Formation and reactivity of the zirconium ethylene complexes [η5-C5H3-1,3-(SiMe2CH 2PR2)2]Zr(η2-CH2= CH2)(X) (R = Me, Pri; X = Br, C5H5)

Article abstract of DOI:10.1139/v00-154

The formation and reactivity of the zirconium ethylene complexes ^R[P₂Cp]Zr(η²-CH₂=CH ₂)(X) (2a: R = Prⁱ, X = Br; 2b: R = Me, X = Br; 4a: R = Prⁱ, X = C₅H₅; ^R[P₂Cp] = (η⁵-C₅H₃-1,3-(SiMe₂CH ₂PR₂)₂)) are described. Ethylene complexes 2a and 2b are prepared from a reaction of ^R[P₂Cp]ZrCl₃ (1a: R = Prⁱ; 1b: R = Me) with 2 equiv of EtMgBr, presumably via β-H elimination from the diethyl intermediate ^R[P₂Cp]ZrCl(CH₂CH₃)₂. The structure of 2b was determined by X-ray crystallography. Addition of carbon monoxide to 16-electron 2 displaces the ethylene ligand to generate the carbonyl complex ^R[P₂Cp]Zr(CO)₂Br (5a: R = Prⁱ; 5b: R = Me), which is stable only under an atmosphere of CO. The corresponding CO reaction with 18-electron 4a to give the metallocene monocarbonyl analogue ^Pr[P₂Cp]Zr(η⁵-C₅H ₅)(CO) (6a) is considerably slower. 2a exhibits fluxional exchange of one carbonyl ligand with bulk ¹³CO in solution; the kinetic parameters for this exchange process are ΔH^? = 9.2(5) kcal mol^-1 and ΔS^? = -17(2) cal mol^-1 K^-1. The addition of diphenylacetylene to 2a yields the alkyne complex ^Pr[P₂Cp]Zr(η²-PhCCPh)Br (7a), which exists in solution as two isomers in equilibrium. A solid-state X-ray structure determination for the minor isomer syn-7a was performed.

Full text of DOI:10.1139/v00-154

536
Formation and reactivity of the zirconium ethylene
complexes [η5-C5H3-1,3-(SiMe2CH2PR2)2]Zr(η2-
CH2=CH2)(X) (R = Me, Pr i; X = Br, C5H5)
Michael D. Fryzuk, Paul B. Duval, and Steven J. Rettig
Abstract: The formation and reactivity of the zirconium ethylene complexes R[P2Cp]Zr(η2-CH2=CH2)(X) (2a: R = Pri,
X = Br; 2b: R = Me, X = Br; 4a: R = Pri, X = C5H5; R[P2Cp] = (η5-C5H3-1,3-(SiMe2CH2PR2)2)) are described. Ethyl-
ene complexes 2a and 2b are prepared from a reaction of R[P2Cp]ZrCl3 (1a: R = Pri; 1b: R = Me) with 2 equiv of
EtMgBr, presumably via β-H elimination from the diethyl intermediate R[P2Cp]ZrCl(CH2CH3)2. The structure of 2b was
determined by X-ray crystallography. Addition of carbon monoxide to 16-electron 2 displaces the ethylene ligand to
generate the carbonyl complex R[P2Cp]Zr(CO)2Br (5a: R = Pri; 5b: R = Me), which is stable only under an atmosphere
of CO. The corresponding CO reaction with 18-electron 4a to give the metallocene monocarbonyl analogue Pr[P2Cp]Zr(
η5-C5H5)(CO) (6a) is considerably slower. 2a exhibits fluxional exchange of one carbonyl ligand with bulk 13CO in so-
lution; the kinetic parameters for this exchange process are ∆H ‡ = 9.2(5) kcal mol–1 and ∆S ‡ = –17(2) cal mol–1 K–1.
The addition of diphenylacetylene to 2a yields the alkyne complex Pr[P2Cp]Zr(η2-PhCCPh)Br (7a), which exists in so-
lution as two isomers in equilibrium. A solid-state X-ray structure determination for the minor isomer syn-7a was per-
formed.
Key words: zirconium, cyclopentadienyl phosphine, alkyne, metallocyclopropane.
Résumé : On décrit la formation et la réactivité de complexes de l’éthylène zirconium R[P2Cp]Zr(η2-CH2=CH2)(X)
(2a : R = iPr, X = Br; 2b : R = Me, X = Br; 4a : R = iPr, X = C5H5; R[P2Cp] = (η5-C5H3-1,3-(SiMe2CH2PR2)2)). On
a préparé les complexes 2a et 2b par réaction du R[P2Cp]ZrCl3 (1a : R = iPr; 1b : R = Me) avec deux équivalents de
EtMgBr, probablement par le biais d’une élimination β d’hydrogène à partir de l’intermédiaire diéthylé
R[P2Cp]ZrCl(CH2CH3)2. On a déterminé la structure du complexe 2b par diffraction des rayons X. L’addition de mo-
noxyde de carbone au complexe 2 à seize électrons conduit à un déplacement du ligand éthylène et à la formation du
complexe carbonylé R[P2Cp]Zr(CO)2Br (5a: R = iPr; 5b: R = Me) qui n’est stable que sous une atmosphère de CO. La
réaction correspondante du CO avec le complexe 4a à dix-huit électrons, qui conduit à la formation du métallocène
monocarbonylé analogue, R[P2Cp]Zr(η5-C5H5)(CO) (6a) est beaucoup plus lente. Le complexe 2a donne lieu à un
échange fluxionnel d’un ligand carbonyle avec le 13CO qui se trouve dans l’ensemble de la solution; les paramètres ci-
nétiques pour ce processus d’échange sont ∆H ‡. = 9,2(5) kcal mol–1 et ∆S ‡ = –17,2 cal mol–1 K–1. L’addition de di-
phénylacétylène au complexe 2a conduit à la formation de R[P2Cp]Zr(η2-PhCCPh)Br (7a) qui, en solution, existe sous
la forme de deux isomères à l’équilibre. Faisant appel à la diffraction des rayons X, on a effectué une détermination de
la structure en phase solide de l’isomère mineur, le syn-7a.
Mots clés : zirconium, cyclopentadiényl phosphine, alcyne, métallocyclopropane.
[Traduit par la Rédaction] Fryzuk et al. 545
Introduction
lytic industrial processes, such as alkene polymerization us-
ing Ziegler–Natta catalysts, olefin metathesis (2, 3) (which
includes both ring-opening (4) and ring-closing (5) metathe-
sis type processes), alkene hydrogenation (6), and the
Wacker process (7). In addition, there is continued interest
in the bonding aspects of the metal-alkene interaction (8),
which has been the focus of theoretical studies (9).
The coordination of ethylene to transition metal centers is
one of the foundations of organometallic chemistry (1) and
dates back to the synthesis of Zeise’s salt (K[Pt(η2-
CH2=CH2)Cl3]) in 1827. Currently, the chemistry of metal-
alkene complexes encompasses numerous important cata-
Received September 5, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on May 23, 2001.
Dedicated to Professor B. R. James, in recognition of his distinguished contributions to homogeneous catalysis, on the occasion of
his 65th birthday.
M.D. Fryzuk1, P.B. Duval, and S.J. Rettig.2 Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, BC V6T 1Z1, Canada.
1Corresponding author (e-mail: fryzuk@chem.ubc.ca).
2Deceased.
Can. J. Chem. 79: 536–545 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-5/6-536

Fryzuk et al.
537
We have been examining the reactivity of zirconium com-
plexes stabilized by the hybrid donor ancillary ligand η5-
C5H3-(SiMe2CH2PR2)2 (R[P2Cp], R = Pri, Me). Deployment
of bulky isopropyl substituents on the side-arm phosphines
has enabled the isolation of Zr(IV) alkylidene complexes,
Pr[P2Cp]Zr=CHR(Cl), (10, 11) from which reaction with eth-
ylene yields the ethylene adduct Pr[P2Cp]Zr(η2-CH2=CH2)Cl
(2c) (11). An interest in studying the reactivity of this prod-
uct as a probe into the nature of the metal-alkene bonding in
this formally Zr(II) species led to a more direct synthetic
route from the starting chloride Pr[P2Cp]ZrCl3 (1a). One ad-
vantage of this alternate preparation is the ability to obtain
the analogous but less bulky Me[P2Cp]Zr ethylene derivative
Scheme 1.
Me2
Si
Me2
Si
2 EtMgBr
Cl
Zr
Cl
P
P
R
- MgCl2
R
R
- MgBrCl
Cl
R
1a R = Pri
1b R = Me
Me2
Si
Me2
Si
Br
Zr
P
P
R
R
R
CH3CH2
R
CH2CH3
(
Me[P2Cp]Zr(η2-CH2=CH2)Br
(2b))
directly
from
Me2
Si
Me2
Si
Me[P2Cp]ZrCl3 (1b).
Br
Zr
P
P
Me2
Si
Me2
Si
Me2
Si
Me2
Si
- CH3CH3
R
R
R
R
H2C
CH2
2a R = Pri
2b R = Me
Cl
Zr
Cl
Zr
P
R
P
P
P
R
R
R
Cl
H2C
CH2
Cl
2c
1a R = Pri
spect to the unique carbon of the cyclopentadienyl (Cp) ring,
as shown below (eq. [1]). Each isomer of 2a displays a sepa-
rate singlet in the 31P NMR spectrum and corresponding res-
1b R = Me
1
onances in the H NMR spectrum.
Although zirconium–alkene complexes are proposed as
intermediates in several important synthetic transformations,
(12, 13) the number of characterizable species has been re-
stricted to a few metallocene-type systems, (14–17) the ma-
jority of which are isolated by the use of a trapping donor
(typically a phosphine) to stabilize these complexes. The
ethylene derivatives introduced in this report expand this
sample to include 16-electron mono(cyclopentadienyl) com-
plexes and a related 18-electron metallocene analogue; in
addition, some substitution studies are included.
Me2
Si
Me2
Si
Me2
Si
Me2
Si
Br
Br
Zr
Zr
P
P
P
P
H2C
CH2
H2C
CH2
syn-2a
anti-2a
The ethylene protons give rise to a complex multiplet cen-
tred near 1.0 ppm in the 1H NMR spectrum, originating
from an AA′BB′XX′ spin system due to additional JPH cou-
pling from the side-arm phosphines. The upfield location of
these resonances in the 1H NMR spectrum is indicative of an
ethylene fragment coordinated to Zr. As with 2c, definitive
assignment of stereochemistry was unfortunately not possi-
R
Preparation of [P2Cp]Zr(η2-CH2=CH2)Br (R = Pri, Me)
3
A toluene solution of the starting Zr(IV) trichloride 1 re-
acts with 2 equiv of EtMgBr at –78°C to give a bright yel-
low solution, which upon warming to room temperature
produces the ethylene complex R[P2Cp]Zr(η2-CH2=CH2)Br
(2a: R = Pri; 2b: R = Me) in excellent yield, either as dark
purple (2a) or dark red (2b) crystals (Scheme 1). The yellow
precursor is presumed to be the diethyl species
R[P2Cp]ZrBr(CH2CH3)2, although no direct proof of its exis-
tence was obtained. The putative reaction sequence is com-
pleted by a β-H abstraction reaction with the elimination of
ethane to generate the ethylene product. A quantitative ha-
lide exchange incorporates a bromide ligand in the final
product, based on elemental analyses and an X-ray structural
determination for 2b (see below).
1
ble (via H NMR NOEDIFF experiments, for example) and
so our designation of syn or anti is presumed.
In contrast to 2a, only a single species is seen in solution
for the Me[P2Cp] derivative 2b; we speculate that the reduced
steric hindrance from the smaller methyl substituents at
phosphorus reduces the tendency for phosphine dissociation
which in turn discourages a syn/anti exchange process in this
complex. The solid-state molecular structure of 2b is shown
in Fig. 1, along with the atom numbering scheme. A sum-
mary of the crystallographic data is given in Table 1; se-
lected bond distances and bond angles are listed in Table 2.
The structure reveals that the ethylene unit is coordinated
parallel to the Cp ring and oriented syn to the unique Cp car-
bon atom C(2), precisely as found for the solid state struc-
ture for 2c (18). Presumably, this orientation reduces steric
interactions between the bound ethylene molecule and the
Cp ring; moreover, it situates the ethylene π*-orbital to
overlap favorably with the filled metal dxy orbital (in this
The 1H and 31P NMR spectra of purple 2a are nearly iden-
tical to that of the crystallographically characterized chloro
derivative Pr[P2Cp]Zr(η2-CH2=CH2)Cl (2c), a red product
formed from the reaction of either alkylidene complex
Pr[P2Cp]Zr=CHR(Cl) (R = Ph, SiMe3) with ethylene (18). As
with 2c, the NMR spectra for 2a show the presence of two
species in a 9:1 ratio assigned as syn (major) and anti (mi-
nor) isomers, designated according to the orientation of the
ethylene ligand as being directed either syn or anti with re-
© 2001 NRC Canada

538
Can. J. Chem. Vol. 79, 2001
Table 1. Crystallographic data.
Fig. 1 Molecular structure of Me[P2Cp]Zr(η2-CH2=CH2)Br (2b);
33% probability thermal ellipsoids are shown. Hydrogen atoms
are omitted for clarity.
Compound
2ba
syn-7ab
Formula
Fw
C25H51ClP2Si2Zr C31H53ClOP2Si2Zr
596.48
686.55
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Triclinic
P-1 (no. 2)
15.432(3)
20.483(3)
20.995(7)
76.50(3)
83.12(7)
88.52(4)
6407(3)
8
1.24
2528
6.0
0.30 × 0.40 ×
0.45
Monoclinic
P21/c (no. 14)
8.666(1)
24.244(1)
17.543(1)
90
100.936(8)
90
3618.9(6)
4
1.260
1448
5.53
0.25 × 0.35 ×
0.45
Z
Dcalc (g cm–3)
F(000)
µ (cm–1)
Crystal size (mm)
Transmission factors
Scan type
0.903–1.00
ω-2θ
0.96–1.00
ω-2θ
Scan range (deg in ω)
Scan speed (deg min–1)
Data collected
0.79 + 0.35 tan θ
16 (up to 9 scans)
±h, +k, ±l
2θmax (deg)
40
60
Crystal decay (%)
Total reflections
Unique reflections
Rmerge
No. with I ≥ 3σ(I)
No. of variables
R (F) (I ≥ 3σ(I))
Rw (F) (I ≥ 3σ(I))
R (F2) (all data)
Rw (F2) (all data)
Gof
2.0
11 928
11 917
0.023
7589
11 425
10 783
0.036
5598
344
0.034
0.031
—
0.055
0.060
0.055
0.060
3.71
Table 2. Selected bond lengths (Å) and bond angles (°) for
Me[P2Cp]Zr(η2-CH2=CH2)Br (2b).
Bond lengths (Å)
Bond angles (°)
—
1.49
Zr(1)—Br(1)
Zr(1)—P(1)
Zr(1)—P(2)
Zr(1)—C(16)
Zr(1)—C(17)
Zr(1)—Cp′
2.6744(6)
2.730(1)
2.703(1)
2.293(5)
2.312(5)
2.23
Br(1)-Zr(1)-P(1)
Br(1)-Zr(1)-P(2)
Br(1)-Zr(1)-C(16)
Br(1)-Zr(1)-C(17)
P(1)-Zr(1)-P(2)
P(1)-Zr(1)-C(16)
P(1)-Zr(1)-C(17)
C(16)-Zr(1)-C(17)
78.28(3)
76.94(3)
109.7(1)
109.7(1)
155.17(4)
75.6(1)
Max ∆/σ (final cycle)
0.000
0.003
–0.35 to 0.31
Residual density (e Å–3) –0.550 to 0.640
aTemperature 294 K, Nonius CAD-4 diffractometer, Mo Kα radiation (λ
= 0.70930 Å), graphite monochromator, takeoff angle 6.0°, aperture 6.0 ×
6.0 mm at a distance of 285 mm from the crystal, stationary background
counts at each end of the scan (scan:background time ratio, 2:1), σ2(F2) =
[S2(C + 4B)]/Lp2 (S = scan rate, C = scan count, B = normalized
background count), function minimized Σw(|Fo| – |Fc|)2 where w =
C(16)—C(17)
1.431(6)
111.0(1)
36.2(2)
2
2
4Fo /σ2(Fo ), R (F) = Σ||Fo| – |Fc||/Σ|Fo|, Rw (F) = (Σw(|Fo| –
|Fc|)2/Σw|Fo|2)1/2, and Gof (F) = [Σw(|Fo| – |Fc|)2/(m – n)]1/2
.
bTemperature 294 K, Rigaku AFC6S diffractometer, Mo Kα radiation (λ
= 0.71069 Å), graphite monochromator, takeoff angle 6.0°, aperture 6.0 ×
6.0 mm at a distance of 285 mm from the crystal, stationary background
counts at each end of the scan (scan:background time ratio, 2:1), σ2(F2) =
[S2(C + 4B)]/Lp2 (S = scan rate, C = scan count, B = normalized
background count), function minimized Σw(|Fo| – |Fc|)2 where w =
backbonding is not significantly altered upon substituting ei-
ther the halide (Br for Cl) or the groups attached to the
phosphines (methyl for isopropyl). However, the phosphines
are bound considerably closer to the metal in 2b, with Zr—P
bond distances averaging 2.72 Å in comparison to 2.86 Å
for 2c. The C—C bond distance of the ethylene ligand is
similar in these complexes to that found for other
monomeric Zr(II) ethylene complexes (14, 16, 21). The
lengthening of this bond compared to 1.337 Å found in free
ethylene (22) is indicative of significant back-bonding from
the metal as represented in the Dewar–Chatt–Duncanson
model (23, 24) and supports a Zr(IV) metallacyclopropane
bonding depiction (A). Also consistent with this formalism
are the short Zr—C bond lengths (average 2.30 Å) in 2b,
which are within the range of typical Zr—C single bonds
2
2
4Fo /σ2(Fo ), R (F) = Σ||Fo| – |Fc||/Σ|Fo|, Rw (F) = (Σw(|Fo| –
|Fc|)2/Σw|Fo|2)1/2, and Gof (F) = [Σw(|Fo| – |Fc|)2/(m – n)]1/2
.
representation the z-axis passes through the Cp-centroid).
Previous investigations into bonding interactions in electron-
rich complexes with piano-stool geometries suggest that the
metal dxy orbital offers the best electron donation to a π-ac-
ceptor ligand (19, 20). The C(16)—C(17) bond distance of
1.431(6) Å in 2b is comparable to the corresponding value
of 1.433(17) Å for 2c, which illustrates that the degree of
© 2001 NRC Canada

Fryzuk et al.
539
Scheme 2.
(12) and considerably shorter than the Zr—C distance of
2.68 (2) in a cationic Zr(IV) olefin complex in which d-π*
back-bonding is unavailable (25).
Pr
2
[P Cp]Zr(η -CH =CH )Me
3a
2
2
2
Ln
M
Ln
M
MeMgBr
H
H
H
H
H
H
H
H
C
C
C
C
Pr
2
[P Cp]Zr(η -CH =CH )Br 2a
2
2
2
B
A
NaCp(DME)
By NMR spectroscopy, only one isomer is seen for 2b in
solution, thus it is assumed that it corresponds to the syn
structure observed in the solid state. The similarities be-
1
tween the H NMR spectrum of the major isomer of 2a with
Pr
2
5
[P Cp]Zr(η -CH =CH )(η -C H )
4a
2
2
2
5 5
that of the single species seen for 2b, particularly in the dis-
tinctive Cp region, suggests that the major isomer of 2a is
the syn isomer as well.
at –5.3 ppm for this ligand system is indicative of an
uncoordinated phosphine, while the other resonance at 42.3
corresponds to a bound phosphine donor. The 1H NMR
spectrum shows a doublet (3JPH = 2 Hz) at 5.20 ppm for the
five protons of the C5H5 ring; the downfield chemical shift
suggests that this ligand is coordinated in an η5-mode. As
the array of ligands in this complex is similar to that found
in the structurally characterized metallocene derivative
Cp2Zr(η2-CH2=CH2)(PMe3), (14, 21) it is reasonable to con-
sider an analogous structure for 4a as shown below.
The formation of the ethylene complexes 2a and 2b di-
rectly from the respective starting trichlorides offers a facile
alternative to the multistep approach to 2c (i.e., via an
alkylidene complex) (18). This general synthetic route has
been employed elsewhere to obtain a number of early metal
alkene complexes (15). Unfortunately, this procedure could
not be extended to other Zr alkene complexes with this sys-
tem. For example, propylene complexes could not be iso-
lated by using Grignard reagents such as PrnMgBr or
PriMgCl, nor was BunLi effective in obtaining a coordinated
butene species. All of these attempts resulted in complete
consumption of the starting materials, but no isolable alkene
complexes were forthcoming; this is reminiscent of the reac-
tions of substituted alkenes with the related alkylidene com-
plexes (18). The apparent instability of these complexes
incorporating higher alkenes may be due to a combination of
greater steric hindrance from the bulky ancillary ligand
along with weaker bonding of these alkenes relative to ethyl-
ene.
Me2
Si
Me2
Si
P
P
Zr
CH2
C
H2
Preparation of Pr[P2Cp]Zr(η2-CH2=CH2)R′ (R′ = Me,
C5H5)
In an attempt to expand the scope of ethylene complexes
for which reactivity studies could be compared, we exam-
ined the preparation of alkyl-ethylene derivatives by stan-
dard metathesis reactions involving the bromide ligand
(Scheme 2).
4a
The characteristic spectroscopic feature of the methyl de-
rivative 3a is the triplet (3JPH = 10 Hz) at –0.67 ppm in the
1H NMR spectrum for the methyl group attached to Zr. The
distinctive Cp region of this spectrum is nearly identical to
that of the major syn isomer of 2, suggesting that the solu-
tion structure of 3a (only one species is seen) is exclusively
the syn isomer. Unfortunately this complex was only charac-
terized in solution as it could not be isolated as a solid, de-
spite many attempts; decomposition with the elimination of
ethylene to give unidentified products was observed.
The Cp derivative 4a could be isolated as reddish-orange
crystals. There are two equal intensity singlets in the 31P
NMR spectrum for 4a at –5.3 ppm and 42.3 ppm; the peak
The asymmetric geometry renders all four ethylene pro-
tons inequivalent, thus generating a complicated second or-
der splitting pattern in the 1H NMR spectrum (with
additional coupling to the coordinated phosphine). This
asymmetry is also evident from the three separate reso-
nances observed for the cyclopentadienyl portion of the
P2Cp ancillary ligand. The presence of only one coordinated
phosphine in 4a supports the view of the C5H5 ligand as η5-
bound; the steric and electronic saturation provided by this
coordination mode yields an 18-electron species which
would force one side-arm to remain dangling, as observed.
The ethylene unit and the coordinated phosphine thus can be
© 2001 NRC Canada

540
Can. J. Chem. Vol. 79, 2001
Fig. 2 Variable temperature 13C NMR spectra of Pr[P2Cp]Zr(CO)2Br
(5a).
considered to be occupying the metallocene wedge, with the
other side-arm left out of the coordination sphere.
Ligand substitution reactions with CO
A purple toluene solution of the ethylene complex 2a re-
acts instantly under an atmosphere of CO to generate a dark
greenish-orange solution that shows a single resonance in
1
the 31P NMR spectrum at 29.1 ppm. In the H NMR spec-
trum the only resonances observed are attributed to the an-
cillary ligand, and a sealed NMR tube reaction reveals a
singlet at 5.25 ppm for the evolution of free ethylene. A sim-
ilar result is obtained in the reaction with 2b. Therefore, it is
apparent in both instances that the coordinated ethylene unit
has been replaced by CO in a ligand substitution reaction to
give a carbonyl complex (eq. [2]).
Me2
Si
Me2
Si
C
O
Br
Zr
– C2H4
P
P
R
R
R
R
H2C
2a R = Pri
2b R = Me
CH2
Me2
Si
Me2
Si
Br
Zr
P
P
R
R
R
R
C
C
O
O
5a R = Pri
5b R = Me
vation suggests that this CO ligand is undergoing an ex-
change process with free CO in solution; the remaining
ligand is either not involved in this process or is at the slow
exchange limit at this temperature, since the peak for this
ligand at 249 ppm is quite sharp in comparison. This ex-
change process was monitored by variable temperature 13C
NMR spectroscopy and a selection of the spectra are shown
in Fig. 2; these spectra demonstrate an increase in line
broadening as the temperature is raised, in agreement with
an increase in the exchange rate between free CO and the
exchanging carbonyl ligand.
For the reaction of 2a with 13CO the downfield region of
the 13C NMR spectrum displays two carbonyl peaks at 241
and 249 ppm for 5a in addition to a third resonance at
184 ppm for excess free 13CO, indicating that an 18-electron
dicarbonyl species has been formed from the 16-electron
precursor. In 5b the carbonyl resonances are located at 240
and 245 ppm, respectively. These complexes have a ligand
arrangement that is similar to known derivatives such as
CpZr(CO)2(dmpe)Cl (26). The presence of a carbonyl ligand
is supported by absorptions in the IR spectra at 1890 and
1898 cm–1 for 5a and 5b, respectively, which shift to 1865
and 1855 cm–1 for the 13C labeled isotopomers. The low νCO
values of the coordinated carbonyl ligands in comparison to
free CO (νCO = 2143 cm–1) are due to strong back-bonding
from the electron-rich Zr(II) centre to the π* orbital of the
CO ligand and is consistent with the stretching frequency in
The NMR spectra were simulated (using DNMR-5)® to
obtain rate constants at each temperature, which were used
in the resulting Eyring plot shown in Fig. 3. The kinetic
parameters ∆H ‡ = 9.2(5) kcal mol–1 and ∆S ‡ = –17(2) cal
mol–1 K–1 were calculated from this plot. The negative en-
tropy term is unexpected, as ligand exchange from this satu-
rated 18-electron species is anticipated to proceed in a
dissociative mechanism that should produce a positive en-
tropy term for a less ordered transition state. A concerted
mechanism involving dissociation of a pendant phosphine
may therefore be a factor in this exchange. An associative
process has also been suggested from a separate kinetic
study into similar CO ligand exchange in other 18-electron
Zr carbonyl species (27, 28).
analogous Zr(II) carbonyls (i.e., CpZr(CO)2(dmpe)Cl, νCO
=
1940 cm–1) (26). Unfortunately, although both carbonyl
complexes 5a and 5b are stable indefinitely in solution in the
presence of excess CO, attempts to isolate these species in
the absence of CO led to decomposition. This instability out-
side of a protective CO atmosphere has been noted with
other Zr carbonyl derivatives (26–28), demonstrating the la-
bile nature of these ligands with early metals.
Ligand exchange is common for early metal carbonyl
complexes. In the related complex Cp*Hf(CO)2(dmpe)Cl
(28) both CO ligands are observed to rapidly and quantita-
tively exchange with excess 13CO. One can speculate as to
A notable feature of the ambient temperature 13C NMR
spectrum of 5a is that the peak for free CO at 184 ppm is
broadened, as is the carbonyl peak at 241 ppm. This obser-
© 2001 NRC Canada

Fryzuk et al.
541
Fig. 3 Eyring plot for the CO ligand exchange process in
Pr[P2Cp]Zr(CO)2Br (5a).
companied by the evolution of free ethylene as observed
with the analogous reaction that forms carbonyl 5. The 31P
NMR spectrum of 6a is similar to that of the precursor 4a,
with one singlet at 58.7 ppm and another at –4.8 ppm. In the
1H NMR spectrum only resonances attributed to the ancil-
lary ligand and the Cp ligand are evident. The presence of
the carbonyl ligand is indicated in the 13C NMR spectrum by
a single downfield resonance at 308 ppm (2JPC = 2 Hz) in
the reaction of 4a with 13CO. As an analogue of the 18-
electron metallocene carbonyl species Cp2Zr(CO)(PMe3)
(31), the proposed structure of 6a is depicted as shown be-
low.
Me2
Si
Me2
Si
P
why only one carbonyl ligand in 5a undergoes this ex-
change. If we assume that the quasioctahedral geometry
shown in eq. [2] holds here, then for such mono(cyclo-
pentadienyl) type complexes the ligand located trans to the
Cp donor is typically weakly bound, (29) with labile coordi-
nation observed in some instances (30). Thus for 5a, it is the
carbonyl ligand located at this labile site that undergoes ex-
change while the other trans to the bromide remains unaf-
fected. To rationalize this observation, one can invoke two
complementary effects: first, the strong labilizing trans ef-
fect of the Cp donor contrasts with the weak trans effect of
the electron withdrawing bromide, and second, and as noted
for 2b, backbonding from the zirconium in monocyclopenta-
dienyl complexes is optimized by the dxy orbital (and to a
lesser extent the dz2 orbital), which is well suited to stabilize
the carbonyl trans to the bromide but of negligible use for
the other carbonyl ligand trans to the Cp group.
The reactivity of the 18-electron ethylene complex 4a
with CO was examined to compare with the results observed
for the 16-electron analogue 2a. As opposed to 2a, which
coordinates CO almost instantly, the reaction of 4a with CO
is considerably slower, requiring more than two weeks for
completion under the same reaction conditions (eq. [3]).
This remarkable difference in reaction times can be traced to
the electronic saturation in 4a, adding further evidence that
the C5H5 donor in this complex is η5-coordinated. As a re-
sult, in lieu of a 20-electron intermediate in this reaction, ei-
ther a ligand must dissociate or alternatively the Cp ring
must undergo η5-η3 ring slippage, prior to the coordination
of a CO molecule.
P
Zr
C
O
6a
In contrast to 5a, the line width of the carbonyl peak for
6a is very narrow, with no evidence for exchange with the
narrow peak at 184 ppm due to excess 13CO. The difference
in the respective lability of the carbonyl ligands in 5a vs. 6a
may originate from the presence of one less CO ligand in
6a, which is not in competition for backbonding and hence
is less labile. The contrasting rapid CO exchange observed
for the similar metallocene analogue Cp2Zr(CO)2 (27) is in
accord with this view, where two carbonyl ligands are in
competition for backbonding from the same filled donor or-
bital (32).
Preparation of Pr[P2Cp]Zr(η2-PhCCPh)Br
Given the facile ligand substitution reaction observed for
the ethylene derivatives 2a and 2b with CO, and the ob-
served tendency for early metal alkene complexes to un-
dergo substitution reactions with alkynes, (33, 34) the
reaction with diphenylacetylene was examined with this sys-
tem. A toluene solution of ethylene 2a reacts with 1 equiv of
diphenylacetylene (PhCCPh) under ambient conditions to
displace the ethylene ligand and generate the alkyne com-
plex Pr[P2Cp]Zr(η2-PhCCPh)Br (7a), which can be isolated
as pale yellow crystals (Scheme 3). In contrast to the imme-
diate displacement of ethylene by CO to give carbonyl 5a,
the corresponding ligand substitution reaction of 2a with
diphenylacetylene is slower, requiring 24 h for completion.
This observation is consistent with steric factors inducing a
slower rate for the bulky alkyne in comparison to the small
linear CO molecule, and points to a probable requirement
that a phosphine must dissociate to allow room for the in-
coming alkyne.
CO
Pr
2
5
[P Cp]Zr(η -CH =CH )(η -C H )
2
2
2
5 5
(- C H )
2
4
4a
Pr
5
[P Cp]Zr(CO)(η -C H )
2
5 5
6a
In solution alkyne 7a exists as two isomers in a 3:2 ratio,
based on the 1H and 31P NMR spectra. There are two
singlets in the 31P NMR spectrum, one each for the major
(8.5 ppm) and minor isomer (11.6 ppm), respectively, and a
The eventual product from the above reaction is the car-
bonyl species Pr[P2Cp]Zr(CO)(η 5-C5H5) (6a), which is ac-
© 2001 NRC Canada

542
Can. J. Chem. Vol. 79, 2001
Scheme 3.
Fig. 4 Molecular structure of Pr[P2Cp]Zr(η2-PhCCPh)Br (syn-7a);
33% probability thermal ellipsoids are shown. Hydrogen atoms
are omitted for clarity.
Me2
Si
Me2
Si
Me2
Si
Me2
Si
Br
Br
Zr
Zr
P
P
P
P
H2C
CH2
H2C
CH2
syn-2a
anti-2a
- C2H4
PhC
CPh
Me2
Si
Me2
Si
Me2
Si
Me2
Si
Br
Br
Zr
C
Zr
C
P
P
P
P
C
C
Ph
Ph
Ph
Ph
syn-7a
anti-7a
Table 3. Selected bond lengths (Å) and bond angles (°) for
Pr[P2Cp]Zr(02-PhCCPh)Br (syn-7a).
Bond lengths (Å)
Bond angles (°)
Zr(1)—Br(1)
Zr(1)—P(1)
Zr(1)—P(2)
Zr(1)—C(24)
Zr(1)—C(25)
Zr(1)—Cp′
2.6771(5)
2.8367(8)
2.8129(8)
2.197(3)
2.182(2)
2.264
Br(1)-Zr(1)-P(1)
Br(1)-Zr(1)-P(2)
Br(1)-Zr(1)-C(24)
Br(1)-Zr(1)-C(25)
P(1)-Zr(1)-P(2)
P(1)-Zr(1)-C(24)
Zr(1)-C(24)-C(26)
C(25)-C(24)-C(26) 126.8(2)
78.61(2)
76.35(2)
102.85(8)
116.88(8)
154.87(4)
77.57(7)
111.0(1)
C(24)—C(25)
C(24)—C(26)
1.322(6)
1.471(4)
degree to which the phenyl rings are pulled back from the
metal as a result of this back-donation. This geometry of the
coordinated alkyne resembles that of a metallacyclopropene
C, rather than a simple π-bound alkyne as shown in D (6).
The double bond character of the coordinated alkyne is indi-
cated by the C(24)—C(25) bond distance of 1.322(6) Å.
These structural parameters are similar to those of the struc-
turally characterized Zr alkyne complex Cp2Zr(η2-
PhCCPh)(PMe3) (34).
1
corresponding set of resonances in the H NMR spectrum
for each species. As found with other complexes having this
ancillary ligand system (18) and observed for 2a, the two
species seen for 7a are likely the syn and anti stereoisomers.
A variable temperature NMR study indicates that these iso-
mers interconvert, as evidenced by increased line broadening
in both peaks in the 31P NMR spectrum as the temperature is
raised. Curiously, a change in the equilibrium ratio of these
species was not detected from this experiment. A 1H
NOEDIFF NMR experiment was performed, from which it
was determined that the anti isomer is the major species in
solution. Irradiation of the resonance assigned to the two
equivalent protons in the Cp ring of the major isomer
showed an enhancement of the signals for the ortho protons
of the phenyl ring; no effect was detected when the unique
Cp proton resonance was irradiated.
Ln
M
Ln
M
C
C
C
C
Ph
Ph
Ph
Ph
C
D
The molecular structure of alkyne 7a was determined by
an X-ray crystal structure analysis and is shown in Fig. 4.
Selected bond lengths and bond angles are listed in Table 3.
The solid-state structure corresponds to the minor syn iso-
mer, with the alkyne coordinated parallel to the Cp ring as
found for the ethylene ligand in 2b. However, in syn-7a the
alkyne unit is tilted from this parallel orientation by approxi-
mately 9°. The short Zr(1)—C(24) and Zr(1)—C(25) bond
lengths of 2.20 Å and 2.18 Å, respectively, are indicative of
strong back-bonding from the metal. This is also reflected in
the C(25)-C(24)-C(26) bond angle of 127° that illustrates the
A comparison of the bonding in syn-7a with that of the
ethylene complex 2b shows in each case that back-bonding
from the metal induces lengthening of the coordinated C—C
bond (compared to the free substrate). In addition, alkynes
have a second π-orbital that can overlap with a metal d or-
bital of suitable symmetry to form a π-bond, such that this
ligand can potentially be represented as a four-electron do-
nor. However, in 7a this interaction is in competition with
one of the π-donor orbitals from the Cp ring, which dimin-
ishes the ability of the alkyne to donate electron density
from this second π-orbital. But the observation that alkyne
© 2001 NRC Canada

Fryzuk et al.
543
7a is unreactive in the presence of CO is in direct contrast to
the rapid reaction seen for ethylene 2a, which otherwise has
the same ligand coordination sphere. Assuming that steric
considerations are minimal for a small nucleophile such as
CO, this difference in reaction times suggests that the alkyne
complex 7a is a fully saturated 18-electron species, implying
that there is at least some component present to the addi-
tional π-bond from the alkyne to the metal centre.
the solution turned to bright yellow upon addition. The mix-
ture was stirred at –78°C for 1 h and allowed to slowly
warm to room temperature, whereupon the colour of the so-
lution gradually changed to a dark reddish purple. The
volatiles were then removed under vacuum and the residue
extracted with pentane and filtered. Dark purple crystals of
2a were obtained from a cold (–40°C) concentrated pentane
solution. Yield: 1.86 g (90%). 1H NMR (20°C, C6D6) δ: 0.21
2
and 0.23 (s, 6H, Si(CH3)2), 0.82 and 0.85 (d, 2H, JPH
=
3
8 Hz, SiCH2P), 0.98 and 1.01 (m, 2H, JPH = 7 Hz,
Experimental section
2
CH2=CH2), 1.08, 1.15, 1.26, and 1.31 (dd, 6H, JHH = 7 Hz,
3
2JPH = 7 Hz, CH(CH3)2), 2.23 and 2.35 (sept, 2H, JHH
=
General considerations
4
7 Hz, CH(CH3)2), 4.37 (t, 1H, JHH = 1 Hz, Cp-H), 6.31 (d,
Unless otherwise stated all manipulations were performed
under an atmosphere of prepurified nitrogen in a Vacuum
Atmospheres HE-553–2 glove box equipped with a MO-40–
2H purification system or in standard Schlenk-type glass-
ware on a dual vacuum–nitrogen line. Reactions involving
gaseous reagents were conducted in Pyrex vessels (bombs)
designed to withstand pressures up to 10 atm (1 atm =
2H, JHH = 2 Hz, Cp-H). 31P NMR (20°C, C6D6) δ: 19.5 (s).
4
Anal. calcd. for C25H51BrP2Si2Zr: C 46.85, H 8.02; found: C
46.89, H 8.06.
Me[P2Cp]Zr(η2-CH2=CH2)Br (2b)
A solution of 3.0 M EtMgBr (1.50 mL, 4.50 mmol) was
diluted in 20 mL of toluene and added dropwise with stir-
ring to a cooled (–78°C) solution of 1b (1.19 g, 2.26 mmol)
dissolved in 40 mL of toluene. The reaction mixture was al-
lowed to warm to room temperature and stirred for 3 h, dur-
ing which a dark reddish brown colour was generated. The
solvent was removed and the residue extracted with hexanes
and filtered. Reducing this solution to 5 mL permitted the
isolation of dark red crystals at –40°C, which were washed
1
101.325 kPa). H NMR spectra (referenced to either C6D5H
or C6D5CD2H for benzene and toluene, respectively) were
performed on one of the following instruments depending on
the complexity of the particular spectrum: Bruker WH-200,
Varian XL-300, or a Bruker AM-500. 13C NMR spectra (ref-
erenced to solvent peaks) were run at 50.323 MHz on the
Bruker WH-200 or at 75.429 MHz on the XL-300 instru-
ment, and 31P NMR spectra (referenced to external P(OMe)3
at 141.0 ppm) were run at 121.421 MHz and 202.33 MHz,
on the XL-300 and Bruker AM-500 instruments, respec-
tively. All chemical shifts are reported in ppm and all cou-
pling constants are reported in Hz. Infrared spectra were
recorded on a BOMEM MB-100 spectrometer. Solution
samples were recorded on a 0.1 mm KBr cell and solid sam-
ples were recorded as pellets. Elemental analyses (Mr. P.
Borda) and GC–MS were performed within this department.
1
with cold pentane and dried. Yield: 0.93 g (79%). H NMR
(20°C, C6D6) δ: 0.09 and 0.16 (s, 6H, Si(CH3)2), 0.84 and
2
1.15 (d, 2H, JHH = 12 Hz, SiCH2P), 0.88 and 1.33 (d, 2H,
2
3JPH = 9 Hz, CH2=CH2), 1.24 and 1.27 (d, 6H, JPH = 7 Hz,
4
P(CH3)2), 4.15 (t, 1H, JHH = 1 Hz, Cp-H), 6.02 (d, 2H,
4JHH = 1 Hz, Cp-H). 31P NMR (20°C, C6D6) δ: 0.0 (s). Anal.
calcd. for C17H35BrP2Si2Zr: C 38.62, H 6.67; found: C
38.54, H 6.74.
Pr[P2Cp]Zr(η2-CH2=CH2)Me (3a)
Reagents
A solution of 1.4 M MeMgBr (0.75 mL, 1.05 mmol) was
diluted in 10 mL of toluene and added dropwise with stir-
ring to a cooled (–78°C) solution of 2a (620 mg, 0.97 mmol)
dissolved in 30 mL of toluene. The mixture was allowed to
slowly warm to room temperature, during which the colour
of the solution gradually changed from a deep purple to a
dark reddish orange. The reaction mixture was stirred for an-
other 24 h, the volatiles were then removed under vacuum
and the residue extracted with hexanes and filtered. An im-
pure pale pink solid was obtained by slow evaporation of the
filtrate, which gradually decomposed with the evolution of
Ethylene and carbon monoxide were obtained as lecture
bottles from Matheson and dried over P2O5 before use. La-
beled 13CO (Cambridge Isotopes) was used as received.
Diphenylacetylene (Aldrich) was sublimed before use.
MeMgBr (1.4 M solution in THF–toluene) and EtMgBr
(3.0 M solution in ether) were purchased from Aldrich and
used as received. NaCp(DME) (DME = dimethoxyethane)
was prepared according to published literature procedure
(35). Hexanes, tetrahydrofuran (THF), and toluene were
predried over CaH2 followed by distillation under argon
from either sodium metal or sodium-benzophenone ketyl;
pentane was distilled from sodium-benzophenone ketyl. The
deuterated solvents C6D6 and C6D5CD3 were dried over so-
dium, vacuum transferred to a bomb, and degassed by
freeze–pump–haw technique before use. The procedures for
1
3
free ethylene. H NMR (20°C, C6D6) δ: –0.67 (t, 3H, JPH
=
10 Hz, ZrCH3), 0.24 and 0.26 (s, 6H, Si(CH3)2), 0.86 (m,
2
3
4H, JPH = 8 Hz, SiCH2P), 1.05 (m, 2H, JPH = 8 Hz,
Zr(CH2=CH2)), 1.10 (m, 24H, CH(CH3)2), 2.09 (sept, 4H,
3JHH = 7 Hz, CH(CH3)2), 4.61 (t, 1H, JHH = 2 Hz, Cp-H),
4
R
the preparation of [P2Cp]ZrCl3 (1a: R = Pri (36); 1b: R =
6.32 (d, 2H, JHH = 2 Hz, Cp-H). 31P NMR (20°C, C6D6) δ:
4
Me3) have been described elsewhere.
20.1 (s).
Pr[P2Cp]Zr(η2-CH2=CH2)Br (2a)
Pr[P2Cp]Zr(η2-CH2=CH2)(η5-C5H5) (4a)
A solution of NaCp(DME) (106 mg, 0.59 mmol) in
30 mL of toluene was added dropwise with stirring to a
cooled (–78°C) solution of 2a (410 mg, 0.59 mmol) dis-
A solution of 3.0 M EtMgBr (2.15 mL, 6.45 mmol) was
diluted in 20 mL of toluene and added dropwise with stir-
ring to a cooled (–78°C) solution of 1a (2.06 g, 3.23 mmol)
dissolved in 100 mL of toluene. The pale yellow colour of
3 M.D. Fryzuk, P.B. Duval, V.J. Young, Jr., and G.P.A. Yap. Manuscript in preparation.
© 2001 NRC Canada

544
Can. J. Chem. Vol. 79, 2001
4
solved in 50 mL of toluene. The mixture was stirred at
−78°C for 2 h, then allowed to slowly warm to room temper-
ature and stirred for another 24 h The colour of the solution
gradually changed from a deep purple to a orange-red. The
volatiles were then removed under vacuum and the residue
extracted with hexanes and filtered. Orange crystals were
obtained from a cold (–40°C) concentrated hexamethyl-
disiloxane–hexanes mixture. The crystals were washed with
cold hexamethyldisiloxane and dried. Yield: 310 mg (84%).
1H NMR (20°C, C6D6) δ: 0.08, 0.13, 0.52 and 0.58 (s, 3H,
5.08 (t, 1H, JHH = 1 Hz, Cp-H). 13C NMR (20°C, C6D6) δ:
240.5 and 245.1 (s, Zr(CO)). 31P NMR (20°C, C6D6) δ:
–12.0 (s).
Pr[P2Cp]Zr(η5-C5H5)(CO) (6a)
A benzene-d6 solution of 4a was placed in a sealable
NMR tube and affixed to a dual vacuum–nitrogen line. A
small portion of solvent was removed under vacuum, and an
atmosphere of carbon monoxide introduced at –78°C. The
contents within the sample were continually mixed under
ambient temperature by a constant inversion of the sample
3
Si(CH3)2), 0.42, 0.75, and 0.85 (ddt, 1H, JHH = 10 Hz (cis
3
1
and trans), JPH = 6 Hz, CHH=CH2), 0.74 and 0.81 (dd, 2H,
by mechanical method. The reaction was monitored by H
2
2JHH = 15 Hz, JPH = 4 Hz, SiCH2P), 0.94, 1.00, 1.02, 1.03,
and 31P NMR spectroscopy until the reaction was complete
(approximately 2 weeks). There was no obvious colour
change associated with this reaction. 6a was observed to be
the major product (90%) based on the NMR spectra. Re-
peating the procedure in a sealed tube with the addition of
13CO yielded a strong diagnostic peak at 308 ppm for the
carbonyl ligand in the 13C NMR spectrum. Conducting the
reaction on a larger scale in toluene generated a thermally
sensitive hydrocarbon-soluble orange oil after workup that
1.04, 1.05, 1.05, and 1.13 (dd, 3H, 3JHH = 7 Hz, 3JPH = 7 Hz,
3
CH(CH3)2), 1.59 and 1.60 (sept, 1H, JHH = 7 Hz,
3
2
CH(CH3)2), 1.74 (d of sept, 1H, JHH = 7 Hz, JPH = 3 Hz,
3
2
CH(CH3)2), 1.98 (d of sept, 1H, JHH = 7 Hz, JPH = 7 Hz,
4
CH(CH3)2), 5.21 (t, 1H, JHH = 2 Hz, Cp-H), 5.35, (s, 5H,
3JPH = 1 Hz, C5H5), 5.61 and 5.62 (dd, 1H, JHH = 6 Hz,
3
4JHH = 2 Hz, Cp-H). 31P NMR (20°C, C6D6) δ: –5.3 (s), 42.3
(s). Anal. calcd. for C30H56P2Si2Zr: C 57.55, H 9.01; found:
C 57.25, H 9.15.
1
gradually decomposed at –40°C. H NMR (20°C, C6D6) δ:
0.13, 0.24, 0.35, and 0.47 (s, 3H, Si(CH3)2), 0.42 and 0.63
2
2
Pr[P2Cp]Zr(CO)2Br (5a)
(dd, 1H, JHH = 11 Hz, JPH = 9 Hz, SiCH2P), 0.69 (d, 2H,
2JPH = 7 Hz, SiCH2P), 0.75, 0.78, 0.90, and 0.91 (dd, 3H,
A benzene-d6 solution of 2a was placed in a sealable
NMR tube and affixed to a dual vacuum–nitrogen line. A
small portion of solvent was removed under vacuum, and an
atmosphere of carbon monoxide introduced at –78°C. The
colour of the solution instantly changed from dark purple to
3
3JHH = 7 Hz, JPH = 7 Hz, CH(CH3)2), 1.05 (m, 12H,
3
CH(CH3)2), 1.29 and 1.55 (sept, 1H, JHH = 7 Hz,
CH(CH3)2), 1.59 (sept, 2H, 3JHH = 7 Hz, CH(CH3)2), 4.95 (s,
5H, 3JPH = 1 Hz, C5H5), 5.08, 5.55, and 5.62 (dd, 1H, 4JHH
=
=
2
1 Hz, Cp-H). 13C NMR (20°C, C6D6) δ: 308.0 (d, JPC
1
dark green. H NMR spectroscopy revealed the evolution of
3 Hz, Zr(CO)). 31P NMR (20°C, C6D6) δ: –4.8 and 58.7 (s).
free ethylene and the formation of one clean product with
resonances associated solely with the ancillary ligand, while
the 31P NMR spectrum showed a single new resonance. Re-
peating the reaction in a sealed tube with the addition of
13CO yielded diagnostic peaks for the carbonyl ligands in
the 13C NMR spectrum. Conducting the reaction on a larger
scale in toluene generated the same colour change in solu-
tion as noted before upon addition of CO, although removal
of the volatiles induced a subtle degradation of the green
colour to this solution. A hydrocarbon-soluble, thermally la-
bile dark orange-green oil was obtained after workup in hex-
Pr[P2Cp]Zr(η2-PhCCPh)Br (7a)
Diphenylacetylene (106 mg, 0.59 mmol) was dissolved in
30 mL of toluene and added dropwise with stirring to a
cooled (–78°C) solution of 2a (410 mg, 0.59 mmol) dis-
solved in 50 mL of toluene. The mixture was slowly warmed
to room temperature and stirred for another 24 h. The colour
of the solution gradually changed from a deep purple to a
dark brown. The volatiles were then removed under vacuum
and the residue extracted with hexanes and filtered. Pale yel-
low crystals were obtained from slow evaporation of a con-
centrated hexamethyldisiloxane–hexanes mixture. The
crystals were washed with two aliquots of cold hexamethyl-
disiloxane and dried. Yield: 313 mg (67%). Anal. calcd. for
C37H57P2Si2Zr: C 56.18, H 7.26; found: C 56.15, H 7.31.
anes. IR (C6D6) νCO (cm–1): 1890, 1865 (13CO). H NMR
1
(20°C, C6D6) δ: 0.12 and 0.28 (s, 6H, Si(CH3)2), 0.51 and
2
2
0.82 (dd, 2H, JHH = 15 Hz, JPH = 9 Hz, SiCH2P), 0.97,
3
3
1.22, 1.29 and 1.32 (dd, 6H, JHH = 7 Hz, JPH = 7 Hz,
CH(CH3)2), 1.95 and 2.48 (sept, 2H, JHH = 7 Hz,
3
4
CH(CH3)2), 5.16 (d, 2H, JHH = 1 Hz, Cp-H), 5.19 (t, 1H,
1
Major isomer (60%): H NMR (20°C, C6D6) δ: 0.27 and
4JHH = 1 Hz, Cp-H). 13C NMR (20°C, C6D6) δ: 241.1 (br s,
0.46 (s, 6H, Si(CH3)2), 0.84 and 1.28 (dd, 2H, 2JHH = 15 Hz,
2JPH = 4 Hz, SiCH2P), 0.80, 0.98, 1.04, and 1.13 (dd, 6H,
Zr(CO)), 249.1 (s, Zr(CO)). 31P NMR (20°C, C6D6) δ: 29.1
(s).
3
3JHH = 7 Hz, JPH = 7 Hz, CH(CH3)2), 1.74 and 2.53 (sept,
3
4
2H, JHH = 7 Hz, CH(CH3)2), 6.75 (d, 2H, JHH = 1 Hz, Cp-
Me[P2Cp]Zr(CO)2Br (5b)
4
H), 6.86 (t, 1H, JHH = 1 Hz, Cp-H), 7.00 (t, 2H, p-C6H5),
7.24 (t, 4H, m-C6H5), 7.45 (d, 4H, o-C6H5). 31P NMR (20°C,
C6D6) δ: 9.3 (s).
The procedure followed was similar to that for 5a, react-
ing ethylene 2b in an atmosphere of CO. The colour of the
solution gradually changed from dark red to a dull purplish
red after the addition of CO. Once again, a scaled up version
of this reaction permitted only the isolation of a thermally
labile oil. IR (C6D6) νCO (cm–1): 1898, 1855 (13CO) 1H NMR
(20°C, C6D6) δ: 0.09 and 0.18 (s, 6H, Si(CH3)2), 0.57 and
1
Minor isomer (40%): H NMR (20°C, C6D6) δ: 0.21 and
0.33 (s, 6H, Si(CH3)2), 0.67 and 1.14 (dd, 2H, 2JHH = 14 Hz,
2JPH = 5 Hz, SiCH2P), 0.75, 1.19, 1.29, and 1.35 (dd, 6H,
3
3JHH = 7 Hz, JPH = 7 Hz, CH(CH3)2), 2.10 and 2.15 (sept,
2
3
4
1.03 (d, 2H, JPH = 7 Hz, SiCH2P), 1.41 and 1.50 (t, 6H,
2H, JHH = 7 Hz, CH(CH3)2), 6.46 (t, 1H, JHH = 1 Hz, Cp-
4
4
2JPH = 5 Hz, P(CH3)2), 4.82 (d, 2H, JHH = 1 Hz, Cp-H),
H), 6.80 (d, 2H, JHH = 1 Hz, Cp-H), 6.94 (t, 2H, p-C6H5),
© 2001 NRC Canada

Fryzuk et al.
545
7.18 (t, 4H, m-C6H5), 7.35 (d, 4H, o-C6H5). 31P NMR (20°C,
C6D6) δ: 6.9 (s).
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Concluding remarks
In this paper a series of zirconium complexes stabilized
by the ancillary P2Cp ligand are presented in which the for-
mal oxidation state can be regarded as Zr(II). The ethylene
complexes 2 and 4a react with CO via loss of ethylene to
generate carbonyl derivatives, and 2a undergoes a similar re-
action with diphenylacetylene to produce alkyne 7a. The
slow reaction of the saturated 18-electron complex 4a with
CO demonstrates that phosphine dissociation is required to
provide a reactive site. The lack of reactivity of alkyne 7a
with CO presumably originates from the same electronic sat-
uration of that species. In comparison, the immediate incor-
poration of CO by the 16-electron ethylene complexes 2a or
2b suggests an associative mechanism. The displacement of
a coordinated ethylene molecule by CO is a rare occurrence
in early metal complexes (33). It might have been antici-
pated that strong back-bonding should confer a degree of
Zr(IV) metallacycle character in the metal—ethylene bond-
ing as shown in A; however, it is apparent from the resulting
ligand substitution reactions that considerable Zr(II) charac-
ter is still present in these ethylene derivatives.
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© 2001 NRC Canada