Interplay of ketenyl and nitrile ligands on d6-transition metal centres. Acetonitrile as an end-on (two-electron) and a side-on (four-electron) ligand

Article abstract of DOI:10.1016/S0022-328X(99)00328-9

Carbyne carbonyl coupling takes place in acetonitrile solutions of the Fischer-type carbyne complexes [tpb′(CO)₂Mo≡CC₆H₄Me-4] (4b) (tpb′ = tris(3,5-dimethylpyrazolyl)hydroborate) and [(C₅Me₅) (CO)₂W≡CSiPh₃] (5). The product [tpb′(CO)(NCMe)Mo{C(C₆H₄Me-4)CO}] (7b) has conventional η¹-nitrile and η²-ketenyl ligands. In contrast, the product [(C₅Me₅)(CO)(NCMe)W{C(SiPh₃)CO}] (8) displays a highly unusual 'four-electron-donor' η²-nitrile and a η¹-ketenyl ligand. The structures of 4b, [tpb′(CO)₂W≡CC₆H₄Me-4] 6, 7b and 8 were determined by X-ray crystallography. Extended Hueckel and density functional theory (DFT) calculations on model compounds show that the observed coordination geometry around W is the electronically preferred for 8. The side-on nitrile in this and other related complexes binds to the metal donating from the two highest occupied orbitals (corresponding to the nitrogen lone pair and the π_⊥), in contrast to two-electron donors where only the HOMO is involved in bonding. Back-donation takes place for all the side-on bound nitriles.

Full text of DOI:10.1016/S0022-328X(99)00328-9

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Journal of Organometallic Chemistry 587 (1999) 233–243
Interplay of ketenyl and nitrile ligands on d6-transition metal
centres. Acetonitrile as an end-on (two-electron) and a side-on
(four-electron) ligand
Hubert Wadepohl a,*, Ulrich Arnold a, Hans Pritzkow a, Maria Jose´ Calhorda b,1,
Lu´ıs F. Veiros c
a Anorganisch-Chemisches Institut der Ruprecht-Karls-Uni6ersita¨t, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
b ITQB, Quinta do Marqueˆs, EAN, Apart. 127, 2781-901 Oeiras, Portugal and Departamento de Qu´ımica e Bioqu´ımica, Faculdade de Cieˆncias,
Uni6ersidade de Lisboa, 1749-016 Lisbon, Portugal
c CQE, Instituto Superior Te´cnico, A6. Ro6isco Pais, 1096 Lisbon Codex, Portugal
Received 12 March 1999; received in revised form 9 April 1999
Dedicated to Professor Dirk Walther on the occasion of his 60th birthday.
Abstract
Carbyne carbonyl coupling takes place in acetonitrile solutions of the Fischer-type carbyne complexes
[tpb%(CO)2MoꢀCC6H4Me-4] (4b) (tpb%=tris(3,5-dimethylpyrazolyl)hydroborate) and [(C5Me5) (CO)2WꢀCSiPh3] (5). The product
[tpb%(CO)(NCMe)Mo{C(C6H4Me-4)CO}] (7b) has conventional h1-nitrile and h2-ketenyl ligands. In contrast, the product
[(C5Me5)(CO)(NCMe)W{C(SiPh3)CO}] (8) displays a highly unusual ‘four-electron-donor’ h2-nitrile and a h1-ketenyl ligand. The
structures of 4b, [tpb%(CO)2WꢀCC6H4Me-4] 6, 7b and 8 were determined by X-ray crystallography. Extended Hu¨ckel and density
functional theory (DFT) calculations on model compounds show that the observed coordination geometry around W is the
electronically preferred for 8. The side-on nitrile in this and other related complexes binds to the metal donating from the two
highest occupied orbitals (corresponding to the nitrogen lone pair and the pÞ), in contrast to two-electron donors where only the
HOMO is involved in bonding. Back-donation takes place for all the side-on bound nitriles. © 1999 Elsevier Science S.A. All
rights reserved.
Keywords: Ketenyl; Nitrile; d6-Transition metal centers
1. Introduction
coupling reactions have been discovered, not only in-
duced by nucleophiles (such as PMe3) but also by
electrophiles or photons [3,4]. The structures and reac-
tions of ketenyl complexes have been reviewed [5].
Coupling of an alkylidyne and a carbonyl ligand is a
general reactivity pattern for low-valent (especially Fis-
cher-type) metal carbyne complexes. This reaction was
first reported in 1976 by Kreißl et al., who obtained the
h1-ketenyl complex 3 (M=W, R=p-tolyl) when tolyl-
carbyne cyclopentadienyl dicarbonyltungsten 1 (M=
W, R=p-tolyl) was treated with two equivalents of
PMe3 [1]. Subsequently, it was shown by the same
group that formation of the h1-ketenyl complexes 3
proceeds via the h2-ketenyl complexes 2 [2]. In the
following years more examples of alkylidyne carbonyl
* Corresponding author. Tel.: +49-6221-54-4827; fax: +49-6221-
54-4197.
E-mail address: bu9@ix.urz.uni-heidelberg.de (H. Wadepohl)
1 Also corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00328-9

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H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
4 days, quantitative conversion of 4b into the ketenyl
In the course of our studies of the hydroboration of
complex 7b was observed. Such a carbyne–carbonyl
coupling reaction to give 7a had previously been ob-
served with the phenylcarbyne derivative 4a upon irra-
diation with Pyrex-filtered UV light [9]. Under similar
conditions, the tungsten complex 6 did not undergo this
reaction. It had been noted earlier that the tris(pyra-
zolyl) derivative of 6, [tpb(CO)2WꢀCC6H4Me-4], while
reacting with PMe3 to afford the ketenyl complex [tpb-
(CO)(PMe3)W{h2-C(p-tolyl)CꢁO}] [10] did not give an
analogous product with the less nucleophilic PPh3 [11].
metal carbon multiple bonds we discovered that during
the reactions of the Fischer-type metal carbyne com-
plexes [X(CO)2MꢀCR] (X=C5H5, C5Me5, tpb, tpb%;
M=Mo, W) [tpb=tris(pyrazolyl)hydroborate, tpb%=
tris(3,5-dimethylpyrazolyl)hydroborate] with hydrobo-
ranes different reaction pathways are followed,
depending on the nature of X [6]. These observations
prompted a more general study of the alkylidyne reac-
tivity of some derivatives [X(CO)2MꢀR] with strongly
electron donating ligands X. Here we wish to report the
differing structures of the products that resulted from
the intramolecular carbyne–carbonyl coupling induced
by acetonitrile in the complexes [tpb%(CO)2-
MoꢀCC6H4Me-4] (4b) and [(C5Me5)(CO)2WꢀCSiPh3]
(5).
2. Results and discussion
2.1. Tris(dimethylpyrazolyl)borate complexes
A
number of carbyne complexes with the
tpb%(CO)2(d6-M) fragment have been reported, includ-
ing the phenylcarbyne molybdenum 4a [7,9] and
p-tolylcarbyne tungsten 6 [8] derivatives. Using a mod-
ification of the procedure published for 6, we prepared
the new complex 4b in 57% yield from
As expected, the spectroscopic data of 7b are very
similar to those of 7a and need not be discussed here.
Single crystals of 4b·MeCN and 7b·MeCN were ob-
tained by fractional crystallisation of the reaction mix-
tures before conversion was complete. Crystals of
6·MeCN were grown from acetonitrile at −20°C.
The crystal structures of 4b·MeCN and of 6·MeCN
are isotypic. In both cases one molecule of acetonitrile
is present in the asymmetric unit as a solvent of crys-
[Br(CO)4MoꢀCC6H4Me-4] and K(tpb%). The IR (wCO
)
and NMR spectroscopic data of 4b closely resemble the
published data for 6. When a saturated acetonitrile
solution of 4b was kept in the daylight, a colour change
from orange–red to dark-green took place. After about
,
Fig. 1. Molecular structure of [tpb%(CO)2MoꢀCC6H4Me-4] (4b). Important bond distances (A) and angles (°) with estimated S.D. in parentheses:
4b: Mo1–C1 1.804(4), Mo1–C9 1.987(4), Mo1–C10 1.987(4), Mo1–N1 2.218(3), Mo1–N3 2.212(3), Mo1–N5 2.306(3), C1–C2 1.449(5), C9–O1
1.145(5), C10–O2 1.140(5); Mo1–C1–C2 163.1(3), Mo1–C9–O1 176.9(4), Mo1–C10–O2 176.3(4). 6: W1–C1 1.829(3), W1–C9 1.985(4),
W1–C10 1.994(4), W1–N1 2.210(3), W1–N3 2.288(3), W1–N5 2.203(3), C1–C2 1.448(4), C9–O1 1.159(5), C10–O2 1.146(5); W1–C1–C2
163.2(3), W1–C9–O1 177.6(4), W1–C10–O2 176.3(4).

H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
235
tallisation. The molecular structure of 4b is shown in
Fig. 1.
The structural parameters of 4b and 6 are quite
similar, with only minor deviations caused by the two
different metals. The distances between the metal atoms
,
,
and the carbyne carbons, 1.80 A (4b) and 1.83 A (6),
fall within the usual range [12]. The carbyne fragments
are noticeably nonlinear on C1 (angle, metal–C1–C2
163°). This is different from carbyne complexes with the
isoelectronic (C5R5)(CO)2M fragment, where linear car-
bynes are generally found [12]. In view of a similar
distortion in [{(pz)4B}(CO)2WꢀCC6H4Me-4] [10] with
the less bulky tetrakis(pyrazolyl)borate [(pz)4B] ligand,
a steric cause for the observed deviation from linearity
is unlikely. A strong trans influence of the carbyne
ligands is evident in the lengthening of the metal–nitro-
,
,
gen bond trans to C1 (2.31 A versus 2.21, 2.22 A).
The linear acetonitrile molecules are situated in pairs
(with an antiparallel orientation, related by a centre of
symmetry) between the metal carbyne molecules. They
do not show any unusually short intermolecular con-
,
tacts. The nitrile N–C distances (1.112(7), 1.125(7) A)
are close to that in free acetonitrile [13,14].
The carbyne–carbonyl coupling product 7b also crys-
tallises as an acetonitrile solvate. The molecular struc-
ture of the ketenyl complex is shown in Fig. 2.
Fig. 2. Molecular structure of [tpb%(CO)(NCMe)Mo{C(C6H4Me-
,
4)ꢁCꢁO}] (7b). Important bond distances (A) and angles (°) with
The p-tolyl substituted ketenyl ligand in 7b adopts
the h2 coordination mode to the molybdenum atom.
The CC and CO bond lengths within the h2-C(R)CꢁO
unit are similar to those of the few other structurally
characterised h2-ketenyl complexes [2a,11,15,16,17,18];
they are somewhat lengthened with respect to free
ketene [19]. The nonlinear ketenyl ligand resembles an
h2-alkyne. The orientation of this ligand with respect to
rotation around the bond axis to the metal is such that
the ketenyl–CO group is placed adjacent to the car-
bonyl ligand C26O2. This is in accord with earlier
theoretical calculations [20] and with the geometry of
the other known examples [3].
estimated S.D. in parentheses: Mo1–C1 2.002(4), Mo1–C2 2.210(4),
Mo1–C26 1.917(5), Mo1–N1 2.173(4), Mo1–N3 2.270(4), Mo1–N5
2.262 (4), Mo1–N7 2.159(4), C1–C2 1.361(6), C1–C3 1.451(6), C2–
O1 1.194(6), C26–O2 1.185(6), N7–C27 1.137(6), C27–C28 1.444(7);
C3–C1–C2 134.3(4), C1–C2–O1 153.2(5), Mo1–C26–O2 176.6(4),
Mo1–N7–C27 179.0(4), N7–C27–C28 178.6(6).
triphenylsilyl carbyne complex 5 also undergoes car-
byne–carbonyl coupling, to give the ketenyl complex 8.
However, the coupling reaction is much more easily
reversible here, and 8 can only be obtained pure in the
crystalline solid state. Dissolved in methylene chloride
or benzene 8 slowly splits off the nitrile ligand to give a
green equilibrium mixture of orange–yellow 5 and blue
8.
Of the two linear acetonitrile molecules in the asym-
metric unit one is h1-bonded to the molybdenum centre
via its nitrogen atom. The other nitrile packs in be-
tween the molecules of 7b as a solvent of crystallisation.
Coordination of the nitrile to the metal results in a
,
lengthening of the NC bond by about 0.03 A. The bond
distances between the molybdenum atom and the pyra-
zolyl nitrogens fall in two groups, a shorter bond trans
,
to the nitrile ligand (2.17 A) and two longer ones trans
2
,
to carbon monoxide (2.26 A) and the h -ketenyl ligand
,
(2.27 A), respectively. Hence, a much larger trans influ-
ence can be assigned to the two latter ligands.
The crystal structure analysis of complex 8 reveals
coordination modes of the nitrile and ketenyl ligands
which are quite different from those in 7b (Fig. 3).
The Ph3Si-substituted ketenyl ligand binds to the
tungsten atom via its carbon atom C1 only. Compared
to the h2-ketenyl in 7b, the bond angles around C1
2.2. Pentamethylcyclopentadienyl complex
When treated with acetonitrile at −20°C and then
warmed to room temperature (r.t.), the tungsten

236
H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
more closely approach the value (120°) expected from
sp2-hybridisation. Despite the somewhat acute angle
W1–C1–C2 (105.2°), the linearity of the C1ꢁC2ꢁO1
WꢂC bonds as well as the long nitrile N–C distances.
The metal–nitrogen bond is shorter than the metal–
carbon bond. The data relative to other side-on bound
nitrile complexes in Table 1 show a tendency for longer
MꢂC and MꢂN bonds, as well as shorter NꢂC bonds in
the 2e-donor ligands. The so-called three-electron
donor nitrile complex 15 is, from a structural point of
view, more similar to the two-electron donors than to
the four-electron donors.
A correlation has been proposed between the elec-
tron donor properties of alkyne ligands and their
13C-NMR chemical shifts [22]. There are fairly well
separated regions on the l scale for alkynes formally
functioning as two, three or four-electron donors, the
latter resonating at the lowest field (highest l). Al-
though much less data are available, a similar be-
haviour for the carbon resonances of side-on
coordinated nitriles appears evident (Table 1). For 8,
the corresponding resonance (l=203.4) is consistent
with a four-electron-donor nitrile. h1-Ketenyl ligands
usually show the 13C(ꢁO) resonance in the low-field
region of the spectrum around l=150…170, and a
high-field signal (l=10…−30) for the metal-bound
carbon atom [2,16]. Carbon resonances due to the
ketenyl ligand in 8 are observed at l=169.3 and 20.7.
In the related h1-ketenyl complex [(C5H5)(CO)2-
(PMe3)W{C(SiPh3)CO}] a signal was found for the
metal-bound ketenyl carbon at a considerably higher
field, l= −49.7 [2c]. During the discussion of the
,
unit and the long distance W1···C2 of 2.808 A rule out
a significant interaction of the metal atom with C2.
The nonlinear (136°) acetonitrile ligand is bonded to
the tungsten atom in a side-on (h2) fashion. The coordi-
nation geometry, notably the short W1–N1 and W1–
C4 distances and the long bond N1–C4, indicates that
this ligand should be considered a four-electron donor,
as required for an 18 valence electron configuration of
the tungsten atom.
Although a number of examples have been reported
[21], the side-on coordination mode is quite unusual for
a nitrile. Even more, unlike the ubiquitous ‘four-elec-
tron-donor’ alkyne ligands [22], h2-coordinated nitriles
only very rarely serve as four-electron donors. To our
knowledge, this property has hitherto been established
in only a few complexes, which are collected in the first
section of Table 1. The second part contains another
nitrile complex, 15, where this ligand has been proposed
to be a three-electron donor. The last section contains a
few examples of unequivocal side-on bound two-elec-
tron-donor nitrile complexes, well characterised by X-
ray diffraction. The most relevant features of all
complexes are given in the table.
The acetonitrile ligand in 8 displays some of the
characteristics of the tungsten phosphine complexes
10–13, notably the above-mentioned short WꢂN and
,
Fig. 3. Molecular structure of [(C5Me5)(CO)(NCMe)W{C(SiPh3)ꢁCꢁO}] (8). Important distances (A) and angles (°) with estimated S.D. in
parentheses: W1–C1 2.174(3), W1–C3 1.942(4), W1–C4 2.058(4), W1–N1 2.020(3), W1–C(C5Me5) 2.295(4)–2.446(4), C1–C2 1.296(5), C1–Si1
1.848(4), C2–O1 1.180(5), C3–O2 1.165(5), C4–N1 1.257(5), C4–C5 1.487(5); C2–C1–Si1 123.4(3), W1–C1–C2 105.2(3), W1–C1–Si1 123.4(3),
C1–C2–O1 178.5(4), W1–C3–O2 174.7(3), N1–C4–C5 136.1(4).

H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
237
Table 1
Structural data of side-on bound nitrile complexes
l(13C)
Ref.
,
,
Complex
M–C (A)
M–N (A)
N–C (°)
N–C–C (°)
Four-electron donors
[W(C5Me5)(CO)(MeCN){C(SiPh3)CO}] (8)
[Mo(dmpe)2Cl(MECN)][BPh4] (9)
[W(bipy)(PMe3)2Cl(MeCN)][PF6] (10)
[W(PMe3)3Cl2(MECN)] (11)
[W(NMe–Paa)(CO)F(C5F5CN)] (12)
[W(NMe–Paa)(CO)F(NCC6F4CN)] (13)
[W(tpb%)(CO)(S2PR2)(MECN)] (14)
2.058(4)
1.981(7)
1.998(5)
1.98(1)
2.009(6)
2.039(9)
2.051(7)
2.020(3)
1.959(7)
2.008(4)
1.99(1)
1.995(5)
2.035(8)
2.033(6)
1.257(5)
1.217(8)
1.267(7)
1.27(2)
1.262(7)
1.28(1)
136
130
130
129
132
130
138
203
This work
[21e]
[21f]
[21g]
[21j]
a
235
230
225
226
[21j]
[21h,k]
1.225(9)
202–204
Three-electron donors
[Nb(tpb%)(CO)(PhCCMe)(C6H5CN)] (15)
2.17(1)
2.139(8)
1.21(1)
135
174–187
[21i]
Two-electron donors
[Mo(C5H5)2(MeCN)] (16)
[Ir(C5H5)(PPh3)(p-ClC6H4CN)] (17)
[Ir(C5Me5)(CO)(p-ClC6H4CN)] (18)
2.124(8)
2.105(23)
2.040(8)
2.219(7)
2.174(15)
2.185(5)
1.20(1)
1.231(28)
1.209(11)
139
171
[21b]
[21c]
[21c]
a
a
a
a
a Not reported.
structure of [(C5H5)(CO)(Et3NCꢀCMe)W{C(p-tolyl)-
CO}] (19), where an electron-rich alkyne plays the role
of the nitrile ligand of 8, Kreißl et al. pointed out the
significance of a possible dipolar resonance structure with
a carbene-type linkage of the ketenyl ligand to the
tungsten centre [23]. This is expected to shorten the
tungsten carbon bond, as is indeed observed in the
molecular structures of 8 and 19. It may also well be the
reason for the unusual chemical shift of the tungsten-
bound ketenyl carbon in 8. Unfortunately, no 13C-NMR
data were reported for 19 which would substantiate this
proposal.
because the energy of the degenerate LUMOs (p* and
Þ
pꢀ*) is relatively high, and, more importantly, because its
largest localisation is on the less electronegative carbon
atom, rather than on the nitrogen. The bending of the
nitrile allows mixing between s and pꢀ, reorienting them
in such a way that a better overlap with metal orbitals
can be achieved, if side-on coordination is considered. The
ligand becomes both a better donor and a better acceptor.
The frontier orbitals include the former lone pair s (6a%),
the two p orbitals, known as pÞ (2a%%) and pꢀ (5a%), and
two empty p* orbitals, known as p* (7a%) and p* (3a%%).
ꢀ
Þ
Side-on coordination allows for an efficient back-dona-
tion, owing to the good overlap and the lower energy of
the LUMO. On the other hand, bending the nitrile is an
energy consuming distortion, so that only for electron rich
metal centres, where back-donation is important, does
this process become favourable.
2.3. Theoretical calculations
There are two major objectives in this section, namely
to rationalise the bonding of the ‘four-electron-donor’
nitriles in complex 8 described above and in some of the
examples from the literature, comparing it with the
bonding in the two-electron-donor nitrile complexes, and
to trace the origin of the preference in 8 for a four-electron
nitrile instead of a four-electron ketenyl [24]. These studies
are based on extended Hu¨ckel calculations [25]. A
complete optimisation of the geometry of a model of
complex 8 was performed using density function theory
(DFT) calculations [26] (ADF program [27]).
Structural data are available for a group of complexes
containing side-on bound nitriles (Table 1). The simplest,
most symmetric, complex with a side-on nitrile considered
a ‘four-electron donor’ is [mer-W(PMe3)3Cl2(NCMe)],
(11), where tungsten exhibits an approximately octahe-
dral coordination. The frontier orbitals of linear and bent
NCH, as well as those of bent NCMe are shown in Scheme
1 in a three-dimensional CACAO representation [28].
When the nitrile is linear, it is a bad p-acceptor, both
The interaction diagram between the W(PH3)3Cl2
fragment and NCH is shown in Fig. 4.
The bonding can be described by donation of electrons
from 6a% (HOMO) and also from 2a%% to two empty d
orbitals of tungsten. There is some mixing with 5a%,
providing a better metal–ligand overlap. This scheme
agrees with our classification of the nitrile as a four-elec-
tron donor, as there are two bonding donation compo-
nents involving orbitals with different symmetry. On the
other hand, back donation occurs from W(PH3)3Cl2 1a
and 2a to the two p* orbitals. We can measure the amount
of donation and back donation by looking at the
occupation of the five orbitals of the nitrile that partici-
pate in binding to the metal. The initially occupied
orbitals (two electrons) become less populated in
the complex (6a% 1.846, 2a%% 1.879, 5a% 1.928), while the
initially empty ones have gained electrons (7a% 1.057, 3a%%

238
H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
Scheme 1.
0.482). In this system, both components of the bonding
help to weaken the NꢂC bond, which in complex 11 is
indeed quite long compared with that of the free nitrile
(see Table 1).
donor nitrile species because more electrons are lost
from bonding orbitals (from two instead of one), and
back-donation is comparable. We add in the table the
corresponding data for the related alkyne complex
[W(C5H5)(CO)(HCCO)(HCCH)], a model for [(C5H5)-
(CO)(Et3NCꢀCMe)W{C(p-tolyl)CO}] (19) [23], where
the two HOMOs donate electrons and the alkyne is a
four-electron-donor. The proposed three-electron-
donor complex 15 appears, from the orbital occupa-
tions (Table 2), to have two nitrile orbitals donating
electrons to the metal, but back-donation from the
metal is small. Therefore, NꢂC bonds are not so weak
and the NꢂC bond distance is in the range of those
observed for two-electron-donor nitriles.
The second aspect concerns [(C5Me5)(CO)(NCMe)-
W{C(SiPh3)ꢁCꢁO}] (8) and why the four electron
donor is the nitrile and not the ketenyl anion, as might
be expected, and as indeed happens for other com-
pounds, such as [tpb%(CO)(NCCH3)Mo{C(C6H4Me-
4)CO}] (7b). In order to trace this preference, we
performed both EH and DFT calculations. The bond-
ing of a side-on nitrile to the metal fragment was
described above. Decomposition of the model complex
[W(C5H5)(CO)(h1-H3SiCCO)(h2-NCMe)] molecule into
different fragments, such as [W(C5H5)(CO)(NCMe)]+
and (H3SiCꢁCꢁO)−, and comparison with the alterna-
tive [W(C5H5)(CO)(h2-H3SiCCO)(h1-NCMe)] employ-
ing similar decompositions is a helpful procedure. Both
h1-NCMe (end on) and h1-(H3SiCꢁCꢁO)− are only s
donors, but the ketenyl forms stronger bonds. The
overlap populations between the relevant fragment
molecular orbitals are 0.23 for the ketenyl and 0.19 for
The bonding in [(C5Me5)(CO)(NCMe)W{C(SiPh3)-
ꢁCꢁO}] (8) is not so easy to understand, owing to the
total lack of symmetry and mixing between all the
orbitals. It has been modelled for the extended Hu¨ckel
calculations by [W(C5H5)(CO)(H3SiCCO)(NCMe)]. The
interaction diagram between the metal fragment and
NCMe, shown in Fig. 5, is qualitatively similar to the
previous one, but mixing makes interpretation more
difficult. The electron occupations of the nitrile are 5a%
1.935, 2a%% 1.875, 6a% (HOMO) 1.894, 7a% (LUMO)
1.012, 3a%% 0.020 electrons, suggesting by comparison
with [W(PH3)3Cl2(NCH)] that two orbitals of the nitrile
are donating electrons and that the nitrile acts as a
four-electron donor. Two donation components are
present, from 2a%% and 6a% to 3a and 4a, but there is only
significant back-donation to one orbital, the nitrile
LUMO, 7a%. The HOMO of the metal fragment does
not have the right symmetry to interact with 3a%%.
Other complexes with side-on bound nitrile ligands
were examined, both four-electron donors and two-
electron donors. The geometry of the NCH model
nitrile was kept constant, to facilitate comparisons
across the series. The occupations of the five relevant
levels of the nitrile after binding to the metal centres are
compiled in Table 2. The two-electron-donor nitrile is
characterised by involvement of only the HOMO 6a% in
bonding to the metal, the participation of 2a%% being
negligible. The NꢂC bond is longer in the four-electron-

H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
239
Fig. 5. Molecular orbital diagram for the interaction between
[(C5H5)(CO)W{C(SiH3)ꢁCꢁO}] and NCMe.
Fig. 4. Molecular orbital diagram for the interaction between
[W(PMe3)3Cl2] and NCH.
coordination preferences of the ligands to be deter-
mined by electronic factors.
the nitrile. One reason for these values is the better
energy match between interacting orbitals in the case of
the ketene, thus remaining h1 bonded in complex 8.
h2-NCMe and h2-H3SiCCO are both good donors and
good p-acceptors.
The structure of the model compound W(C5H5)-
(CO)(HCCO)(NCH) was fully optimised using DFT
calculations. The final geometry is shown in Fig. 6, in
two different views. The agreement between the calcu-
lated and the observed structure is quite good, espe-
cially considering that the model is significantly less
bulky.
We performed a similar geometry optimisation con-
straining the nitrile ligand to stay coordinated through
the N atom. The final energy is 0.66 eV higher than for
the previous calculation, showing that it is a local
minimum.
Each of the two molecules W(h5-C5H5)(CO)
(h1-HCCO)(h2-NCH) (A) and W(h5-C5H5)(CO)
(h2- HCCO)(h1-NCH) (B) was decomposed into three
fragments, respectively W(h5-C5H5)(CO), the nitrile and
the ketenyl. The bonding energy of each molecule calcu-
Some distances and angles can be compared. For
,
instance, the W–N distances (A) are 2.020 and 2.106
(calculated values in italics), W–C(nitrile) 2.058 and
2.107, C–N 1.257 and 1.248, W–C(CO) 1.942 and
1.936, C–O 1.165 and 1.171, W–C(ketenyl) 2.174 and
2.123, W–C%(ketenyl) 2.809 and 2.794, C–C%(ketenyl)
1.298 and 1.315, C–O(ketenyl), 1.180 and 1.176, while
some relevant angles (°) are the following: N–C–C/H
136.1 and 135.6, W–C–O 174.7 and 175.9, CꢁC%ꢁO
178.5 and 177.4.
Fig. 6. DFT optimised geometry of [(C5H5)(CO)W(NCH)-
This lowest-energy structure is very similar to the
X-ray-determined structure of complex 8, showing the
(HCꢁCꢁO)], in a side view (left) and a top view (right). Bond
,
distances (A) are given in normal typeface, angles (°) in italics.

240
H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
Table 2
Occupation of the frontier orbitals of side-on bound nitrile in several model complexes
Complex
Model for complex
3a%%
7a%
6a%
2a%%
5a%
Four-electron donors
[W(C5H5)(CO)(NCH){C(H)CO}]
[Mo(PH3)4Cl(HCN)]+
[W(PH3)3Cl2(HCN)]
8
9
11
0.020
0.347
0.482
1.012
1.008
1.057
1.894
1.840
1.846
1.875
1.866
1.879
1.935
1.925
1.928
Three-electron donors
[Nb(tpb%)(CO)(HCCH)(HCN)]
15
0.030
0.683
1.803
1.878
1.905
Two-electron donors
[Mo(C5H5)2(HCN)]
[Ir(C5H5)(PH3)(HCN)]
16
17
0.017
0.059
0.620
0.904
1.853
1.833
1.963
1.996
1.936
1.890
Four-electron donor (alkyne)
[W(C5H5)(CO)(HCCH){C(H)CO}]
19
0.030
0.866
1.764
1.750
1.935
−20°C for 80 min and then cooled to −80°C. The
supernatant was removed and the residue suspended in
cold (−40°C) CH2Cl2. Solid Ktpb% (3.0 g, 8.9 mmol)
was added and the mixture was stirred at −20°C for
36 h. After removal of solvent under reduced pressure,
the dark-yellow residue was chromatographed two
times at r.t. on Al2O3 with 4:1 n-hexane–CH2Cl2 as the
mobile phase. The product was obtained from the
orange–yellow fraction after removal of solvent under
reduced pressure. Recrystallisation from MeCN at 4°C
affords orange–yellow crystals of 4b·MeCN (2.8 g,
57%). IR (in THF) wCO (cm−1): 1982(m), 1899(s) —
1H-NMR (in CDCl3): l 2.32 (s, 3H, Me-4), 2.34 (s, 3H,
tpb%ꢂCH3), 2.36 (s, 6H, tpb%ꢂCH3), 2.41 (s, 3H,
tpb%ꢂCH3), 2.49 (s, 6H, tpb%ꢂCH3), 5.74 (s, 1H,
tpb%ꢂCH), 5.83 (s, 2H, tpb%ꢂCH), 7.09 and 7.44 [(AB)2-
lated relative to that of the fragments was −7.97 eV
for A and −6.86 eV for B. The bonding energy can be
decomposed into several terms. Among the most im-
portant we can identify the Pauli repulsion, which
amounts to 23.23 eV for A and 17.26 eV for B. This
term is compensated both by the stronger electrostatic
interaction in A (−13.66 vs. −9.22 eV) and the more
favourable orbital interactions in A (−17.64 vs.
−15.13 eV). We can therefore assign the most stable
geometry to stronger electrostatic and covalent interac-
tions, in spite of a larger repulsion between closed
shells.
3. Experimental
3
system, JHH=8.1 Hz, 4H, C6H4) — 13C{1H}-NMR
3.1. General procedures
(in CDCl3): l 12.6, 12.7, 14.6, 15.8 (tpb%ꢂCH3), 21.6
(Me-4), 106.1, 106.2 (tpb%ꢂCH), 128.8, 129.1 (C6H4),
138.6, 143.8, 144.4, 144.8, 151.1, 151.2, (tpb%ꢂCCH3,
C6H4ꢂC-1, C-4), 225.8 (CO); 288.9 (carbyne-C) —
11B{1H}-NMR (CDCl3): l −8.8 — MS: m/z (relative
intensity) 554 (8, M+), 526 (36, [M−CO]+), 498 (100,
[M−2CO]+).
All operations were carried out under an atmosphere
of purified argon (BASF R3-11 catalyst) using Schlenk
techniques. Solvents were dried by conventional meth-
ods. The compounds [(CO)5Mo{C(OMe)C6H4Me-4}]
[29], [tpb%(CO)2WCC6H4Me-4] (6) [8] and [Br(CO)4-
WCSiPh3] [30] were prepared as described in the litera-
ture. NMR spectra were obtained on a Bruker AC-200
instrument (200.1 MHz for H, 50.3 MHz for 13C). H
and 13C chemical shifts are reported versus SiMe4 and
were determined by reference to internal SiMe4 or
residual solvent peaks. The assignment of the 13C reso-
nances was corroborated by DEPT spectra. IR spectra
were measured with a Bruker IFS-28 Fourier transform
spectrometer in CaF2 cells.
1
1
3.3. Preparation of [(C5Me5)(CO)2WCSiPh3] (5)
Complex 5 was prepared from 3 g (4.6 mmol) of
[Br(CO)4WCSiPh3] and 1.2 g (6.9 mmol) of K(C5Me5)
following the procedure described for the C5H5 deriva-
tive in Ref. [30]. Yield, 880 mg (30%) orange microcrys-
tals. IR (in CH2Cl2) wCO (cm−1): 1987 (s), 1909 (vs) —
1H-NMR (in CD2Cl2): l 2.13 (s, 15H, C5Me5), 7.48 (m,
15H, SiPh3) — 13C{1H}-NMR (in CD2Cl2): l 11.3
(C5Me5), 105.7 (C5Me5), 128.1, 129.7, 135.2, 136.0 (Ph),
227.5 (CO), 355.1 (carbyne-C) — MS: m/z (relative
intensity) 646 (46, M+), 618 (36, [M−CO]+), 590
(100, [M−2CO]+).
3.2. Preparation of [tpb%(CO)2MoCC6H4Me-4] (4b)
A 3.0 g (12.0 mmol) sample of BBr3 was slowly
added to a cold (−15°C) suspension of 3.28 g (8.9
mmol) [(CO)5Mo{C(OMe)C6H4Me-4}] in 125 ml of
cold pentane. The mixture was stirred vigorously at

H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
241
Table 3
3.4. Preparation of
Details of the crystal structure determinations of the complexes
[tpb%(CO)2MCC6H4Me-4] (4b) (M=Mo) and (6) (M=W)
[tpb%(CO)(NCMe)Mo{C(C6H4Me-4)CO}] (7b)
A 200 mg (0.4 mmol) sample of 4b was dissolved in
the minimum amount of acetonitrile at 60°C. The dark
solution was allowed to stand in the daylight at r.t. for
2 days. Upon cooling to 0°C most of the unreacted 4b
crystallises. The more soluble product 7b (60 mg, 28%)
is separated as blue–green crystals from residual or-
ange–red 4b by fractional crystallisation of the dark-
green supernatant at −20°C. IR (in THF) wCO (cm−1):
4b
6
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/c
Monoclinic
P21/c
,
a (A)
10.740(2)
14.307(3)
19.363(4)
103.11(2)
2897.7
4
1.360
1224
0.49
10.639(5)
14.244(7)
19.286(10)
102.78(4)
2850.2
4
1.588
1352
4.09
,
b (A)
,
c (A)
i (°)
3
,
V (A )
Z
1
1887(vs), 1773(m, br) — H-NMR (in CD2Cl2): l 1.30,
1.51, 2.28, 2.35, 2.40, 2.48, 2.53, 2.73 (each s, 3H;
tpb%ꢂCH3, CH3CN and tolyl-CH3), 5.67 (s, 1H,
tpb%ꢂCH), 5.80 (s, 1H, tpb%ꢂCH), 5.95 (s, 1H, tpb%ꢂCH),
Dcalc. (g cm−3
F(000)
)
v (Mo–Ka) (mm−1
)
3
,
X-Radiation, u (A)
Mo–Ka, graphite monochromated,
0.71069
6.82 and 7.03 [(AB)2-system, JHH=7.9 Hz, 4H, C6H4)
—
13C{1H}-NMR (in CD2Cl2): l 5.1 (CH3CN), 12.8,
Data collection temperature Ambient
(K)
203
13.1, 13.7, 15.2 (tpb%ꢂCH3), 21.6 (Me-4), 106.4, 106.8
(tpb%ꢂCH), 125.9, 129.3 (C6H4), 137.7, 138.3 (C6H4ꢂC-
1, C-4), 142.7 (CH3CN), 144.9, 145.7, 145.9, 150.6,
151.3, 154.0 (tpb%ꢂCCH3), 209.0 (CꢁCꢁO), 231.3 (CO),
231.6 (CꢁCꢁO) — 11B{1H}-NMR (CD2Cl2): l −8.1
2rmax (°)
hkl-Range
50
58
0/12, 0/17,
−23/22
5373
5126
3980
Empirical
361
1.112
−14/14, 0/19,
0/26
7892
7591
6653
Empirical
360
1.049
Reflections measured
Unique
Observed [I]2|(I)]
Absorption correction
Parameters refined
GOF
—
MS: m/z (relative intensity) 554 (10, [M−
CH3CN]+), 526 (38, [M−CH3CNꢂCO]+), 498 (100,
[M−CH3CNꢂ2CO]+), 297 (56, [tpb%]+), 28 (57, CO).
R (observed reflections
only)
0.041
0.028
3.5. Preparation of
[(C5Me5)(CO)(NCMe)W{C(SiPh3)CO}] (8)
wR2 (all reflections) a
0.094
0.075
A 120 mg (0.19 mmol) sample of 5 was added to 10
ml of cold (−20°C) acetonitrile. The stirred mixture is
slowly warmed to r.t. After 2 h the volume of the
solution was reduced to 3 ml under reduced pressure.
The product 8 (55 mg, 42%) precipitated from the green
solution at −25°C as dark blue crystals. The mother
liquor mainly contained complex 5. IR (in CH2Cl2) wCO
(cm−1): 2024(s), 1887(s); wCN (cm−1): 2073(w) — 1H-
NMR (in CD2Cl2): l 1.83 (s, 15H, C5Me5), 2.95 (s, 3H,
NCCH3), 7.46 (m, 15H,SiPh3) — 13C{1H}-NMR (in
a w=1/[|2(Fo2)+(AP)2+BP], P=max(Fo2, 0)+2F2c)/3, where A=
0.0338 and 0.043 for 4b and 6, respectively, and B=1.78 and 2.42 for
4b and 6, respectively.
Lorentz, polarisation and absorption effects (Tables 3
and 4). The structures were solved by the heavy atom
or direct methods and refined on F2 by full-matrix
least-squares using all measured unique reflections. All
non-hydrogen atoms were given anisotropic thermal
parameters.
The hydrogen atoms on boron were located from
difference Fourier maps and refined with isotropic
atomic displacement parameters. All other hydrogen
atoms were input in calculated positions.
CD2Cl2):
l
10.2 (C5Me5), 18.8 (NCCH3), 20.7
(CꢁCꢁO), 109.6 (C5Me5), 128.1, 129.6, 136.2, 137.1
(Ph), 169.3 (CꢁCꢁO), 203.4 (NCCH3), 232.9 (CO) —
MS: m/z (relative intensity) 646 (48, [M−CH3CN]+),
618 (38, [M−CH3CNꢂCO]+), 590 (100, [M−
CH3CNꢂ2CO]+) — Anal. Calc. for C33H33NO2SiW
(687.57): C, 57.64; H, 4.83; N, 2.03. Found: C, 57.13;
H, 5.14; N, 2.91.
The calculations were performed using the programs
SHELXS-86 [31] and SHELXL-93 [32]. Graphical repre-
sentations were drawn with the SCHAKAL-88 program
[33].
3.6. X-ray crystal structure determination of
[tpb%(CO)2MꢀCC6H4Me-4] (M=Mo, 4b), (M=W, 6),
[tpb%(CO)(NCCH3)Mo{C(C6H4Me-4)CO}] (7b) and
[(C5Me5)(CO)(NCMe)W{C(SiPh3)CO}] (8)
3.7. Molecular orbital calculations
Extended Hu¨ckel calculations were performed using
the extended Hu¨ckel method [25] with modified Hij
values [34]. The basis set for the metal atoms consisted
of ns, np and (n-1)d orbitals. The s and p orbitals were
described by single Slater-type wave functions, and the
d orbitals were taken as contracted linear combinations
Intensity data were collected with Syntex R3 (com-
plex 4b) and Siemens–Stoe AED2 four-circle diffrac-
tometers (complexes 6, 7b and 8) and corrected for

242
H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
the ketenyl, the carbonyl and the C5H5 ligand were
taken from the structure.
of two Slater-type wave functions. Only s and p orbitals
were considered for Cl. The parameters used for W
were (Hii (eV), n): 6s −8.26, 2.341; 6p −5.17, 2.309;
5d −10.37, 4.982, 2.068 (2), 0.6685 (C1), 0.5424 (C2);
for Cl: (Hii (eV), z): 3s −30.00, 2.033; 3p −15.00,
2.033. Standard parameters were used for other atoms.
Interaction diagrams were drawn using the program
CACAO [28].
The iridium compound 17 was modelled by a two-
legged piano stool with C5H5–Ir–L angles of 135°,
respectively. Bond distances of the (C5H5)Ir moiety and
the Ir–L distances were taken from the structure of 17.
DFT calculations [26] were carried out using the
Amsterdam Density Functional (ADF) program [27]
developed by Baerends and co-workers [35]. Vosko,
Wilk and Nusair’s local exchange correlation potential
was used [36], with Becke’s nonlocal exchange and
Perdew’s correlation corrections [37]. The geometry op-
timisation procedure was based on the method devel-
oped by Versluis and Ziegler [38], using the non-local
correction terms in the calculation of the gradients. A
full geometry optimisation of the model compound
[W(C5H5)(CO)(HCCO)(NCH)] was performed, starting
from an idealised geometry, based on the X-ray deter-
mined structure, where both the nitrile and the ketene
were half way between h1 and h2. The relativistic
effects were treated by a quasi-relativistic method where
Darwin and mass–velocity terms are incorporated [39].
The core orbitals were frozen for W ([1–5]s, [2–5]p,
[3–4]d) and C, N, O (1s). Triple-z Slater-type orbitals
(STO) were used for H 1s, C, N, O 2s and 2p, W 6s and
6p. A set of polarisation functions was added: H (single
z, 2p), C, N, O (single z, 3d).
The calculations were performed on model complexes
with idealised geometries. NCH was used as the nitrile
,
ligand, with bond distances C–N, 1.27, C–H, 1.03 A
and the N–C–H angle set to 129°. Substituents on
other ligands (CH3, Ph, SiPh3) were replaced by hydro-
gen. Dmpe was approximated by two Ph3 ligands. The
geometry of 9 and 11 was forced to regular octahedral,
with the bond distances taken from the real structures.
The model for complex 8 has approximately a piano
stool geometry with a C5H5–W–C(carbonyl) angle of
118°, a C5H5–W–C(ketenyl) angle of 115° and a
C5H5–W–C(nitrile) angle of 122°. Bond distances for
Table 4
Details of the crystal structure determinations of the complexes
[tpb%(CO)(NCMe)Mo {C(C6H4Me-4)CO}] (7b) and [(C5Me5)(CO)-
(NCMe)W{C(SiPh3)CO}] (8)
7b
8
Crystal system
Space group
Unit cell dimensions
Triclinic
Triclinic
(
(
P1
P1
,
a (A)
10.145(7)
10.441(8)
15.729(12)
109.10(6)
93.87(6)
91.16(6)
1569.2
2
10.123(5)
12.172(6)
13.808(7)
64.16(3)
85.00(3)
69.88(3)
1433.9
2
4. Supplementary material
,
b (A)
,
c (A)
h (°)
i (°)
u (°)
V (A )
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC-
132488 (4b), -132489 (6), -132490 (7b) and -132491 (8).
Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
3
,
Z
Dcalc. (g cm−3
F(000)
)
1.343
656
0.46
1.592
684
4.10
v(Mo–Ka) (mm−1
)
,
X-radiation, u (A)
Mo–Ka, graphite monochromated,
0.71069
Data collection temperature 203
(K)
203
2rmax (°)
hkl-range
52
50
−12/10, −12/ −11/12, −12/14,
12, 0/19
6000
6000
0/16
5053
5043
Reflections measured
Unique
Acknowledgements
Observed [I]2|(I)]
Absorption correction
Parameters refined
GOF
5187
Empirical
389
1.077
0.062
4724
Empirical
352
1.040
0.022
This work was supported by the Deutsche
Forschungsgemeinschaft under the auspices of the Son-
derforschungsbereich 247 of the Universita¨t Heidelberg
and by the Fonds der Chemischen Industrie. The au-
thors thank the Deutscher Akademischer Austauschdi-
enst and the Conselho de Reitores das Universidades
Portuguesas for a scientific exchange grant within the
Acc¸o˜es Integradas Luso-Alema˜s scheme. A Heisenberg
fellowship to H.W. is gratefully acknowledged.
R (observed reflections
only)
wR2 (all reflections) a
0.183
0.057
a w=1/[|2(Fo2)+(AP)2+BP], P=max(Fo2, 0)+2F2c)/3, where A=
0.1332 and 0.0392 for 7b and 8, respectively, and B=0.58 and 0.55
for 7b and 8, respectively.

H. Wadepohl et al. / Journal of Organometallic Chemistry 587 (1999) 233–243
243
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