated to 65 °C under a gentle argon purge
for 8 h. The red reaction mixture was stripped of all volatiles, and
the soluble portion was redissolved in 150 mL of CCl4. The reaction
mixture was filtered, and the filtrates were concentrated to ∼15
mL. The red solution was cooled to 0 °C for 30 min and filtered to
give 3.09 g of 8 (40%). Anal. calcd for C24H54Cl4MoO2Si2: C,
43.11; H, 8.14. Found: C, 42.77; H, 8.39.
4. [(silox)2Mo]2(µ:η2η2-C2Me2) (6). A NMR tube attached to a
needle valve adaptor was charged with a solution of 3 (30 mg,
0.028 mmol) in 0.65 mL of C6D6. The tube was fitted with a 180°
needle valve and freeze-pump-thaw degassed three times. 2-Bu-
tyne (176 Torr, 0.28 mmol) was transferred via a calibrated 30.1
mL gas bulb. The tube was sealed and heated for 48 h at 100 °C.
9. mer-(silox)2MoCl3(THF) (10). Procedure 6 was followed to
obtain 8/9 in a ∼1:4 ratio. A small portion of the mixture (0.250
g) was taken up in 25 mL of THF, and the resulting solution was
filtered, leaving behind a dark red solid (presumably 8). The red
solution was allowed to evaporate at room temperature until crystals
were seen to form. Small orange-red crystals of 10 were isolated
by decanting the remaining mother liquor (0.088 g, 40%). The
1
Partial conversion (80%) was seen by H NMR spectroscopy, but
20% of the starting material remained even after continued
thermolysis (8 h, 120 °C).
1
reaction was not optimized. H NMR (C6D6, 300 MHz): δ 0.95
(27 H, ν1/2 ≈ 117 Hz), 1.92 (4 H, ν1/2 ≈ 102 Hz). 13C{1H} NMR
(C6D6, 500 MHz): δ 37.35 (C(CH3)3 ν1/2 ≈ 625 Hz). Anal. calcd
for C28H62Cl3MoO3Si2: C, 47.68; H, 8.86. Found: C, 47.31; H, 8.43.
10. (silox)2MoCl3(PMe3) (11). Procedure 6 was followed to
obtain 8/9 in a ∼1:4 ratio. The product mixture was redissolved in
300 mL of CCl4 and filtered, leaving behind a small amount of
dark solid. The CCl4 was removed in Vacuo, and the reaction
mixture was suspended in 100 mL of Et2O. The transfer of 2 equiv
5. Mo(NMe2)4. To a 1 L three-neck round-bottom flask contain-
ing 30 g (0.59 mol) of LiNMe2 was vacuum-transferred 300 mL
of Et2O. The LiNMe2 solution was cooled to 0 °C, and a suspension
46
of MoCl4(OEt2)2 (55 g, 0.142 mol) in 150 mL of Et2O was
cannulated into the ice-cold LiNMe2 solution. The reaction mixture
was stirred at 0 °C for 4 h and then stirred at 23 °C for 48 h. The
Et2O was removed from the brown reaction mixture in Vacuo, and
the flask was fitted with a condenser. The surface of the condenser
was cooled with a dry ice/acetone mixture, and the residue was
heated to 80 °C for 2 h at 10-3 Torr. The resulting purple solid
t
of PMe3 (stoichiometry based on Bu3SiOH consumed) via cali-
brated gas bulb into the red suspension at -78 °C resulted in a
bright orange solid and a green solution upon warming to 23 °C.
The reaction mixture was stirred for 1 h and filtered, and the filter
cake was washed to give bright orange (tBu3SiO)2MoCl3(PMe3)
(47) (a) Evans, D. F. J. Chem. Soc. 1959, 2003–2005. (b) Schubert, E. M.
J. Chem. Educ. 1992, 69, 62.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10551

Kuiper et al.
(11; 12.29 g, 75%). The formulation is tentative and based on EA
and reactivity studies. Anal. calcd for C36H82MoO3Si3: C, 45.72;
H, 8.95. Found: C, 44.70; H, 8.02.
dissolved in minimal Et2O and recrystallized at -30 °C over 12 h
to give 1.72 g of 14 (2.24 mmol, 85%). Anal. calcd for
C30H72O2P2Si2W: C, 46.99; H, 9.46. Found: C, 46.55; H, 9.66.
14. (tBu3SiO)2Mo(PMe3)2 (17). To a 100 mL round-bottom flask
containing 1.83 g of 12 (2.72 mmol) and 13.83 g of Na/Hg amalgam
(2.1 equiv, 5.71 mmol Na) was vacuum-transferred ∼50 mL of
Et2O at -78 °C. PMe3 (3 equiv, 8.16 mmol) was transferred to the
reaction mixture via calibrated gas bulb. The reaction mixture was
warmed to 23 °C and stirred for 24 h. The blue reaction mixture
slowly turned red throughout the course of the reaction. All volatiles
were removed in vacuo, and the Hg0 was decanted. Pentane (30
mL) was transferred to the reaction mixture, and the deep red
solution was filtered. The red filtrates were concentrated to ∼2 mL,
and the red product was crystallized by slow evaporation of the
saturated solution at -30 °C for 18 h. The small amount of solution
11. (silox)2MoCl2(PMe3) (12). To a 100 mL round-bottom flask
containing 2.43 g of 11 (3.43 mmol) and 9.12 g of Na/Hg amalgam
(1.1 equiv, 3.77 mmol) was vacuum-transferred ∼40 mL of Et2O
at -78 °C. PMe3 (3 equiv, 10.29 mmol) was transferred to the
reaction mixture via calibrated gas bulb. The reaction mixture was
warmed to 23 °C and stirred vigorously for 4 h. The orange
suspension slowly turned green and then blue during the course of
reaction. The blue suspension was stripped of all volatiles, and the
Hg0 was decanted. To the reaction mixture was transferred 50 mL
of toluene. The blue solution was filtered, and the filter cake was
washed with toluene until no trace of color remained. The blue
filtrates were stripped of all volatiles; 20 mL of Et2O was transferred
into the flask, and the suspension was filtered to give microcrys-
talline 12 (2.08 g, 90%). Anal. calcd for C27H63MoO2PSi2: C, 48.13;
H, 9.42. Found: C, 47.81; H, 9.13.
1
remaining was removed, yielding 1.26 g (68%) of 17. H NMR
(C6D6, 300 MHz): δ 2.33 (ν1/2 ) 75 Hz), 11.44 (ν1/2 ) 120 Hz).
13C{1H} NMR (C6D6, 500 MHz): δ 91.40 (C(CH3)3). Anal. calcd
for C27H63O2PSi2W: C, 42.58; H, 8.34 Found: C, 42.19; H, 8.02.
µeff (293 K) ) 2.6 µB(C6D6).
12. (silox)2WCl2(PMe3) (16). To a 100 mL flask containing
1.00 g of (silox)2WCl4 (15; 1.32 mmol) and 6.72 g of Na/Hg
amalgam (2.1 equiv, 2.78 mmol Na) was vacuum-transferred ∼40
mL of Et2O at -78 °C. PMe3 (3 equiv, 3.96 mmol) was transferred
to the reaction mixture via calibrated gas bulb. The reaction mixture
was warmed to 23 °C and stirred vigorously for 4 h. The red
suspension slowly turned purple throughout the course of the
reaction. The purple suspension in purple solution was stripped of
all volatiles, and the Hg0 was decanted. To the reaction mixture
was transferred 50 mL of toluene. The purple solution was filtered,
and the filter cake was washed with toluene until no trace of color
remained. The purple filtrates were stripped of all volatiles; 20 mL
of Et2O was transferred into the flask, and the suspension was
filtered to give microcrystalline 26 (0.724 g, 72%). Anal. calcd for
C27H63O2PSi2W: C, 42.58; H, 8.34. Found: C, 42.19; H, 8.02.
13.(silox)2HW(η2-CH2PMe2)PMe3 (14).a.From(silox)2WCl2-
(PMe3) (16). To a 100 mL round-bottom flask containing 1.00 g
of 16 (1.16 mmol) and 5.73 g of Na/Hg (2.37 mmol) was vacuum-
transferred 50 mL of Et2O at -78 °C. PMe3 (10 equiv, 11.6 mmol)
was vacuum-transferred into the reaction mixture via calibrated gas
bulb. The reaction mixture was allowed to warm to 23 °C and stir
for 24 h. The red solution with white precipitate was stripped of
all volatiles; the mercury was decanted, and the reaction mixture
was dissolved and filtered in 50 mL of pentane. The removal of
solvent in vacuo resulted in a mixture of red solid 14 and
NaOSitBu3. Pure red 14 (0.133 g, 0.173 mmol) was isolated by
fractional crystallization from a slowly evaporating Et2O solution
at -30 °C.
b. From (silox)2WCl4(15). To a 100 mL round-bottom flask
containing 2.00 g of 15 (2.64 mmol) and 26.2 g of Na/Hg (10.8
mmol) was vacuum-transferred 50 mL of Et2O at -78 °C. PMe3
(3 equiv of 7.92 mmol) was vacuum-transferred into the -78 °C
reaction mixture via calibrated gas bulb. The dark red suspension
was allowed to warm to 23 °C and stirred for 12 h. Within the first
hour of the reduction, a dramatic color change to a green-brown
solution with orange precipitate and then to a purple solution with
a purple precipitate occurred. The reaction mixture gradually turned
to red over the remaining reaction time. The reaction mixture was
stripped of all volatiles; the Hg0 was decanted, and the red solid
was dissolved in 50 mL of pentane. The reaction mixture was
filtered and the pentane removed in vacuo. The red solid was
15. Single-Crystal X-Ray Diffraction Studies. Upon isolation,
the crystals were covered in polyisobutylene and placed under a
173 °C N2 stream on the goniometer head of a Siemens P4 SMART
CCD area detector system (graphite-monochromated Mo KR
radiation, λ ) 0.71073 Å). The structures were solved by direct
methods (SHELXS). All non-hydrogen atoms were refined aniso-
tropically unless stated, and hydrogen atoms were treated as
idealized contributions (Riding model).
16. mer-(silox)2MoCl3(THF) (10). A yellow block (0.6 × 0.35
× 0.15) was obtained from tetrahydrofuran. A total of 14 811
reflections were collected with 4409 being symmetry-independent
(Rint ) 0.0406), and 3302 were greater than 2σ(I). The data were
corrected for absorption by SADABS, and the refinement utilized
2
2
w-1 ) σ2(Fo ) + (0.0399p)2 + 0.2684p, where p ) (Fo
2Fc )/3).
+
2
17. (silox)2MoCl2(PMe3) (12). A blue rod (0.5 × 0.4 × 0.3)
was obtained from diethyl ether. A total of 29 927 reflections were
collected, with 8586 being symmetry-independent (Rint ) 0.0252),
and 7143 were greater than 2σ(I). The data were corrected for
2
absorption by SADABS, and the refinement utilized w-1 ) σ2(Fo )
2
+ (0.0316p)2 + 0.6198p, where p ) (Fo + 2Fc2)/3).
18. (silox)2HW(η2-CH2PMe2)PMe3 (14). A red rod (0.2 × 0.05
× 0.03) was obtained from diethyl ether. A total of 32 273
reflections were collected, with 6351 being symmetry-independent
(Rint ) 0.0914), and 4775 were greater than 2σ(I). The data were
corrected for absorption by SADABS, and the refinement utilized
2
2
w-1 ) σ2(Fo ) + (0.0470p)2 + 0.0000p, where p ) (Fo + 2Fc2)/
3). The PMe3 group was partially disordered by rotation along the
W-P bond. The hydride H atom was not visible on a difference
Fourier map. It was placed using a “sterically most feasible” model.
19. (tBu3SiO)2Mo(PMe3)2 (17). A red plate (0.2 × 0.15 × 0.03)
was obtained from pentane. A total of 25 756 reflections were
collected, with 4052 being symmetry-independent (Rint ) 0.0728),
and 3529 were greater than 2σ(I). The data were corrected for
2
absorption by SADABS, and the refinement utilized w-1 ) σ2(Fo )
2
+ (0.0403p)2 + 16.2911p, where p ) (Fo + 2Fc2)/3).
Computational Methods. Calculations were performed on full
silox-decorated models using the Gaussian 03 package.38 DFT,
specifically the BLYP functional, was utilized for all simulations.39
The transition metals and heavy main group atoms (Si and P) were
described with the Stevens effective core potentials and attendant
valence basis sets (VBSs).40 This scheme, dubbed CEP-31G, entails
a valence triple-ꢁ description for the transition metals, and a
10552 Inorganic Chemistry, Vol. 47, No. 22, 2008

Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2
double-ꢁ VBS for the main group elements. The 6-31G(d) all-
electron basis set was used for C, H, and O atoms. All main group
VBSs are augmented with a d polarization function (C(ꢂd) ) O(ꢂd)
) 0.8; Si(ꢂd) ) 0.3247; P(ꢂd) ) 0.37). This level of theory was
selected on the basis of a series of test calculations on the singlet
and triplet states of Nb(OH)3, Ta(OH)3,1 and their olefin adducts.41
Full silox models were studied using hybrid QM/MM techniques
within the ONIOM framework.42 The QM region of M(silox)3(X)
complexes contained the transition metal, the O and Si atoms of
the silox group, and the entire X group (if present). The QM level
of theory employed is that described above. The remainder of the
molecule, that is, the tert-butyl groups of silox, was modeled with
the universal force field.43
energy were calculated using harmonic vibrational frequencies
determined at the same level of theory employed for geometry
optimization and are calculated at 1 atm and 298.15 K.
Closed- and open-shell species were described with the restricted
and unrestricted Kohn-Sham formalisms, respectively, with no
evidence of spin contamination for the latter.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0415506, (PTW)), the US Dept. of Energy (DE-
FG02-03ER15490 (TRC)), and Cornell University for fi-
nancial support.
Full geometry optimizations without any metric or symmetry
restrictions were employed to obtain the minima in this research.
All of the resultant stationary points were characterized as true
minima (i.e., no imaginary frequencies) by calculation of the energy
Hessian. Enthalpic and entropic corrections to the total electronic
Supporting Information Available: CIF files of 10, 12, 14, and
17. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC8011958
Inorganic Chemistry, Vol. 47, No. 22, 2008 10553