Ligand substitution in the mixed-metal cluster PhCCo2Ni(CO) 6Cp by 2,3-bis(diphenylphosphino)maleic anhydride (bma): An intimate picture involving the stepwise conversion of the trinuclear cluster PhCCo 2Ni(CO)4(η2-bma)Cp to the mononuclear compound CpNi[PPh2CPhCC(PPh2)C(O)OC(O)]

Article abstract of DOI:10.1016/j.jorganchem.2005.01.069

Heating the mixed-metal cluster PhCCo₂Ni(CO)₆Cp with the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) in 1,2-dichloroethane proceeds by CO loss and formation of the cobalt-bridged cluster PhCCo₂Ni(CO)₄(η²-bma)Cp (2). The bma ligand is fluxional in solution and is in equilibrium with the cobalt-chelated isomer, as demonstrated by VT IR and ³¹P NMR measurements. The van't Hoff parameters (ΔH = 1.49 ± 0.02 kcal/mol; ΔS = 12.0 ± 0.1 eu) for the chelate-to-bridge equilibrium have been evaluated from IR band-shape analyses of the in-phase anhydride carbonyl stretching band over the temperature range 173-116 K. Cluster 2 readily loses CO to furnish the cluster PhCCo₂Ni(CO)₃(μ, η²-bma)Cp (3), where the 6e- donor bma ligand chelates one of the cobalt centers via the phosphine groups and is tethered to the second cobalt center by the maleic anhydride π bond. Continued heating of cluster 3 is followed by the formation of 50e- cluster Co₂Ni(CO) ₄Cp[μ₂,η²,η¹-C(Ph) CC(PPh₂)C(O)OC(O)](μ₂-PPh₂) (4), which in turn gives the mononuclear complex CpNi[PPh₂CPhCC(PPh ₂)C(O)OC(O)] (5) as the end-product of the thermolysis reaction. Each of these new compounds has been isolated and their thermolysis reactivity independently examined, allowing for the unequivocal sequence associated with the decomposition of PhCCo₂Ni(CO)₄(η²-bma) Cp to be established. Compounds 2-5 have been fully characterized in solution by IR and NMR(¹H, ¹³C, ³¹P) spectroscopies, and the solid-state structures of all four products have been determined by X-ray crystallography. The solution spectroscopic data of the new products are compared with the X-ray diffraction structures and the structural highlights of each compound are discussed. The coordination of the maleic anhydride π bond in PhCCo₂Ni(CO)₃(μ,η²-bma)Cp (3) provides crucial insight into one of the necessary requirements for P-C bond cleavage of the bma ligand at a tetrahedral cluster. The reactivity of the heterometallic cluster PhCCo₂Ni(CO)₄(η²- bma)Cp is contrasted with its homometallic analogue PhCCo₃(CO) ₇(η²-bma).

Full text of DOI:10.1016/j.jorganchem.2005.01.069

Journal of Organometallic Chemistry 690 (2005) 3067–3079
www.elsevier.com/locate/jorganchem
Ligand substitution in the mixed-metal cluster PhCCo2Ni(CO)6Cp
by 2,3-bis(diphenylphosphino)maleic anhydride (bma): An
intimate picture involving the stepwise conversion
of the trinuclear cluster PhCCo2Ni(CO)4(g2-bma)Cp to the
mononuclear compound CpNi[PPh2CPhC@C(PPh2)C(O)OC(O)]
q
a,
b
b,
*
Simon G. Bott *, Kaiyuan Yang , Michael G. Richmond
a
Department of Chemistry, University of Houston, Houston, TX 77204, USA
b
Department of Chemistry, University of North Texas, Denton, TX 76203, USA
Received 4 September 2004; revised 27 January 2005; accepted 28 January 2005
Available online 23 May 2005
Abstract
Heating the mixed-metal cluster PhCCo2Ni(CO)6Cp with the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride
(bma) in 1,2-dichloroethane proceeds by CO loss and formation of the cobalt-bridged cluster PhCCo2Ni(CO)4(g2-bma)Cp (2).
The bma ligand is fluxional in solution and is in equilibrium with the cobalt-chelated isomer, as demonstrated by VT IR and 31P
NMR measurements. The vanꢀt Hoff parameters (DH = 1.49 0.02 kcal/mol; DS = 12.0 0.1 eu) for the chelate-to-bridge equilib-
rium have been evaluated from IR band-shape analyses of the in-phase anhydride carbonyl stretching band over the temperature
range 173–116 K. Cluster 2 readily loses CO to furnish the cluster PhCCo2Ni(CO)3(l,g2-bma)Cp (3), where the 6eꢀ donor bma
ligand chelates one of the cobalt centers via the phosphine groups and is tethered to the second cobalt center by the maleic anhydride
p
bond. Continued heating of cluster 3
is followed by the formation of 50eꢀ cluster Co2Ni(CO)4Cp[l2,g2,g1-
C(Ph)C@C(PPh2)C(O)OC(O)](l2-PPh2) (4), which in turn gives the mononuclear complex CpNi[PPh2CPhC@C(PPh2)C(O)OC(O)]
(5) as the end-product of the thermolysis reaction. Each of these new compounds has been isolated and their thermolysis reactivity
independently examined, allowing for the unequivocal sequence associated with the decomposition of PhCCo2Ni(CO)4(g2-bma)Cp
to be established. Compounds 2–5 have been fully characterized in solution by IR and NMR(1H, 13C, 31P) spectroscopies, and the
solid-state structures of all four products have been determined by X-ray crystallography. The solution spectroscopic data of the
new products are compared with the X-ray diffraction structures and the structural highlights of each compound are discussed.
The coordination of the maleic anhydride p bond in PhCCo2Ni(CO)3(l,g2-bma)Cp (3) provides crucial insight into one of the nec-
essary requirements for P–C bond cleavage of the bma ligand at a tetrahedral cluster. The reactivity of the heterometallic cluster
PhCCo2Ni(CO)4(g2-bma)Cp is contrasted with its homometallic analogue PhCCo3(CO)7(g2-bma).
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Mixed-metal clusters; P–C bond cleavage; Diphosphine fluxional behavior; VT IR
q
Dedicated to Prof. Charles U. Pittman, Jr. (aka CUP) for his work
in polymer and metal cluster chemistry on the occasion of his 66th
1. Introduction
birthday.
*
Corresponding authors. Tel.: +713 743 2771 (S.G. Bott), Tel.: +940
565 3548; fax: +940 564 4318 (M.G. Richmond).
The ligand substitution behavior of the homometallic
clusters RCCo3(CO)9 has been thoroughly examined
over the last three decades. Monodentate phosphines
E-mail addresses: sbott@uh.edu (S.G. Bott), cobalt@unt.edu
(M.G. Richmond).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.069

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S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
react with RCCo3(CO)9 to furnish the expected
substituted derivatives RCCo3(CO)9ꢀxPx, with the de-
gree of ligand incorporation (x = 1–3) strongly depen-
dent upon the size of the incoming P ligand. In the
case of multiple CO replacements, phosphine addition
takes place sequentially at Co(CO)3 centers leading ulti-
mately to the trisubstituted clusters RCCo3(CO)6P3 [1]
CO substitution reactions have also been studied with
bi- and tridentate phosphine ligands. This latter family
of phosphine ligands gives RCCo3(CO)6P3 compounds
through the face-capping of the three cobalt atoms via
axial coordination [2], with the substitution chemistry
using bidentate ligands being more diverse and less pre-
dictable. There diphosphine ligands have been found to
bridge adjacent cobalt centers and/or chelate to a single
cobalt atom [3].
We have previously presented our findings concern-
ing the reactivity of the redox-active diphosphine ligand
2,3-bis(diphenylphosphino)maleic anhydride (bma) with
RCCo3(CO)9 (where R = Ph, Fc) to afford RCCo3-
(CO)7(g2-bma) [4]. Of surprise to us was: (1) the coordi-
native flexibility exhibited by the ancillary bma ligand
when tethered to the tricobalt cluster, which manifests
itself in the rapid equilibration of the bridging and che-
lating bma isomers of RCCo3(CO)7(g2-bma), and (2)
the subsequent ligand/cluster fragmentation reactivity
that is depicted in Scheme 1. Our findings here reveal
that the nature of the capping alkylidyne moiety does in-
deed influence the course of ligand reactivity in these
thermolysis reactions [5].
of HCCo2Ni(CO)6Cp with added bma produces the
putative species HCCo2Ni(CO)4(g2-bma)Cp, followed
by bma attack at both the methylidyne cap and the
CpNi center, as shown in Eq. (1). Continued heating
of this product leads to P–C bond cleavage and forma-
tion of the 50eꢀ hypho cluster Co2NiCp(CO)4[l2,g2,r-
C(H)PPh2C@CC(O)OC(O)](l2-PPh2) (not shown) [6].
ð1Þ
Herein, we report our results on the reaction between
PhCCo2Ni(CO)6Cp and bma, which yields PhCCo2Ni
(CO)4(g2-bma)Cp (2). The thermodynamics for
chelate-to-bridge ligand isomerization in 2 have been as-
sessed by VT IR measurements. Cluster 2 is thermally
unstable and gives PhCCo2Ni(CO)3(l,g2-bma)Cp (3),
followed by Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)-
C(O)OC(O)](l2-PPh2) (4). Continued heating of cluster
4
is accompanied by cluster fragmentation and
formation of mononuclear nickel complex CpNi[PPh2-
CPhC@C(PPh2)C(O)OC(O)] (5) as the end-product.
Clusters 2–5 have been isolated and fully characterized
in solution by standard spectroscopic methods and the
solid-state structures of all four products have all been
determined by X-ray crystallography. The reactivity of
the heterometallic cluster PhCCo2Ni(CO)4(g2-bma)Cp
is discussed relative to its homometallic counterpart
PhCCo3(CO)7(g2-bma).
In an attempt to gain a greater understanding of the
scope and generality associated with the reactivity of the
bma ligand at other tetrahedrane clusters, we have ex-
plored the reaction of the mixed-metal clusters RCCo2-
Ni(CO)6Cp (where R = H, Ph) with bma. Thermolysis
Scheme 1.

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3069
2. Experimental
183 K) d 201.03 (chelating isomer), 203.41 (s, 2C, bridg-
ing isomer), 209.75 (2C, bridging isomer). 31P NMR
(CDCl3; 298K): d 24.24 (b, bridging isomer); (THF;
173 K) d 25.48 (s, bridging isomer), 51.45 (s, chelating
isomer), 59.50 (s, chelating isomer). Anal. Calc. (found)
for C44H30Co2NiO7P2: C, 58.10 (57.89); H, 3.32 (3.38).
2.1. General methods
The bma ligand was synthesized from 2,3-dichloro-
maleic anhydride, which was purchased from Aldrich
Chemical Co. and used as received, and Ph2PTMS [7].
The starting cluster PhCCo2Ni(CO)6Cp was prepared
from PhCCo3(CO)9 [8] and nickelocene [9] according
to the general procedure of Vahrenkamp [10]. All reac-
tion and NMR solvents were distilled under argon from
a suitable drying agent and stored in Schlenk storage
vessels [11]. The combustion analyses were performed
by Altantic Microlab, Norcross, GA.
2.3. Thermolysis of PhCCo2Ni(CO)4(g2-bma)Cp (2) to
give PhCCo2Ni(CO)3(l,g2-bma)Cp (3) and Co2Ni
(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)]
(l2-PPh2) (4)
To a Schlenk tube containing 0.30 g (0.33 mmol) of
PhCCo2Ni(CO)4(g2-bma)Cp was added 25 mL of
DCE under argon flush. The solution was then heated
at reflux for 3.0 h, at which time maximum conversion
to clusters 3 and 4 was found by TLC analysis. The
DCE solvent was removed and the components in the
reaction mixture separated by column chromatography
over silica gel using CH2Cl2 as the eluent. The first green
band isolated from the column belonged to 2, followed
by a second green band corresponding to PhCCo2Ni
CO)3(l,g2-bma)Cp (3), and finally a brown band due
Routine solution infrared spectra were recorded on a
Nicolet 20 SXB FT-IR spectrometer in amalgamated
0.1 mm NaCl cells, using PC control and OMNIC soft-
ware. The VT IR spectra were recorded on the above
spectrometer with a Specac Model P/N 21.000 IR cell
equipped with inner and outer CaF2 windows and mod-
ified for flow sampling conditions. The coolant employed
in the VT IR studies was either liquid nitrogen (<143 K)
or a pentane/liquid nitrogen slush (173–143 K). The re-
ported cell temperatures, taken to be accurate to 2 ꢁC,
were determined with a Cu-constantan thermocouple.
to
Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)O-
C(O)] (l2-PPh2) (4). Cluster 3 was recrystallized from
a CH2Cl2 solution that had been layered with hexane,
while cluster 4 was recrystallized from a benzene solu-
tion that had been layered with hexane. These recrystal-
lizations furnished X-ray quality crystals and the
samples for combustion analysis. Yield of PhCCo2Ni
(CO)3(l,g2-bma)Cp: 65 mg (21%). IR (CH2Cl2;
298 K): m(CO) 2006 (s), 1954 (vs), 1912 (s, semibridging
CO), 1789 (m, symm anhydride carbonyl), 1740 (m,
antisymm anhydride carbonyl) cmꢀ1. 1H NMR (CDCl3;
298 K): d 4.69 (s, Cp), 6.80–9.70 (m, phenyls). 13C
(THF; 223 K) d 201.31 (1C) 209.46 (1C), 223.79 (1C,
semibridging CO). 31P NMR (THF; 178 K): d 25.30 (s,
b), 29.57 (s, b). Anal. Calc. (found) for C43H30Co2-
NiO6P2: C, 58.59 (58.32); H, 3.45 (3.56). Yield of Co2N-
i(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-
PPh2): 75 mg (25%). IR (CH2Cl2; 298 K): m(CO) 2032
(s), 2000 (vs), 1985 (m), 1962 (m) 1798 (m, symm anhy-
dride carbonyl), 1739 (m, antisymm anhydride carbonyl)
1
The H NMR spectra were recorded at 200 MHz on a
Varian Gemini-200 spectrometer, and the 13C and 31P
NMR spectra were recorded at 75 and 121 MHz, respec-
tively, on a Varian 300-VXR spectrometer in the proton
decoupled mode. The reported 31P chemical shifts are ref-
erenced to external H3PO4 (85%), taken to have d = 0.0.
2.2. Synthesis of PhCCo2Ni(CO)4(g2-bma)Cp (2) from
PhCCo2Ni(CO)6Cp (1)
To 0.30 g (0.60 mmol) of PhCCo2Ni(CO)6Cp and
0.30 g (0.64 mmol) of bma in a large Schlenk tube was
added 30 mL of 1,2-dichloroethane (DCE) by syringe,
after which the vessel was sealed and heated at 75–
80 ꢁC for ca. 0.5 h. Yields of PhCCo2Ni(CO)4(g2-
bma)Cp were maximized when the reaction solution
was monitored by TLC analysis and heating terminated
when clusters 3 and 4 and decomposition material at the
origin was observed. Upon such time the solvent was re-
moved under vacuum and the product cluster was puri-
fied by flash column chromatography over silica gel
using a 1:1 mixture of CH2Cl2/petroleum ether as the
eluent. Recrystallization of PhCCo2Ni(CO)4(g2-bma)-
Cp from a CH2Cl2 solution that had been layered with
hexane afforded the analytical sample and single crystals
of 2 suitable for X-ray diffraction analysis. Yield of
green PhCCo2Ni(CO)4(g2-bma)Cp: 0.35 g (67%). IR
(CH2Cl2; 298 K): m(CO) 2012 (s), 1989 (vs), 1971 (s),
1821 (w, symm anhydride carbonyl), 1767 (m, antisymm
anhydride carbonyl) cmꢀ1. 1H NMR (CDCl3; 298 K): d
5.15 (s, Cp), 6.75–7.75 (m, phenyls). 13C (CH2Cl2;
1
cmꢀ1. H NMR (CDCl3; 298 K): d 4.83 (s, Cp), 7.00–
8.50 (m, phenyls). 13C (THF; 223 K) d 201.74 (s, b)
206.10 (s, b), 206.55 (s, b) 209.67 (s, b). 31P NMR
(THF; 178 K): d 63.88 (Ni–P), 170.94 (s, phosphido).
Anal. Calc. (found) for C44H30Co2NiO7P2 Æ C6H6: C,
60.82 (60.53); H, 3.67 (3.70).
2.4. Thermolysis of Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@
C(PPh2)C(O)OC(O)](l2-PPh2) to CpNi[PPh2CPhC@
C(PPh2)C(O)OC(O)] (5)
The thermolysis was carried out with 0.30 g
(0.33 mmol) of cluster 4 in 25 mL of DCE. The solution

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S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
was heated at reflux in a Schlenk vessel under argon for
ca. 2 days until only a trace of starting cluster remained,
as determined by TLC analysis. The DCE solvent was
removed under vacuum and the sole observable product
of the reaction was isolated by chromatography over sil-
ica gel using a 9:1 mixture of CH2Cl2/acetone. The
resulting CpNi[PPh2CPhC@C(PPh2)(O)OC(O)] that
was obtained was recrystallized from CH2Cl2 and hex-
ane by allowing the former solvent to evaporate slowly
under argon protection. Yield of black 5: 30 mg (13%).
IR (CH2Cl2): m(CO) 1746 (m, symm anhydride car-
bridging and chelating isomer in solution over the
temperature range of 173–116 K. The temperature-
dependent behavior associated with the in-phase anhy-
dride carbonyl stretching band of each isomer was
analyzed by VT IR spectroscopy, with the area of each
band quantified by band-shape analysis using the com-
mercially available program Peakfit and where the area
under each carbonyl absorption was assumed to be pro-
portional the relative amount of each isomer. All spectra
were subjected to base-line correction and were success-
fully fitted by using Gaussian band shapes. The thermo-
dynamic parameters for the chelate ꢀ bridge exchange
were obtained from a plot of ln Keq vs. Tꢀ1, from which
the slope and intercept furnished the DH and DS, respec-
tively. The error limits for the thermodynamic data were
calculated by using the available least-squares regression
program and do not reflect uncertainties in sample prep-
aration, band-area intensities, or temperature control
but rather the deviation of the data points about the
least-squares line [12].
bonyl), 1714 (s, antisymm anhydride carbonyl) cmꢀ1
.
1H NMR (CDCl3; 298 K): d 4.92 (s, Cp), 6.70–7.35
(m, phenyls). 13C NMR (CDCl3; 298 K): d 94.75 (Cp).
31P NMR (CDCl3, 298 K): d 12.22 (d, JP–P = 76 Hz),
42.07 (d, JP–P = 76 Hz). Anal. Calc. (found) for
C40H30NiO3P2: C, 70.66 (70.08); H, 4.43 (4.53).
2.5. X-ray crystallography
Table 2 contains the X-ray data and processing
parameters for the structures 2–5. Selected crystals of
the four compounds for X-ray diffraction analysis were
grown as described above and were each sealed inside
a Lindemann capillary, followed by mounting on an En-
raf-Nonius CAD-4 diffractometer. After the cell con-
stants were obtained for all four samples, intensity
data in the range of 2ꢁ 6 2h 6 32ꢁ (2), 2ꢁ 6 2h 6 25ꢁ
(3), 2ꢁ 6 2h 6 28ꢁ (4), and 2ꢁ 6 2h 6 26ꢁ (5) were col-
lected at 298 K and were corrected for Lorentz, polari-
zation, and absorption (DIFABS). All four structures
were solved by using SHELX-86. All non-hydrogen atoms
were located with difference Fourier maps and full-
matrix least-squares refinement and were refined aniso-
tropically in cluster 2. The data reduction in 3 proceeded
similarly with the Ni, Co, P, O, and Cp groups being re-
fined anisotropically, and with the exception of the phe-
nyl carbon atoms in cluster 4, all other non-hydrogen
atoms were refined anisotropically. In the case of the
compound 5 only the Ni and P atoms were refined
3. Results and discussion
3.1. Reaction of PhCCo2Ni(CO)6Cp with bma, spectro-
scopic evidence for diphosphine isomerization, and X-ray
diffraction structure of PhCCo2Ni(CO)4(g2-bma)Cp
Heating an equimolar mixture of PhCCo2Ni(CO)6Cp
(1) with the diphosphine ligand bma in DCE at 75–80 ꢁC
promotes the loss of CO and formation of the substi-
tuted cluster PhCCo2Ni(CO)4(g2-bma)Cp (2) without
the presence of other products, provided the reaction
time is kept short. Maximum yields of 2 were realized
when the reaction was monitored by IR or TLC analysis
and terminating the reaction at the first sign of by-prod-
ucts. Cluster 2 was isolated by chromatography over sil-
ica gel as a mildly air-sensitive green solid. The reaction
between cluster 1 and bma is shown in Eq. (2).
anisotropically. Refinement on
2
converged at
R = 0.0318 and Rw = 0.0334 for 3619 unique reflections
with I > 3r(I), while for 3 refinement converged at
R = 0.0426 and Rw = 0.0459 for 2551 unique reflections
with I > 3r(I). The refinement for 4 and 5 afforded con-
vergence values of R = 0.0466 and Rw = 0.0507 for 2902
unique reflections with I > 3r(I) and R = 0.0934 and
Rw = 0.1094 for 2676 unique reflections with I > 3r(I),
respectively.
ð2Þ
The room temperature 31P NMR spectrum of 2 re-
vealed the presence of a broad resonance at ca. d 24
suggesting the presence of a bridging bma ligand [13].
Since it was not immediately clear whether the broad-
ened resonance was due to scalar coupling between the
31P and 59Co nuclei or dynamic isomerism of the bma
about the cluster polyhedron [14], we examined the 31P
2.6. Thermodynamic assessment of the equilibrium
between the bridging and chelating isomers of
PhCCo2Ni(CO)4(g2-bma)Cp
2-MeTHF solutions of PhCCo2Ni(CO)4(g2-bma)Cp
(ca. 7.0 · 10ꢀ3 M) were used to measure the amount of

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3071
NMR spectrum at reduced temperature. The resonance
at d 24 sharpened considerably as the temperature was
dropped to 248 K and was accompanied by the appear-
ance of two very weak resonances at ca. d 50 and 60.
Further cooling to 173 K, the lowest attainable temper-
ature on our spectrometer, led to an increase in the
intensity of the latter two low-field resonances at the
expense of the bridging resonance at d 25.48, and in
keeping with the bma isomerization behavior found
in PhCCo3(CO)7(g2-bma), the two equal intensity reso-
nances at d 51.45 and 59.50 are assigned to the chelat-
ing isomer of PhCCo2Ni(CO)4(g2-bma)Cp. Integration
of the 31P NMR data at 173 K gives a Keq of 5.7 in fa-
vor of the bma-bridged cluster 2. The equilibrium con-
stant calculated for 2 is identical to that reported by us
for PhCCo3(CO)7(g2-bma), [4a] where the bma-bridged
cluster was found to be favored under the same condi-
tions. While this may appear fortuitous, we would not
expect the dynamics for bma ligand fluxionality to be
substantially different between PhCCo2Ni(CO)4(g2-
bma)Cp and PhCCo3(CO)7(g2-bma) given the struc-
tural/electronic similarities of the cobalt centers that
participate in the isomerization sequence. The isomeric
forms of PhCCo2Ni(CO)4(g2-bma)Cp that contribute
to this equilibrium are shown in the top portion of
Scheme 2.
The 13C NMR spectrum of 13CO enriched 2 exhibited
two broad terminal carbonyl resonances at d 203 and
209 with an integral ratio of 1:1 at 298 K. These groups
are readily assigned to the pairwise equivalent axial and
equatorial Co–CO groups in the bma-bridged cluster
possessing Cs symmetry. Recording the same sample at
Scheme 2.

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S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
183 K led to a sharpening of the two CO groups of the
bridged isomer and produced a small, broad carbonyl
signal at d 201. This behavior is fully reversible and
the chemical shift at d 201 is ascribed to the three termi-
nally bound Co–CO groups in the chelating isomer of 2,
on the basis of chemical shift arguments and the VT 13
C
NMR data reported for PhCCo3(CO)7(g2-bma). Not
observed in the chelating isomer of 2 is the expected
low-field resonance for the lone CO group center that
is bound at the phosphine-tethered cobalt center. The
absence of this CO group may be due to the rapid intra-
molecular scrambling of all the CO ligands in the chelat-
ing isomer or 59Co quadrupolar broadening. In either
case spectral observation would be problematic.
Additional proof for the isomerization of the bma li-
gand in PhCCo2Ni(CO)4(g2-bma)Cp and thermody-
namic assessment of the chelate ¢ bridge equilibrium
were provided by VT FT-IR measurements. The room
temperature IR spectrum of 2 in CH2Cl2 solvent displays
three prominent terminal m(CO) bands at 2012, 1989, and
1971 cmꢀ1, along with two weaker m(CO) bands at 1821
and 1767 belonging to the vibrationally coupled anhy-
dride moiety [15]. Recording the same spectrum right
above the freezing point of CH2Cl2 revealed additional
small but observable anhydride carbonyl stretching
bands. Since the chelating isomer is favored at low tem-
perature and that we were temperature limited in
CH2Cl2, we employed 2-MeTHF as a solvent. The IR
spectra of 2 in 2-MeTHF at temperatures <173 K
showed a sharpening of the terminal CO groups associ-
ated with the two cobalt atoms, but the three observed
carbonyl bands exhibited insufficient resolution to make
them of any use in the study of the two isomers. How-
ever, the two m(CO) bands belonging to the anhydride
ring did provide the means by which this equilibrium
could be studied. Fig. 1 shows the IR spectra for the
anhydride m(CO) bands of cluster 2 recorded at 173
and 116 K. At 173 K in 2-MeTHF the higher energy
in-phase m(CO) band at 1821 cmꢀ1 of the ligand-bridged
isomer is accompanied by a weaker intensity shoulder at
1830 cmꢀ1. This doubling of m(CO) bands was also found
with the lower energy antisymmetric m(CO) band; here
m(CO) bands at 1761 (bridge) and 1770 (chelate) cmꢀ1
were recorded. As the temperature was progressively
lowered, the bands at 1821 and 1761 cmꢀ1 decreased in
intensity as the other two m(CO) bands of the chelating
isomer increased. The changes were reversible, as con-
firmed by several independent experiments and repeated
cycling of the temperature over the range of 173–116 K.
The IR spectra were subjected to band-area measure-
ments in order to determine the area under each in-
phase carbonyl stretching band [16]. Table 1 lists the
Keq values for the equilibrium involving chelate-to-
bridge exchange, from which the vanꢀt Hoff plot that
accompanies Fig. 1 afforded values of 1.49 0.02 kcal/
mol for DH and 12.0 0.1 eu for DS. The opposing
Fig. 1. VT FT-IR spectra of PhCCo2Ni(CO)4(g2-bma)Cp (2) in the
anhydride carbonyl region at 173 (black) and 116 K (red) in 2-MeTHF
solvent, with inclusion of the vanꢀt Hoff plot as an inset.
Table 1
Equilibrium data for the conversion of chelating 2 into bridging 2a
103/T, Kꢀ1
Chelating (%)
Bridging (%)
ln Keq
b
5.78
6.62
6.85
7.09
7.35
7.63
7.94
8.26
8.62
15.0
24.3
28.0
33.6
35.7
42.0
49.2
53.0
59.0
85.0
74.7
72.0
66.4
64.3
58.0
50.8
47.0
41.0
1.73
1.12
0.94
0.68
0.59
0.32
0.032
ꢀ0.12
ꢀ0.36
DH = 1.49 0.02 kcal/mol; DS = 12.0 0.1 eu.
a
From 7.0 · 10ꢀ3 M PhCCo2Ni(CO)4(g2-bma)Cp in 2-MeTHF by
following the changes in the area of the in-phase carbonyl stretching
bands at 1821 and 1830 cmꢀ1
.
Defined as the area of [bridged isomer]/[chelated isomer].
b
enthalpic and entropic contributions reveal that above
ca. 125 K the predominant isomer in solution will corre-
spond to the cluster with a bridging bma ligand. Other
than our original study on PhCCo3(CO)7(g2-bma)
where bma ligand fluxionality was verified by 31P
NMR spectroscopy, our current studies have provided
additional evidence for the isomerization of the ancillary
bma ligand through both 31P and IR spectroscopies. To
our knowledge, the only other report for related biden-
tate ligand fluxionality derives from the demonstrated
racemization reactivity involving the ancillary ligands
in the tetrahedrane clusters Me2CHO2CCCo3(CO)7
(L–L) (where L–L = dppe, arphos). In the case of the ar-
phos ligand (Ph2PCH2CH2 AsPh2), it has been sug-
gested that the observed racemization results from the
migration of the arsine moiety between the cobalt
centers [17], with the ancillary ligand maintaining its

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3073
Table 2
X-ray crystallographic data and processing parameters for the compounds 2–5
Compound
2
3
4
5
CCDC entry no.
Crystal system
Space group
244754
Monoclinic
P21/n
244755
Monoclinic
P21/c
244756
Monoclinic
P21/n
245326
Triclinic
ꢀ
P1
Unit cell dimensions
˚
a (A)
˚
12.3179(9)
16.382(1)
19.662(1)
14.188(2)
14.344(1)
19.547(3)
13.618(1)
22.736(3)
14.5181(9)
14.633(7)
14.826(8)
17.798(2)
69.04(3)
68.97(3)
65.29(5)
3174(3)
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
94.461(6)
109.54(1)
100.886(8)
ꢀ3
˚
V (A
)
3966.3(5)
C44H30Co2NiO7P2
909.25
3749.0(8)
C43H30Co2NiO6P2
881.24
4414.2(6)
C50H36Co2NiO7P2
987.36
Mol. formula
Fw
C40H30NiO2P2
679.34
4
Formula units per cell (Z)
Dcalc (g/cm3)
4
1.523
4
1.561
4
1.486
1.421
0.71073
7.50
˚
k (Mo Ka) (A)
0.71073
14.26
0.018
0.71073
15.05
0.031
0.71073
12.88
0.026
Absorption coeff. (cmꢀ1
Rmerge
)
–
Abs. corr. factor
Total reflections
Independent reflections
Data/res/parameters
R
0.84–1.19
5310
3619
0.80–1.14
5036
2551
0.89–1.06
5840
2902
0.68/1.26
7193
2676
3619/0/505
0.0318
0.0334
2551/0/312
0.0426
0.0459
2902/0/387
0.0466
0.0507
2676/0/389
0.0934
0.1094
1.08
Rw
GOF on F2
Weights
0.93
[0.04F2 + (rF)2]ꢀ1
1.02
[0.04F2 + (rF)2]ꢀ1
1.01
[0.04F2 + (rF)2]ꢀ1
[0.04F2 + (rF)2]ꢀ1
bridging coordination mode upon completion of the
equilibration sequence [18,19].
The molecular structure of 2 was established by X-
ray crystallography and with it the bridging of the two
cobalt centers by the bma ligand. Fig. 2 shows the the
ORTEP diagram of PhCCo2Ni(CO)4(g2-bma)Cp, with
selected bond distances and angles reported in Table 3.
The overall structure of 2 is unchanged from the parent
cluster 1, insomuch that it contains 48 valence electrons
and a nido Co2Ni(l3-C) core [20]. Cluster 2, to our
knowledge, represents only the second structurally char-
acterized PhCCo2Ni(CO)4 (P–P)Cp cluster to date [21].
The Co(1)–Co(2), Ni–Co(1), and Ni–Co(2) bond dis-
˚
tances of 2.4639(9), 2.3742(9), and 2.3811(9) A, respec-
tively, are unremarkable in comparison to related
Co2Ni and CoNi2 clusters [22], and are consistent with
metal–metal single-bond distances. The C(11)–C(15) p
bond of the maleic anhydride residue reveals a distance
Fig. 2. ORTEP drawing of PhCCo2Ni(CO)4(g2-bma)Cp (2) showing
the thermal ellipsoids at the 50% probability level.
˚
of 1.353(6) A that is in agreement with the C@C bond
distance of simple alkenes [23]. The two phosphorus
groups are nearly trans to the CpNi moiety given the
bond angles of 156.87(5)ꢁ and 150.57(5)ꢁ found for the
Ni–Co(2)–P(2) and Ni–Co(1)–P(1) linkages, respec-
tively. The plane defined by the core atoms of the maleic
anhydride ring is distinctly tipped out and away from
the plane of the Co2Ni atoms based on the observed
dihedral angle of ca. 62ꢁ, which in turn causes the ancil-
lary phenyl rings comprised by the atoms C(111)–C(116)
and C(211)–C(216) to adopt an essentially coplanar
arrangement given the observed dihedral angle of ca. 5ꢁ.
3.2. Thermolysis of PhCCo2Ni(CO)4(g2-bma)Cp to give
PhCCo2Ni(CO)3(l,g2-bma)Cp and Co2Ni(CO)4Cp-
[l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-PPh2),
and X-ray diffraction evidence for bma p-bond
coordination and metal core expansion
Thermolysis of PhCCo2Ni(CO)4(g2-bma)Cp in
refluxing DCE furnishes the new clusters PhCCo2Ni
(CO)3(l,g2-bma)Cp (3) and Co2Ni(CO)4Cp[l2,g2,
g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-PPh2) (4) in nearly
equal amounts during the early stages of the reaction

3074
S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
Table 3
a
˚
Selected bond distances (A) and angles (ꢁ) in the compounds 2–5
PhCCo2Ni(CO)4(g2-bma)Cp (2)
Bond distances
Ni–Co(1)
2.3742(9)
1.748(8)
2.211(1)
1.792(5)
1.753(5)
1.496(6)
1.490(7)
Ni–Co(2)
2.3811(9)
2.4639(9)
1.769(5)
2.192(1)
1.802(6)
1.353(6)
Ni–Cp(centroid)
Co(1)–P(1)
Co(1)–C(2)
Co(2)–C(3)
C(11)–C(12)
C(14)–C(15)
Co(1)–Co(2)
Co(1)–C(1)
Co(2)–P(2)
Co(2)–C(4)
C(11)–C(15)
Bond angles
Ni–Co(1)–P(1)
150.57(5)
96.4(2)
99.4(2)
Co(2)–Co(1)–P(1)
P(1)–Co(1)–C(2)
Ni–Co(2)–P(2)
106.68(4)
P(1)–Co(1)–C(1)
C(1)–Co(1)–C(2)
Co(1)–Co(2)–P(2)
P(2)–Co(2)–C(4)
Co(1)–P(1)–C(11)
Co(1)–C(1)–O(1)
Co(2)–C(3)–O(3)
111.8(2)
156.87(5)
102.1(2)
102.6(3)
111.5(1)
173.6(5)
176.9(5)
99.61(4)
99.4(2)
P(2)–Co(2)–C(3)
C(3)–Co(2)–C(3)
Co(2)–P(2)–C(15)
Co(1)–C(2)–O(2)
Co(2)–C(4)–O(4)
115.5(1)
176.7(5)
174.2(5)
PhCCo2Ni(CO)3(l,g2-bma)Cp (3)
Bond distances
Ni–Co(1)
2.491(2)
2.114(7)
2.405(1)
2.249(2)
1.821(9)
2.025(9)
1.50(1)
Ni–Co(2)
2.420(2)
1.737(9)
2.178(3)
1.760(8)
1.78(1)
Ni–C(2)
Co(1)–Co(2)
Ni–Cp(centroid)
Co(1)–P(1)
Co(1)–C(1)
Co(2)–C(3)
Co(2)–C(15)
C(11)–C(15)
Co(1)–P(2)
Co(2)–C(2)
Co(2)–C(11)
C(11)–C(12)
2.064(8)
1.43(1)
C(14)–C(15)
1.447(9)
Bond angles
Co(1)–Ni–C(2)
Ni–Co(1)–P(1)
92.8(2)
132.61(8)
110.9(3)
76.96(7)
87.83(9)
113.1(3)
115.4(3)
123.2(2)
150.4(3)
81.3(2)
Co(2)–Ni–C(2)
Ni–Co(1)–P(2)
46.7(3)
99.76(7)
77.70(7)
167.9(3)
108.5(3)
57.7(2)
141.2(2)
103.8(3)
79.2(2)
97.3(4)
95.7(4)
117.2(4)
178(1)
Ni–Co(1)–C(1)
Co(2)–Co(1)–P(2)
P(1)–Co(1)–P(2)
P(2)–Co(1)–C(1)
Ni–Co(2)–C(3)
Ni–Co(2)–C(15)
Co(1)–Co(2)–C(3)
Co(1)–Co(2)–C(15)
C(2)–Co(2)–C(11)
C(3)–Co(2)–C(11)
C(11)–Co(2)–C(15)
Ni–C(2)–Co(2)
Co(2)–C(2)–O(2)
Co(2)–Co(1)–P(1)
Co(2)–Co(1)–C(1)
P(1)–Co(1)–C(1)
Ni–Co(2)–C(2)
Ni–Co(2)–C(11)
Co(1)–Co(2)–C(2)
Co(1)–Co(2)–C(11)
C(2)–Co(2)–C(3)
C(2)–Co(2)–C(15)
C(3)–Co(2)–C(15)
Co(1)–C(1)–O(1)
Ni–C(2)–O(2)
136.2(4)
99.6(4)
40.9(3)
75.5(3)
156.3(7)
128.0(7)
179(1)
Co(2)–C(3)–O(3)
Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-PPh2) (4)
Bond distances
Ni–Co(1)
2.434(2)
1.76(1)
2.595(2)
1.76(1)
1.946(8)
1.78(1)
2.019(8)
1.46(1)
1.45(1)
Ni–P(1)
Ni–C(16)
2.165(3)
2.018(8)
2.164(3)
1.806(9)
2.167(3)
1.80(1)
Ni–Cp(centroid)
Co(1)–Co(2)
Co(1)–C(1)
Co(1)–C(16)
Co(2)–C(3)
Co(2)–C(11)
C(11)–C(12)
C(14)–C(15)
Co(1)–P(2)
Co(1)–C(2)
Co(2)–P(2)
Co(2)–C(4)
Co(2)–C(15)
C(11)–C(15)
2.067(8)
1.45(1)
Bond angles
Co(1)–Ni–P(1)
Ni–Co(1)–P(2)
Ni–Co(1)–C(2)
Co(2)–Co(1)–P(2)
96.89(8)
150.87(9)
93.5(3)
P(1)–Ni–C(16)
Ni–Co(1)–C(1)
Ni–Co(1)–C(16)
Co(2)–Co(1)–C(1)
90.1(3)
105.3(3)
53.5(2)
53.25(8)
149.1(3)

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3075
Table 3 (continued )
Co(2)–Co(1)–C(2)
P(2)–Co(1)–C(2)
C(1)–Co(1)–C(2)
Co(1)–Co(2)–C(3)
Co(1)–Co(2)–C(11)
P(2)–Co(2)–C(3)
P(2)–Co(2)–C(11)
C(3)–Co(2)–C(4)
C(3)–Co(2)–C(15)
C(4)–Co(2)–C(15)
Co(1)–C(1)–O(1)
Co(2)–C(3)–O(3)
99.2(3)
99.3(3)
98.1(4)
P(2)–Co(1)–C(1)
P(2)–Co(1)–C(16)
Co(1)–Co(2)–P(2)
Co(1)–Co(2)–C(4)
Co(1)–Co(2)–C(15)
P(2)–Co(2)–C(4)
P(2)–Co(2)–C(15)
C(3)–Co(2)–C(11)
C(4)–Co(2)–C(11)
C(11)–Co(2)–C(15)
Co(1)–C(2)–O(2)
Co(2)–C(4)–O(4)
98.7(3)
105.1(3)
53.13(7)
98.2(3)
71.9(2)
96.2(3)
104.0(3)
106.8(4)
106.6(4)
41.5(4)
175(1)
146.5(3)
91.7(2)
98.59(3)
140.8(3)
102.7(4)
103.8(4)
143.7(4)
173.3(8)
177.2(9)
177.2(9)
CpNi[PPh2CPhC@C(PPh2)C(O)OC(O)] (5)
Bond distances
Molecule A
molecule B
Ni(2)–P(3)
Ni(2)–P(4)
Ni(1)–P(1)
Ni(1)–P(2)
2.152(9)
2.137(6)
1.72(1)
1.78(2)
1.79(2)
1.38(3)
1.45(3)
1.45(3)
2.147(9)
2.160
Ni(1)–Cp(centroid)
P(1)–C(11)
P(2)–C(16)
Ni(2)–Cp(centroid)
P(3)–(31)
P(4)–C(36)
1.74(2)
1.75(2)
1.79(2)
1.42(3)
1.38(3)
1.51(3)
C(11)–C(12)
C(11)–C(15)
C(14)–C(15)
C(31)–C(32)
C(31)–C(35)
C(34)–C(35)
Bond angles
P(1)–Ni(1)–P(2)
Ni(1)–P(1)–C(11)
Ni(1)–P(2)–C(16)
P(1)–C(11)–C(12)
P(1)–C(11)–C(15)
93.8(3)
112.8(9)
116.0(7)
125(2)
P(3)–Ni(2)–P(4)
Ni(2)–P(3)–C(31)
Ni(2)–P(4)–C(36)
P(3)–C(31)–C(32)
P(3)–C(31)–C(35)
P(4)–C(36)–C(35)
93.4(3)
112(1)
117.7(7)
120(2)
132(2)
116(2)
128(2)
123(2)
P(2)–C(16)–C(15)
a
Numbers in parentheses are estimated standard deviations in the least significant digits.
(3–5 h), as determined by TLC analysis. Prolonged
heating of the reaction solution led to complete con-
sumption of cluster 2, coupled with the gradual diminu-
tion of 3 and increase in cluster 4. The two new clusters
were isolated by column chromatography over silica gel
and characterized in solution and by X-ray crystallogra-
phy. Scheme 1 show the relationship between clusters
2–4.
The IR spectrum for 3 in CH2Cl2 displayed two
strong terminal m(CO) bands at 2006 (s) and 1954 (vs)
cmꢀ1, along with an intense semibridging CO group at
1912 cmꢀ1. The two m(CO) bands of the anhydride moi-
ety appear at 1789 and 1740 cmꢀ1. These two anhydride
CO bands of cluster 3 are shifted ca. 30 cmꢀ1 to lower
energy in comparison to cluster 2 in concert with the
coordination of the p bond of the bma ligand to an adja-
cent cobalt center and increased electron density to the
anhydride moiety. The 31P NMR spectrum of 3 re-
corded at 178 K revealed the presence of inequivalent
31P groups at d 25.30 and 29.57 consistent with the
structure of 3 (vide infra). The broadness of both reso-
nances is due to quadrupolar coupling to the 59Co center
and not an exchange process involving the bma ligand
since the recorded chemical shifts were temperature
invariant over from 298 to 178 K. The high-field shift
exhibited by this type of P,P,p-chelated bma ligand is
not unusual compared to analogous compounds pre-
pared by our groups [24]. The 13C NMR spectrum of
a
13CO enriched sample of PhCCo2Ni(CO)3(l,g2-
bma)Cp displayed three carbonyl resonances at d
201.31, 209.46, and 223.79 in a 1:1:1 integral ratio at
223 K. The assignment for these CO groups is shown
below, with the chemical shifts in keeping with accepted
trends reported for other metal cluster compounds [25].
The P,P,p-coordination of the bma ligand to the clus-
ter frame in 3 is readily seen in the ORTEP diagram of 3
that is shown in Fig. 3. The bma ligand functions as a
6eꢀ donor ligand through chelation of the Co(1) center
with the P(1) and P(2) centers and coordination of the
Co(2) with the C(11)–C(15) p bond of the maleic anhy-
dride moiety. The total valence electron count in
PhCCo2Ni(CO)3(l,g2-bma)Cp is unchanged relative to
its precursor cluster 2 and the nido polyhedral core is
˚
maintained. The Ni–Co(1) [2.491(2) A], Ni–Co(2)
˚
˚
[2.420(2) A], and Co(1)–Co(2) [2.405(1) A] bond dis-
tances are in agreement with Ni–Co and Co–Co sin-
gle-bond distances. The Ni–Co(2) bond best viewed as
a donor–acceptor bond or a zwitterionic covalent bond
when conventional electron formalism and the EAN
rule are considered, as depicted below. The existence

3076
S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
revealed that the linewidth of the high-field resonance
remained sharp over all temperatures, ruling out a Co-
bound phosphine moiety but not that of phosphine at-
tack on the benzylidyne cap or the nickel center. Since
precedents for both of these latter modes of ligand at-
tack have been found in this genre of cluster by us [6],
the nature of the bma-derived fragment and its coordi-
nation to the metallic frame in 4 were resolved by
X-ray analysis.
Fig. 3. ORTEP drawing of PhCCo2Ni(CO)3(l,g2-bma)Cp (3) showing
the thermal ellipsoids at the 50% probability level.
The ORTEP diagram of 4 is shown in Fig. 4, where the
presence of a phosphido moiety and an opening of the tri-
angular metal frame constitute the prominent structural
features in this 50-valence electron cluster. The structure
adopted by Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)-
C(O)OC(O)](l2-PPh2) is readily understood through
application of Polyhedral Skeletal Electron Pair (PSEP)
theory. Here, the product cluster 4 may by viewed as a
four-vertex hypho cluster that possesses 8 SEP and whose
polyhedral shape may be traced back to the parent four-
vertex nido cluster 3 before the formal association of
two additional SEP. The observed Ni–Co(1)
of a semibridging C(2)O(2) group that spans the Ni–
Co(2) vector is confirmed by the Ni–C(2) and Co(2)–
˚
C(2) bond distances of 2.114(7) and 1.821(9) A,
respectively, and the bond angles of 128.0(7) and
156.3(7)ꢁ adopted by Ni–C(2)–O(2) and Co(2)–C(2)–
O(2) linkages, respectively [26]. The adoption of a semi-
bridging Ni–C(2)–O(2) interaction in 3 allows for unfa-
vorable intramolecular contacts between this particular
CO group and the phenyl group comprised of the
C(217)–C(222) atoms to be eliminated. The bond dis-
˚
˚
[2.434(2) A] and Co(1)–Co(2) [2.595(2) A] bond distances
are unremarkable when compared to other structurally
characterized Ni- and Co-containing polynuclear com-
˚
tances found for the Co(2)–C(11) [2.025(9) A], Co(2)–
˚
˚
C(15) [2.064(8) A], and C(11)–C(15) [1.43(1) A] vectors
are in good agreement with distances reported for other
p-bound Co(alkene) compounds [27].
˚
plexes. The Niꢁ ꢁ ꢁCo(2) distance of 3.825(2) A clearly pre-
cludes any bonding interactions between these two metal
centers. The l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)
ligand functions as a 6eꢀ donor ligand and is attached
The observed 13C NMR spectrum of cluster 4 was
unexpected in that it contained four terminal carbonyl
groups at d 201.77, 206.10, 206.55, and 209.67 that inte-
grate for one CO group each. The 31P NMR spectrum of
cluster 4 signaled the cleavage of a P–C bond in the bma
ligand based on the two 31P chemical shifts at d 63.88
and 170.94 at 178 K. The lower field resonance is as-
cribed to a bridging phosphido moiety [28], with the
assignment of the remaining resonance still equivocal
at this point. Examination of cluster 4 from 289–178 K
Fig. 4. ORTEP drawing of Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@
C(PPh2)C(O)OC(O)](l2-PPh2) (4) showing the thermal ellipsoids at
the 50% probability level. The solvent molecule has been omitted for
clarity.

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3077
to the all three metal atoms in a fashion analogous to that
found by us in the tricobalt cluster Co3(CO)6[l2,g2,
CO loss asthe rate-limiting step and that obeysthe general
rate law [30,31]:
4
g1-C(R)C@C(PPh2)C(O)OC(O)](l2-PPh2). The struc-
rate ¼ k1k2½PhCCo2NiðCOÞ ðg2-bmaÞCpꢂ=k ½COꢂ þ k2
ꢀ1
4
ture of 4 unequivocally establishes the attack of the
PPh2 moiety in this 6eꢀ ligand to the CpNi center, a fact
fully consistent with the temperature invariant high-field
31P resonance. The Co(1)–P(2) and Co(2)–P(2) distances
The stoichiometry in going from 3 to 4 mandates the
capture of an extra CO ligand as cluster 4 possesses one
additional CO group relative to PhCCo2Ni(CO)3(l,g2-
bma)Cp. Provided that cluster 3 serves as a precursor
to cluster 4 and functions as the source of CO to the
same, the amount of product that can be formed in any
thermolysis reaction is limited to no more than 75%.
These aspects pertaining to the role of PhCCo2Ni
(CO)3(l,g2-bma)Cp were tested by an independent ther-
molysis reaction employing pure 3. Refluxing a known
amount of cluster 3 in DCE for ca. 5 h led to the near
complete consumption of 3 and the production of cluster
4 as the major product by TLC analysis, along with a
noticeable amount of material that remained at the ori-
gin of the TLC plate. Cluster 4 was subsequently isolated
by chromatography in ca. 50% of its theoretical yield.
˚
of 2.164(3) and 2.167(3) A, respectively, and the Co(1)–
P(2)–Co(2) bond angle of 73.61(9)ꢁ agree well with other
structurally characterized cobalt-bound phosphido
groups [29]. The torsion angle of ca. 20ꢁ for the P(2)–
Ni–Co(1)–Co(2) linkage reveals the nearly planar orien-
tation of these four atoms. The bonding of the maleic
anhydride C(11)–C(15) p bond to the Co(2) center and
the four terminal CO groups are unexceptional and re-
quire no comment.
We next sought to establish the relationship between
PhCCo2Ni(CO)3(l,g2-bma)Cp and Co2Ni(CO)4Cp[l2,
g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-PPh2). Indepen-
dent thermolysis reactions employing isolated samples of
cluster 3 did indeed afford cluster 4 when the reaction was
monitored by IR and TLC analyses and fully validated
the reaction pathway shown in Scheme 1. The kinetics
for the conversion of PhCCo2Ni(CO)4(g2-bma)Cp to
PhCCo2Ni(CO)3(l,g2-bma)Cp were also investigated to
determine whether a dissociative CO loss mechanism or
an associative attack of the maleic anhydride p bond, cou-
pled with CO loss, was responsible for the formation of
cluster 3. These reactions were studied by IR spectroscopy
in toluene solution over the temperature range of
92–107 ꢁC by following the change in concentration in
antisymmetric m(CO) band of the anhydride moiety at
1765 cmꢀ1. All reactions followed first-order kinetics for
at least three half-lifes, as verified from linear plots of ln
At vs time. The slopes of such plots afforded the first-order
rate constants given in Table 4. The effect of added CO on
the reaction was explored (entry 5), where it is found that
the rate of the reaction is retarded by ca. a factor of 10 with
respect to the reaction run under argon. The calculated
3.3. Thermolysis of Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@
C(PPh2)C(O)OC(O)](l2-PPh2) and decomposition to
the mononuclear complex CpNi[PPh2CPhC@
C(PPh2)C(O)OC(O)]
Prolonged thermolysis of the phosphido-bridged clus-
ter Co2Ni(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)O-
C(O)](l2-PPh2) (4) in refluxing DCE yields the
mononuclear compound CpNi[PPh2CPhC@C(PPh2)-
C(O)OC(O)] (5) as the final degradation product in the
reaction between PhCCo2Ni(CO)6Cp (1) and bma. Com-
pound 5 was isolated in low yield due to the extensive
decomposition that accompanies the thermolysis of 4.
1
A Cp resonance at d 4.92 in the H NMR spectrum of
5 confirmed the existence of a CpNi group, while the
observation of two m(CO) bands at 1746 and 1714 cmꢀ1
supported the presence of an anhydride moiety in the iso-
lated material. The two sharp 31P doublets found at d
12.22 and 42.07 in the 31P NMR spectrum ruled out
the presence of cobalt-bound phosphine or phosphido
groups. The identity of compound 5 was established by
X-ray diffraction analysis. The molecular structure of
5, as shown in Fig. 5, consists of two independent mole-
cules of 5 that may be viewed as originating from the for-
mal insertion of the carbyne fragment ‘‘PhC’’ into one of
the P–C bonds of the mononuclear compound CpNi
(bma). Both molecules of CpNi[PPh2CPhC@C(PPh2)-
C(O)OC(O)] consist of a six-membered ring, where the
pairwise equivalent Ni(1)–P(1) and Ni(2)–P(3) bonds re-
¼
activation parameters for 2 ! 3 (DH = 34.8 0.9 kcal/
¼
mol andDS = 18 3 eu)and the observed CO inhibition
support a unimolecular process involving a dissociative
Table 4
Experimental rate constants for the transformation of PhCCo2Ni-
(CO)4(g2-bma)Cp (2) to PhCCo2Ni(CO)3(l,g2-bma)Cp (3)a
Entry no.
T (ꢁC)
105 kobsd (sꢀ1
)
1
2
3
4
5b
a
92.3
97.3
9.74 0.70
16.5 1.1
33.1 4.0
58.7 2.9
5.82 0.18
102.3
106.6
106.6
˚
veal a mean distance of 2.150 A and the Ni(1)–P(2) and
Ni(2)–P(4) bonds exhibit an average bond length of
From ca. 5.50 · 10ꢀ3 M PhCCo2Ni(CO)4(g2-bma)Cp in toluene
solvent by following the decrease in the m(CO) band at 1765 cmꢀ1. All
kinetic data represent the average of two, independent measurements.
˚
2.149 A. The phosphorus–ylide P(2)–C(16) and P(4)–
C(36) bond distances closely match the distances found
for the P(1)–C(11) and P(3)–C(31) vectors, where a mean
b
Reaction conducted in the presence of 1 atm of CO.

3078
S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
lowed by a rapid coordination of the maleic anhydride p
bond and formation of PhCCo2Ni(CO)3(l,g2-bma)Cp
(3). Thermolysis of 3 furnishes the 8 SEP cluster Co2Ni-
(CO)4Cp[l2,g2,g1-C(Ph)C@C(PPh2)C(O)OC(O)](l2-
PPh2) (4), which in turn gives the mononuclear complex
CpNi[PPh2CPhC@C(PPh2)C(O)OC(O)] (5) as the final
product of the thermolysis reaction. The isolation and
structural characterization of compounds 2–5 have al-
lowed us to study and chart each step of the thermal deg-
radation of the original bma-substituted cluster 2.
5. Supporting information available
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 244754 for 2; 244755 for 3;
244756 for 4; and 245326 for 5. Copies of this informa-
tion may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ UK
[fax: +44(1223)336-033; e-mail: deposit@ccdc.ac.uk or
http://www:ccdc.cam.ac.uk].
Fig. 5. ORTEP drawing of one of the two molecules of
CpNi[PPh2CPhC@C(PPh2)C(O)OC(O)] (5) in the unit cell showing
the thermal ellipsoids at the 50% probability level.
Acknowledgment
˚
bond distance of 1.78 A is found for all four bonds. The
Financial support from the Robert A. Welch Foun-
dation (B-1093-M.G.R.) is greatly appreciated.
similarity found in these distances suggests that these
four phosphorus–carbon bonds possess significant sin-
gle-bond character. The maleic anhydride carbon–
carbon p bond in each molecule of 5 [C(11)–C(15)
References
˚
˚
1.45(3) A; C(31)–C(36) 1.38 A] is lengthened slightly
with respect to a simple alkene linkage but is in excellent
agreement to the analogous distance found in the X-ray
structure of CpNi[Ph2PC@C(C5H6PPh2)C(O)OC(O)],
which has been obtained from the reaction between nick-
elocene and bma [32]. The dihedral angle of ca. 60ꢁ found
between the planes comprised by the maleic anhydride
moiety and the cyclopentadienyl carbon atoms in both
molecules of 5 confirms that these moieties do not adopt
an orthogonal orientation. The remaining bond dis-
tances and angles are unexceptional and require no
comment.
[1] (a) P.A. Dawson, B.H. Robinson, J. Simpson, J. Chem. Soc.,
Dalton Trans. (1979) 1762;
(b) K. Hinkelmann, J. Heinze, H.-T. Schacht, J.S. Field, H.
Vahrenkamp, J. Am. Chem. Soc. 111 (1989) 5078;
(c) G.J. Bezems, P.H. Rieger, S. Visco, J. Chem. Soc., Chem.
Commun. (1981) 265;
(d) A.L. Downard, B.H. Robinson, J. Simpson, Organometallics
5 (1986) 1132–1140;
(e) S.B. Colbran, B.H. Robinson, J. Simpson, Organometallics 2
(1983) 952;
(f) T.W. Matheson, B.H. Robinson, W.S. Tham, J. Chem. Soc. A
(1971) 1457.
[2] (a) M.F. DꢀAgostino, C.S. Frampton, M.J. McGlinchey, Organo-
metallics 10 (1991) 1383;
(b) Y. Takahashi, M. Akita, Y. Moro-oka, Chem. Commun.
(1997) 1557;
4. Conclusions
(c) J.T. Mague, S.E. Dessens, J. Organomet. Chem. 262 (1984)
347.
[3] (a) G. Balavoine, J. Collin, J.-J. Bonnet, G. Lavigne, J. Organo-
met. Chem. 280 (1985) 429;
The reaction between the heterometallic cluster
PhCCo2Ni(CO)6Cp (1) and 2,3-bis(diphenylphosph-
ino)maleic anhydride has been studied and found to give
initially the diphosphine-substituted cluster PhCCo2Ni
(CO)4(g2-bma)Cp (2), where the bma ligand tethers the
two cobalt centers. VT IR and 31P NMR measurements
have revealed that the bma ligand is fluxional in solution
and is in equilibrium with the cobalt-chelated isomer.
Heating cluster 2 leads to a rate-limiting loss of CO, fol-
(b) A.J. Downard, B.H. Robinson, J. Simpson, Organometallics 5
(1986) 1122;
(c) M.-J. Don, M.G. Richmond, W.H. Watson, R.P. Kashyap,
Acta Crystallogr. 47C (1991) 20;
(d) S. Onaka, A. Mizuno, S. Takagi, Chem. Lett. (1989) 2037;
(e) S. Onaka, T. Moriya, S. Takagi, A. Mizuno, H. Furuta, Bull.
Chem. Soc. Jpn. 65 (1991) 1415;
(f) A.J. Downard, B.H. Robinson, J. Simpson, J. Organomet.
Chem. 447 (1993) 281;

S.G. Bott et al. / Journal of Organometallic Chemistry 690 (2005) 3067–3079
3079
(g) G.A. Acum, M.J. Mays, P.R. Raithby, H.R. Powell, G.A.
Solan, J. Chem. Soc., Dalton Trans. (1997) 3427;
(h) M.-J. Don, M.G. Richmond, W.H. Watson, M. Krawiec,
R.P. Kashyap, J. Organomet. Chem. 418 (1991) 231;
(i) W.H. Watson, A. Nagl, S. Hwang, M.G. Richmond, J.
Organomet. Chem. 445 (1993) 163;
Douglas, L. Manojlovic-Muir, K.W. Muir, R.J. Puddephatt,
Organometallics 9 (1990) 409;
(b) R.D. Adams, B. Captain, W. Fu, P.J. Pellechia, Chem.
Commun. (2000) 937;
(c) E. Gullo, S. Detti, G. Laurenczy, R. Roulet, J. Chem. Soc.,
Dalton Trans. (2002) 4577;
(j) K. Yang, S.G. Bott, M.G. Richmond, J. Organomet. Chem.
454 (1993) 273;
(d) G. Laurenczy, G. Bondietti, R. Ros, R. Roulet, Inorg. Chim.
Acta 247 (1996) 65.
(k) G.A. Acum, M.J. Mays, P.R. Raithby, G.A. Solan, J.
Organomet. Chem. 508 (1996) 137;
[19] (a) The possibility of bma fluxionality in cluster 2 arising from a
CO loss, ligand rearrangement, CO capture scheme can be
eliminated from consideration based on the extrapolated rate
constant for dissociative CO loss of ca. 10ꢀ9 sꢀ1 at 273 K.;
(b) cf.: the extrapolated value for dissociative CO loss of ca.
3 · 10ꢀ7 sꢀ1 in PhCCo3(CO)9 at 273 K.A. Cartner, R.G. Cun-
ninghame, B.H. Robinson, J. Organomet. Chem. 92 (1975) 49.
[20] D.M.P. Mingos, D.J. Wales, Introduction to Cluster Chemistry,
Prentice-Hall, Englewood Cliffs, NJ, 1990.
(l) S.G. Bott, J.C. Wang, M.G. Richmond, J. Chem. Crystallogr.
28 (1998) 401;
(m) S. Aime, M. Botta, R. Gobetto, D. Osella, J. Organomet.
Chem. 320 (1987) 229.
[4] (a) K. Yang, J.M. Smith, S.G. Bott, M.G. Richmond, Organo-
metallics 12 (1993) 4779;
(b) H. Shen, S.G. Bott, M.G. Richmond, Inorg. Chim. Acta 250
(1996) 195.
[21] For the structural details on PhCCo2Ni(CO)4 [(Z)-
Ph2PCH@CHPPh2]Cp, see: S.G. Bott, K. Yang, S.-H. Huang,
M.G. Richmond, J. Chem. Crystallogr. 34 (2004) 885.
[22] (a) R. Blumhofer, K. Fischer, H. Vahrenkamp, Chem. Ber. 119
(1986) 194;
[5] The reactivity study of the hydrogen- and formyl-capped clusters
with bma and the related ligand bpcd has recently been completed.
W.H. Watson, J. Liu, M.G. Richmond, unpublished results.
[6] S.G. Bott, K. Yang, K.A. Talafuse, M.G. Richmond, Organo-
metallics 22 (2003) 1383.
(b) L.R. Byers, V.A. Uchtman, L.F. Dahl, J. Am. Chem. Soc. 103
(1981) 1942;
[7] (a) D. Fenske, H. Becher, Chem. Ber. 107 (1974) 117;
(b) D. Fenske, Chem. Ber. 112 (1979) 363.
(c) J.J. Maj, A.D. Rae, L.F. Dahl, J. Am. Chem. Soc. 104 (1982)
3054;
[8] M.O. Nestle, J.E. Hallgren, D. Seyferth, Inorg. Synth. 20 (1980) 226.
[9] R.B. King Organometallic Syntheses, vol. 1, Academic Press, New
York, 1965.
(d) H. Beurich, H. Vahrenkamp, Angew. Chem., Int. Ed. Engl. 17
(1978) 863.
[10] (a) H. Beurich, R. Blumhofer, H. Vahrenkamp, Chem. Ber. 115
(1982) 2409;
[23] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 56th ed.,
CRC Press, Cleveland, OH, 1975.
(b) R. Blumhofer, K. Fischer, H. Vahrenkamp, Chem. Ber. 119
(1986) 194.
[24] (a) H. Shen, S.G. Bott, M.G. Richmond, Organometallics 14
(1995) 4625;
[11] D.F. Shriver, The Manipulation of Air-sensitive Compounds,
McGraw-Hill, New York, 1969.
(b) K. Yang, J.A. Martin, S.G. Bott, M.G. Richmond, Organo-
metallics 15 (1996) 2227;
[12] A.J. Gordon, R.A. Ford, The Chemistꢀs Companion. A Hand-
book of Practical Data Techniques and References, Wiley, New
York, 1976.
(c) S.G. Bott, K. Yang, J.C. Wang, M.G. Richmond, Inorg.
Chem. 39 (2000) 6051.
[25] (a) M.G. Richmond, J.K. Kochi, Inorg. Chem. 25 (1986) 1334;
(b) S. Aime, L. Milone, R. Rossetti, P.L. Stanghellini, Inorg.
Chim. Acta 25 (1977) 103;
[13] (a) For differentiation of bridging versus chelating diphosphine
compounds through 31P NMR spectroscopy, see: P.E. Garrou,
Chem. Rev. 81 (1981) 229;
(c) D.J. Darensbourg, B.S. Peterson, R.E. Schmidt Jr., Organo-
metallics 1 (1982) 306.
(b) M.R. Churchill, R.A. Lashewycz, J.R. Shapley, S.I. Richter,
Inorg. Chem. 19 (1980) 1277;
[26] C.P. Horwitz, D.F. Shriver, Adv. Organomet. Chem. 23 (1984)
219.
(c) M.G. Richmond, J.K. Kochi, Organometallics 6 (1987) 245.
[14] (a) For the effect of scalar coupling on a cobalt-bound phosphorus
center, see: M. Schwartz, M.G. Richmond, A.F.T. Chen, G.E.
Martin, J.K. Kochi, Inorg. Chem. 27 (1988) 4698;
(b) M.-J. Don, M.G. Richmond, Inorg. Chem. 30 (1991) 1703.
[15] (a) W.G. Dauben, W.W. Epstein, J. Org. Chem. 24 (1959) 1595;
(b) N.B. Colthup, Appl. Spectr. 39 (1985) 1030.
[27] A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Watson,
R. Taylor, J. Chem. Soc., Dalton Trans. (1989) S1.
[28] A.J. Carty, S.A. MacLaughlin, D. Nucciarone, in: J.G. Verkade,
L.D. Quin (Eds.), Phosphorus-31 NMR Spectroscopy in Stereo-
chemical Analysis, VCH Publishers, Deerfield Beach, FL, 1987
(Chapter 16).
[16] In theory both the symmetric and antisymmetric carbonyl
stretching bands of the anhydride moiety in the isomers of cluster
2 could be used in the band-area measurements. However, the area
under the antisymmetric stretch of each isomer exhibited poor
linear behavior over the temperature range investigated and was
not utilized in the vanꢀt Hoff treatment of the IR data. Moreover,
analysis of the area under the two antisymmetric anhydride bands
at 173 K afforded a bridging:chelating ratio of 66:34 in poor
agreement with the ratio determined by 31P NMR spectroscopy.
The use of the symmetric CO band to quantify the isomeric
composition was strengthened by a calculated bridging:chelating
ratio at 173 K that was identical with the 31P NMR data.
[17] (a) K.A. Sutin, J.W. Kolis, M. Mlekuz, P. Bougeard, B.G. Sayer,
M.A. Quilliam, R. Faggiani, C.J.L. Lock, M.J. McGlinchey, G.
Jaouen, Organometallics 6 (1987) 439;
[29] (a) T.A. Albright, S.-K. Kang, A.M. Arif, A.J. Bard, R.A. Jones,
J.K. Leland, S.T. Schwab, Inorg. Chem. 27 (1988) 1246;
(b) A.D. Harley, R.R. Whittle, G.L. Geoffroy, Organometallics 2
(1983) 60;
(c) R. Regragui, P.H. Dixneuf, N.J. Taylor, A.J. Carty, Organo-
metallics 3 (1984) 1020;
(d) S. Onaka, H. Muto, Y. Katsukawa, S. Takagi, J. Organomet.
Chem. 543 (1997) 241.
[30] (a) J.D. Atwood, Inorganic and organometallic reaction mecha-
nisms, Brooks/Cole Publisher Co., Monterey, CA, 1985;
(b) R.B. Jordan, Reaction Mechanisms of Inorganic and Orga-
nometallic Systems, second ed., Oxford University Press, New
York, 1998.
[31] The rigorous rate law in the absence of added CO (k2 > kꢀ1[CO])
reduces to simple first-order expression: rate = k1[PhCCo2Ni
CO)4(g2-bma)Cp].
[32] W. Bensmann, D. Fenske, Angew. Chem., Int. Ed. Engl. 17 (1978)
462.
(b) M.F. DꢀAgostino, M.J. McGlinchey, Polyhedron 7 (1988) 807.
[18] (a) For some examples of intramolecular phosphine exchange
reactions in other polynuclear systems, see: A.M. Bradford, G.