Synthesis, structure, and bonding of the unusual μ-σ,σ-allenylidene complex [Rh2(μ-OOCCH3)(μ-σ,σC=C=CPh 2)(CO)2(PCy3)2]BF4

Article abstract of DOI:10.1021/om960164h

The binuclear complex [Rh(μ-OOCCH₃)(CO)(PCy₃)]₂ (1) reacts with 1,1-diphenyl-2-propyn-1-ol to give [Rh₂(μ-OOCCH₃)(μ-η¹:η ²-C₂C(OH)R₂)(CO)₂(PCy ₃)₂] (2), which affords [Rh₂(μ-OOCCH₃)(μ-σ,σ-C=C=CPh ₂)(CO)₂(PCy₃)₂]BF₄ ^·1/₂CH₂Cl₂ (3) by protonation with HBF₄·OEt₂. The molecular structure of 3 can be described as a coordinatively unsaturated (30 valence electrons according to the EAN rule) dinuclear species, containing a single rhodium-rhodium bond (d(Rh-Rh) = 2.723(1) A?). Additionally, the metal centers are bridged by an allenylidene ligand and a carboxylate group. The C_α-C_β and C_β-C_γ distances of the former are 1.30(1) and 1.32(1) A?, while the C_α-C_β-C_γ angle is 174.4(12)°. The electronic structure of the model cations [Rh₂(μ-OOCH)(μ-σ,σ-C=C=CH₂)(CO) ₂(PH₃)₂]⁺ (5) and [Rh₂(μ-OOCH)(μ-C=CH₂)(CO)₂-(PH ₃)₂]⁺ (6) have been studied by means of approximate EHT MO calculations. The interaction of the vinylidene ligand with the bimetallic unit is similar to that of the allenylidene. For both cases the unsaturated η¹-carbon ligand has a net acceptor behavior. In the allenylidene ligand the highest negative charge is located on the C_α carbon atom; this charge decreases toward the C_γ carbon atom. In contrast, for the complex 6, the C_α bridging carbon atom is the least negatively charged.

Full text of DOI:10.1021/om960164h

3556
Organometallics 1996, 15, 3556-3562
Syn th esis, Str u ctu r e, a n d Bon d in g of th e Un u su a l
µ-σ,σ-Allen ylid en e Com p lex
[Rh 2(µ-OOCCH3)(µ-σ,σ-CdCdCP h 2)(CO)2(P Cy3)2]BF 4
Andrew J . Edwards, Miguel A. Esteruelas,* Fernando J . Lahoz, J avier Modrego,
Luis A. Oro,* and J o¨rg Schrickel
Departamento de Quı´mica Inorga´nica, Instituto de Ciencias de Materiales de Arago´n,
Universidad de Zaragoza, CSIC, 50009 Zaragoza, Spain
Received March 5, 1996X
The binuclear complex [Rh(µ-OOCCH3)(CO)(PCy3)]2 (1) reacts with 1,1-diphenyl-2-propyn-
1-ol to give [Rh2(µ-OOCCH3)(µ-η1:η2-C2C(OH)R2)(CO)2(PCy3)2] (2), which affords [Rh2(µ-
OOCCH3)(µ-σ,σ-CdCdCPh2)(CO)2(PCy3)2]BF4‚1/2CH2Cl2 (3) by protonation with HBF4‚OEt2.
The molecular structure of 3 can be described as a coordinatively unsaturated (30 valence
electrons according to the EAN rule) dinuclear species, containing a single rhodium-rhodium
bond (d(Rh-Rh) ) 2.723(1) Å). Additionally, the metal centers are bridged by an allenylidene
ligand and a carboxylate group. The CR-Câ and Câ-Cγ distances of the former are 1.30(1)
and 1.32(1) Å, while the CR-Câ-Cγ angle is 174.4(12)°. The electronic structure of the model
cations [Rh2(µ-OOCH)(µ-σ,σ-CdCdCH2)(CO)2(PH3)2]+ (5) and [Rh2(µ-OOCH)(µ-CdCH2)(CO)2-
(PH3)2]+ (6) have been studied by means of approximate EHT MO calculations. The
interaction of the vinylidene ligand with the bimetallic unit is similar to that of the
allenylidene. For both cases the unsaturated η1-carbon ligand has a net acceptor behavior.
In the allenylidene ligand the highest negative charge is located on the CR carbon atom;
this charge decreases toward the Cγ carbon atom. In contrast, for the complex 6, the CR
bridging carbon atom is the least negatively charged.
In tr od u ction
cyanoethylene;8 (iv) treatment of binuclear compounds
containing the acetylenic dianion -CtCC(O-)tBu2 with
COCl2.5,7 Surprisingly, terminal alkynols have not been
used to prepare the µ-σ,σ-allenylidene complexes.
We have previously reported the reaction of the
binuclear complex [Rh(µ-OOCCH3)(CO)(PCy3)]2 with
terminal alkynes HCtCR gives [Rh2(µ-OOCCH3)(µ-η1:
η2-C2R)(CO)2(PCy3)2] (R ) Ph, CO2CH3, SiMe3), where
the asymmetric coordination of the alkynyl ligands was
Transition-metal vinylidene complexes, MdCdCR2,
have been the subject of intensive study in recent years.
In contrast, the chemistry of the related allenylidene
derivatives, MdCdCdCR2, is less developed.1 Although
several synthetic routes have been described for the
generation of the MdCdCdCR2 unit,2 the most versa-
tile appears to involve terminal alkynols HCtCCR2OH
as starting materials.3 From a mechanistic point of
view, it has been proposed that, following alkyne
coordination, a rearrangement occurs to give a CR2OH-
substituted vinylidene compound as an intermediate,
which spontaneously dehydrates to form the final
product.3a,b
The binuclear µ-σ,σ-allenylidene derivatives are rare.
The complexes of this type previously reported are
neutral species of 34 valence electrons of W,4 Mn,5,6 and
Fe,7,8 which have been prepared by some of the following
methods: (i) thermal decomposition of mononuclear
allenylidene compounds;4 (ii) addition of M(CO)n frag-
ments to mononuclear allenylidene complexes;4,6 (iii)
reactions of vinylidene-bridged complexes with tetra-
(3) (a) Selegue, J . P. Organometallics 1982, 1, 217. (b) Selegue, J .
P. J . Am. Chem. Soc. 1983, 105, 5921. (c) Le Bozec, H.; Ouzzine, K.;
Dixneuf, P. H. J . Chem. Soc., Chem. Commun. 1989, 219. (d) Selegue,
J . P.; Young, B. A.; Logan, S. L. Organometallics 1991, 10, 1972. (e)
Wolinska, A.; Touchard, D.; Dixneuf, P. H.; Romero, A. J . Organomet.
Chem. 1991, 420, 217. (f) Pirio, N.; Touchard, D.; Toupet, L.; Dixneuf,
P. H. J . Chem. Soc., Chem. Commun. 1991, 980. (g) Pilette, D.; Ouzzine,
K.; Le Bozec, H.; Dixneuf, P. H.; Rickard, C. E. F.; Roper, W. R.
Organometallics 1992, 11, 809. (h) Lomprey, J . R.; Selegue, J . P.
Organometallics 1993, 12, 616. (i) Pirio, N.; Touchard, D.; Dixneuf, P.
H. J . Organomet. Chem. 1993, 462, C18. (j) Schwab, P.; Werner, H. J .
Chem. Soc., Dalton Trans. 1994, 3415. (k) Cadierno, V.; Gamasa, M.
P.; Gimeno, J .; Borge, J .; Garcia-Granda, S. J . Chem. Soc., Chem.
Commun. 1994, 2495. (l) Cadierno, V.; Gamasa, M. P.; Gimeno, J .;
Lastra, E.; Borge, J .; Garcia-Granda, S. Organometallics 1994, 13, 745.
(m) Werner, H.; Rappert, T.; Wiedemann, R.; Wolf, J .; Mahr, N.
Organometallics 1994, 13, 2721. (n) Cadierno, V.; Gamasa, M. P.;
Gimeno, J .; Lastra, E. J . Organomet. Chem. 1994, 474, C27. (o)
Touchard, D.; Pirio, N.; Dixneuf, P. H. Organometallics 1995, 14, 4920.
(p) Werner, H.; Stark, A.; Steinert, P.; Gru¨nwald, C.; Wolf, J . Chem.
Ber. 1995, 128, 49. (q) Peron, D.; Romero, A.; Dixneuf, P. H. Organo-
metallics 1995, 14, 3319. (r) Braun, T.; Steinert, P.; Werner, H. J .
Organomet. Chem. 1995, 488, 169.
(4) Berke, H.; Ha¨rter, P.; Huttner, G.; Zsolnai, L. Chem. Ber. 1982,
115, 695.
(5) Berke, H.; Ha¨rter, P.; Huttner, G.; Zsolnai, L. Chem. Ber. 1984,
117, 3423.
(6) Kolobova, N. E.; Ivanov, L. L.; Zhvanko, O. S.; Aleksandrov, G.
G.; Struchkov, Yu. T. J . Organomet. Chem. 1982, 228, 265.
(7) Berke, H.; Gro¨ssmann, U.; Huttner, G.; Zsolnai, L. Chem. Ber.
1984, 117, 3432.
(8) (a) Etienne, M.; Toupet, L. J . Chem. Soc., Chem. Commun. 1989,
1110. (b) Etienne, M.; Talarmin, J .; Toupet, L. Organometallics 1992,
11, 2058.
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
(1) Bruce, M. I. Chem. Rev. 1991, 91, 197.
(2) (a) Fischer, E. O.; Kalder, H. J .; Ko¨hler, F. H.; Huttner, G. Angew.
Chem., Int. Ed. Engl. 1976, 15, 623. (b) Berke, H. Chem. Ber. 1980,
113, 1370. (c) Berke, H.; Ha¨rter, P.; Huttner, G.; Zsolnai, L. Z.
Naturforsch., B: Anorg. Chem., Org. Chem. 1981, 36, 929. (d) Berke,
H.; Huttner, G.; von Seyerl, J . Z. Naturforsch., B: Anorg. Chem., Org.
Chem. 1981, 36, 1277. (e) Berke, H.; Ha¨rter, P.; Huttner, G.; von Seyerl,
J . J . Organomet. Chem. 1981, 219, 317. (f) Kolobova, N. E.; Ivanov, L.
L.; Zhvanko, O. S.; Checkulina, I. N.; Derunov, V. V. Izv. Akad. Nauk
SSSR, Ser. Khim. 1982, 2632. (g) Binger, P.; Mu¨ller, P.; Wenz, R.;
Mynott, R. Angew. Chem., Int. Ed. Engl. 1990, 29, 1037. (h) Deutsch,
M.; Stein, F.; Lackmann, R.; Pohl, E.; Herbst-Irmer, R.; de Meijere, A.
Chem. Ber. 1992, 125, 2051.
S0276-7333(96)00164-1 CCC: $12.00 © 1996 American Chemical Society

An Unusual Allenylidene Complex
Organometallics, Vol. 15, No. 16, 1996 3557
Sch em e 1
the binuclear complex [Rh(µ-OOCCH3)(CO)(PCy3)]2 (1)
with phenylacetylene, methyl propiolate, and (trimeth-
yl)silylacetylene to give [Rh2(µ-OOCCH3)(µ-η1:η2-C2R)-
(CO)2(PCy3)2] (R ) Ph, CO2CH3, SiMe3), the treatment
of 1 with 1.1 equiv of 1,1-diphenyl-2-propyn-1-ol in
hexane at room temperature leads to the derivative
[Rh2(µ-OOCCH3)(µ-η1:η2-C2C(OH)Ph2)(CO)2(PCy3)2] (2,
Scheme 1), which was isolated as a green solid in 82%
yield.
The presence of the alkynyl ligand was established
from the 13C{1H} NMR spectrum in chloroform-d1,
which shows two resonances corresponding to the acety-
lenic carbon atoms at about 111 and 99 ppm and a
singlet at 76.1 ppm due to the -C(OH)Ph2 carbon atom.
Interestingly, the 31P{1H} NMR spectrum exhibits a
doublet at 44.1 ppm with a Rh-P coupling constant of
147 Hz, suggesting a rapid oscillation of the alkynyl
group between the two metal atoms in solution. As
found for the related [Rh2(µ-OOCCH3)(µ-η1:η2-C2R)(CO)2-
(PCy3)2] (R ) Ph, CO2CH3, SiMe3) complexes, the motion
is still rapid at -60°C. The dynamic oscillation process
of these alkynyl groups between the two rhodium atoms
most probably involves a symmetrical Rh2(µ-η1-C2R)
bonding mode. An EHT-MO calculation on the fluxional
movement in the model compound [Rh2(µ-OOCH)(µ-η1:
η2-C2H)(CO)2(PH3)2] shows that the process has a low
activation barrier, ca. 11.53 kcal‚mol-1 at the EHT
level.9
The addition of 1 equiv of HBF4‚OEt2 to a diethyl
ether solution of 2 affords a violet solid from which a
crystalline solid analyzing for [Rh2(µ-OOCCH3)(µ-σ,σ-
CdCdCPh2)(CO)2(PCy3)2]BF4‚1/2CH2Cl2 (3) was isolated
in 78% yield after crystallization from dichloromethane-
diethyl ether. From a mechanistic point of view the
µ-σ,σ-allenylidene complex 3 is most probably the result
of the spontaneous dehydration of a hydroxyvinylidene-
bridged intermediate, which is generated by electro-
philic attack of the proton of the acid to the Câ carbon
atom of alkynyl 2 (eq 1).
established by an X-ray investigation on a monocrystal
of [Rh2(µ-OOCCH3)(µ-η1:η2-C2Ph)(CO)2(PCy3)2].9 The
protonation of these compounds with HBF4‚OEt2 affords
the corresponding vinylidene-bridged rhodium deriva-
tives [Rh2(µ-OOCCH3)(µ-CdCHR)(CO)2(PCy3)2]+, which
are the first examples of cationic vinylidene-bridged
derivatives, and homobinuclear vinylidene-bridged com-
pounds with 30 valence electrons.10 The same reaction
with disubstituted terminal alkynol should afford the
related µ-σ,σ-allenylidene derivative; we have thus
carried out the reaction with 1,1-diphenyl-2-propyn-1-
ol.
We report the synthesis, X-ray characterization, and
comparative EHT-MO calculations of the unusual µ-σ,σ-
allenylidene derivative [Rh2(µ-OOCCH3)(µ-σ,σ-CdCd
CPh2)(CO)2(PCy3)2]+, which is the first cationic, unsat-
urated (30 valence electrons according to EAN), and
group nine µ-σ,σ-allenylidene complex.
The IR spectrum of 3 in Nujol exhibits two ν(CO)
bands at 2026 and 1995 cm-1 as well as the νasym(OCO)
(1538 cm-1) and νsym(OCO) (1440 cm-1) absorptions
corresponding to the carboxylate group, which are
separated by 98 cm-1, in agreement with the bridging
bidentate coordination mode of this ligand.11 The most
Resu lts a n d Discu ssion
P r ep a r a t ion of [R h 2(µ-OOCCH 3)(µ-σ,σ-CdCd
CP h 2)(CO)2(P Cy3)2]BF 4. Similarly to the reactions of
(9) Esteruelas, M. A.; Lahuerta, O.; Modrego, J .; Nu¨rnberg, O.; Oro,
L. A.; Rodriguez, L.; Sola, E.; Werner, H. Organometallics 1993, 12,
266.
(10) Esteruelas, M. A.; Lahoz, F. J .; On˜ate, E.; Oro, L. A.; Rodriguez,
L.; Organometallics 1993, 12, 4219.
(11) (a) Mitchell, R. W.; Ruddick, J . D.; Wilkinson, G. J . J . Chem.
Soc. A 1971, 3224. (b) Robinson, S. D.; Uttley, M. F. J . Chem. Soc.,
Dalton Trans. 1973, 1912. (c) Deacon, G. B.; Phillips, R. J . Coord.
Chem. Rev. 1980, 33, 327. (d) Lahoz, F. J .; Martin, A.; Esteruelas, M.
A.; Sola, E.; Serrano, J . L.; Oro, L. A. Organometallics 1991, 10, 1794.

3558 Organometallics, Vol. 15, No. 16, 1996
Edwards et al.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p lex 3
Rh(1)-Rh(2)
Rh(1)-P(1)
Rh(1)-C(1)
Rh(1)-C(5)
Rh(1)-O(3)
C(5)-C(6)
C(7)-C(11)
O(3)-C(3)
C(1)-O(1)
2.723(1)
2.364(2)
1.799(10)
1.988(10)
2.082(7)
1.30(1)
1.47(2)
1.28(1)
1.17(1)
C(3)-C(4)
Rh(2)-P(2)
Rh(2)-C(2)
Rh(2)-C(5)
Rh(2)-O(4)
C(6)-C(7)
C(7)-C(21)
O(4)-C(3)
C(2)-O(2)
1.51(2)
2.369(2)
1.838(11)
1.997(10)
2.080(7)
1.32(1)
1.49(2)
1.24(1)
1.14(1)
Rh(1)-C(5)-Rh(2)
86.2(4)
C(5)-C(6)-C(7)
174.4(12)
P(1)-Rh(1)-Rh(2) 169.38(7) P(2)-Rh(2)-Rh(1) 171.99(7)
O(3)-Rh(1)-Rh(2)
C(1)-Rh(1)-Rh(2)
C(5)-Rh(1)-Rh(2)
P(1)-Rh(1)-C(5)
P(1)-Rh(1)-O(3)
P(1)-Rh(1)-C(1)
O(3)-Rh(1)-C(1)
Rh(1)-C(5)-C(6)
Rh(1)-O(3)-C(3)
O(3)-C(3)-C(4)
O(3)-C(3)-O(4)
C(6)-C(7)-C(11)
82.6(2)
98.7(3)
47.0(3)
139.6(3)
88.3(2)
90.4(3)
178.7(3)
135.4(8)
123.9(6)
O(4)-Rh(2)-Rh(1)
C(2)-Rh(2)-Rh(1)
C(5)-Rh(2)-Rh(1)
P(2)-Rh(2)-C(5)
P(2)-Rh(2)-O(4)
P(2)-Rh(2)-C(2)
O(4)-Rh(2)-C(2)
Rh(2)-C(5)-C(6)
Rh(2)-O(4)-C(3)
83.9(2)
95.9(3)
46.8(3)
136.7(3)
88.6(2)
91.4(3)
176.7(4)
137.2(8)
123.4(7)
118.8(10)
F igu r e 1. (a) observed 31P{1H} NMR (CDCl3) spectrum
of [Rh2(µ-OOCCH3)(µ-σ,σ-CdCdCPh2)(CO)2(PCy3)2]BF4 (3).
(b) Simulated spectrum (AA′XX′ system, A ) P, X ) Rh;
J P-P′ ) 9 Hz, J P-Rh ) 134 Hz, J P-Rh′ ) 116 Hz).
115.1(10) O(4)-C(3)-C(4)
125.9(10) C(11)-C(7)-C(21) 121.1(9)
118.6(10) C(6)-C(7)-C(21) 120.3(10)
The cation can be described as a dinuclear species of
30 valence electrons, containing a single rhodium-
rhodium bond bridged by an allenylidene and an car-
boxylate ligand. Each rhodium is five coordinate and
completes its coordination with one terminal phosphine
and one carbonyl group. Four of the five atoms directly
bonding each metal form a slightly distorted square
planar geometry, while the fifth (C(5) from the allen-
ylidene ligand) is located approximately 1.45(1) Å
outside the plane. The atoms P(1), C(1), Rh(2), and
O(3), thus, describe the square planar environment
around the Rh(1) atom, with P(1) trans disposed to Rh-
(2) (P(1)-Rh(1)-Rh(2) ) 169.38(7)°). Meanwhile, P(2),
C(2), Rh(1), and O(4) form the square planar environ-
ment around Rh(2), with P(2) trans disposed to Rh(1)
(P(2)-Rh(2)-Rh(1) ) 171.99(7)°). The CR carbon atom
of the allene ligand (C5) is symmetrically bonded to both
metals with Rh-C distances of 1.998(10) and 1.997(10)
Å, similar to the related bond lengths in the vinylidene-
bridged cation [Rh2(µ-OOCCH3)(µ-CdCHPh)(CO)2-
(PCy3)2]+ (4, 1.984(5) and 2.015(5) Å).10 The Rh2-µ-C
plane is roughly perpendicular to the (Cipso)2-C plane
(87.5(2)°), while the Rh(1)-C(5)-Rh(2) angle (86.2(4)°)
agrees well with the related vinylidene-bridged ligand
in 4 (83.2(2)°)10 and the carbonyl-bridged ligands in
[Rh 4(µ-OOCCH 3)4(µ-CO)4(CO)4(NC5H 4CH dNC6H 4-
OC14H29)4] (84.6(3) and 85.1(3)°).11d The symmetric
disposition of the allenylidene ligand leads to statisti-
cally identical rhodium-phosphine, rhodium-carbonyl,
and rhodium-carboxylate bond lengths (see Table 1).
This is in contrast with that observed for 4, where the
asymmetry of the unsaturated η1-carbon ligand pro-
duces two significantly different Rh-P distances (2.396-
(1) and 2.329(1) Å) and Rh-Rh-P bond angles (171.99-
(4)° and 139.30(4)°). The allenylidene ligand is almost
linear (C(5)-C(6)-C(7) ) 174.4(12)°, while the CR-Câ
distance (C(5)-C(6) ) 1.30(1) Å) is statistically identical
with the Câ-Cγ bond length (C(6)-C(7) ) 1.32(1) Å),
which agree well with average carbon-carbon bond
lengths for a carbon-carbon double bond (1.32(1) Å).12
F igu r e 2. Molecular diagram of [Rh2(µ-OOCCH3)(µ-σ,σ-
CdCdCPh2)(CO)2(PCy3)2]+ (3), with 50% thermal ellipsoids
shown.
interesting feature of this spectrum is the presence of
a strong band at 1914 cm-1
. This absorption is at-
tributed to the vibration of the allene bridging ligand
and is also observed in the other µ-σ,σ-allenylidene
complexes,4-8 as well as in mononuclear allenylidene
compounds.1 We note that frequencies are higher for
mononuclear complexes (between 1920 and 1961 cm-1
)
than for the bimetallic derivatives (between 1835 and
1914 cm-1). In addition, the absorption due to the
[BF4]- anion with Td symmetry, centered at 1054 cm-1
,
indicates that, although the metallic centers of 3 are
coordinatively unsaturated, the anion is not coordinated
to the rhodium atoms. In the 13C{1H} NMR spectrum
the CR carbon atom of the unsaturated η1-carbon ligand
is observed at 174.2 ppm as a triplet of triplets due to
coupling with the rhodium (J Rh-C ) 52 Hz) and the
phosphorus (J P-C ) 36 Hz) atoms. The Câ carbon atom
gives a non-phosphorus-coupled resonance and, there-
fore, appears as a triplet at 181.34 ppm with a Rh-C
coupling constant of 6 Hz, while the Cγ carbon atom
gives rise to a singlet at 134.2 ppm. The 31P{1H} NMR
spectrum, shown in Figure 1a, can be simulated using
an AA′XX′ (A ) P, X ) Rh) system (Figure 1b) with J A-A′
) 9 Hz, J A-X ) 134 Hz, and J A-X′ ) 116 Hz.
X-r a y Str u ctu r e of 3. Figure 2 shows the molecular
structure of 3. Selected bond distances and angles are
listed in Table 1.
(12) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Taylor,
R. J . Chem. Soc., Dalton Trans. 1989, S1.

An Unusual Allenylidene Complex
Organometallics, Vol. 15, No. 16, 1996 3559
Finally, the Rh-Rh distance (2.723(1) Å) lies in the
region to invoke a single metal-metal bond, slightly
longer than that found in the related vinylidene-bridged
cation [Rh2(µ-OOCCH3)(µ-CdCHPh)(CO)2(PCy3)2]+ (4;
2.6557(5) Å)10 and in the complexes [Rh2(η5-C5H5){µ-
η4-C4(CF3)2HtBuCO}(µ-CdCHtBu)] (2.625(2) Å),13 [Rh2-
(η5-C9H7)2(µ-CdCH2(CO)2] (2.691(1) Å),14 and [Rh2(η5-
C5H5)(µ-CdCH3)2(CO)2] (2.684(0) Å).15
EHT-MO Ca lcu la tion s for 3. A study of the bond-
ing in the allenylidene and vinylidene complexes was
carried out due to the complexity of the interactions
involving these ligands within the respective rhodium
complexes. The electronic structure of complex 3 has
been studied by means of approximate EHT-MO16
calculations using the model compound [Rh2(µ-OOCH)-
(µ-σ,σ-CdCdCH2)(CO)2(PH3)2]+ (5). These results are
compared with calculations for the model vinylidene
complex [Rh2(µ-OOCH)(µ-CdCH2)(CO)2(PH3)2]+ (6)
(which represents the bent compound [Rh2(µ-OOCCH3)-
(µ-CdCHPh)(CO)2(PCy3)2]+ (4) and the linear complex
[Rh2(µ-OOCCH3)(µ-CdCH2)(CO)2(PCy3)2]+ (7)10). Gen-
eral studies on the bonding of mononuclear vinylidene
and allenylidene complexes have previously been pub-
lished.17
The electronic structure of the binuclear fragment
[Rh2(µ-OOCH)(CO)2(PH3)2]+ is shown in Figure 3a. Its
orbital scheme is related to the interaction of two planar
ML3 units with some mixing among the d orbitals, due
to the lower symmetry of the rhodium environment in
the binuclear fragment compared with an ideal square
planar geometry.18 The occupied orbitals correspond to
pairs of bonding and antibonding interactions between
the d orbitals of each metallic atom; this leads to a
formal bond order of zero between both metal atoms,
consistent with a reduced overlap population of only
0.05 electrons between the rhodium atoms. The 10 d
orbitals are populated by a total of 16 electrons; thus,
the LUMO could act as the possible acceptor on the
addition of the two remaining electrons donated by the
interacting allene fragments. The fragment orbital
energy levels for the ligand µ-σ,σ-CdCdCH2 are pre-
sented in Figure 3b.
F igu r e 3. Molecular orbital diagram of complex [Rh2(µ-
OOCH)(µ-σ,σ-CdCdCH2)(CO)2(PH3)2]+ (5) (center) showing
the interaction of ligand CdCdCH2 (a) with fragment [Rh2-
(µ-OOCH)(CO)2(PH3)2]+ (b).
Interaction between the bimetallic and the ligand
fragments involves the orbital 6a′ (Figure 4a), which
acts as a σ donor. This orbital overlaps with the 19a′
(empty) and 16a′ (occupied) fragment orbitals of the
bimetallic moiety forming a typical three-center-four-
electron interaction which, according to a Mu¨lliken
analysis, transfers 0.47 electrons from the organic
ligand to the binuclear fragment. In addition, orbital
2a′′ of the ligand (Figure 4b) acts as a π acceptor of 0.9
electrons from the binuclear fragment. The Mu¨lliken
analysis of the fragment electron population shows a
F igu r e 4. Some of the fragment orbitals of ligand µ-σ,σ-
CdCdCH2 in model complex [Rh2(µ-OOCH)(µ-σ,σ-
CdCdCH2)(CO)2(PH3)2]+ (5): σ donor 6a′ (a); π acceptor
2a′′ (b); the fully π bonding combination of carbon px
orbitals 1a′′ (c).
(13) Dickson, R. S.; Fallon, G. D.; Mehere, F. I.; Nesbit, R. J .
Organometallics 1987, 6, 215.
(14) Al-Obaidi, Y. N.; Green, M.; White, N. D.; Taylor, G. E. J . Chem.
Soc., Dalton Trans. 1982, 319.
(15) Herrmann, W. A.; Weber, C.; Ziegler, M. L.; Serhadli, O. J .
Organomet. Chem. 1985, 297, 245.
(16) (a) Hoffmann, R. J . Chem. Phys. 1963, 39, 1397. (b) Hoffmann,
R.; Lipscomb, W. N. J . Chem. Phys. 1962, 36, 3179. (c) Hoffmann, R.;
Lipscomb, W. N. J . Chem. Phys. 1962, 37, 2872.
(17) (a) Stegmann, R.; Neuhaus, A.; Frenking, G. J . Am. Chem. Soc.
1993, 115, 11930. (b) Silvestr, J .; Hoffmann, R. Helv. Chim . Acta 1985,
68, 1461. (c) Kostic, N. M.; Fenske, R. F. Organometallics 1982, 1, 974.
(d) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J . Am. Chem.
Soc. 1979, 101, 585.
net acceptor behavior for the ligand, with a total charge
transfer of 0.32 electrons from the binuclear fragment
to the organic ligand. Table 2 shows the distribution
of partial charges among the C atoms of the ligand and
their valence orbitals. The largest negative charge is
located on the CR atom, decreasing toward the Cγ atom.
The partial population change on the s and py orbitals
(18) Albright, T. A.; Burdett, J . K.; Whangbo M. H. Orbital Interac-
tions in Chemistry; J ohn Wiley & Sons: New York, 1985.

3560 Organometallics, Vol. 15, No. 16, 1996
Edwards et al.
Ta ble 2. Distr ibu tion of Ch a r ge on th e Ca r bon Atom s of th e Allen ylid en e a n d Vin ylid en e Liga n d s in Mod el
Com p lexes 5 a n d 6 a n d in th e F r a gm en ts (in P a r en th eses)
ligand
C atom
tot charge (e-)
S orbital
px
py
pz
allenylidene
CR
Câ
Cγ
CR
Câ
-0.196 (-0.656)
-0.133 (0.101)
-0.096 (0.424)
-0.07 (-0.21)
-0.30 (-0.02)
1.34 (1.51)
1.17 (1.16)
1.19 (1.19)
1.35 (1.52)
1.20 (1.19)
0.86 (0.51)
0.96 (0.98)
1.04 (0.51)
0.81 (0.16)
0.92 (0.91)
0.99 (1.50)
0.89 (0.86)
0.95 (0.96)
1.01 (1.53)
0.96 (0.93)
1.00 (1.13)
1.12 (0.91)
0.92 (0.92)
0.90 (1.00)
1.23 (1.00)
vinylidne
of the CR atom upon coordination reflects the role of
these orbitals as σ donors in the sp hybrid orbital 6a′.
The variation on the partial population on the px orbital
of the CR and Cγ atoms clearly shows its π acceptor
capability.
between what is basically a ligand carbon sp hybrid
orbital (4a′, Figure 6a) and the 19a′ (empty) and 16a′
(occupied) orbitals of the bimetallic unit. Furthermore
there is a π acceptor interaction which transfers 0.8
electrons from the binuclear fragment to orbital 2a′′
(Figure 6b) of the ligand. The total interaction is also
very similar to the case of the allene ligand, leading to
a net charge transfer of 0.25 electrons to the vinylidene
ligand. As in the case of the allenylidene complex, the
analysis of the reduced overlap population between the
rhodium atoms shows an increase upon coordination of
the organic bridge, from 0.06 electrons in the fragment
to 0.16 electrons in the complex.
The difference in the charge distribution over the C
atoms of both ligands in the model complexes 5 and 6
is significant as shown in Table 2. In the case of the
vinylidene complex, the CR bridging carbon atom is the
least negative position of the organic ligand, whereas
in the allenylidene bridge the CR atom carries the most
negative charge. As inferred from Table 2, this differ-
ence results from the role of the orbitals px in their
respective fragments. In the vinylidene ligand the CR
px orbital takes part only in the π acceptor LUMO on
coordination to the metals, while, the CR px orbital in
the allenylidene ligand is partially occupied in the free
fragment, because it is involved in the π bonding
combination 1a′′ (Figure 4c). This significant amount
of charge is incremented through back-donation from
the binuclear moiety following coordination.
In contrast to the essentially linear complexes 3 and
7, complex 4 is distorted and possesses one nonlinear
P-Rh-Rh arrangement. In agreement with this ob-
servation, an EHT study at different values for the
P-Rh-P angle of the model complex 6 shows there is
a very flat potential for the pivoting movement of the
phosphine ligand around the rhodium atom (as little as
3 kcal/mol is necessary to drive the phosphine 45° out
of linearity). Thus, as seen in the molecular structure
of complex 4, the distortion seems to result from steric
interactions between the phosphine and the vinylidene
ligands.
The electron donation from the ligand into acceptor
orbital 19a′ of fragment [Rh2(µ-OOCH)(CO)2(PH3)2]+
populates this metal-metal bonding combination of d
orbitals whereas the corresponding antibonding combi-
nation remains empty, leading to a formal bond order
of one between the metal atoms, as suggested from the
X-ray crystal structure analysis. The final reduced
orbital population between the rhodium atoms is 0.16
electrons. The group of 10 d orbitals in complex 5 are
now formally populated by a total of 18 electrons.
However, the bonding and antibonding combinations of
the py orbitals of the rhodium atoms remain empty and
are not involved in bonding, a result of the unsaturated
coordination of these atoms. This allows rationalization
of a 30 valence electron count, in accord with the EAN
rule, since only 15 of the 18 available metal valence
orbitals have electron density and are involved in
bonding.
It is interesting to compare the bonding characteris-
tics of the allenylidene 5 with those of the related
vinylidene 6. Thus, we have carried out a set of MO
calculations on the model complex [Rh2(µ-OOCH)(µ-
CdCH2)(CO)2(PH3)2]+ (6). The interaction of the vi-
nylidene ligand with the binuclear moiety is similar to
that of the allenylidene (Figure 5). The main bonding
interaction between the organic ligand and the binuclear
fragment is a three-center-four-electron σ interaction
F igu r e 5. Molecular orbital diagram of complex [Rh2(µ-
OOCH)(µ-σ,σ-CdCH2)(CO)2(PH3)2]+ (6) (center) showing
the interaction of ligand CdCH2 (a) with fragment [Rh2-
(µ-OOCH)(CO)2(PH3)2]+ (b).
F igu r e 6. Some of the fragment orbitals of ligand µ-σ,σ-
CdCH2 in model complex [Rh2(µ-OOCH)(µ-σ,σ-CdCH2)-
(CO)2(PH3)2]+ (6): σ donor 4a′ (a); π acceptor 2a′′(b).

An Unusual Allenylidene Complex
Con clu d in g Rem a r k s
Organometallics, Vol. 15, No. 16, 1996 3561
Ta ble 3. Cr ysta llogr a p h ic Da ta for Com p lex 3
formula
mol wt
a, Å
b, Å
c, Å
C55H79BF4O4P2Rh2‚0.5(CH2Cl2)
We have revealed that the terminal alkynols are
useful starting materials, not only to prepare mono-
nuclear allenylidene complexes but also to obtain ho-
mobimetallic µ-σ,σ-allenylidene compounds. Thus, we
show the reaction of [Rh(µ-OOCCH3)(CO)(PCy3)]2 with
1,1-diphenyl-2-propyn-1-ol leads to [Rh2(µ-OOCCH3)(µ-
η1:η2-C2C(OH)Ph2)(CO)2(PCy3)2], which affords [Rh2(µ-
OOCCH3)(µ-σ,σ-CdCdCPh2)(CO)2(PCy3)2]BF4 by pro-
tonation with HBF4‚OEt2. This complex is the first
cationic µ-σ,σ-allenylidene derivative of this type for a
group nine metal compound that is coordinatively
unsaturated (30 valence electrons according to the EAN
rule) and fully structurally characterized.
The X-ray structural characterization of Rh2(µ-
OOCCH3)(µ-σ,σ-CdCdCPh2)(CO)2(PCy3)2]BF4 shows a
rhodium-rhodium distance in agreement with a single
bond. EHT-MO calculations on the model cation con-
firm this metal-metal bond order and rationalize the
valence electron count. They show a net acceptor
behavior for the unsaturated η1-carbon ligand and
reveal the electronic nature, suggesting nucleophilic
attack at the ligand is unlikely.
1201.21
12.802(3)
17.560(5)
13.726(3)
97.66(1)
3058.1(13)
monoclinic
P21
â, deg
V, Å3
cryst syst
space group
Z
temp, °C
scan method
θ (min-max), deg
octants
no. measd refls
no. indep refls
no. of params
GOOF
2
-100.0(2)
ω/2θ
1.5-25
-h,-k,(l; +h,+k,(l
11 725
10 046
611
0.962
0.710 73
1.305
λ, Å
F
calcd, g cm-1
µ, mm-1
F(000)
R(int)
0.687
1246
0.0583
0.0616
a
R1
wR2
a
0.1979
several times with diethyl ether and dried under vacuum.
Recrystallization from dichloromethane/diethyl ether gave
dark violet crystals. Yield: 126 mg (78%). Anal. Calcd for
Exp er im en ta l Section
C
55H68BF4O4P2Rh2‚1/2CH2Cl2: C, 55.48; H, 6.66. Found: C,
55.10; H, 6.79. IR (Nujol, cm-1): ν(CO) 2026, 1995; ν(CdCdC)
1914; ν(OCOasym) 1538, ν(OCOsym) 1440; ν(BF4) 1054. 1H NMR
(300 MHz, CDCl3, 20 °C, δ): 7.8-7.2 (m, 10 H, Ph); 1.9 (s, 3H,
CH3); 2.2-1.2 (m, 33H, C6H11). 13C{1H} NMR (75.45 MHz,
Gen er a l Con sid er a tion s. All reactions were carried out
under an argon atmosphere using Schlenk tube techniques.
Solvents were dried and purified by known procedures and
distilled under argon prior to use. 1,1-Diphenyl-2-propyn-1-
ol was recrystallized from n-pentane. The starting complex
[Rh(µ-OOCCH3)(CO)(PCy3)]2 (1) was prepared by a published
method.9 Elemental analyses were performed with a Perkin-
Elmer 240 XL microanalyzer. NMR spectra were recorded on
Varian UNITY 300 or Bruker AXR 300 instruments. Chemical
shifts are expressed in parts per million, upfield from Si(CH3)4
(13C{1H}, 1H) and 85% H3PO4 (31P{1H}). Infrared spectra were
obtained from a Perkin-Elmer 783 instrument as Nujol mulls
on polyethylene sheets or in solution using NaCl cells.
P r ep a r a tion of [Rh 2(µ-OOCCH3)(µ-η1:η2-C2C(OH)P h 2)-
(CO)2(P Cy3)2] (2). A suspension of 1 (100 mg, 0.106 mmol)
in n-pentane (20 mL) was treated with 1,1-diphenyl-2-propyn-
1-ol (0.024 g, 0.116 mmol). The mixture was stirred for 2 h
and changed from the original yellow suspension to a clear
green solution. The volume was reduced to ca. 3 mL and
cooled in dry ice to give a dark green microcrystalline solid.
The solvent was decanted and the product washed several
times with cold pentane. The crystals contain 1 mol of
1
CDCl3, 20 °C, δ): 186.9 (AA′MM′XX′ system, J Rh-C ) 74 Hz;
2J Rh-C ) 2.2 Hz; J P-C ) 13.5 Hz); 186.2 (s, OCO); 181.3 (m,
2
2J Rh-C ) 6 Hz, Câ); 174.2 (tt, J Rh-C ) 52 Hz; J P-C ) 36 Hz;
2
2
CR); 140.6 (d, J P-C ) 61.4 Hz, Cipso-Ph); 134.2 (s, Cγ); 130.0 (d,
para-Ph); 129.2 (2d, Cortho-Ph, Cmeta-Ph); 34.4 (d, CH2CHP); 30.1
C
(s, CH2CHP); 27.3(d, CH2CHP); 25.8 (s, CH2), 23.3 (s, CH3).
31P{1H} NMR (121.45 MHz, CDCl3, 20 °C, δ): 47.7 (AA′XX′
system, J P-P′ ) 9 Hz, J P-Rh ) 134 Hz, J P-Rh′ ) 116 Hz).
X-r a y St r u ct u r e An a lysis of C55H 79BF 4O4P 2R h 2‚
0.5CH2Cl2 (3). A summary of crystal data and refinement
parameters is reported in Table 3. Crystals suitable for the
X-ray diffraction study were obtained from a dichloromethane/
ether solution. An oil-coated rapidly cooled deep violet crystal-
line rectangular block of approximate dimensions 0.60 × 0.42
× 0.40 mm was mounted onto the tip of a glass fiber directly
from solution.19 A set of randomly searched reflections were
indexed to monoclinic symmetry and accurate unit cell dimen-
sions determined by least-squares refinement of 24 carefully
centred reflections (25 e 2θ e 30). Data were collected on a
Siemens P4 diffractometer, with graphite-monochromated Mo
KR radiation by the 2θ/ω scan method. Three orientation and
intensity standards were monitored every 55 min of measuring
time throughout data collection; no variation was observed.
Data were corrected for Lorentz and polarization effects, and
for absorption using DIFABS.20 The structure was solved by
direct methods (SHELXTL PLUS)21 and conventional Fourier
techniques and refined by full-matrix least-squares on F2
(SHELXL-93);22 see Table 3. All non-hydrogen atoms were
refined anisotropically except fluorine atoms of the BF4 coun-
terion. All hydrogens were included in fixed idealized posi-
tions. Half a solvent molecule of dichloromethene was located
in the asymmetric unit. The Flack parameter was refined to
-0.16(5), indicating the correct absolute structure determi-
nation. The largest peak and hole in the final difference map
pentane/mol of 2. Yield: 94 mg (82%). Anal. Calcd for C55H80
O5P2Rh2‚C5H12: C, 62.08; H, 7.93. Found: C, 61.94; H, 7.77.
IR (pentane, cm-1): ν(CO) 1985. IR (Nujol, cm-1): ν(OCOasym
-
)
1554, ν(OCOsym) 1442. 1H NMR (300 MHz, CDCl3, 20 °C, δ):
7.8-7.0 (m, 10 H, Ph); 3.69 (s, 1 H, OH); 1.77 (s, 3H, CH3);
2.0-1.0 (m, 33H, C6H11). 13C{1H} NMR (75.45 MHz, CDCl3,
-30 °C, δ): 190.6 (dd, J Rh-C ) 75 Hz, J P-C ) 18 Hz, CO);
181.96 (s, O2CCH3); 128.3 (s, Ph); 127.6 (s, Ph); 126.5 (s, Ph);
111.2 (br, CR); 99.3 (br, Câ); 76.12 (s, tCPh2OH); 34.3 (d, J P-C
) 33 Hz, CH2CHP); 30.3 (d, OCCCH3); 28.2 (d, J Rh-C ) 19.5
Hz, CH2CHP); 26.9 (s, CH2). 31P{1H} NMR (121.45 MHz,
CDCl3, 20 °C, δ): 44.1 (d, J Rh-P ) 147 Hz).
P r ep a r a t ion of [R h 2(µ-OOCCH 3)(µ-σ,σ-CdCdCP h 2)-
(CO)2(P Cy3)2]BF 4 (3). A solution of 2 (150 mg, 0.14 mmol)
in diethyl ether (20 mL) was treated with HBF4‚OEt2 (10 µl,
0.14 mmol). The color of the solution immediately turned from
green to dark violet, and a solid precipitated from the solution.
The mixture was stirred for 0.5 h and the volume reduced to
ca. 5 mL. The solvent was decanted and the solid washed
were measured as 2.245 and -0.797 eÅ-3
.
(20) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 1581.
(21) Sheldrick, G. M. SHELXTL PLUS. Siemens Analytical X-ray
Instruments, Inc., Madison, WI, 1990.
(19) Kottke, T.; Stalke, D. J . Appl. Crystallogr. 1993, 26, 615.
(22) Sheldrick, G. M. SHELXL-93, Go¨ttingen, Germany, 1993.

3562 Organometallics, Vol. 15, No. 16, 1996
Edwards et al.
Ack n ow led gm en t. We thank the DGICYT (Projects
PB96-0806 and PB94-1186, Programa de Promocio´n
General del Conocimiento) and the EU (Project: Selec-
tive Processes and Catalysis Involving Small Molecules)
for financial support. A.J .E. and J .S. thank the EU
(Human, Capital and Mobility programme, CT93-0347)
for grants.
(PH3)2]+ using standard geometrical parameters and an
idealized CS symmetry based in the experimental
structure of complex 3 and assuming a perfect linear
arrangement for the P-Rh-Rh-P atoms. The calcula-
tions have been performed using the program CACAO23
with the supplied atomic Hii parameters.
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
lengths and angles, anisotropic thermal parameters, and
complete atomic coordinates and isotropic thermal parameters
(8 pages). Ordering information is given on any current
masthead page.
Ap p en d ix
The extended Hu¨ckel calculations have been carried
out on model complexes [Rh2(µ-OOCH)(µ-σ,σ-CdCdCH2)-
(CO)2(PH3)2]+ and [Rh2(µ-OOCH)(µ-σ,σ-CdCH2)(CO)2-
(23) Mealli, C.; Proserpio, D. M. J . Chem. Educ. 1990, 67, 39.
OM960164H