Chemistry of unsaturated group 6 metal complexes with bridging hydroxy and methoxycarbyne ligands. 1. Synthesis, structure, and bonding of 30-electron complexes

Article abstract of DOI:10.1021/om700562g

The new cationic alkoxy and hydroxycarbyne complexes [M₂Cp ₂(μ-COR)(μ-PR′₂)₂]BF₄ (Cp = μ⁵C₅H₅; M = W, R = Me, R′ = Ph; M = Mo, R = Me and H, R′ = Et) and [Mo₂Cp₂(μ-COR) (μ-COR′)(μ-PCy₂)]BF₄ (R = Me; R′ = H, Me, Et) are obtained in high yield by the reaction of the corresponding neutral monocarbonyl precursors with either [Me₃O]BF₄ or HBF ₄-OEt₂ in dichloromethane. While the methoxycarbyne complexes are stable at room temperature, the analogous hydroxycarbyne species are thermally unstable, and above ca. 253 K they experience a hydrogen migration from oxygen to metal, to give the new hydride carbonyl complexes [Mo ₂Cp₂(H)(μ-PEt₂)₂(CO)]BF ₄ and [Mo₂Cp₂(H)(μ-COMe)(μ-PCy ₂)(CO)]BF₄ as major products. The electronic structure and bonding in some of these face-sharing bioctahedral complexes were studied by means of density functional theory. These calculations allow us to describe the intermetallic interaction present in the carbonyl-bridged 30-electron complexes by a configuration of type σ²δ⁴, with the δ orbitais being involved in π back-bonding to the carbonyl bridges. Upon methylation of the latter ligands, these δ-bonding orbitais become more delocalized over the Mo₂C(carbyne) triangle and constitute the π-bonding component of the metal-carbyne bond. Therefore, a partial reduction of the direct metal-metal overlap occurs upon formation of these methoxycarbyne complexes, which still retain some multiplicity in the corresponding C-O bonds. The topological analysis of the electron density under the AIM scheme supports the above description of the Mo-Mo, Mo-C, and C-O bonds in these complexes.

Full text of DOI:10.1021/om700562g

4930
Organometallics 2007, 26, 4930-4941
Chemistry of Unsaturated Group 6 Metal Complexes with Bridging
Hydroxy and Methoxycarbyne Ligands. 1. Synthesis, Structure, and
Bonding of 30-Electron Complexes
M. Esther Garc´ıa,† Daniel Garc´ıa-Vivo´,† Miguel A. Ruiz,*,† Santiago Alvarez,‡ and
Gabriel Aullo´n‡
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, E-33071 OViedo,
Spain, and Departamento de Qu´ımica Inorga´nica, UniVersidad de Barcelona, E-08028 Barcelona, Spain
ReceiVed June 8, 2007
The new cationic alkoxy and hydroxycarbyne complexes [M2Cp2(µ-COR)(µ-PR′2)2]BF4 (Cp ) η5-
C5H5; M ) W, R ) Me, R′ ) Ph; M ) Mo, R ) Me and H, R′ ) Et) and [Mo2Cp2(µ-COR)(µ-COR′)-
(µ-PCy2)]BF4 (R ) Me; R′ ) H, Me, Et) are obtained in high yield by the reaction of the corresponding
neutral monocarbonyl precursors with either [Me3O]BF4 or HBF4‚OEt2 in dichloromethane. While the
methoxycarbyne complexes are stable at room temperature, the analogous hydroxycarbyne species are
thermally unstable, and above ca. 253 K they experience a hydrogen migration from oxygen to metal, to
give the new hydride carbonyl complexes [Mo2Cp2(H)(µ-PEt2)2(CO)]BF4 and [Mo2Cp2(H)(µ-COMe)-
(µ-PCy2)(CO)]BF4 as major products. The electronic structure and bonding in some of these face-sharing
bioctahedral complexes were studied by means of density functional theory. These calculations allow us
to describe the intermetallic interaction present in the carbonyl-bridged 30-electron complexes by a
configuration of type σ2δ4, with the δ orbitals being involved in π back-bonding to the carbonyl bridges.
Upon methylation of the latter ligands, these δ-bonding orbitals become more delocalized over the Mo2C-
(carbyne) triangle and constitute the π-bonding component of the metal-carbyne bond. Therefore, a
partial reduction of the direct metal-metal overlap occurs upon formation of these methoxycarbyne
complexes, which still retain some multiplicity in the corresponding C-O bonds. The topological analysis
of the electron density under the AIM scheme supports the above description of the Mo-Mo, Mo-C,
and C-O bonds in these complexes.
relatively little work has been carried out on the corresponding
alkoxy-substituted carbyne (COR) or hydroxycarbyne (COH)
complexes, even though Shriver and co-workers reported the
first complex displaying a bridging alkoxycarbyne ligand
(obtained by O-alkylation of a bridging carbonyl in the anion
[Fe3(CO)11(H)]-) only 2 years after the pioneering work of
Fischer.6 Besides a few complexes displaying terminal coordi-
nation of the COR moiety,7 most of the alkoxycarbyne
complexes described so far contain this ligand acting as a
bridging group (in a µ2 or µ3 fashion) on electron-precise di- or
trinuclear metal centers, and it is on these substrates that the
reactivity of the alkoxycarbyne ligand has been mostly inves-
tigated.8 As a result, little is known of the behavior of these
ligands when bridging multiple metal-metal bonds.
The chemistry of hydroxycarbyne (COH) complexes is even
more undeveloped than that of the alkoxycarbyne complexes,
due to the lack of available compounds with enough thermal
stability. In fact the only stable complexes of this type so far
reported are the unsaturated cations [Mo2Cp2(µ-COH)(µ-
PCy2)2]+,9 [W2Cp2(µ-COR)(µ-L2)(CO)2]+ (Cp ) η5-C5H5; L2
) Ph2PCH2PPh2, Me2PCH2PMe2; R ) H, Me),10 and [M2Cp2-
(µ-COR)(µ-PPh2)2]+ (M ) Mo, W; R ) H, Me),11 all of them
Introduction
Since the discovery of the first metal-alkylidyne complexes
at E. O. Fischer’s laboratory in 1973,1 this area within
organometallic chemistry has developed tremendously.2 These
complexes are of interest not only because of their great
reactivity but also for their relevance in the context of some
important industrial reactions such as the Fischer-Tropsch
process and other metal-catalyzed hydrogenations of CO,3,4 or
the alkyne metathesis.5
Compared to the extensively developed chemistry of com-
plexes containing the alkylidyne moiety CR (R ) alkyl, aryl),
† Universidad de Oviedo.
‡ Universidad de Barcelona.
(1) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; Mu¨ller, J.; Hu¨ttner, G.;
Lorentz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 564.
(2) See for example: (a) Stone, F. G. A. LeaVing No Stone Unturned.
Pathways in Organometallic Chemistry; American Chemical Society:
Washington, DC, 1993. (b) Fischer, H.; Hoffmann, P.; Kreissl, F. R.;
Schrock, R. R.; Schubert, U.; Weiss, K. Carbyne Complexes; VCH:
Weinheim, Germany, 1988. (c) Mays, A.; Hoffmeister, H. AdV. Organomet.
Chem. 1991, 32, 259. (d) Angelici, R. J.; Heesook, K. P. AdV. Organomet.
Chem. 1987, 27, 51. (e) Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1984,
23, 89.
(3) (a) Maitlis, P. M. J. Organomet. Chem. 2004, 689, 4366. (b) Maitlis,
P. M. J. Mol. Catal. A 2003, 204-205, 55. (c) Dry. M. E. Catal. Today
2002, 71, 227. (d) Campbell, I. M. Catalysis at Surfaces; Chapman and
Hall: New York, 1988. (e) Bell, A. T. Catal. ReV. Sci. Eng. 1981, 23, 203.
(4) Nicholas, K. M. Organometallics 1982, 1, 1713.
(6) Shriver, D. F.; Lehman, D.; Strope, D. J. Am. Chem. Soc. 1975, 97,
1594.
(7) (a) Stone, K. C.; Jamison, G. M.; White, P. S.; Templeton, J. L.
Organometallics 2003, 22, 3083. (b) Stone, K. C.; White, P. S.; Templeton,
J. L. J. Organomet. Chem. 2003, 684, 13. (c) Peters, J. C.; Odon, A. L.;
Cummins, C. C. Chem. Commun. 1997, 1995. (d) Carnahan, E. M.;
Protasiewicz, J. D.; Lippard, S. J. Acc. Chem. Res. 1993, 26, 90. (e) Jamison,
G. M.; White, P. S.; Templeton, J. L. Organometallics 1991, 10, 1954.
(5) For some recent reviews see: (a) Mori, M.; Kitamura, K. In
ComprehensiVe Organometallic Chemistry III; Crabtree, R. H., Mingos,
D. M. P., Eds.; Elsevier: Oxford, UK, 2007; Vol. 11, Chapter 8. (b)
Mortreux, A.; Coutelier, O. J. Mol. Catal. A 2006, 254, 96. (c) Schrock, R.
R. Chem. Commun. 2005, 2773.
10.1021/om700562g CCC: $37.00 © 2007 American Chemical Society
Publication on Web 08/29/2007

Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4931
Chart 1
prepared in our laboratory. The related methoxycarbyne complex
cations are more robust thermally, but still they are the first
binuclear alkoxycarbyne complexes with electron counts of 30
or 32, and the chemistry of all these species remains largely
unexplored. We have noted previously that the study of the
reactivity of hydroxy- and alkoxycarbyne ligands at unsaturated
binuclear centers might be of interest not only because of the
unusual transformations that can be induced by the coexistence
of different multiple bonds (metal-metal and metal-carbon)
in the same substrate but also in the context of the metal-
catalyzed hydrogenation processes of carbon monoxide.10
Apart from their interest as synthetic intermediates, the
unsaturated dimolybdenum and ditungsten complexes mentioned
above, for which we can formulate metal-metal bond orders
(BO) of 2 and 3 according to the effective atomic number (EAN)
rule, are good candidates for theoretical studies as well, both
because of the presence of multiple metal-metal bonding and
because of the presence of the alkoxycarbyne bridge itself.
Indeed the nature of the intermetallic interaction in unsaturated
binuclear transition metal complexes has been the subject of
intense debate, and a lot of experimental and theoretical research
has been carried out, but it still remains an active area of
research.13 Special problems are found in complexes containing
bridging π-acid ligands, since the metal-metal interaction is
extensively delocalized over the bridges.14 This obviously can
be the case of the hydroxycarbyne or alkoxycarbyne ligands,
but this matter has not been previously studied using modern
quantum mechanical methods. To our knowledge, the only
previous study on the electronic structure and bonding of an
alkoxycarbyne complex was carried out some years ago by
Farrugia and Aitchison using extended-Hu¨ckel methods.15
In this paper we report the synthesis and structural charac-
terization of new unsaturated hydroxy and alkoxycarbyne
complexes having formally triple metal-metal bonds. These
have been prepared via O-alkylation or protonation of the
bridging carbonyl ligands in the neutral complexes [W2Cp2(µ-
PPh2)2(µ-CO)] (1),16 [Mo2Cp2(µ-PEt2)2(µ-CO)] (2),16 and [Mo2-
Cp2(µ-COR)(µ-PCy2)(µ-CO)] [R ) Me (3), Et (4)].17 Although
the reactivity of the new cationic complexes will be reported
separately, we here include the results of a complete study of
the electronic structure and bonding in these complexes based
on density functional theory (DFT) calculations, with special
attention given to the nature of the intermetallic interaction and
the changes induced upon alkylation of the carbonyl ligands.
This computational study will also give us a basis for the
interpretation of the reaction patterns of these complexes.
Some of the synthetic work here discussed, as well as some
unusual transformations experienced by the new unsaturated
cations (that involve inter alia the formation and cleavage of
C-C, C-O, and O-H bonds), was previously reported in a
short communication.18
(8) For some studies of reactivity of alkoxycarbyne-bridged compounds
see: (a) Hersh, W. H.; Fong, R. H. Organometallics 2005, 24, 4179. (b)
Peters, J. C.; Odon, A. L.; Cummins, C. C. Chem. Commun. 1997, 1995.
(c) Bronk, B. S.; Protasiewicz, J. D.; Pence, L. E.; Lippard, S. J.
Organometallics 1995, 14, 2177. (d) Seyferth, D.; Ruschke, D. P.; Davis,
W. H. Organometallics 1994, 13, 4695. (e) Chi, Y.; Chuang, S. H.; Chen,
B.-F.; Peng, S.-M.; Lee, G.-H. J. Chem. Soc., Dalton Trans. 1990, 3033.
(f) Friedman, A. E.; Ford, P. C. J. Am. Chem. Soc. 1989, 111, 551. (g)
Keister, J. B. Polyhedron 1988, 7, 847. (h) Friedman, A. E.; Ford, P. C. J.
Am. Chem. Soc. 1986, 108, 7851. (i) Farrugia, L. J.; Miles, A. D.; Stone,
F. G. A. J. Chem. Soc., Dalton Trans. 1985, 2437. (j) Beanan, L. R.; Keister,
J. B. Organometallics 1985, 4, 1713. (k) Nuel, D.; Dahan, F.; Mathieu, R.
J. Am. Chem. Soc. 1985, 107, 1658. (l) Shapley, J. R.; Yeh, W.-Y.; Churchill,
M. R.; Li, Y.-J. Organometallics 1985, 4, 1898. (m) Green, M.; Mead, K.
A.; Mills, R. M.; Salter, I. D.; Stone, F. G. A.; Woodward, P. J. J. Chem.
Soc., Chem. Commun. 1982, 51.
(9) Alvarez, M. A.; Garc´ıa, M. E.; Martinez, M. E.; Ramos, A.; Ruiz,
M. A.; Sa´ez, D.; Vaissermann, J. Inorg. Chem. 2006, 45, 6965.
(10) (a) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Robert,
F. Organometallics 2002, 21, 1177. (b) Alvarez, M. A.; Garc´ıa, M. E.;
Riera, V.; Ruiz, M. A. Organometallics 1999, 18, 634. (c) Alvarez, M. A.;
Bois, C.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A. Angew. Chem., Int. Ed.
Engl. 1996, 35, 102.
Results and Discussion
Synthesis and Structural Characterization of New Cat-
ionic Methoxycarbyne Complexes. The neutral compounds
1-4 are methylated easily at the oxygen atom of their carbonyl
bridges upon reaction with [Me3O]BF4 in dichloromethane
solution, to give the new cationic methoxycarbyne complexes
[M2Cp2(µ-COMe)(µ-PR2)2]BF4 [M ) W, R ) Ph (5); M ) Mo,
R ) Et (6)] and [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 [R
) Me (7), Et (8)], respectively, in high yields (Chart 1). All
these 30-electron complexes, for which we can formulate a
metal-metal triple bond according to the EAN rule, exhibit a
face-sharing bioctahedral geometry, with the bridging positions
being occupied by either two phosphide and one methoxycar-
byne or one phosphide and two alkoxycarbyne ligands.
Spectroscopic data for the bis(phosphide) complexes 5 and
6 (Table 1 and Experimental Section) are similar to those
previously reported by us for the triflate salt of compound 5,
[W2Cp2(µ-COMe)(µ-PPh2)2](CF3SO3), a complex obtained
through the slower reaction of 1 with CF3SO3Me and character-
ized through a single-crystal X-ray study.11 Therefore we can
propose for the cation in compounds 5 and 6 a structure
essentially identical to that determined for the above triflate salt,
(11) Garc´ıa, M. E.; Riera, V.; Rueda, M. T.; Ruiz, M. A.; Halut, S. J.
Am. Chem. Soc. 1999, 121, 1960.
(12) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds between
Metal Atoms, 3rd ed.; Springer Science: New York, 2005.
(13) See for example: (a) Brynda, M.; Gagliardi, L.; Widmark, P.-O.;
Power, P. P.; Roos, B. O. Angew. Chem., Int. Ed. 2006, 45, 3804. (b) Radius,
U.; Breher, F. Angew. Chem., Int. Ed. 2006, 45, 3006. (c) Nguyen, T.;
Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power, P. P. Science
2005, 310, 844.
(14) For some representative studies on this subject see: (a) Wang H.;
Xie, Y.; King, R. B.; Schaefer, H. F., III. Inorg. Chem. 2006, 45, 3384. (b)
Kenny, J. P.; King, R. B.; Schaefer, H. F., III. Inorg. Chem. 2001, 40, 900.
(c) Xie, Y.; Schaefer, H. F., III; King, R. B. J. Am. Chem. Soc. 2000, 122,
8746. (d) Low, A. A.; Hall, M. B. Inorg. Chem. 1993, 32, 3880. (e) Low,
A. A.; Kunze K. L.; MacDougall, P. J.; Hall, M. B. Inorg. Chem. 1991, 30,
1079.
(16) Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sa´ez, D.
Organometallics 2002, 21, 5515.
(17) Garc´ıa, M. E.; Melo´n, S.; Ramos, A.; Riera, V.; Ruiz, M. A.; Belletti,
D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983.
(18) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa, M. E.; Garc´ıa-Vivo´, D.;
Ruiz, M. A. Organometallics 2005, 24, 4122.
(15) Aitchison, A.; Farrugia, L. J. Organometallics 1987, 6, 819.

4932 Organometallics, Vol. 26, No. 20, 2007
Garc´ıa et al.
Table 1. Selected NMR Data for New Compoundsa
other 30-electron dimolybdenum complexes having dialkyl- or
1
diarylphosphide bridges.9,16-19 However, while the H NMR
compound
δP
δµ-C (JCP)
spectrum of complex 7 would be fully consistent with the C2
symmetry found in the crystal for that complex, the cation in 8
cannot have this symmetry element; therefore the presence of
only one resonance for the inequivalent Cp ligands suggests
the occurrence of a dynamic process generating an effective
symmetry plane. Actually, a similar process must be operative
even for compound 7, since each of the two inequivalent
cyclohexyl groups gives rise to four (instead of six) 13C NMR
resonances. Thus we have to assume again the occurrence of
fast rotation (on the NMR time scale) of the alkoxy groups
around the corresponding O-C(carbyne) bonds. This process
does not affect the carbyne C atoms, which give rise to the
characteristic strongly deshielded 13C NMR resonances (ca. 365
ppm).
Synthesis and Structural Characterization of New Cat-
ionic Hydroxycarbyne Complexes and Their Hydride Iso-
mers. Just as the neutral complexes 2 and 3 experience an easy
O-methylation at their bridging carbonyls, they are protonated
at the same position by using HBF4‚OEt2, then giving the
corresponding hydroxycarbyne derivatives [Mo2Cp2(µ-COH)-
(µ-PEt2)2]BF4 (9) and [Mo2Cp2(µ-COH)(µ-COMe)(µ-PCy2)]BF4
(10), respectively (Chart 2). In contrast to the alkoxycarbyne
complexes 5-8, the dichloromethane solutions of the hydroxy-
carbyne complexes 9 and 10 are thermally unstable, and they
rearrange above ca. 273 K (9) or 253 K (10) to give initially
the corresponding hydride-carbonyl isomers [Mo2Cp2(H)(µ-
PEt2)2(CO)]BF4 (11) and [Mo2Cp2(H)(µ-COMe)(µ-PCy2)(CO)]-
BF4 (12) in a quite selective way (Chart 2). This rearrangement
is unexpected, since the related hydroxycarbyne complexes [M2-
Cp2(µ-COH)(µ-PR2)2] (M ) W, R ) Ph;11 M ) Mo, R ) Cy9)
do not experience a similar transformation, but just a slow and
generalized decomposition. We can quote, however, a related
behavior in the 32-electron complex [W2Cp2(µ-COH)(µ-Ph2-
PCH2PPh2)(CO)2]BF4, which undergoes a rather complex,
solvent-assisted transformation into a mixture of the hydride
derivatives [W2Cp2(µ-H)(µ-Ph2PCH2PPh2)(CO)x]BF4 (x ) 2 and
4).10b In any case, the hydrides 11 and 12 are yet unstable species
that decompose progressively upon manipulation and therefore
could not be isolated as pure solids.
The structural characterization of compounds 9 and 10 can
be made easily by comparison of their spectroscopic data (Table
1) with those of the related diphenyl- and dicyclohexylphos-
phide-bridged analogues [M2Cp2(µ-COH)(µ-PR2)2]BF4 (M )
W, R ) Ph;11 M ) Mo, R ) Cy9) and with those of the
methoxycarbyne compounds 6 and 7, therefore not needing a
detailed discussion. The most relevant data are the appearance
of characteristic strongly deshielded resonances in the corre-
sponding 1H (ca. 13 ppm) and 13C (ca. 365 ppm) NMR spectra,
these clearly denoting the presence of the hydroxycarbyne
ligand.
Compounds 11 and 12 are, on the other hand, quite unusual
in having their hydride ligands terminally bound to a metal atom
involved in a triple intermetallic bond, since 30-electron
binuclear complexes usually display bridging hydride
ligands.10b,12,21 In fact, the only reported structural analogue of
these compounds is the cationic complex [Mo2Cp2(H)(µ-PCy2)2-
[W2Cp2(µ-COMe)(µ-PPh2)2]BF4 (5)
191.7b
262.2
264.2
263.8
256.7c
261.5e
285.4d
294.5c
[Mo2Cp2(µ-COMe)(µ-PEt2)2]BF4 (6)
[Mo2Cp2(µ-COMe)2(µ-PCy2)]BF4 (7)
[Mo2Cp2(µ-COEt)(µ-COMe)(µ-PCy2)]BF4 (8)
[Mo2Cp2(µ-COH)(µ-PEt2)2]BF4 (9)
367.9(14)
366.0(13)
365.7(13)
368.9(11)d
365.8(13)e
259.8(7)d
[Mo2Cp2(µ-COH)(µ-COMe)(µ-PCy2)]BF4 (10)
[Mo2Cp2(H)(µ-PEt2)2(CO)]BF4 (11)
[Mo2Cp2(H)(µ-COMe)(µ-PCy2)(CO)]BF4 (12)
a Recorded in CD2Cl2 solution at 290 K and 121.50 (31P) or 75.47 (13C)
MHz, unless otherwise stated; δ in ppm relative to internal TMS (13C) or
external 85% aqueous H3PO4 (31P); J in Hz. b J(31P-183W) ) 363.
c Recorded at 243 K. d Recorded at 213 K. e Recorded at 233 K.
with the methoxyl substituent of the carbyne ligand placed in
the plane defined by the metal atoms and the bridgehead carbon
(Chart 1), a common structural feature of bridging alkoxycar-
byne ligands.10b
The 31P NMR spectra of compounds 5 and 6 show highly
deshielded resonances (191.7 and 262.2 ppm, respectively), this
being a characteristic of 30-electron dimolybdenum and ditung-
sten complexes having dialkyl- or diarylphosphide bridges.9,16-19
The great difference in their chemical shifts (ca. 70 ppm) can
be mainly attributed to the distinct deshielding effect of the metal
(tungsten vs molybdenum) and, to a lesser extent, to the higher
deshielding influence of the bulkier Cy substituents (compared
to phenyl) on the phosphorus atom.16 On the other hand, the
bridging methoxycarbyne ligands in 5 and 6 give rise to a
characteristic highly deshielded 13C NMR resonance at ca. 370
ppm, a shift similar to those reported for other complexes
displaying bridging alkoxycarbyne ligands.8,11,17 In addition,
these compounds were found to exhibit dynamic behavior in
1
solution, as deduced in each case from the presence in the H
and 13C NMR spectra of just one resonance for both cyclopen-
tadienyl rings, which are inequivalent in the static structure
(Chart 1). This fluxional behavior can be attributed to the fast
rotation, on the NMR time scale, of the OCH3 groups around
the O-C(carbyne) bond, and it has been previously studied in
detail for other alkoxycarbyne complexes;10b,20 therefore this
was not investigated further.
The bis(alkoxycarbyne) complexes 7 and 8 are isoelectronic
and isostructural to the bis(phosphide) complexes 5 and 6, with
a bridging PR2 group being replaced by a COR ligand (Chart
1). The solid-state structure of complex 7 was confirmed through
an X-ray study (see ref 18 and Table 2) and confirms the
expected analogies. In particular, the very short intermetallic
length of 2.474(1) Å is almost the same as that found for the
neutral complex 4 (see ref 17 and Table 2) and is consistent
with the triple Mo-Mo bond formulated for these 30-electron
cations. However, the nature of the intermetallic interaction in
all these unsaturated species will be discussed in detail later on
the basis of the DFT calculations carried out on these complexes.
Spectroscopic data for compounds 7 and 8 (Table 1) are
essentially consistent with the structure found in the solid state
for 7, but once more they suggest that a dynamic process occurs
in solution. The phosphorus nuclei (δP ca. 264 ppm) exhibit
the characteristic highly deshielded NMR resonances found for
(19) Garc´ıa, G.; Garc´ıa, M. E.; Melo´n, S.; Riera, V.; Ruiz, M. A.;
Villafan˜e, F. Organometallics 1997, 16, 624.
(20) For examples of fluxional behavior, see for example: (a) Bavaro,
L. M.; Keister, J. B. J. Organomet. Chem. 1985, 287, 357. (b) Keister, J.
B.; Payne, M. W.; Muscatella, M. J. Organometallics 1983, 2, 219. (c)
Johnson, B. F. G.; Lewis, J.; Orpen, A. G.; Raithby, P. R.; Su¨ss, G. J.
Organomet. Chem. 1979, 173, 187. (d) Keister, J. B. J. Chem. Soc., Chem.
Commun. 1979, 214. (e) Gavens, P. D.; Mays, M. J. J. Organomet. Chem.
1978, 162, 389.
(21) (a) Su¨ss-Fink, G.; Therrien, B. Organometallics 2007, 26, 766. (b)
Alvarez, C. M.; Alvarez, M. A.; Garc´ıa, M. E.; Ramos, A.; Ruiz, M. A.;
Lanfranchi, M.; Tiripicchio, A. Organometallics 2005, 24, 7. (c) Suzuki,
H. Eur. J. Inorg. Chem. 2002, 1009. (d) Kang, B. S.; Koelle, U.; Thewalt,
U. Organometallics 1991, 10, 2569. (e) Forrow, W. J.; Knox, S. A. R. J.
Chem. Soc., Chem. Commun. 1984, 679. (f) Hoyano, J. K.; Graham, W. A.
G. J. Am. Chem. Soc. 1982, 104, 3722.

Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4933
Table 2. Selected DFT-Optimized Bond Lengths (Å) and Angles (deg)a
parameter
2b
6c
A
3d
7e
Mo1-Mo2
Mo1-C*
2.536 {2.515(2)}
2.537 {2.532(1)}
1.998 {1.97(1)}
2.499
2.502 {2.479(2)}
1.997 {1.974(5)}
2.501 {2.474(1)}
2.035 {2.00(1)}
2.006 {1.99(1)}
2.006 {2.00(1)}
2.035 {2.01(1)}
Mo2-C*
Mo1-CO
Mo2-CO
C*-OMe
C-O
2.034 {1.97(1)}
2.026 {2.011(5)}
2.117 {2.091(5)}
2.113 {2.073(6)}
1.324 {1.332(5)}
1.193 {1.190(6)}
1.436 {1.443(6)}
2.110 {2.08(1)}
2.108 {2.08(1)}
2.120
2.089
2.091
2.118
1.307 {1.39(1)}
1.446 {1.40(1)}
1.305 {1.31(1)}
1.305 {1.33(1)}
1.193 {1.201}
1.206
1.206
O-Me
1.449 {1.47(2)}
1.449 {1.48(1)}
2.466 {2.405(3)}
2.469 {2.409(3)}
Mo-P
2.415 {2.374(3)}
2.409 {2.389(3)}
2.409 {2.373(3)}
2.416 {2.384(3)}
2.433 {2.372(3)}
2.423 {2.372(3)}
2.428 {2.372(3)}
2.437 {2.372(3)}
120.5 {114(1)}
2.437
2.439
2.445 {2.392(2)}
2.450 {2.395(2)}
C-O-Me
118.4 {118.2(4)}
120.0 {116(1)}
119.9 {117(1)}
a Experimental values, as derived from X-ray data for the same or related species, in brackets; the Mo1 label corresponds to the metal atom positioned
away from the methyl group of the methoxycarbyne ligand, except for compounds 2, A, and 7, for which both metal centers are equivalent; C* refers to
the bridgehead atom of the carbyne ligand. b Experimental values for [Mo2Cp2(µ-PPh2)2(µ-CO)] (see ref 50). c Experimental values for the cation in
[W2Cp2(µ-COMe)(µ-PPh2)2](CF3SO3) (see ref 11). d Experimental values for the ethoxycarbyne complex 4 (see ref 17). e Experimental values for the cation
in [Mo2Cp2(µ-COMe)2(µ-PCy2)]BF4 (see ref 18).
ppm).17,21b On the other hand, the proposal of a semibridging
Chart 2
character for the carbonyl ligand in 11 follows from its relatively
low C-O stretching frequency (1843 cm-1) and high 13C
chemical shift (δC ) 259.8 ppm). We note that the related
complex [Mo2Cp2(H)(µ-PCy2)2(CO)]BF4 displays a semibridg-
ing carbonyl in the solid state and exhibits similar IR and 13C
NMR features in solution (νCO ) 1838 cm-1, δC ) 260.6 ppm).9
Complex 11 exhibits fluxional behavior in solution, as
deduced from the appearance of single 1H or 13C NMR
resonances for the inequivalent Cp ligands and from the
equivalence of the diastereotopic 13C cyclohexyl resonances.
A similar behavior was found for the complex [Mo2Cp2(H)(µ-
PCy2)2(CO)]BF4 and was explained by assuming the occurrence
of a concerted scrambling of the CO and H ligands between
both metal atoms, this effectively generating the required
(CO)]BF4, whose structure was confirmed through an X-ray
symmetry plane relating both metal centers.9 This seems to be
study. This complex could be prepared recently in our laboratory
the case also for compound 11. Indeed, on lowering the
by UV-visible irradiation of dichloromethane solutions of a
temperature, its hydride resonance remains unchanged, but the
mixture of the dicarbonyl tautomers [Mo2Cp2(µ-PCy2)(µ-κ2-
cyclopentadienyl 1H NMR resonance at 5.75 ppm first broadens
HPCy2)(CO)2]BF4 and [Mo2Cp2(µ-H)(µ-PCy2)2(CO)2]BF4.9 In-
and then eventually splits into two well-separated resonances
terestingly, this hydride complex cannot be formed via thermal
(δH ) 5.83 and 5.73 ppm at 213 K), in agreement with the
or photochemical rearrangement of the corresponding hydroxy-
rigid structure shown in Chart 2. From the corresponding
coalescence temperature (ca. 250 K) we can estimate for the
carbyne isomer [Mo2Cp2(µ-COH)(µ-PCy2)2]BF4.
Spectroscopic data in solution for 11 (Table 1 and Experi-
corresponding fluxional process an activation barrier of ca. 52-
mental Section) are in general agreement with those previously
((1) kJ mol-1 22
to be compared with a barrier of 43((1) kJ
,
reported for [Mo2Cp2(H)(µ-PCy2)2(CO)]BF4.9 For example, the
phosphorus nuclei give rise to a similar 31P NMR resonance
[285.4 vs 305.2 ppm for the µ-PCy2 analogue], which is even
more deshielded than those of 30-electron phosphide-bridged
dimolybdenum complexes having three bridging ligands, such
as the complexes 1-10. The presence of a terminal hydride in
11 is denoted by the appearance of a poorly shielded hydride
resonance [δH ) -1.47 vs -1.60 ppm for the µ-PCy2 analogue],
to be compared with the more shielded resonances characteristic
of bridging hydrides, such as those in the electron-precise
tetracarbonyls [M2Cp2(µ-H)(µ-PR2)(CO)4] (M ) Mo, W; R )
Cy, Et, Ph) (ca. -15 ppm).16 Moreover, the hydride shielding
in 11 is still considerably lower than that observed in the
isoelectronic hydride-bridged complex [Mo2Cp2(µ-H)(µ-PCy2)-
(CO)2], also having a triple intermetallic bond (δH ) -6.94
mol-1 calculated for [Mo2Cp2(H)(µ-PCy2)2(CO)]BF4.9 This
difference can be explained by assuming that the lower steric
demands of the PEt2 bridges (compared to PCy2 bridges), would
allow for a higher stabilization of the ground-state structure
(puckered Mo2P2 central framework, with closer mutual ap-
proach of substituents on different P atoms) relative to the
transition state (flat Mo2P2 framework).
The spectroscopic data for compound 12 (Table 1 and
Experimental Section) are very similar to those just discussed
for 11, and they need not be analyzed in detail. Thus, the
structures for these hydride complexes are assumed to be
essentially identical, by just replacing one of the phosphide
groups by the isoelectronic methoxycarbyne ligand. The latter
is expected to be a poorer σ-donor and better π-acceptor ligand

4934 Organometallics, Vol. 26, No. 20, 2007
Garc´ıa et al.
than a dialkylphosphide ligand, which is consistent with the
observed C-O stretching frequency for 12 (1873 cm-1), some
35 cm-1 above that of 11. Interestingly, complex 12 also exhibits
fluxional behavior in solution, presumably also involving a
concerted scrambling of the CO and H ligands between both
metals. From the corresponding coalescence temperature of the
1H NMR cyclopentadienyl resonances (ca. 263 K) we can
estimate an activation barrier of ca. 55((1) kJ mol-1 22
marginally above that calculated for 11.
,
only
Computational Studies. Thanks to the rapid progress in
computing power and the development of new methodologies,
quantum mechanical calculations for transition metal complexes
are today accessible for much more complex systems. In
particular, the great computational efficiency and chemical
accuracy of the DFT methods allow the study of real (rather
than model) molecules of the size of the binuclear metal
complexes discussed here.23
We have carried out DFT calculations (see Experimental
Section for further details) for the new cationic methoxycarbyne
complexes [Mo2Cp2(µ-COMe)(µ-PEt2)2]+ (6) and [Mo2Cp2(µ-
COMe)2(µ-PCy2)]+ (7), as well as for their carbonyl-bridged
precursors [Mo2Cp2(µ-PEt2)2(µ-CO)] (2), [Mo2Cp2(µ-PCy2)(µ-
CO)2]- (A), and [Mo2Cp2(µ-COMe)(µ-PCy2)(µ-CO)] (3). This
will allow us to better understand not only the metal-metal
and metal-carbon bonding in these unsaturated complexes but
also the electronic changes accompanying the process of
methylation at the oxygen atom of a bridging carbonyl ligand.
All these complexes exhibit a face-sharing bioctahedral structure
(assuming the Cp groups as equivalent to three terminal ligands)
and have a total of 30 electrons for metal binding, this leading
to the formulation of a triple metal-metal bond, according to
the EAN rule. First, we will discuss the optimized geometries
calculated for these molecules. This will be followed by an
analysis of the electronic structure and bonding in these
unsaturated complexes from two distinct points of view: the
properties of the relevant molecular orbitals and the topological
properties of the electron density as managed in the AIM
theory.24
Figure 1. Optimized geometries for complexes 2, 6, A, 3, and 7,
with hydrogen atoms omitted for clarity.
The intermetallic distances are all found within the range
2.499-2.537 Å and are almost unaffected by the methylation
at the carbonyl bridges, with changes being on the order of just
0.001 Å. The nature of the donor atoms at the bridging positions
is more relevant in this respect: as expected, complexes with
one P-donor and two C-donor bridging ligands exhibit distances
shorter (by ca. 0.035 Å) than those complexes with one C-donor
and two P-donor bridges, doubtless as a consequence of the
lower covalent radius of carbon when compared to that of
phosphorus.
The Mo-C bond lengths follow also the expected trends,
with the Mo-C(carbonyl) lengths (ca. 2.10 Å) longer than the
Mo-C(carbyne) distances (ca. 2.01 Å), in agreement with the
change in the formal order of the corresponding bonds, which
increase from 1 in the bridging carbonyls to 1.5 in the bridging
methoxycarbyne ligands. The latter exhibit a slightly asymmetric
coordination, being closer to the metal center placed away from
the methyl group (Mo1 in Table 2), which is possibly a genuine
steric effect. We note finally the excellent fit of the calculated
C-O lengths and C-O-C angles with the corresponding
experimental data, with the larger deviation in the case of 6
being considered as not significant, due to the low quality of
the experimental data available, with a disorder in the meth-
oxycarbyne ligand.11
Optimized Geometries. The most relevant parameters de-
rived from the geometry optimization of the anion [Mo2Cp2-
(µ-PCy2)(µ-CO)2]- (A), the cations in 6 and 7, and the neutral
compounds 2 and 3 can be found in Table 2, with the
corresponding views being collected in Figure 1. The experi-
mental data (from X-ray diffraction studies) for the same or
closely related species are also collected in that table. As it can
be seen from inspection of all these data, the optimized bond
lengths are in quite good agreement with the corresponding
experimental data, although those involving the metal atoms
tend to be slightly greater (less than 0.05 Å) than the corre-
sponding values measured in the solid state for the same or
strongly comparable compounds by X-ray diffraction. This is a
common tendency with the functionals currently used in the
DFT computations of transition metal compounds.12,23a,25
Frequency analysis was carried out in all the stationary points
to ensure that the optimized geometries correspond to global
minima of the potential energy surface (PES). For all complexes
but 6 and 7 this analysis showed no imaginary frequencies, this
allowing us to classify the corresponding geometries as real
minima in the PES. For the cations in 6 and 7, however, the
geometry optimization converged with one imaginary vibrational
frequency of low value (ca. 10i cm-1), which we attribute to
the internal rotation of one of the cyclopentadienyl ligands. We
thus concluded that there is a genuine minimum of energy
identical to or very close to the stationary point in question
regarding the geometry of the Mo2PxCy core, and therefore we
(22) Calculated using the modified Eyring equation ∆G# ) 19.14Tc[9.97
+ log(Tc/∆ν)] (J mol-1). See: Gu¨nther, H. NMR Spectroscopy; John
Wiley: Chichester, UK, 1980; p 243.
(23) (a) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density
Functional Theory, 2nd ed.; Wiley-VCH: Weinheim, 2002. (b) Ziegler, T.
Chem. ReV. 1991, 91, 651. (c) Foresman, J. B.; Frisch, Æ. Exploring
Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.:
Pittsburgh, 1996.
(24) (a) Bader, R. F. W. Atoms in Molecules-A Quantum Theory; Oxford
University Press: Oxford, 1990. (b) Bader, R. F. W. Chem. ReV. 1991, 91,
893.
(25) Cramer, C. J. Essentials of Computational Chemistry, 2nd ed.;
Wiley: Chichester, UK, 2004.

Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4935
Table 3. Calculated and Experimental C-O Stretching
MO calculations carried out on the cluster [Co3(µ3-COMe)-
(CO)9].15 In any case, however, the highest positive charges for
all these complexes are located at the phosphorus atoms, in
agreement with the good electron-donor properties of the PR2
ligands, while the highest negative charges are still located at
the oxygen atoms.
MO Analysis. Molecular orbital theory is the most widely
used tool for studying the chemical bond in molecules.25,29 In
the DFT-based framework, Kohn-Sham (KS) orbitals, when
compared to classical MOs, are not only associated with the
one-electron potential which includes all nonclassical effects
but are also consistent with the exact ground-state density.
Moreover, the power of the KS orbitals has been recognized
by many authors as being similar to that of the classical MOs
for the analysis of chemical bonding, at least in a qualitative
way.25,30 Accordingly, a detailed analysis of the corresponding
KS orbitals has been performed to obtain information about the
bonding in our unsaturated molecules.
The electronic structure of face-sharing bioctahedral com-
plexes has received considerable attention from computational
chemists, which, however, have been mainly concerned with
non-organometallic compounds.31 The metal-metal bonding in
these systems follows from the occupation of one σ- and two
δ-bonding MOs. Using this scheme as a first approach, we can
therefore expect an occupation up to the δ-bonding orbitals
(σ2δ4) for our 30-electron organometallic complexes, and thus
formally a triple metal-metal bond.12,32 This situation, however,
can be much more complicated in our complexes, since in low-
symmetry systems a mixture between δ-bonding orbitals and
high-energy π counterparts (from the metals or the bridging
ligands present there) can occur to yield more complicated
bonding patterns. With this in mind, we will first analyze the
molecular orbitals of the bis(diethylphosphide) complexes 2 and
6 and then those of the dicyclohexylphosphide complexes A,
3, and 7. The first couple illustrates the effect of a single
methylation at a bridging carbonyl, while the latter triad
corresponds to two successive methylation steps and allows an
internal comparison between carbonyl and bridging methoxy-
carbyne ligands.
Molecular Orbitals of Compounds 2 and 6. The most
relevant orbitals of compounds 2 and 6 are depicted in Figure
2, with their associated energy and prevalent bonding character.
The highest occupied molecular orbital (HOMO, MO 95) of
the monocarbonylic complex 2 corresponds essentially to one
of the two components of the metal-carbonyl σ bond, involving
metal d orbitals. This is quite surprising since for these highly
unsaturated complexes we should expect the HOMO to be a
metal-metal bonding δ orbital, as noted above. Indeed the next
three orbitals are those accounting for the metal-metal bond-
ing: two of them can be clearly identified as the σ (MO 92)
and one of the δ (MO 94) components of the expected triple
metal-metal bond.33 However, the second δ component of the
triple bond (MO 93) shows an important participation of the
Frequenciesa
νst(C-O)/cm-1
d
compd
calcdb
exptc
%
A
1770(34)
1756(100)
1826
1643(m, sh)
1624(vs)
1697
7.7
8.1
7.6
7.6
2
3
1829e
1315
1700e
6
7
1349
1367
1347
1271f
1308f
1273f
6.1
4.5
5.8
a Values corresponding to the CO or COMe ligands, with relative
intensities in brackets; experimental data for complexes A, 2, and 3 taken
from refs 16 and 17. b Uncorrected. c In THF solution, unless otherwise
stated. d (calcd - expt)/expt. e Carbonyl ligand. f Recorded in Nujol mull.
did not follow any further the imaginary eigenvector in search
of a stationary point with no imaginary vibrational frequencies.
The calculated and experimental values of the C-O stretching
frequencies for both the carbonyl and the methoxycarbyne
ligands in these complexes are shown in Table 3. As it can be
appreciated, the calculated values overestimate the experimental
figures by roughly 7%, which is a normal bias for DFT-derived
IR stretching frequencies.26 In any case, the calculated values
for the methoxycarbyne complexes allow us to unambiguously
assign the experimental IR bands. Bearing in mind that for
simple molecules such as organic ethers the C-O stretching
frequencies tend to appear in the range 1150-1250 cm-1, even
when including sp2-hybridized carbon atoms, the high values
measured for our methoxycarbyne complexes, ca. 1290 cm-1
,
might be considered as indicative of the persistency of a
significant π-bonding interaction at the C-OMe bond, also
consistent with the conformation of the ligand (arrangement in
the Mo2C plane and C-O-C angles close to 120°). As it will
be discussed later on, this kind of interaction is fully consistent
with the MO and AIM analysis of the bonding in these
methoxycarbyne complexes.
The electronic charges calculated for the different complexes
are shown in Table S1 (see Supporting Information), which
includes the Mulliken charges,27 as well as those derived from
the NBO analysis (NPA charges).28 The complexes having
bridging carbonyls (2, A, and 3) display the highest negative
atomic charges at the corresponding oxygen atoms, in agreement
with the preferential addition of the hard cations Me+ or H+ at
this site. Since the high-energy occupied molecular orbitals of
the carbonyl complexes do not involve their oxygen atoms (see
below), then we can conclude that the formation of the hydroxy-
and methoxycarbyne complexes discussed in this paper are
essentially charge-controlled reactions. Upon formation of the
methoxycarbyne ligands, the atomic charges indicate a decrease
of electron density at the Mo atoms and a significant increase
of ca. 0.16e at the C(carbyne) atoms, with charges of ca. -0.02e
(Mulliken) or +0.26e (NPA), to be compared with the values
of ca. +0.14e (Mulliken) or +0.42e (NPA) in the carbonyl-
bridged precursors. Incidentally, we note that low atomic charges
were also found for the C(carbyne) atom after the semiempirical
(29) (a) Jean, Y. Molecular Orbitals of Transition Metal Complexes;
Oxford University Press: Oxford, UK, 2005. (b) Jean, Y.; Volatron, F.;
Burdett, J. An Introduction to Molecular Orbitals; Oxford University
Press: Oxford, UK, 1993.
(26) Yu, L.; Srinivas, G. N.; Schwartz, M. J. Mol. Struct. (THEOCHEM)
2003, 625, 215.
(27) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833.
(28) Mulliken population analyses fail to give a useful and reliable
characterization of the charge distribution in many cases, especially when
highly ionic compounds and diffuse basis functions are involved. Charges
calculated according to the natural population analysis (NPA) do not show
these deficiencies and are more independent of the basis set: (a) Reed, A.
E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (b) Reed,
A. E.; Curtis, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(30) See, for example: (a) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys.
Chem. 1996, 100, 12974. (b) Baerends, E. J.; Gritsenko, O. V. J. Phys.
Chem. A 1997, 101, 5383. (c) Stowasser, R.; Hoffmann, R. J. Am. Chem.
Soc. 1999, 121, 3414. (d) Baerends, E. J. Theor. Chem. Acc. 2000, 103,
265.
(31) Petrie, S.; Stranger, R. Inorg. Chem. 2003, 42, 4417, and references
therein.
(32) (a) Collman, J. P.; Boulatov, R. Angew. Chem., Int. Ed. 2002, 41,
3948. (b) Summerville, R. H.; Hoffmann, R. J. Am. Chem. Soc. 1979, 101,
3821.

4936 Organometallics, Vol. 26, No. 20, 2007
Garc´ıa et al.
Figure 3. Canonical forms used to describe the bonding in
methoxycarbyne complexes.
and co-workers for the complexes [M2Cp2(CO)4] (M ) Cr, Mo,
W), in which the transition of the carbonyl ligands from terminal
to semibridging was calculated to cause a decrease in the
intermetallic π-bonding interaction.34 In summary, we can
describe the intermetallic interaction in complex 2 as a triple
metal-metal bond somewhat weakened due to back-bonding
(δMM to π*CO) to the bridging carbonyl.
As for the methoxycarbyne complex 6, the frontier MOs are
similar to those of its precursor 2, except for some significant
differences. The most striking effect of the incorporation of the
methyl group is the stronger stabilization (compared to that of
the other frontier MOs) of the frontier σM-C orbital (the HOMO
in 2), now appearing as MO 95, below the three orbitals
accounting for the intermetallic interaction. The latter includes
the HOMO of the molecule (MO 99), now corresponding to
one of the two δ components of the triple metal-metal bond.
This is followed by the δ orbital originally involved in the π
back-bonding to the CO bridge, now exhibiting a more
pronounced delocalization over the Mo2C triangle (MO 98), and
then by the σ-bonding orbital (MO 97). The orbital MO 98
therefore retains significant metal-metal δ-bonding character
while becoming the π component of the metal-carbyne bond
(the two σ components of that bond are described by MO 95
and the low-energy orbital MO 57). As concerning the bonding
within the methoxycarbyne ligand, the nature of MO 81
indicates the persistence of significant π-bonding interaction
in the C-O bond of this ligand (a neat π bond in the carbonyl
precursor). This interaction must compete necessarily with the
πMC-bonding interaction implied by MO 98, since both molec-
ular orbitals share the same atomic orbital of the carbon atom.
Thus a stronger π-bonding interaction between the C(carbyne)
and O atoms must imply a weaker Mo-C π-bonding interaction
and vice versa. This description parallels the two canonical
forms traditionally proposed to explain the metal-alkoxycar-
byne bond in binuclear complexes, which imply partial double-
bond character in both the M-C and C-O bonds (Figure 3).
The retention of a significant (but only partial) π-bonding
interaction between the C and O atoms explains the short values
(ca. 1.30 Å, Table 2) of the corresponding C-O bonds in these
systems, with figures somewhat shorter than those found for
comparable single bonds, such as the C(sp2)-O bonds of organic
molecules (ca. 1.35 Å), but still far from the reference values
for double C-O bonds (ca. 1.21 Å).35 In summary, we can
describe the intermetallic interaction in complex 6 also as a triple
metal-metal bond, but significantly weakened now due to π
bonding to the bridgehead carbon atom of the carbyne ligand,
which in turn is partially involved in a π-bonding interaction
with its oxygen atom.
Figure 2. Selected molecular orbitals of complexes 2 and 6, with
their energies and main bonding character indicated below.
We note finally that the lowest unoccupied molecular orbitals
(LUMO) of compounds 2 and 6 are largely metal-based
molecular orbitals and exhibit δ*M-M-antibonding character.
These orbitals are relevant to the understanding of the reactivity
of these compounds and lead us to the prediction that the
π*C-O orbital of the bridging carbonyl and, therefore, implies
the presence of significant metal-to-carbonyl π back-bonding.
The corresponding electrons are thus substantially delocalized
over the Mo2C triangle, in detriment of the direct metal-metal
bonding. A similar effect was previously reported by Hoffman
(34) Jennis, E. D.; Pinhas, A. R.; Hoffmann, R. J. Am. Chem. Soc. 1980,
102, 2576.
(35) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(33) The characterization of these orbitals as σ, π, or δ bonding is
arbitrary to some extent, but nevertheless it is a useful distinction based on
the predominant type of orbital overlap.

Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4937
Figure 4. Selected molecular orbitals of complexes A, 3, and 7, with their energies and main bonding character indicated below.
incorporation of electron-donor ligands to the dinuclear center
under orbital-controlled conditions is accompanied by a reduc-
tion of the intermetallic bond order, as expected for such highly
unsaturated compounds.
intermetallic bond in these complexes is also made up of a σ
and two δ components, but now only the σ component [MO
109 (A), 114 (3), and 117 (7)] has pure metal character (in
complex 7 this orbital even appears somewhat mixed with a
Molecular Orbitals of Compounds A, 3, and 7. The relevant
orbitals for the anion [Mo2Cp2(µ-PCy2)(µ-CO)2]- (A) and its
methoxycarbyne derivatives 3 and 7 are collected in Figure 4,
and an orbital correlation diagram is shown in Figure 5. The
general trends in these complexes are similar to those previously
discussed for compounds 2 and 6. The successive methylation
at the oxygen atoms of the carbonyl ligands causes again a
strong and progressive stabilization of the (now two) frontier
MOs having σ metal-carbon bonding character. The triple
σ
MC-bonding orbital, MO 119). The δ components of this bond
are mixed with the appropriate carbon orbitals, thus implying
the presence of significant π back-bonding to the carbonyls (A),
π bonding with the methoxycarbynes (7), or a mixture of both
tricentric interactions (3), all of them a detriment to the direct
intermetallic bond. As for the bonding within the methoxycar-
byne ligand, the situation is identical to that described for
complex 6, so that low-energy orbitals representing a partial

4938 Organometallics, Vol. 26, No. 20, 2007
Garc´ıa et al.
to 7 are comparable to (even slightly higher than) the figure
calculated for the triply bonded [Mo2Cp2(CO)4] (0.576 e Å-3),
which exhibits linear semibridging carbonyls. A further observa-
tion of interest is that the intermetallic F is only marginally
reduced (by ca. 0.005 e Å-3) upon formation of the carbyne
ligands via methylation of the carbonyl bridges. Thus, it is
tempting to conclude that the extent of the π interactions with
either the carbonyl or methoxycarbyne bridges in compounds
2 to 7 might be comparable, but perhaps this is just a reflection
of the very low contribution of the implied bonding electrons
to the total electron density at the intermetallic bcp, due to their
tricentric distribution.
The values of 32F at the intermetallic bcp for our complexes
are moderately positive (1.90-2.92 e Å-5), a feature that seems
to be a characteristic of metal-metal interactions in transition
metal complexes,38 attributed to the diffuse character of the
electrons involved in the bonding.39 These values are similar
to that calculated for [Mo2Cp2(CO)4], but significantly lower
than those for the triply bonded complexes [Mo2X6] in Table
5. This is surely just a consequence of the higher internuclear
distance for the 30-electron cyclopentadienyl complexes, this
implying by force a smoother change of the electronic density
along the intermetallic vector.
Figure 5. Correlation diagram of the frontier molecular orbitals
of complexes A, 3, and 7. Those of compounds 3 and 7 are
represented 2.5 and 5.0 eV above their actual energy, respectively.
π
CO-bonding interaction for each of these ligands are found for
both 3 and 7 (see Supporting Information).
Topological Analysis of the Electron Density. The quantum
theory of atoms in molecules provides an alternative way of
analyzing the chemical bond by inspection of the topology of
the total electron density of a molecule.24 We have collected in
Table 4 the values of the electron density (F) and its Laplacian
32F at several bond critical points (bcp) of complexes 2 to 7.
Figure 6 shows electron density maps of the planes M-C(O)-M
and M-C(OMe)-M in complex 3, which are quite representa-
tive of the density maps for all other complexes in the planes
involving the bridging carbonyl and carbyne ligands.
As for the M-C bonds within the carbonyl and methoxy-
carbyne ligands, the AIM analysis is fully consistent with the
geometric features and orbital analysis discussed above. As
expected, the values of F at the M-C bcp’s are significantly
higher for the M-C(carbyne) bonds (in the range 0.83 to 0.93
e Å-3) than for the M-C(carbonyl) bonds (ca. 0.71 e Å-3).
We note that the latter figure is comparable to the values
reported for the bridging carbonyls at [Co2(CO)8],39 but we are
not aware of any previous topological analysis reported for a
carbyne-bridged complex that could be used for comparative
purposes. Even so, by taking into account that the progressive
increase (from 1 to 3) of the formal M-C bond order in the
series CrCH3/CrCH2/CrCH is reflected in a rather modest
increase of F at the corresponding Cr-C bcp’s (0.769, 0.904,
and 1.174 e Å-3, respectively),40 then we conclude that the
average value of F of ca. 0.88 e Å-3 in our complexes is
consistent with a substantial multiplicity of the corresponding
Mo-C(carbyne) bonds. We note also the disappearance of the
minimum of the electron density in the center of the M-C-M
triangle involving the methoxycarbyne ligand (right in Figure
6) compared to the analogous plane containing a carbonyl ligand
(left in Figure 6), this reflecting the accumulation of π-electron
density in this triangle upon methylation of the bridging
carbonyls. Finally, the presence of the methoxyl group in the
same plane containing the metal and carbyne atoms causes an
asymmetry in the coordination of the methoxycarbyne ligand,
as already discussed. This causes the electron density at the
bcp of the shorter bond to be higher (ca. 0.90 e Å-3) than the
corresponding value of the longer bond (ca. 0.83 e Å-3), as
expected.
The first point of interest is the localization of the corre-
sponding bcp for the metal-metal bond in all cases. This is
not granted in dinuclear complexes of transition metals having
bridging ligands with π-bonding ability such as µ-CO or µ-CR;
for example the intermetallic bcp could not be localized for the
singly bonded complexes [Fe2(CO)9]36 or [Co2(CO)8].14e To gain
a better understanding of the values of F obtained at the
intermetallic bcp’s obtained for our complexes, we carried out
DFT computations, using the same functional and basis, on some
prototype complexes having no bridging ligands that could
contribute to the delocalization of the metal-metal interaction
(Table 5). We also computed the triply bonded [Mo2Cp2(CO)4],
a representative example of the unsaturated cyclopentadienyl
complexes displaying semibridging carbonyls along a formally
triple intermetallic bond.37 As expected, the reduction of the
formal metal-metal bond order is accompanied by a strong
decrease of the electronic density at the intermetallic bcp for
the model compounds. The values of F at the intermetallic bcp
in our complexes (ca. 0.60 e Å-3) are lower that those calculated
for the triply bonded complexes [Mo2X6] (ca. 0.98 e Å-3), which
is not surprising in view of the much shorter intermetallic lengths
in the latter complexes and the absence of bridging ligands.
Clearly, the presence in compounds 2 to 7 of bridging carbonyl
or carbyne ligands involved in π-bonding interactions with the
δ-bonding orbitals of the metals has the effect of subtracting
electronic density from the intermetallic vector and, thus, causes
a lowering of the values of F at the bcp. Yet, the figures for
compounds 2 to 7 are much higher than those calculated for
singly bonded complexes having no carbonyl bridges such as
[Mo2Cp2(CO)6] and [Mo2(CO)10]2- (ca. 0.15 e Å-3, Table 5).
Most interestingly, the intermetallic F values for compounds 2
As for the C-O bond within the methoxycarbyne ligand, we
find that the electron density at the corresponding bcp (ca. 2.10
e Å-3) has a value intermediate between those at the single C-O
bond of the methoxyl group (ca. 1.58 e Å-3) and those at the
double C-O bond of the carbonyl bridges (ca. 2.75 e Å-3).
This can be taken as independent evidence for the presence of
(38) Gervasio, G.; Bianchi, R.; Marabello, D. Chem. Phys. Lett. 2004,
387, 481, and references therein.
(39) Macchi, P.; Sironi, A. Coord. Chem. ReV. 2003, 238-239, 383.
(40) Vidal, I.; Melchor, S.; Dobado, J. A. J. Phys. Chem. 2005, 109,
7500.
(36) Bo, C.; Sarasa, J. P.; Poblet, J. M. J. Phys. Chem. 1993, 97, 6362.
(37) Winter, M. J. AdV. Organomet. Chem. 1989, 29, 101.

Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4939
Table 4. Topological Properties of the Electron Density at the Bond Critical Pointsa
2
6
A
3
7
2
2
2
2
2
bond
r
∇ F
r
∇ F
r
∇ F
r
∇ F
r
∇ F
Mo1-Mo2
Mo1-C*
0.588
1.90
0.584
0.935
2.14
6.08
0.615
2.38
0.609
0.925
2.52
6.65
0.606
0.920
0.831
0.920
0.832
2.128
2.129
1.553
1.554
2.72
6.21
6.90
6.20
6.90
1.87
1.89
-8.19
-8.19
Mo2-C*
C*-OMe
O-Me
0.832
2.116
1.562
6.75
1.42
0.835
2.027
1.637
0.704
0.715
2.793
7.29
0.81
-8.21
-10.38
5.53
Mo1-CO
Mo2-CO
C-O
0.711
0.715
2.793
5.56
5.56
0.698
0.743
0.739
0.702
2.714
2.713
5.58
5.85
5.85
5.58
14.90
14.84
5.44
17.63
17.72
a Values of the electron density at the bond critical points (F) are given in e Å-3; values of the Laplacian of the electron density at these points (32F) are
given in e Å-5; the Mo1 label corresponds to the metal atom positioned away from the methyl group of the methoxycarbyne ligand, except for compounds
2, A, and 7, for which both metal centers are equivalent; C* refers to the bridgehead atom of the carbyne ligand.
this function exhibits an even higher value (32F ) 20.29 e
Å-5).39 We trust that the above differences in the values of the
Laplacian of the electron density at the C-O bcp’s are another
indication of the persistence of significant π-bonding interaction
in the corresponding bond of the methoxycarbyne ligand.
Concluding Remarks
The carbonyl-bridged complexes 1 to 4 react with either
Figure 6. Electron density maps of complex 3 in the planes Mo-
[Me3O]BF4 or HBF4‚OEt2 to give the corresponding methoxy-
C(O)-Mo (left) and Mo-C(OMe)-Mo (right). Nuclear positions
or hydroxycarbyne derivatives 5 to 8 as a result of a charge-
(b), bond critical points (2), bond critical paths (bold lines), and
controlled addition of the electrophile at the oxygen atom of
ring critical points (+) are also indicated.
the carbonyl bridges. The DFT method used in all these
complexes leads to face-sharing bioctahedral optimized geom-
Table 5. Properties of the Electron Density in Some
Prototypal Dinuclear Complexesa
etries that are in excellent agreement with those experimentally
determined through X-ray diffraction. The analysis of the
molecular orbitals in these 30-electron complexes gives support
to the triple intermetallic bond proposed according to the EAN
rule, which is made up of one σ and two δ components; the
latter orbitals, however, are involved in π back-bonding with
the bridging carbonyls and, to a greater extent, in π bonding
with the carbyne ligands. These two interactions imply a
delocalization of the bonding electrons over the Mo2C triangles
and thus a partial reduction of the direct intermetallic binding.
In addition, in the methoxycarbyne complexes there is some
residual π-bonding interaction at the C-O bond of the carbyne
ligand (originally a bridging carbonyl), which competes with
the metal-carbyne π bonding. The topological analysis of the
electron density within these unsaturated molecules is fully
consistent with the MO description of the bonding just given.
The electron density at the intermetallic bond critical points
decreases only slightly upon methylation of the carbonyl bridges
and still keeps a high value of ca. 0.6 e Å-3, comparable to
that calculated for the triply bonded [Mo2Cp2(CO)4]. At the same
time, the electron densities at the Mo-C (carbyne) bcp’s (ca.
0.88 e Å-3 on average) are substantially increased above the
figures for the Mo-C(carbonyl) bonds (ca. 0.71 e Å-3), in
agreement with the increased order of those Mo-C bonds. Yet,
the persistence of substantial multiplicity in the C(carbyne)-O
bond of the methoxycarbyne ligand in the complexes studied
can be clearly deduced from the values of electron density and
their Laplacian at the corresponding bcp’s, which are intermedi-
ate between the corresponding figures of the bridging carbonyls
and those of the methoxyl groups.
2
compd
BOb
∆
MsM/Å
r
∇ F
[Mo2(HCO2)4]
[Mo2Me6]
[Mo2(NMe2)6]
[Mo2Cp2(CO)4]
[Mo2(CO)10]2-
[Mo2Cp2(CO)6]
4
3
3
3
1
1
2.122
2.212
2.243
2.519
3.334
3.343
1.186
0.996
0.962
0.576
0.140
0.171
13.34
10.08
8.37
2.51
0.29
0.21
a Values of the electron density at the intermetallic bond critical points
(F) are given in e Å-3; values of the Laplacian of the electron density at
these points (32F) are given in e Å-5 b Formal order of the intermetallic
.
bond.
significant multiplicity in the C-O bond, consistent with the
MO analysis made above, but the figures cannot be fully
compared due to the differences in the carbon environments
(trigonal vs tetrahedral) and the implied geometrical effects,
already noticed. However, we must note that the distinct types
of C-O bonds also exhibit quite different values of the
Laplacian of F at the corresponding bcp’s. For the carbonyl
bridges this function takes quite high and positive values (17.72
> 32F > 14.84 e Å-5), whereas these values are still positive,
but much lower, for the C(carbyne)-O bonds (0.88 > 32F >
1.89 e Å-5), and finally the O-Me bonds have values of 32F
as expected from covalent bonds, that is, with figures that are
negative (-8.19 > 32F > -10.38 e Å-5). Actually, the latter
are not very different from those previously reported for simple
organic molecules such as dimethyl ether (32F ) -9.94 e
Å-5).39 The presence of positive values is a characteristic of
the carbonyl ligands coordinated to metal atoms, being derived
from their singular electron distribution, and in the free molecule

4940 Organometallics, Vol. 26, No. 20, 2007
Garc´ıa et al.
a 54% Et2O solution, 0.06 mmol) were stirred in CH2Cl2 (5 mL)
or CD2Cl2 (0.6 mL) at 243 K for 2 min to give brownish solutions
shown (by NMR) to contain essentially pure compound 9. These
solutions decomposed progressively upon storage or manipulation
above ca. 273 K to give compound 11 and then other yet
uncharacterized products. Spectroscopic data for compound 9: 1H
NMR (243 K): δ 12.65 (s, br, 1H, COH), 5.96 (s, 10H, Cp), 2.12
(m, 4H, CH2), 2.02 (m, 4H, CH2), 1.06 (dt, JHP ) 17, JHH ) 7, 6H,
CH3), 0.31 (dt, JHP ) 20, JHH ) 7, 6H, CH3) ppm. 13C{1H} NMR
(213 K): δ 368.9 (t, JCP ) 11, µ-COH), 94.0 (s, Cp), 36.4 (m,
Experimental Section
General Procedures and Starting Materials. All manipulations
and reactions were carried out under a nitrogen (99.995%)
atmosphere using standard Schlenk techniques. Solvents were
purified according to literature procedures and distilled prior to
use.41 Petroleum ether refers to that fraction distilling in the range
65-70 °C. Compounds [W2Cp2(µ-PPh2)2(µ-CO)] (1),16 [Mo2Cp2-
(µ-PEt2)2(µ-CO)] (2),16 and [Mo2Cp2(µ-COR)(µ-PCy2)(µ-CO)] [R
) Me (3), Et (4)]17 were prepared as described previously. All other
reagents were obtained from the usual commercial suppliers and
used as received. Filtrations were carried out using diatomaceous
earth. IR stretching frequencies were measured in solution or Nujol
mulls and are referred to as ν(solvent) or ν(Nujol), respectively.
Nuclear magnetic resonance (NMR) spectra were routinely recorded
at 300.13 (1H), 121.50 (31P{1H}), or 75.47 (13C{1H}) at 290 K in
CD2Cl2 solutions unless otherwise stated. Chemical shifts (δ) are
given in ppm, relative to internal tetramethylsilane (1H, 13C) or
external 85% aqueous H3PO4 (31P). Coupling constants (J) are given
in Hz.
AA′X, |JCP + JCP′| ) 27, 2 × CH2), 30.0 (m, AA′X, |JCP + JCP′
|
) 16, 2 × CH2), 13.9 (s, 2 × CH3), 10.6 (s, 2 × CH3) ppm.
Spectroscopic data for compound 11: IR ν(CH2Cl2) 1843 (C-O)
1
cm-1. H NMR: δ 5.75 (s, 10H, Cp), 2.49 (m, 4H, PCH2), 2.04
(m, 4H, PCH2), 1.32 (dt, JHP ) 18, JHH ) 7, 6H, CH3), 0.73 (dt,
JHP ) 20, JHH ) 8, 6H, CH3), -1.21 (t, JHP ) 45, 1H, Mo-H)
1
ppm. H NMR (253 K): δ 5.76 (s, br, 10H, Cp), 2.63 (s, br, 2H,
PCH2), 2.31 (s, br, 2H, PCH2), 2.06 (s, br, 4H, PCH2), 1.31 (dt,
JHP ) 18, JHH ) 7, 6H, CH3), 0.70 (dt, JHP ) 20, JHH ) 8, 6H,
CH3), -1.34 (t, JHP ) 45, 1H, Mo-H) ppm. 1H NMR (213 K): δ
5.83 (s, 5H, Cp), 5.73 (s, 5H, Cp), 2.69 (s, br, 2H, PCH2), 2.19 (s,
br, 2H, PCH2), 1.99 (s, br, 4H, PCH2), 1.31 (m, br, 6H, CH3), 0.65
(m, br, 6H, CH3), -1.47 (t, JHP ) 45, 1H, Mo-H) ppm. 13C{1H}
NMR (213 K): δ 259.8 (t, JCP ) 7, CO), 94.5 (s, Cp), 92.5 (s,
Cp), 37.4 (m, AA′X, |JCP + JCP′| ) 28, 2 × PCH2), 31.9 (m, AA′X,
2 × PCH2), 13.4 (s, 2 × CH3), 12.9 (s, 2 × CH3) ppm.
Preparation of [W2Cp2(µ-COMe)(µ-PPh2)2]BF4 (5). A dichlo-
romethane solution (10 mL) of compound 1 (0.100 g, 0.112 mmol)
was stirred with an excess of [Me3O]BF4 (ca. 0.050 g, 0.34 mmol)
for 2 h to give a brown solution. This solution was filtered, and
then petroleum ether (15 mL) was added to this filtrate. Removal
of the solvents under vacuum and washing of the residue with
petroleum ether gave compound 5 as a brown powder (0.103 g,
92%). Anal. Calcd for C36H33BF4OP2W2: C, 43.32; H, 3.33.
Found: C, 43.25; H, 3.39. IR ν(Nujol): 1281 (s, C-OMe), 1051
Preparation of Solutions of [Mo2Cp2(µ-COH)(µ-COMe)(µ-
PCy2)]BF4 (10). Compound 3 (0.030 g, 0.05 mmol) and HBF4‚
OEt2 (8 µL of a 54% Et2O solution, 0.06 mmol) were stirred in
CH2Cl2 (5 mL) or CD2Cl2 (0.6 mL) at 233 K for 2 min to give
yellow solutions shown (by NMR) to contain essentially pure
compound 10. These solutions decomposed progressively upon
storage or manipulation above ca. 253 K to give compound 12 and
then other yet uncharacterized products. Spectroscopic data for
compound 10: 1H NMR (233 K): δ 13.16 (s, br, 1H, COH), 6.18
(s, 10H, Cp), 3.94 (s, 3H, OMe), 2.2-0.3 (m, 22H, Cy) ppm. 13C-
{1H} NMR (233 K): δ 365.8 (d, br, JCP ) 13, µ-COH), 363.0 (d,
1
(s, br, B-F) cm-1. H NMR: δ 8.0-7.1 (m, 20H, Ph), 6.30 (s,
10H, Cp), 4.06 (s, 3H, OCH3) ppm.
Preparation of [Mo2Cp2(µ-COMe)(µ-PEt2)2]BF4 (6). The
procedure is identical to that described for 5, but using compound
2 (0.300 g, 0.568 mmol) instead. This gives compound 6 (0.347 g,
97%) as a brown powder. Anal. Calcd for C20H33BF4Mo2OP2: C,
38.12; H, 5.28. Found: C, 38.23; H, 5.40. IR ν(Nujol): 1271 (s,
1
C-O), 1045 (s, br, B-F) cm-1. H NMR: δ 6.01 (s, 10H, Cp),
3.85 (s, 3H, OCH3), 2.17 (m, 4H, PCH2), 2.00 (m, 4H, PCH2),
1.10, 0.34 (2 × dt, JHP ) 18, JHH ) 8, 2 × 6H, CH3) ppm. 13C-
{1H} NMR: δ 367.9 (t, JCP ) 14, µ-COMe), 93.8 (s, Cp), 68.6 (s,
JCP ) 13, µ-COMe), 97.1 (s, Cp), 68.9 (s, OCH3), 40.1 [d, JCP
)
18, C1(Cy)], 39.0 [d, JCP ) 18, C1(Cy)], 33.4 [s, 4 × C2(Cy)], 26.98
[d, JCP ) 12, 2 × C3(Cy)], 26.95 [d, JCP ) 12, 2 × C3(Cy)], 25.79,
25.74 [2 × s, 2 × C4(Cy)] ppm. Spectroscopic data for compound
OCH3), 36.5 (m, AA′X, 2 × PCH2), 31.2 (false t, AA′X, |JCP
+
1
12: ν(CH2Cl2) 1873 (C-O) cm-1. H NMR (243 K): δ 6.00 (s,
JCP′| ) 9, 2 × PCH2), 13.0 (s, 2 × CH3), 10.5 (s, 2 × CH3) ppm.
Preparation of [Mo2Cp2(µ-COMe)2(µ-PCy2)]BF4 (7). The
procedure is identical to that described for 5, but using compound
3 (0.100 g, 0.169 mmol) instead. This gives compound 7 (0.107 g,
92%) as a brown powder. Anal. Calcd for C26H38BF4Mo2O2P‚CH2-
Cl2: C, 41.72; H, 5.18. Found: C, 41.68; H, 5.25. IR ν(Nujol):
1308 (w, C-Oasym), 1273 (s, C-Osym) cm-1. 1H NMR: δ 6.22 (s,
10H, Cp), 4.00 (s, 6H, OCH3), 2.1-0.4 (m, 22H, Cy) ppm. 13C-
{1H} NMR: δ 366.0 (d, JCP ) 13, µ-COMe), 97.1 (s, Cp), 69.4 (s,
5H, Cp), 5.91 (s, 5H, Cp), 4.16 (s, 3H, OCH3), 2.1-0.4 (m, 22H,
Cy), -1.40 (d, JHP ) 32, 1H, Mo-H) ppm. 31P{1H} NMR (243
K): δ 294.5 (s) ppm.
Computational Details. All computations were carried out using
the GAUSSIAN03 package,42 in which the hybrid method B3LYP
was applied with the Becke three-parameter exchange functional43
and the Lee-Yang-Parr correlation functional.44 Effective core
potentials (ECP) and their associated double-ú LANL2DZ basis
set were used for the molybdenum and phosphorus atoms,45
OMe), 40.5 [d, JCP ) 17, C1(Cy)], 33.5 [s, C2(Cy)], 27.1 [d, JCP
)
13, C3(Cy)], 25.9 [s, C4(Cy)] ppm.
Preparation of [Mo2Cp2(µ-COEt)(µ-COMe)(µ-PCy2)]BF4 (8).
The procedure is identical to that described for 5, but using
compound 4 (0.100 g, 0.165 mmol) instead. This gives compound
8 as a brownish-yellow powder (0.109 g, 94%). Anal. Calcd for
C27H40BF4Mo2O2P: C, 45.92; H, 5.71. Found: C, 45.99; H, 5.61.
1H NMR: δ 6.21 (s, 10H, Cp), 4.20 (q, JHH ) 7, 2H, OCH2), 4.00
(s, 3H, OCH3), 1.43 (t, JHH ) 7, 3H, CH3), 2.1-0.4 (m, 22H, Cy)
ppm. 13C{1H} NMR: δ 365.7 (d, JCP ) 13, µ-COR), 363.5 (d, JCP
) 13, µ-COR), 96.9 (s, Cp), 80.4 (s, OCH2), 69.2 (s, OCH3), 40.3
[d, JCP ) 18, C1(Cy)], 33.2 [s, C2(Cy)], 26.8 [d, JCP ) 15, C3-
(Cy)], 25.7 [s, C4(Cy)], 14.6 (s, CH3) ppm.
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M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
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Synthesis, Structure, and Bonding of 30-Electron Complexes
Organometallics, Vol. 26, No. 20, 2007 4941
supplemented by an extra d-polarization function in the case of
P.46 The light elements (O, C, and H) were described with the
6-31G* basis.47 Geometry optimizations were performed under no
symmetry restrictions, using initial coordinates derived from X-ray
data of the same or comparable complexes, and frequency analyses
were performed to ensure that a minimum structure with no
imaginary frequencies was achieved in each case. In the case of
complexes 6 and 7, the geometry optimization converged with one
negative vibrational frequency of low value (ca. 10i cm-1), which
was assigned to the internal rotation of one of the cyclopentadienyl
ligands. At this point we did not follow the imaginary eigenvector
in search of a stationary point with no imaginary vibrational
frequencies. In all other cases, imaginary frequencies were absent.
For interpretation purposes, natural population analysis (NPA)
charges28b were derived from the natural bond order (NBO) analysis
of the data.28b Molecular orbitals and vibrational modes were
visualized using the Molekel program.48 The topological analysis
of the electron density F was carried out with the Xaim routine.49
Acknowledgment. We thank the MEC of Spain for a grant
(to D.G.) and financial support (Projects BQU2003-05471 and
CTQ2005-08123-C02-02BQU). The computing resources at the
Centre de Supercomputacio´ de Catalunya (CESCA) were made
available to us through a grant from Fundacio´ Catalana per a la
Recerca (FCR) and Universitat de Barcelona.
Supporting Information Available: Mulliken and NPA charges,
molecular orbitals, and data from the AIM topological analysis for
complexes 2, 3, A, 6, and 7. This material is available free of charge
via the Internet at http://pubs.acs.org.
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