1
Tungsten: chemical element from group six of the periodic system. A heavy metal, Z 74, with natural isotopes with mass numbers 184 (30.64%), 186 (28.41%), 182
(26.41%), 183 (14.40%), 180 (0.135%); atomic mass 183.5, valence usually VI, but also
V, IV, III, II, 0, density 19.26 melting point 3410 C, boiling point 5660C, electrical conductivity 17.8 sm mm
-2
, standard electrode potential (W/W
3+
) –0.11V.
Properties: Tungsten is shiny, white metal which, when very pure, is ductile and easily shaped, it crystallizes in a cubic body-centered lattice. Even small amounts of carbon or oxygen makes the metal very hard and brittle. At room temperature, it is completely stable in the presence of dry air. At red heat, it burns to tungsten(IV)oxide,
WO
3
. Steam will oxidize glowing tungsten to tungsten(IV) oxide, WO
2
. Tungsten does not react easily with nitrogen, but fluorine oxidizes it even at room temperature to tungsten hexafluoride, WF
6
. The other halogens react with tungsten only at higher temperatures, forming tungsten hexachloride, pentabromide, or triiodide, respectively.
Tungsten powder reacts with ammonia at higher temperatures, forming the nitride. Acids, even aqua regia and hydrofluoric acid, reacts only slowly with tungsten, due to passivation. However, tungsten is soluble in a mixture of hydrofluoric and concentrated nitric acids, and also in alkali hydroxide melts.
In tungsten compounds, the most common state is +6, but +5, +4, +3, +2 are also important; they are represented by tungsten(V) bromide, octacyanotungstate(IV), and the cluster compounds W
6
Cl
18
and W
6
Cl
12
. The oxidation states 0 and –2 are present in the
1 Concise Encyclopedia Chemistry, p. 1128-1129.

carbonyl compounds W(CO)
6
and [W(CO)
5
]
-2
. In tungsten complexes, the most common coordination numbers are 6 to 8.
Analysis: An important indicator reaction for tungsten is based on the reduction of tungsten(IV) compounds to tungsten blue. For colorimetric determination of tungsten, tungstate(VI) are reduced with tin(II) chloride in the presence of potassium thiocyanate, forming the yellow thiocyanatotungstates(V). For gravimetric determination of tungsten(VI) oxide, mercury(I) tungstate and tungsten oximates are used.
Occurrence: tungsten makes up 0.00012% of the earth’s crust. The most important ores are tungstates, e.g. wolframite (Mn, Fe)(WO
4
), an isomorphic mixture of berberite, FeWO
4
and hubnerite, MnWO
4
, scheelite, CaWO
4
, and stolzite, PbWO
4
.
Tungstite (tungsten ocher) is a weathering product of wolframite, WO
3 x H
2
O.
Production: tungsten ores, usually wolframite or scheelite, are first enriched by flotation or an electromagnetic process. They are solublized by melting them with sodium carbonate in a furnace at 800C, and the sodium tungstate formed in this step is then leached with water, or pressure leached with aqueous sodium hydroxide solution.
Tungstic acid is either precipitated directly by addition of hydrochloric acid to the sodium tungstate solution, or calcium chloride is first added to obtain calcium tungstate as an intermediate, which is then converted by reaction with hydrochloric acid to tungsten(VI) oxide hydrate. This is filtered off, dehydrated by heating, and reduced by hydrogen to tungsten at 1000 to 1200C:
WO
3
+ 3 H
2

W + 3 H
2
O
In this reaction, tungsten is produced as a powder with a purity of about 98%; the metal can be obtained in compact form by sintering in a hydrogen atmosphere and hammering.

Tungsten can also be obtained by reaction of tungsten(VI) sulfide with calcium oxide in and electric arc. The metal produced in this way is not completely pure, and is therefore hard and brittle. It can be made ductile by hammering, and then drawn into then wires.
Pressed threads, when heated to about 2500C in the presence of thorium dioxide, can form meter long single crystal fibers which are very flexible. The tungsten used to make tungsten steel is made in the form of ferrotungsten by reduction of tungsten and iron mixtures with carbon in an electric furnace.
Applications: about 90% of the world production of tungsten is utilized in the form of ferrotungsten to make tungsten steels. Of the metals, tungsten has the highest melting point are reasons for its use to make lightbulb filaments and cathodes and anticathodes for electronic tubes. Tungsten is used for thermoelements, rocket valves and heat shields for space vehicles. Highly stressed electric contacts are usually made of tungsten, to which copper and silver are added before sintering. Tungsten wires are used for production of group IVb and Vb metals by the vacuum deposition method. The use of tungsten to make hard metals and cutting tools is of special importance. In its alloys, tungsten transfer its properties to these systems, to a greater or lesser extent.
Historical: in 1781, scheele detected tungsten(VI) oxide as a component of scheelite. The Spanish scientists Fausto and Joseph d’Elhujar discovered the oxide in wolframite as well, and in 1783, prepared the element. The name wolfram was coined by berzelius, while the name tungsten was supplied by the Swede Gronstedt, who in 1768 named a heavy tungsten mineral “heavy stone” in Swedish, “tungstein.”
Tungsten alloys: multicomponent systems in which tungsten is the main component. High-tungsten alloys with silver, copper or nickel are used for electrical

contacts, and because of their high densities, are used for counterweights, flywheels or damping members in physics apparatus. They were used as drive masses in automatic wristwatches. These alloys are produced by sintering powdered mixtures or impregnation of previously sintered tungsten matrix with metal. To prevent recrystallization of tungsten wires in light bulbs, small amounts of oxides (thorium, silicon, aluminum oxides) are added. Tungsten is used as an alloy element in steels; it is added in the form of ferrotungsten, which consist of 81 to 83% tungsten.

2
The diamagnetic tungsten(VI) complex [W(

-C
5
Me
5
)Cl{2,2
’
-
(NC
6
H
4
)
2
(CH
2
CH
2
)}] is obtained upon reaction of [W(

-C
5
Me
5
)Cl
4
] with 2 equivalents of the diamine 2,2
’
-(H
2
NC
6
H
4
)
2
C
2
H
4
in refluxing toluene. Brown-orange blocks suitable for an X-ray diffraction study were grown from acetonitrile and the molecular structure is shown below. Bond lengths and angles are given in table 1. The coordination geometry is best described as distorted tetrahedral with the C
5
Me
5
ring occupying one position; angles at W range from 99.8 to 117.6.
(CH
2
)
2
N
W
N
Cl complex 2
Ph
2
P
N
W
Cl
N
PPh
2 complex 1
For the chelating diimide ligand the C-N-W angles are 163.4 and 166.3 with corresponding W-N distances of 1.795 and 1.776 Å. An approximate noncrystallographic mirror plane bisects the molecule passing through C(19), C(20), W, Cl and the center of the ethanediyl bridge, the main distortion from the symmetry being in this bridge. The only other structurally characterized compound containing a chelating bis(imido) ligand derived from 2,2
’
-(H
2
NC
6
H
4
)
2
C
2
H
4
is [MoCl
2
{2,2
’
-(NC
6
H
4
)
2
CH
2
CH
2
}
(dme = 1, 2-dimethoxyethane), for which the bite angle is 107.13. There are clear evidence of C
5
Me
5
ring slippage in complex 1; the W-C distances differ significantly,
2 J. Chem. Soc., Dalton Trans., 1997, pages 3343-3347.

from 2.359 for C(16) to 2.498 Å for C(20). A similar situation occurs in the complex
[W(

-C
5
Me
5
)Cl
2
{1,2-(HN)
2
C
6
H
4
}]; interestingly here the use of 1,2-phnylenediamine results only in a diamido chelate most probably due to the restricted angles at the metalbound nitrogen atoms which prevent the pseudo-linear M-N-C imido ligand arrangement being attained.
O-diphenylphosphine-substituted aniline might afford a chelating imidophosphine or alternatively an amidophosphine spices. The reaction of [W(

-C
5
Me
5
)Cl
4
] with 1,2-
(H
2
N)(Ph
2
P)C
6
H
4
in refluxing toluene gave after work-up orange needles in ca. 30% isolated yield. Analytical and spectroscopic data were not consistent with either an imido- or amido-phosphine chelate complex and hence its structure of complex 2.
The molecule has a pseudo-tetrahedral geometry with a non-crystallographic mirror plane passing through Cl, W and the C
5
Me
5
ring. Interestingly, the imido ligand is not a chelate, preferring instead to form a linear WNR group with a pendant and noncoordinated Ph
2
P group [W-N-C 157.9 and 165.1; W-N 1.783 and 1.775Å]. Its structure bears close resemblance to the bis(arylimido) complex [W(

-C
5
Me
5
)(NR)
2
Cl] (R =
C
6
H
3
Pr i
2
). Dilworth and co-workers have structurally characterized a rhenium complex.
[ReCl
2
(NC
6
H
4
PPh
2
-2)(HNC
6
H
4
PPh
2
-2)], which is found to contain a bidentate bonding mode for this ligand, though
31
P NMR data indicate that the monodentate coordination mode prevails in solution. For complex 2 we observe the opposite trend, the solid-state structure showing the monodentate coordination mode, the solution
31
P NMR spectrum revealing two resonances, at δ +39.5 and –15.0, consistent with the presence of one coordinated and one non-coordinated phosphine group.

Table 1
W-N(1)
W-Cl
W-C(24)
W-C(18)
W-Cen
N(2)-C(14)
Complex 1
1.795
2.35
2.366
2.466
2.105
1.38
N(2)-W-N(1) 107.2
W-N(2)
W-C(16)
W-C(22)
W-C(20)
N(1)-C(1)
N(1)-W-Cl
N(2)-W-Cl 99.8
1.776
2.359
2.45
2.498
1.367
W-N(1)
W-Cl
W-C(39)
W-C(41)
W-Cen
N(2)-C(19)
102.8 N(2)-W-N(1)
Cen-W-N(1) 117.6 N(2)-W-Cl
Complex 2
1.783
2.326
2.427
2.47
2.1
W-N(2)
W-C(37)
W-C(45)
W-C(43)
N(1)-C(1)
N(1)-W-Cl
1.775
2.319
2.427
2.475
1.368
1.378
104 103.8
103.4 Cen-W-N(1) 117.3
Cen-W-N(2) 115.2 Cen-W-Cl 112.1 Cen-W-N(2) 117.7 Cen-W-Cl 108.9
C(1)-N(1)-W 163.4 C(14)-N(2)-W 166.3 C(1)-N(1)-W 157.9 C(19)-N(2)-W 165.1
Note: Cen is the centroid of the C
5
Me
5
ring. Bond lengths in Å, and angles in degrees.
Preparation:
[W(

-C
5
Me
5
)Cl{2,2
’
-(NC
6
H
4
)
2
(CH
2
CH
2
)}]: the complex [W(

-C
5
Me
5
)Cl
4
] (1.0g,
2.16 mmol) and the diamine 2,2
’
-(H
2
NC
6
H
4
)
2
C
2
H
4
(0.92 g, 4.33 mmol) were refluxed in toluene (40 cm
3
) for 12h. Volatiles were removed in vacuo, and the residue was extracted into hot MeCN (50 cm
3
) affording on cooling to room temperature (1-2 d) large brown – red prisms. Yield 0.56g, 46% (found: C, 51.2; H, 4.9; N, 5.0. C
24
H
27
ClN
2
W requires C,
51.0; H, 4.9; N, 5.2%). IR: 2360m, 2342m, 1588w, 1352m, 1315m, 1261s, 1089s, 1022s,
939w, 867w, 800s, 764m, 741m, 723m, and 669w cm
-1
.
1
H NMR [(CD
3
)
2
SO]:

7.35-
6.88 (m, 8H, aryl H), 2.80 (m, 2H, CH
2
), 2.14 (m, 2H, CH
2
) and 2.08 (s, 15H, C
5
Me
5
).
[W(

-C
5
Me
5
)Cl(NC
6
H
4
PPh
2
-2)
2
]: Analogous conditions were employed to those described for the preparation of the previous complex using [W(

-C
5
Me
5
)Cl
4
] (0.25g,
0.54 mmol) and 2-diphenylphosphinoaniline (0.3 g, 1.08 mmol). Recrystallization from
MeCN or CH
2
Cl
2
afforded as orange needles. Yield 0.16g, 30% (found: C, 57.4; H, 4.7;
N, 2.8. C
48
H
46
ClN
2
P
2
WCH
2
Cl
2 requires C, 56.9; H, 4.6; N, 2.8%). IR: 1615w, 1585w,
1568w, 1435m, 1351w, 1321m, 1261s, 1092s, 1021s, 977w, 940w, 896w, 867w, 800s,
766m, 747m, 721w, 709w, 696m, 566w, 543w, 520w, 497w, 485w and 468w cm -1 . 1 H

NMR [CD
3
CN]:

7.68-6.61 (m, 28H, aryl H), 2.23 (s, 15H, C
5
Me
5
). 31 p –{ 1 H} NMR
(CD
3
CN):

39.5 (s) and –15.0 (s).
3
The majority of isolated nitrene complexes contain linear, unreactive imido ligands, a property compatible with their role as ligands for the stablization of high metal oxidation states. Bending of an imido ligand occurs when lone pair donation would result in a complex with an electron count exceeding eighteen. These linear and bent bonding modes suggest structural analogies with oxo, carbene, and carbyne ligands. The nitrene ligand may be nucleophilic or electrophilic at nitrogen. Many nucleophilic imido species react with aldehydes or ketones to give organic imines. For example, the putative zerovalent complex (CO)
5
W(NPh) reacts with a variety of electrophilic aldehydes, ketones, and thioketones to produce free organic imines. Bending of the imido ligand is generally believed to increase its nucleophilicity. The classic example of this behavior is the protonation (or methylation) of a single imido ligand of (Et
2 dtc)
2
Mo(NPh)
2
which contains one linear and one bent imido unit. A reactive low valent linear nitrene complex,
Cp
*
IrNBu t
, has been reported which reacts with MeI, CO, and CNBu t
and also undergoes
2+2 reactions with CO
2
and an alkyne. Deuteration of the amido protons of
(Bu t
3
SiNH)
2
(Et
2
O)Ti=NSiBu t
3
in the presence of C
6
D
6
is proposed to occur via the transient imido species [(Bu t
3
SiNH)
2
Ti=NSiBu t
3
].
3 Organometallics, vol. 13, No. 5, 1994, pages 1851-1864.

Nitrene complexes with reactivity patterns that indicate electrophilic character at nitrogen are rare. The amination of olefins by osmium imido complexes has been proposed to proceed by initial association of the olefin with an electron deficient nitrene ligand. An example of apparent electrophilicity at nitrogen is the reaction of
(Me
3
SiO)
2
Cr(NBu t
)
2 with Ph
2
Zn which yields tert-butylaniline upon hydrolysis. Sulfuric or hydrochloric acid decomposition of [Ir(NH
3
)
5
N
3
]
2+
to give [Ir(NH
3
)
5
NH
2
OSO
3
]
2+
or
[Ir(NH
3
)
5
NH
2
Cl] 3+ , respectively, is proposed to process through an electrophilic nitrene intermediate [Ir(NH
3
)
5
NH]
3+
which reacts with HSO
4
-
or Cl
-
, respectively. A number of electrophilic nitrene complexes add phosphine at nitrogen to form phosphine imides.
Ambiphilic (CO)
5
W(NPh), is trapped with PPh
3
to form the coordinated phosphine imide. The related heteroatom stabilized nitrene (CO)
5
W(NNMe
2
) undergoes CO substitution with PPh
3
and DPPE in preference to phosphine addition at nitrogen although reaction with DPPM yields the metallacyclic phosphinimine,
(CO)
4
W(PPh
2
CH
2
PPh
2
)NNMe
2
. The putative nitrene intermediate from the reaction of fac-Mo(CO)
3
(NCCH
3
)
2
PPh
3
with 8-azidoquinoline is trapped by phosphine to yield
(CO)
4
Mo[N(PPh
3
)(C
9
H
6
N)]. Neutral amido complexes, Tp
’
(CO)
2
W(NHR) [R = Bu t
(1a),
Ph (1b), Bu n (1c), CH
2
Ph (1d), and H (1e)], serve as precursors to both cationic,
Tp
’
(CO)
2
W(NR)
+
, and anionic, Tp
’
(CO)
2
W(NR)
-
, imido complexes.

Synthesis and Reactivity of Tp
’
(CO)
2
W(N(R)R
’
)
Reaction of Tp
’
(CO)
3
WI [Tp
’
= hydridotris(3,5-dimethylpyrazolyl)borate] with excess amine [NH
2
Bu t
or NH
2
Ph) in refluxing THF leads to the formation of amido(dicarbonyl)tungsten(II)d
4
complexes Tp
’
(CO)
2
W(NHR) [R = Bu t
and Ph]
N
N
H
B
N
N
N
N
=
N
B
N
N
Figure 1: Hydridotris(3,5-dimethylpyrazolyl)borate
Tp
’
(CO)
3
WI

[ NH
2
R, -CO, THF, and reflux ]

Tp
’
(CO)
2
W

NHR + [NH
3
R]I
This reaction most probably proceeds through an unsaturated dicarbonyl iodide species that is observed by IR spectroscopy (1944, 1844 cm
-1
) during the course of the reaction.
These stretching frequencies are identical to those observed when Tp
’
(CO)
3
WI is refluxed in THF in the absence of amine to form a paramagnetic 16 electron species,
Tp
’
(CO)
2
WI, which can be isolated as an orange microcrystalline solid.
Tp
’
(CO)
3
WI

[ -CO, THF, and reflux ]

Tp
’
(CO)
2
WI
Tp
’
(CO)
2
WI reacts with amines under relatively mild conditions and, for this reason, is more satisfactory than Tp
’
(CO)
3
WI for the synthesis of Tp
’
(CO)
2
W(NHR).
These complexes are stable for long periods of time when stored under nitrogen, except for the NH
2
derivative which decomposes after 1 or 2 weeks even at –20C.
Tp
’
(CO)
2
WI

[ NH
2
R ]

Tp
’
(CO)
2
W

NHR + [NH
3
R]I

Complex
1 a
1 b
1 c
1 d
1 e
R
Bu t
Ph
Bu n
CH
2
Ph
H
Conditions
CH
2
Cl
2
, 25C
CH
2
Cl
2
, 25C
THF, 25C
CH
2
Cl
2
, 25C
CH
2
Cl
2
, -50C
Spectroscopic studies of 1a-e support the formulation as neutral amido complexes with C s
symmetry. Carbonyl stretching frequencies near 1910 and 1790 cm
-1
for 1a-e are compatible with expectations for neutral d
4
complexes. The presence of a molecular mirror plane is evident from both the 2:1 pattern for the pyrazole rings in the
1
H and
13
C
NMR spectra and single 13 C resonance for the two carbonyl ligands. C s
symmetry on the
NMR time scale is compatible with either a static NHR fragment that lies in the mirror plane or with rapid rotation around the W-N bond. If the amido ligand is static and lies in the mirror plane, there are two possible isomers, one with R oriented away from the bulky Tp
’
ligand (anti) and one with R located near the Tp
’
ligand (syn).
B B
N
N N
CO
W
H
CO
N
R anti
N
N N
CO
W
R
CO
N
H syn
Figure 2: Anti and syn isomers of Tp'(CO)2W(NHR).

1
H NMR signals in the range 10-16 ppm are diagnostic for coordinated amido protons (N H R). Tp
’
(CO)
2
W(NHBu n ) (1c) and Tp
’
(CO)
2
W(NHCH
2
Ph) (1d) exist as a 6:1 mixtures of two isomers in solution at room temperature, as indicated by their
1
H NMR signals for the amido hydrogens at 13.7 and 11.9 ppm (1c) and 13.6 and 11.8 ppm (1d), respectively. Tp
’
(CO)2W(NHPh) (1b) also exists as a mixture of two isomers in solution.
In this case, the major isomer can be separated by crystallization and obtained as a solid.
Kinetics experiments on formation of the second isomer of 1b have been carried out at
22C. Upon dissolution of crystals of the major isomer of 1b (15.3 ppm, N H ) in CD
2
Cl
2
, the minor isomer (13.0 ppm, N H ) is observed to grow in over 5 days. The minor isomer accounts for 11.8% of the total material at equilibrium which corresponds to K eq
= 0.13.
The interconversion of the two isomers is a first order process with rates of interconversion

1
= 7.0 X 10
-6
s
-1
and

-1
= 5.2 X 10
-5
s
-1 corresponding to

G
*
= 24.2 kcal/mol and

G
*
= 23.0 kcal/mol, respectively.
B
N
N
CO
W
N
H
CO
N
Ph k1 k-1
B
N
N
CO
W
N
Ph
CO
N
H

For the Tp’(CO)
2
W(NHR) complexes it is expected that restricted rotation around the tungsten amide multiple bond is the mode of isomerization. When R is very bulky
(Bu n
, 1a), only the favored isomer is observed. Complex 1a displays a single N H resonance at 14.15 ppm. A static anti geometry is likely to exist for this complex based on the steric hindrance of the bulky Bu t
group compared to the hydrogen atom. When R is somewhat less bulky (Ph, 1b; Bu n
, 1c; CH
2
Ph, 1d), two isomers, syn and anti, are observed in solution.
Two broad signals at 13.5 and 11.6 ppm in the room temperature
1
H NMR spectrum of the parent amido complex Tp’(CO)
2
W(NH
2
) (1e) are assigned to the two amido protons. These signals coalesce at 103C, corresponding to

G
*
= 17 kcal/mol.
Thus this barrier to rotation is significantly lower than the barriers for interconversion of the syn and anti isomers of Tp’(CO)
2
W(NHPh) (1b). Rotation around the tungstennitrogen bond in Tp’(CO)
2
W(NHPh) (1b) may be expected to be more restricted than that in Tp’(CO)
2
W(NH
2
) (1e) based on the steric encumbrance of the phenyl group which must rotate past the Tp’ methyl groups.
Deprotonation of Tp’(CO)
2
W(NHR) amido complexes 1b, 1d, and 1e with LDA
(1d and 1e) or Bu t
Li (1b) yields reactive dicarbonyl species. On the basis of the low carbonyl stretching frequencies (1750 and 1652 cm
-1
, 1b; 1858 and 1720 cm
-1
, 1d; 1861 and 1724 cm
-1
, 1e), most probably anionic tungsten nitrene complexes,
[Li][Tp’(CO)
2
W(NR)] (3a-c), are formed.
Tp’(CO)
2
W

NHR

[ LDA Or Bu t
, THF ]

[Tp
’
(CO)
2
W=NR
-
Li
+
NRR’ 
Tp’(CO)
2
W

[ R’X ]
 

Complex
3a
3b
3c
4a
4b
R
Ph
CH
2
Ph
H
Ph
CH
2
Ph
R’X
PhCH
MeI
2
Br
These anionic dicarbonyl nitrene complexes are susceptible to protonation by traces of moisture to reform the starting material, Tp’(CO)
2
W(NHR). When quenched with alkylating agents at low temperature, however, dialkyl substituted amido complexes 4a-c are formed. In the reaction of Tp’(CO)
2
W(NH
2
) (1e) with base followed by MeI, double alkylation occurs.
Tp’(CO)
2
W

NH
2

[ 1. Base, 2. MeI (excess) ]

Tp’(CO)
2
W

NMe
2
Under these conditions, perhaps any Tp’(CO)
2
W(NHMe) formed is susceptible to deprotonation which is followed by methylation by excess MeI.
B
N
N N
CO
W
CO
N
R'
R
Schematic representation of amido orientation
Spectroscopic data for 4a-c are similar to those observed for 1a-e. consequently, similar structural formulation is proposed for 4a-c.

Single crystal X-ray diffraction studies of Tp’(CO)
2
W(N(Ph)CH
2
Ph) (4a) and
Tp’(CO)
2
W(Nme
2
) (4c) unequivocally establish the geometry of these complexes and confirm that the NRR’ moiety lies in the molecular mirror plane. The 1.981Å (4a) and
1.956 Å (4c) W-N distances are appropriate for tungsten-nitrogen double bonds.
Although tungsten hydrazido species are sometimes considered to have metal-nitrogen double bonds, the two coordinate nitrogen in W-N-NR
2
systems exhibits W-N distances well below 2.0 Å and more in the range typical of tungsten nitrenes. The amide nitrogens are roughly sp
2
hydridized on the basis of bond angles: W(1)-N(3)-C(4), 122.7; W(1)-
N(3)-C(21), 125.6; and C(4)-N(3)-C(5), 111.1 (4a) and W(1)-N(3)-C(4), 128.7; W(1)-
N(3)-C(5), 121.2; and C(4)-N(3)-C(5), 110.1 (4c). The WNRR’ skeletons are planar and are located in the mirror plane of the molecules. The phenyl group of 4a is situated anti to the Tp’ ligand. x
B
N
N
R'
N W N y
CO CO
R dyz py
Dπ-nitrogen pπ interactions for Tp’(CO)
2
W(NRR’) z
For acute OC-M-CO angles optimal π-bonding occurs when the amide (NRR’) is in the mirror plane. This vertical arrangement allows donation from the nitrogen (p y
) into the lone vacant dπ orbital (d yz
). The two filled dπ orbitals (d xy
and d x
2
-y
2 ) are stabilized by back-bonding to the two π-acid carbonyl ligands. Preferential stabilization of the filled d xz orbital occurs when the angle between the carbonyls is decreased to less than 90 in order

to increase the overlap of the CO π
*
orbitals with the filled d xz
orbital while overlap with the vacant d yz
orbital decreases. Intensity measurements for the carbonyl infrared stretches of this series of Tp’(CO)
2
W(N(R)R’) complexes indicate that the acute angle between the carbonyls is maintained in solution (80, 1b; 86, 1d; 85, 4b); by x-ray diffraction the angles between the carbonyls of 4a and 4c are observed to be 74.6 and
71.8, respectively, in the solid state
IR data in cm
-1
for complexes 1a-e, 2, 3a-c, and 4a-c. b
CH
2
Cl
2
solution. c
Nujol mull. v (BH) c
Complex
1a, Tp’(CO)
2
WNHBu t
)
1b, Tp’(CO)
2
WNPh) v (CO) b
1910, 1782
1900, 1786
Other
3260, 3121 (N-H)
3231, 3138 (N-H) c c
1c, Tp’(CO)
2
WNHBu n
)
1d, Tp’(CO)
2
WNHCH
2
Ph)
1e, Tp’(CO)
2
WNH
2
)
2, Tp’(CO)
2
WI
1911, 1794
1910, 1784
1915, 1793
1944, 1844
3a, [Li][Tp’(CO)
2
W(NPh)] 1750, 1652
3b, [Li][Tp’(CO)
2
W(NCH
2
Ph)] 1858, 1720
3c, [Li][Tp’(CO)
2
W(NH)] 1861, 1724
3260, 3125 (N-H)
3311, 3261 (N-H)
3395, 3305 (N-H) c c c
4a, Tp’(CO)
2
W(N(Ph)CH
2
Ph) 1914, 1787
4b, Tp’(CO)
2
W(N(Me)CH
2
Ph) 1904, 1777
4a, Tp’(CO)
2
W(NMe
2
) 1904, 1777
2546
2548
2540
2544
2540
2540
2536

0
5
5
5
2
+
4
Organoimido complexes of the transition metals have been implicated in catalytic processes such as propylene ammoxidation and nitrile reduction and have been shown to function as imido transfer intermediates in the aziridination and amination of olefins.
Recent achievements in imido chemistry include the generation of reactive M=NR ligands which can serve as sites for C-H bond activation (e.g. ( t
Bu
3
SiNH)
2
Ti(=NSi t
Bu
3
) and ( t
Bu
3
SiNH)
2
Zr(=NSi t
Bu
3
)) or for cycloaddition chemistry (e.g. (η
5
-C
5
Me
5
)Ir(=NR),
(η 6
-arene)Os(=NR), and (η
5
-C
5
H
5
)
2
Zr(=NR)). One important aspect of these compounds is the metal’s coordination by multiple π donor, a feature which may contribute to destabilizing strong metal-ligand d{π}

p{π} interactions and which has prompted efforts in multiple imido chemistry.
Preparation and properties of (η 5
-C
5
Me
5
)W(=NAr)
2
Cl and its derivatives: Upon reaction of W(=NAr)
2
Cl
2
(THF)
2
with Li[C
5
Me
5
] in refluxing THF / toluene, burgundy crystals of compound 1 are obtained in high yield after workup (Scheme 1).
Spectroscopic data and elemental analyses for 1 are consistent with its formulation as (η 5 -
C
5
Me
5
)W(=NAr)
2
Cl. The
1
H and
13
C NMR spectra for 1 reveal a compound with a molecular plane of symmetry and free rotation about the W-N-C linkage as characterized by the single C
5
Me
5
and C H Me
2
resonances but two CH Me
2
environments. (η
5
-
C
5
Me
5
)W(=NAr)
2
Cl (1) is soluble in alkanes, and is quite stable thermally as samples of
1 show no decomposition in refluxing C
6
D
6
even after several days. Formally, 1 is related to the tris(imido) anion [W(=NAr)
3
Cl]
-
in that this neutral analog maintains a
4 Organometallics, vol. 12, No. 1, 1993, pages 91-97.

coordination sphere of three 1σ, 2π donor ligands. Tungsten tris(imido) complexes are susceptible to electrophilic attack at the imido nitrogens; thus, the reaction of (η 5 -
C
5
Me
5
)W(=NAr)
2
Cl (1) with MeI and OCNR (R = t
Bu, Ph, 2,6-C
6
H
3
Me
2
) were carried out. However, in no case was a reaction observed, even under severe conditions
(refluxing C
6
D
6
, sealed tube, 5 days).
(η 5
-C
5
Me
5
)W(=NAr)
2
Cl (1) is functionalized using MeLi or PhLi in toluene to provide moderate yields of orange red (η 5 -C
5
Me
5
)W(=NAr)
2
Me (2) and red brown (η 5 -
C
5
Me
5
)W(=NAr)
2
Ph (3), respectively.
SCHEME 1
1
Cl
O
N
W
Cl
N
O
Li [C5Me5]
THF, Toluene, heat
W
N
Cl
N
2
N
W
CH
3
N
MeLi, toluene, heat
3
W
N
Ph
N
PhLi toluene heat
LiBEt3H
THF heat
W
N
H
N
4

These reactions require forcing conditions to proceed to completion ( ≥ 90C for days), which indicates the thermal stability of complexes 2 and 3. Attempts to prepare alkyl derivatives from Grignard reagents and attempts to prepare alkoxide and phenoxide compounds with LiOR provide no reaction. (η 5
-C
5
Me
5
)W(=NAr)
2
Cl (1) also reacts with
LiBEt
3
H in refluxing THF to afford cherry red crystals of the hydride derivative (η
5
-
C
5
Me
5
)W(=NAr)
2
H (4) in 88% yield. Complex 4 is characterized by a hydride resonance at δ 6.65 (C
6
D
6
) in its 1 H NMR spectrum and a v (W-H) at 1938 cm -1 in its infrared spectrum (CsI). This is confirmed by the disappearance of the δ 6.65 signal from the
1
H
NMR spectrum of the deuterated analog (η 5
-C
5
Me
5
)W(=NAr)
2
D and the isotopic shift observed in the infrared spectrum of 4-d where v (W-D) appears at 1395 cm
-1
.
5
1
5
4
2
2
Upon reaction of (η 5
-C
5
Me
5
)W(=NAr)
2
Cl (1) with 1 equivalent of LiNHAr
(refluxing THF), orange crystalline 5 is obtained in high yield. The
1
H and
13
C NMR spectra of 5 reveal a structure with a molecular plane of symmetry, but without a coordinated NHAr ligand. The NMR data and elemental analysis all support the formulation of 5 as the “tucked-in” complex (η 5 ,η 1
-C
5
Me
4
CH
2
)W(=NAr)
2
. Thus, three
Cp
* 1 H NMR resonances are observed with the lowest field signal at δ 3.97 (C
6
D
6
) integrating for 2 protons and showing doublet satellites with (
183
W-
1
H) = 11.0 Hz.

5
N
W
Cl
N
LiNH
THF, heat
N
W
N tBuLi
THF, -30C
4
N
W
N
room temp.
W
N
H
N
The formation of 5 proceeds quite slowly and, like the preparations of 2-4, requires forcing conditions. The reaction is extremely clean as examining solutions of 1
+
LiNHAr at intermediate reaction times reveals only the presence of 1 and 5.

A
NR = N
+ LiNHR
- LiCl
NR
NR
W
N
R
H
B
- H
2
NR
1
+ LiNHR
- H
2
NR
CH
2
NR
NR
W
Cl
W
N
W
Cl
N
N
C
+ LiNHR
- H2NR
- LiCl
W
NR
NHR
Cl
SCHEME 3
Scheme 3 presents possible mechanisms for the formation of 5 which include: (i) chloride displacement by [NHAr] to form A, followed by the deprotonation of a Cp * methyl by [NHAr]
-
; (ii) intermolecular deprotonation of a η
5
-C
5
Me
5 methyl group by
[NHAr]
-
to generate B, followed by chloride displacement by a formal η
5
-C
5
Me
4
CH
2 carbanion; and (iii) intramolecular addition of a Cp
*
C-H bond across W=NR, generating
(η 5 ,η 1
-C
5
Me
4
CH
2
)W(=NAr)(NHAr)Cl (C ), followed by its (inter- or intramolecular) reaction with LiNHAr to remove “HCl.”
N
5
These results are most consistent with 5 arising via pathway i. The reaction of 1 to generate 4 in scheme 2 appears to prove that the reagents used served as nucleophiles to displace chloride at the metal.

Discussion:
The cyclopentadienyl anion [C
5
R
5
] , imido dianion [NAr] 2, and acetylene dianion
[RC=CR]
2-
, as well as oxo O
2-
, nitrido N
3-
, and alkylidyne [CR]
3-
ligands, may all be described as “1δ, 2π” donors. There are several examples where these ligands can be interchanged to afford congeners within or between groups. For example, the series
Re(=NR) n
(RC≡CR)
3-n
X has been reported for n = 3, 2, 1, 0. This analogy is best illustrated with 3-fold symmetry. For example, the electronic similarities between the classic “20-electrons” W(RC≡CR)
3
L species and imido complexes W(=NAr)
3
L and
Os(=NR)
3
, each of these complexes is characterized by a ligand based, nonbonding MO comprised of a set of ligand π orbitals oriented roughly perpendicular to the molecule’s
C
3
axis. Therefore while each of these compounds may be considered a 20-electron species if the ligand donates their full complement of electrons to the metal, in 3-fold symmetry that occupation of this nonbonding MO results in these compounds being more accurately described as 18-electron complexes.
Since the alkyl derivatives (η 5
-C
5
Me
5
)W(=NAr)
2
R (3 and 4) are so thermally stable, the instability of (η 5
-C
5
Me
5
)W(=NAr)
2
(NHAr) must relate the availability of the amido lone pair, i.e. this lone pair is not engaging in bonding to the metal center. Such a notion is consistent with the orbital picture developed for M(1δ, 2π)
3
“π-loaded” metal centers. The orbital interactions which allow the [(η 5
-C
5
Me
5
)W(=NAr)
2
]
+
functional group to be described as a formal 16-electron fragment effectively restrict the metal from accepting more than 2 additional electrons from another ligand. Therefore, the amido ligand in η 5 -C
5
Me
5
)W(=NAr)
2
(NHAr) does not appear to π donate to the already “πloaded” metal center. The amido lone pair in such a species will be highly accessible for

deprotonating a η 5
-C
5
Me
5 methyl group which is consistent with the instability of inferred
(η 5 -C
5
Me
5
)W(=NAr)
2
(NHAr) with respect to (η 5 ,η 1 -C
5
Me
4
CH
2
)W(=NAr)
2
(5).
5
O
O
Ph
Ph
1
Ph
NH
NH
Ph
2
2 i
O
Ph
O
Ph
2
Ph
Cl
N
W
N
Cl
Ph
O
O
Synthesis of Tungsten complex: Reagents and conditions: i, Na
2
WO
4
, 1, Me
3
SiCl, Et
3
N, dme (W: 1: Me
3
SiCl:Et
3
N = 1:1:20:20), 12h, 85C. These conditions are expected to work for Tungsten, based on the assumption, Mo and W will behave in very similar ways in this reaction.

This report contained a brief introduction of Tungsten, few examples of imido and amido complexes of tungsten. Finally, the first chiral diimido chelate complexes of tungsten is added. There are many journal articles studying specific complexes of tungsten. Few articles were extra helpful, i.e. Synthesis and reactivity of low-valent amido, imido, azavinylidene, and nitrido complexes of tungsten by K. R. Powell et al .
The structures and diagrams included in this paper were created using ISIS draw, the origins of these drawings and schemes are in the references of the footnotes.
The most important lesson learned is the appreciation of the broad range of research opportunities in the field of inorganic chemistry. Also, the interesting knowledge of the uses of tungsten and its importance to humans.
5 Chem. Commun., 1999, 2381-2382.

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Brookhart, M. et al. Synthesis and Reactivity of Low-Valent Amido, Imido,
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