Chemistry of Dimetallaboranes Derived from the Reaction of [Cp*MCl2]2 with Monoboranes (M = Ru, Rh; Cp* = η5-C5Me5)

Article abstract of DOI:10.1021/ja982954j

In a single step, from [Cp*RuCl₂]₂ (Cp* = η⁵-C₅Me₅) and Li[BH₄], nido-1,2-(Cp*Ru)₂(μ-H)₂B₃H ₇, 1, is produced in high yield. Addition of BH₃·THF to 1 results in conversion to nido-1,2-(Cp*Ru)₂(μ-H)B₄H₉, 2. Reaction of BH₃·THF directly with [Cp*RuCl₂]₂ yields a mixture of 1 and 2. In two steps, a rhodium analogue, nido-2,3-(Cp*Rh)₂B₃H₇, 9, is accessible by the reaction of [Cp*RhCl₂]₂ and Li[BH₄] to exclusively produce (Cp*Rh)₂B₂H₆, 8, which adds BH₃·THF to give 9 as the major product in a mixture. Reaction of BH₃·THF directly with [Cp*RhCl₂]₂ yields the chloro derivative of 9, nido-1-Cl-2,3-(Cp*Rh)₂B₃H₆, 11, in high yield via the intermediate positional isomer, nido-3-Cl-1,2-(Cp*Rh)₂B₃H₆, 10. With high concentrations of Co₂(CO)₈, 1 reacts with Co₂(CO)₈ to give nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO)₂(μ ₃-CO)B₃H₆,3, whereas low concentrations permit competitive degradation of 1 to yield arachno-(Cp*Ru)(CO)(μ-H)B₃H₇, 4. On the other hand, reaction of 11 with Co₂(CO)₈ gives closo-1-Cl-6-{Co(CO)₂}-2,3-(Cp*Rh)₂(μ ₃-CO)B₃H₃, 12. Mild thermolysis of 3 results in loss of hydrogen and the formation of closo-6-Co(CO)₂-2,3-(Cp*Ru)₂(μ-CO)(μ ₃-CO)B₃H₄, 5, whereas thermolysis of 2 results in loss of hydrogen and formation of pileo-2,3-(Cp*Ru)₂B₄H₈, 6, with a BH-capped square pyramidal structure. Finally, 6 reacts with Fe₂(CO)₉ to yield pileo-6-Fe(CO)₃-2,3-(Cp*Ru)₂(μ ₃-CO)B₄H₄, 7, with a BH-capped octahedral cluster structure. The overall isolated yield of 7, formed in four steps from [Cp*RuCl₂]₂, is ≈50% and evidences good control of reactivity.

Full text of DOI:10.1021/ja982954j

J. Am. Chem. Soc. 1999, 121, 1275-1287
1275
Chemistry of Dimetallaboranes Derived from the Reaction of
[Cp*MCl2]2 with Monoboranes (M ) Ru, Rh; Cp* ) η5-C5Me5)
Xinjian Lei, Maoyu Shang, and Thomas P. Fehlner*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556-5670
ReceiVed August 17, 1998
Abstract: In a single step, from [Cp*RuCl2]2 (Cp* ) η5-C5Me5) and Li[BH4], nido-1,2-(Cp*Ru)2(µ-H)2B3H7,
1, is produced in high yield. Addition of BH3‚THF to 1 results in conversion to nido-1,2-(Cp*Ru)2(µ-H)B4H9,
2. Reaction of BH3‚THF directly with [Cp*RuCl2]2 yields a mixture of 1 and 2. In two steps, a rhodium
analogue, nido-2,3-(Cp*Rh)2B3H7, 9, is accessible by the reaction of [Cp*RhCl2]2 and Li[BH4] to exclusively
produce (Cp*Rh)2B2H6, 8, which adds BH3‚THF to give 9 as the major product in a mixture. Reaction of
BH3‚THF directly with [Cp*RhCl2]2 yields the chloro derivative of 9, nido-1-Cl-2,3-(Cp*Rh)2B3H6, 11, in
high yield via the intermediate positional isomer, nido-3-Cl-1,2-(Cp*Rh)2B3H6, 10. With high concentrations
of Co2(CO)8, 1 reacts with Co2(CO)8 to give nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO)2(µ3-CO)B3H6, 3, whereas
low concentrations permit competitive degradation of 1 to yield arachno-(Cp*Ru)(CO)(µ-H)B3H7, 4. On the
other hand, reaction of 11 with Co2(CO)8 gives closo-1-Cl-6-{Co(CO)2}-2,3-(Cp*Rh)2(µ3-CO)B3H3, 12. Mild
thermolysis of 3 results in loss of hydrogen and the formation of closo-6-Co(CO)2-2,3-(Cp*Ru)2(µ-CO)(µ3-
CO)B3H4, 5, whereas thermolysis of 2 results in loss of hydrogen and formation of pileo-2,3-(Cp*Ru)2B4H8,
6, with a BH-capped square pyramidal structure. Finally, 6 reacts with Fe2(CO)9 to yield pileo-6-Fe(CO)3-
2,3-(Cp*Ru)2(µ3-CO)B4H4, 7, with a BH-capped octahedral cluster structure. The overall isolated yield of 7,
formed in four steps from [Cp*RuCl2]2, is ≈50% and evidences good control of reactivity.
Introduction
and these have been particularly successful for monometallic
compounds.11-17 There are two deficiencies in this approach:
activated polyboranes are required and polymetallic species are
difficult to make selectively. The former requires the synthesis
or acquisition of the appropriate polyborane, and the low yields
and time-consuming separations associated with the latter stymie
the development of derivative chemistry. Both inhibit consid-
eration of potential applications.
In recent years we have been exploring the reactions of
monocyclopentadienylmetal chloride dimers with commercially
available monoboranes as a general route to dimetallaboranes.
Full descriptions of the Co,18 Cr,19 and Mo20 systems have been
reported to date. The metal dimer serves as a scaffold upon
which a polyborane fragment assembles under mild conditions.
Under kinetic control, the monoborane activates the metal dimer
by removing Cl (as Cl- for [BH4]- or, with the exception of
Cr, BH2Cl for BH3‚THF) and high yields of single compounds
are observed. Thus, in a single step both M-B and B-B bonds
are formed with high selectivity. Others have used monoboranes
Metallaborane chemistry echoes organometallic chemistry in
that borane and transition metal fragments are mutually stabi-
lized through their bonding interactions.1 Just as with the
hydrocarbon and transition metal fragments of an organometallic
compound,2 the mutual perturbation of electronic structure and
properties promotes (or inhibits) reactions of the borane fragment
and, likewise, the metal fragment. At the level of electron-
counting rules,3,4 the principles governing geometric structure
of metallaboranes are verified in numerous examples5-9 and
our level of understanding approaches that of structural orga-
nometallic chemistry. The same cannot be said for synthetic
metallaborane chemistry in that the proven methods for the
formation of compounds containing metal-boron bonds are
more limited than those for metal-carbon bonds.7,10 Hence,
much of the richness metal variability adds to organometallic
chemistry is still lacking in metallaborane chemistry.
Previous synthetic approaches have emphasized the reactions
of polyboranes, often anionic, with metal fragment precursors,
(11) Greenwood, N. N.; Ward, I. M. Chem. Soc. ReV. 1974, 3, 231.
(12) Wegner, P. A. In Boron Hydride Chemistry; Muetterties, E. L., Ed.;
Academic Press: New York, 1975; p 431.
(1) Inorganometallic Chemistry; Fehlner, T. P., Ed.; Plenum: New York,
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(2) Elschenbroich, C.; Salzer, A. Organometallics; VCH: New York,
1989.
(3) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1.
(4) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry;
Prentice Hall: New York, 1990.
(5) Kennedy, J. D. Prog. Inorg. Chem. 1984, 32, 519.
(6) Kennedy, J. D. Prog. Inorg. Chem. 1986, 34, 211.
(7) Housecroft, C. E. Boranes and Metalloboranes; Ellis Horwood:
Chichester, U.K., 1990.
(8) Housecroft, C. E. AdV. Organomet. Chem. 1991, 33, 1.
(9) Housecroft, C. E. Cluster Molecules of the p-Block Elements; Oxford
University Press: Oxford, U.K., 1994.
(13) Shore, S. G. Pure Appl. Chem. 1977, 49, 717.
(14) Grimes, R. N. Acc. Chem. Res. 1978, 11, 420.
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Eds.; Pergamon: Oxford, U.K., 1980; p 73.
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21, 57.
(17) Grimes, R. N. In Metal Interactions with Boron Clusters; Grimes,
R. N., Ed.; Plenum: New York, 1982; p 269.
(18) Nishihara, Y.; Deck, K. J.; Shang, M.; Fehlner, T. P.; Haggerty, B.
S.; Rheingold, A. L. Organometallics 1994, 13, 4510.
(19) Ho, J.; Deck, K. J.; Nishihara, Y.; Shang, M.; Fehlner, T. P. J. Am.
Chem. Soc. 1995, 117, 10292.
(10) Woollins, J. D. Non-Metal Rings, Cages and Clusters; Wiley: New
York, 1988.
(20) Aldridge, S.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1998,
120, 2586.
10.1021/ja982954j CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/27/1999

1276 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
to good effect in the formation of metallaboranes. Particularly
noteworthy are the dimetallaboranes of Ta21 and Nb,22 the
metallaborane analogues of mononuclear ethylene complexes,23-27
the formation of a chloroborohydride complex,28 the chemistry
of a triosmaborane,29-32 and the metallaborane chemistry
associated with certain Mo- and W-based organometallic
compounds.27,33-35
answers to these questions would serve to forge another
connection between metallaborane chemistry and organometallic
chemistry.
Experimental Section
Syntheses. (a) General Information. All operations were conducted
under argon atmosphere using standard Schlenk techniques. Solvents
were distilled before use under N2 as follows: sodium benzophenone
ketyl for hexane, diethyl ether, and tetrahydrofuran. BH3‚THF (1.0 M
in THF), LiBH4(2.0 M in THF), [Cp*RhCl2]2, and Fe2(CO)9 were used
as received from Aldrich. Likewise [Cp*RuCl2]2 and Co2(CO)8 (Strem)
were used as received. Note that although Cp*RuCl2 is described as a
polymer in the solid state, it is thought to exist as a dimer in solution.46
Silica gel (60-200 mesh) was purchased from J. T. Baker Inc. and
predried at 140 °C before use. Chromatography was carried out on 3
cm of silica gel in a 2.5 cm diameter column. NMR spectra were
recorded on a 300 or 500 MHz Varian FT-NMR spectrometer. Residual
protons of solvent were used as reference (δ, ppm, benzene, 7.15) while
a sealed tube containing [(Me4N)(B3H8)] in acetone-d6 (δΒ, ppm, -29.7)
was used as the external reference for the 11B NMR. Infrared spectra
were obtained on a Nicolet 205 FT-IR spectrometer. Mass spectra (m/
z) were obtained on a JEOL JMS-AX505HA mass spectrometer using
the EI ionization mode or the FAB mode in a 3-nitrobenzyl alcohol
matrix. Perfluorokersene was used as the standard for the high-resolution
EI mass spectra. Elemental analysis was performed by M-H-W
Laboratories, Phoenix, AZ.
Herein we describe the application of our approach to dimetal
ruthena- and rhodaboranes.36 A large number of metallaboranes
containing one or more group 8 M(CO)3 fragments have been
characterized,5-7,37but there are few crystallographically char-
acterized examples containing simple group 8 CpM fragments,
Cp ) η5-C5R5.24,33,38-41 Likewise, a number of mononuclear
rhodaboranes are known,42,43 but dinuclear rhodaboranes are
virtually unstudied.44
The present work was initiated with three questions in mind.
The first concerned synthesis. Would a route based on the
reactions of monoboranes with monocyclopentadienylmetal
chorides provide useful syntheses of metallaboranes containing
diruthenium and dirhodium fragments? The second involved
structure. Fragments such as M(CO)3 (M ) Fe, Ru, Os, CpM;
M ) Co, Rh) which are three-orbital, two-electron species,
readily participate in cluster bonding.14,45 As the CpRu fragment
has one electron less than the CpRh fragment, what structural
features would compensate in related dimetallaboranes contain-
ing the two metals? The third, and most important one,
concerned reactivity. Given the characteristic compositional and
structural features of the Rh and Ru metallaboranes, what
similarities and differences in reactivity would result? Clear
(b) nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1. In a typical reaction,
[Cp*RuCl2]2 (0.40 g, 0.64 mmol) was placed in a 100 mL Schlenk
tube with 30 mL of freshly distilled THF to generate a red suspension.
The mixture was chilled to -40 °C, and LiBH4 (1.6 mL, 3.2 mmol)
was added slowly by syringe. The reaction mixture was allowed to
warm slowly to room temperature. The red slurry became a red solution
within 10 min, along with release of gas. After 1 h of stirring, the
THF was removed under vacuum, and the resulting residue was
extracted with hexane. Column chromatography was applied, and
elution with hexane gave a yellow band from which 0.27 g of 1 was
isolated (82% based on the Ru). MS(FAB): P+ ) 516, 3 B, 2 Ru atoms;
calcd for weighted average of isotopomers lying within the instrument
resolution, 516.1418; obsd, 516.1404. NMR: 11B (hexane, 22 °C), δ
-0.5 (d, JB-H ) 120 Hz, {1H}, s, 1B), -2.5 (dd, JB-H ) 100, 60 Hz,
(21) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1989, 111, 3449.
(22) Brunner, H.; Gehart, G.; Meier, W.; Wachter, J.; Wrackmeyer, B.
J. Organomet. Chem. 1992, 436, 313.
(23) Medford, G.; Shore, S. G. J. Am. Chem. Soc. 1978, 100, 3953.
(24) Plotkin, J. S.; Shore, S. G. J. Organomet. Chem. 1979, 182, C15.
(25) Coffy, T. J.; Medford, G.; Plotkin, J.; Long, G. J.; Huffman, J. C.;
Shore, S. G. Organometallics 1989, 8, 2404.
(26) Coffy, T. J.; Shore, S. G. J. Organomet. Chem. 1990, 394.
(27) Grebenik, P. D.; Leach, J. B.; Green, M. L. H.; Walker, N. M. J.
Organomet. Chem. 1988, 345, C31.
(28) Kawano, Y.; Shimoi, M. Chem. Lett. 1998, 935.
(29) Jan, D.-Y.; Hsu, L.-Y.; Workman, D. P.; Shore, S. G. Organome-
tallics 1987, 6, 1984.
(30) Krause, J. K.; Jan, D.-Y.; Shore, S. G. J. Am. Chem. Soc. 1987,
109, 4416.
(31) Jan, D.-Y.; Workman, D. P.; Hsu, L.-Y.; Krause, J. A.; Shore, S.
G. Inorg. Chem. 1992, 31, 5123.
(32) Chung, J.-H.; Boyd, E. P.; Liu, J.; Shore, S. G. Inorg. Chem. 1997,
36, 4778.
(33) Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Leach, J. B.;
Mountford, P.; Stringer, G.; Walker, N. M.; Wong, L.-L. J. Chem. Soc.,
Chem. Commun. 1988, 799.
(34) Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Leach, J. B.;
Mountford, P. J. Chem. Soc., Chem. Commun. 1989, 1387.
(35) Bullick, H. J.; Grenbenik, P. D.; Green, M. L. H.; Hughes, A. K.;
Leach, J. B.; McGowan, P. C. J. Chem. Soc., Dalton Trans. 1995, 67.
(36) Lei, X.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1998, 120,
2686.
(37) Housecroft, C. E. In Inorganometallic Chemistry; Fehlner, T. P.,
Ed.; Plenum Press: New York, 1992; p 73.
(38) Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Mountford, P.;
Leach, J. B. New J. Chem. 1992, 16, 19.
1
{1H}, s, 2B); H (C6D6, 22 °C), 3.28 (partially collapsed quartet, pcq,
1H, B-Ht), 2.66 (pcq, 2H, B-Ht), 1.93 (s, 15H, C5Me5), 1.79 (s, 15H,
C5Me5), -4.05 (s, br, 2H, B-H-B), -11.25 (pcq, 2H, B-H-Ru),
-13.55 (s, 2H, Ru-H-Ru). IR (hexane, cm-1): 2498 m, 2450 m, 2426
w (B-Ht). Anal. Calcd for C20H39B3Ru2: C, 46.73; H, 7.65. Found:
C, 46.65; H, 7.79.
(c) nido-1,2-(Cp*Ru)2(µ-H)B4H9, 2. Two equivalents of BH3‚THF
(0.38 mL, 0.38 mmol) was added to 1 (0.10 g, 0.19 mmol) in 10 mL
of THF, and the solution was stirred at 60 °C for 10 h. Filtration and
removal of the THF afforded 0.096 g of orange crystals of 2 (95%
based on the Ru). MS(FAB): P+ ) 528, 4 B, 2 Ru atoms; calcd for
weighted average of isotopomers lying within the instrument resolution,
528.1589; obsd, 528.1602. NMR: 11B (hexane, 22 °C), δ 17.1 (dd,
1
JB-H ) 110 Hz, 56 Hz,{1H}, s), 3.9 (d, JB-H ) 120 Hz, {1H}, s); H
(C6D6, 22 °C), 3.36 (pcq, 2H, B-Ht), 2.70 (pcq, 2H, B-Ht), 1.85 (s,
15H, C5Me5), 1.81 (s, 15H, C5Me5), -3.62 (s,br, 1H, B-H-B), -3.93
(s, br, 2H, B-H-B), -13.39 (br, 2H, B-H-Ru), -13.50 (s, 1H, Ru-
H-Ru). IR (hexane, cm-1): 2500 m, 2434 m. Anal. Calcd for C20H40B4-
Ru2: C, 45.68; H, 7.67. Found: C, 45.84; H, 7.44.
(d) nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO)2(µ3-CO)B3H6, 3. 1
(0.13 g, 0.25 mmol) and Co2(CO)8 (0.13 g, 0.38 mmol) in 10 mL of
hexane were stirred for 15 h at room temperature to give a greenish
brown solution. A 11B NMR spectrum of the resulting solution showed
a major boron-containing product plus small amounts of other boron-
contaning species. Column chromatography with hexane removed a
dark brown band (Co4(CO)12) and a bright orange band ([Cp*Ru-
(CO)2]2).47 Elution with ether then afforded a greenish brown solution
(39) Suzuki, H.; Takaya, Y.; Tada, K.; Kakigano, T.; Igarashi, M.;
Tanaka, M. In Symposium on Organometallic ChemistrysAbstracts; Kinki
Chemical Society: Tokyo, 1992; p 154.
(40) Weiss, R.; Grimes, R. N. J. Am. Chem. Soc. 1977, 99, 8087.
(41) Weiss, R.; Grimes, R. N. Inorg. Chem. 1979, 18, 3291.
(42) Boocock, S. K.; Toft, M. A.; Inkrott, K. E.; Hsu, L.-Y.; Huffman,
J. C.; Folting, K.; Shore, S. G. Inorg. Chem. 1984, 23, 3084.
(43) Fontaine, X. L. R.; Fowkes, H.; Greenwood, N. N.; Kennedy, J.
D.; Thornton-Pett, M. J. Chem. Soc., Dalton Trans. 1987, 2417.
(44) Fontaine, X. L. R.; Greenwood, N. N.; Kennedy, J. D.; MacKinnon,
P. I.; Thornton-Pett, M. Chem. Commun. 1986, 1111.
(45) Fehlner, T. P. New J. Chem. 1988, 12, 307.
(46) Ko¨lle, U.; Kossakowski, J. J. Organomet. Chem. 1989, 362, 383.
(47) King, R. B.; Iqbal, M. Z.; King, A. D. J. Organomet. Chem. 1979,
171, 53.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1277
1
which gave 0.10 g of 3 (61% based on Ru). MS(EI): P+ ) 684, 3 B,
2 Ru atoms; fragment peaks corresponding to sequential loss of 4 CO;
calcd for weighted average of isotopomers lying within the instrument
resolution, 684.0312; obsd, 684.0339. NMR: 11B (hexane, 22 °C), δ
65.9 (d, JB-H ) 154 Hz, {1H}, s, 1B), 20.0 (dd, JB-H ) 60, 130 Hz,
{1H}, s, 1B); H (C6D6, 22 °C), δ 8.52 (pcq, 2H, B-Ht), 6.52 (pcq,
1H, B-Ht), 3.11 (pcq, 1H, B-Ht), 1.64 (s, 30H, C5Me5). IR (KBr,
cm-1): 2576 w, 2474 w, sh, 2454 w (B-H); 2006 vs, 1952 s, 1927 s,
1744 s (CO). IR (hexane, cm-1): 2566 w, 2495 w (B-H); 2020 vs,
1971 s, 1955 s, 1731 s (CO). Anal. Calcd for C24H34O4B4FeRu2: C,
41.91; H, 4.98. Found: C, 42.16; H, 5.13.
1
{1H}, s, 1B), 2.0 (d, JB-H ) 150 Hz, {1H}, s, 1B); H (C6D6, 22 °C),
δ 6.20 (pcq, 1H, B-Ht), 3.80 (pcq, 1H, B-Ht), 2.89 (pcq, 1H, B-Ht),
1.58 (s, 15H, C5Me5), 1.50 (s, 15H, C5Me5), 0.84 (br, 1H, B-H-M),
-2.37 (br, 1H, B-H-B), -4.89 (br, 1H, B-H-M); 13C{1H} (Tol-d8,
22 °C), δ 212.6 (CO), 102.9 (C5Me5), 102.9 (C5Me5), 9.6 (C5Me5), 9.5
(C5Me5). IR (KBr, cm-1): 2511 w, 2443 w (B-H); 2010 vs, 1961 s,
1911 s, 1897 s, sh, 1679 s (CO). Anal. Calcd for C24H36O4B3CoRu2:
C, 42.26; H, 5.32. Found: C, 42.40; H, 5.67.
(i) (Cp*Rh)2B2H6, 8. [Cp*RhCl2]2 (0.11 g, 0.17 mmol) was placed
in a 100 mL Schlenk tube, and 15 mL of freshly distilled THF was
added to generate an orange suspension. The mixture was chilled to
-40 °C, and LiBH4 (0.43 mL, 0.86 mmol) was added slowly by syringe.
The reaction mixture was allowed to warm slowly up to room
temperature. The orange slurry became a red-brown solution within
10 min. After 1 h of stirring, the THF was removed under vacuum,
and the resulting residue was extracted with hexane. The solution was
(e) arachno-(Cp*Ru)(CO)(µ-H)B3H7, 4. 1 (0.10 g, 0.19 mmol) and
Co2(CO)8 (0.06 g, 0.18 mmol) were placed in a 100 mL Schlenk tube.
A 20 mL portion of freshly distilled hexane was added, and the reaction
mixture was stirred for 10 h at room temperature. Column chroma-
tography was performed. Elution with hexane (dark brown solution)
followed by ether afforded 3. The dark brown solution was kept at
-40 °C overnight. Black crystals (Co4(CO)12) formed and were removed
by filtration. The solution was then concentrated and again kept at -40
°C overnight. The procedure was repeated until the solution IR indicated
the absence of Co4(CO)12. The resulting light brown solution was dried
to give 0.015 g of 4 (26% based on Ru). MS(EI): P+ ) 306, 3 B, 1
Ru atoms; fragment peak corresponding to loss of 1 CO; calcd for
weighted average of isotopomers lying within the instrument resolution,
306.1087; obsd, 306.1057. NMR: 11B (hexane, 22 °C), δ 0.8 (d, t,
JB-H ) 140, 40 Hz, {1H}, s, 1B), -2.8 (t, JB-H ) 120 Hz,{1H}, s,
1B), -11.7 (t, JB-H ) 110 Hz, {1H}, s, 1B); 1H (C6D6, 22 °C), δ 3.18
(pcq, 2H, B-Ht), 2.49 (overlaping pcq, 2H, B-Ht), 2.28 (overlaping
pcq, 1H, B-Ht), 1.44 (s, 15H, C5Me5), -3.23 (br, 1H, B-H-M), -4.09
(br, 1H, B-H-B), -11.79 (br, 1H, B-H-M). IR (C6D6, cm-1): 2521
w, 2482 w, 2435 w, 2422 w (B-H); 1990 vs (CO).
(f) closo-6-Co(CO)2-2,3-(Cp*Ru)2(µ-CO)(µ3-CO)B3H4, 5. 3 (0.05
g, 0.07 mmol) was dissolved in 10 mL of freshly distilled toluene.
The resulting green brown solution was stirred for 30 min at 80 °C.
Column chromatography with hexane gave an orange solution contain-
ing [Cp*Ru(CO)2]2. Elution with ether afforded a red-brown solution,
which was dried to give 0.03 g of red microcrystals of 5 (66%). Note
that longer heating time resulted in decomposition of 5 to [Cp*Ru-
(CO)2]2. MS (EI): P+ ) 682, 3 B, 2 Ru atoms; fragment peaks
corresponding to sequential loss of 4 CO; calcd for weighted average
of isotopomers lying within the instrument resolution, 682.0192; obsd,
682.0133. NMR: 11B (toluene, 22 °C), δ 82.4 (d, JB-H ) 130 Hz, {1H},
s, 1B), 49.1 (d, JB-H )132 Hz, {1H}, s, 2B); 1H (C6D6, 22 °C), δ 8.50
(pcq, 1H, B-Ht), 6.30 (pcq, 2H, B-Ht), 1.56 (s, 30H, C5Me5), -10.25
(br, 1H, Co-H). IR (C6D6, cm-1): 2495 w, 2490 w (B-H); 2035 vs,
1991 s, 1825 s, 1755 s (CO).
dried to afford 0.07 g of dark brown 8 (82% based on Rh). MS(EI):
12
P+ ) 504, 2 B, 2 Rh atoms; calcd for
C
1H3611B2103Rh2, 504.1121;
20
obsd, 504.1119. NMR: 11B (hexane, 22 °C), δ -2.2 (d, JB-H ) 150
1
Hz, {1H}, s); H (C6D6, 22 °C), 9.20 (pcq, 2H, B-Ht), 1.92 (s, 30H,
C5Me5), -15.64 (s, 4H, B-H-Rh).
(j) nido-2,3-{Cp*Rh}2B3H7, 9. To a solution of 8 (0.08 g, 0.16
mmol) was added 2 equiv of BH3‚THF (0.32 mL, 0.32 mmol) at -40
°C. After the mixture was stirred at room temperature for 1 h and then
at 60 °C for 20 h, the solvent was removed under vacuum. Hexane
extraction, filtration, and column chromatography at -40 °C afforded
a dark red solution. Crystallization at -40 °C gave 0.03 g of dark red
crystals of 9 (36%). MS(EI): P+ ) 516, 3 B, 2 Rh atoms; calcd for
12
C
20
1H3711B3103Rh2, 516.1295; obsd, 516.1304. NMR: 11B (hexane, 22
°C), δ 43.9 (d, JB-H ) 150 Hz, {1H}, 1B), 14.9 (d, JB-H ) 130 Hz,
{1H}, s, 2B); H (C6D6, 22 °C), δ 5.34 (pcq, 1H, B-Ht), 4.86 (pcq,
1
2H, B-Ht), 1.83 (s, 30H, C5Me5), -2.04 (br, 1H, B-H-B,), -12.09
(br, 2H, Rh-H-B), -16.58 (t, JRh-H )25 Hz, 1H, Rh-H-Rh). IR
(hexane, cm-1): 2497 m, 2455 w (B-Ht). Anal. Calcd for C20H37B3-
Rh2: C, 46.58; H, 7.23. Found: C, 46.83; H, 7.40.
(k) nido-3-Cl-2,3-{η5-C5Me5Rh}2B3H6, 10. [Cp*RhCl2]2 (0.10 g,
0.16 mmol) was placed in a 100 mL Schlenk tube, and 10 mL of freshly
distilled THF was added to generate an orange suspension. The mixture
was chilled to -40 °C, and BH3‚THF (1.12 mL, 1.12 mmol) was added
slowly by syringe. The reaction mixture was allowed to warm slowly
to room temperature. The color of the mixture changed from orange to
deep blue and then to red-orange. A homogeneous red-orange solution
was observed after a period of 30 min. After the solution was stirred
for another 2 h, THF was removed under vacuum, and the resulting
residue was extracted with hexane. Low-temperature column chroma-
tography (-20 °C) was applied. Elution with 5% ether in hexane gave
a dark red solution, which was dried to afford 0.09 g of 10 (84% based
on Rh). MS(EI): P+ ) 550, 3 B, 1 Cl, 2 Rh atoms; calcd for
12
C
20
1H3611B3103Rh235Cl, 550.0907; obsd, 550.0896. NMR: 11B (hexane,
22 °C), δ 48.0 (s, {1H}, 1B), 4.0 (d, JB-H ) 150 Hz, {1H}, s, 1B),
-5.3 (d, JB-H ) 122 Hz, {1H}, s, 1B); 1H (C6D6, 22 °C), δ 3.78 (pcq,
2H, B-Ht), 1.93 (s, 15H, C5Me5), 1.73 (s, 15H, C5Me5), -4.01 (br,
2H, B-H-B), -14.00 (br, 1H, Rh-H-B,), -14.83 (t, br, JRh-H ) 25
Hz, 1H, Rh-H-Rh). IR (hexane, cm-1): 2510 w (B-Ht).
(g) pileo-2,3-{Cp*Ru}2(µ-H)B4H7, 6. 2 (0.10 g, 0.19 mmol) was
disolved in 5 mL of freshly distilled toluene, and the solution was stirred
at 95 °C for 20 h. Filtration and removal of the toluene afforded 0.098
g of orange solid 6 (98%). MS(FAB): P+ ) 526, 4 B, 2 Ru atoms;
12
calcd for
C
1H3811B4102Ru2, 526.1433; obsd, 526.1432. NMR: 11B
(l) nido-1-Cl-2,3-{Cp*Rh}2B3H6, 11. [Cp*RhCl2]2 (0.18 g, 0.28
mmol) was loaded in a 100 mL Schlenk tube, and 20 mL of freshly
distilled THF was added to generate an orange suspension. The mixture
was chilled to -40 °C, and 1.96 mL (1.96 mmol) of BH3‚THF in THF
was added slowly by syringe. The reaction mixture was allowed to
warm slowly to room temperature. The color of the mixture changed
from orange to deep blue and then to red-orange. A homogeneous red-
orange solution was observed over a period of 30 min. The solution
was then heated to ca. 65 °C and stirred for 6 h. THF was removed
under vacuum, and the resulting residue was extracted with hexane.
Column chromatography was applied, and elution with ether gave a
red-orange solution, from which 0.14 g of 11 was isolated (91% based
20
(hexane, 22 °C), δ 122.5 (d, JB-H ) 110 Hz,{1H}, s, 1B), 21.3 (d,
JB-H ) 120 Hz, {1H}, s, 2B), -32.8 (d, JB-H ) 130 Hz, {1H}, s, 1B);
1H (C6D6, 22 °C), 10.01 (pcq, 1H, B-Ht), 5.46 (pcq, 2H, B-Ht), 1.75
(s, 15H, C5Me5), -0.27 (s, br, 1H, B-H-B), -2.37 (q, JB-H ) 130
Hz, 1H, B-Ht), -12.37 (br, 2H, B-H-Ru), -15.08 (s, 1H, Ru-H-
Ru). IR (hexane, cm-1): 2492 w, 2528 m. Anal. Calcd for C20H38B4-
Ru2: C, 45.85; H, 7.31. Found: C, 46.52; H, 7.25.
(h) pileo-6-{Fe(CO)3}-2,3-{Cp*Ru}2-(µ3-CO)B4H4, 7. 6 (0.10 g,
0.19 mmol) and an excess of Fe2(CO)9 were placed in a 100 mL Schlenk
tube. A 10 mL portion of freshly distilled hexane was added to the
mixture by syringe. The reaction mixture was stirred for 2 h at 60 °C.
Filtration through Celite and column chromatography afforded a red
solution, which was dried to give 0.09 g of red microcrystals of 7 (70%
based on Ru). MS(EI): P+ ) 690, 4 B, 2 Ru atoms; fragment peaks
corresponding to sequential loss of 4 CO; calcd for weighted average
of isotopomers lying within the instrument resolution, 690.0311; obsd,
690.0293. NMR: 11B (hexane, 22 °C), δ 98.1 (d, JB-H ) 160 Hz, {1H},
s, 1B), 79.0 (d, JB-H )132 Hz,{1H}, s, 2B), 5.2 (d, JB-H ) 125 Hz,
on Rh). MS(EI): P+ ) 550, 3 B, 1 Cl, 2 Rh atoms; calcd for
12
C
20
1H3611B3103Rh235Cl, 550.0907; obsd, 550.0896. NMR: 11B (hexane,
22 °C), δ 45.5 (s, {1H}, 1B), 11.5 (d, JB-H ) 120 Hz, {1H}, s, 2B); 1H
(C6D6, 22 °C), δ 4.62 (pcq, 2H, B-Ht), 1.74 (s, 30H, C5Me5), -2.48
(br, 1H, B-H-B,), -11.63 (br, 2H, Rh-H-B), -15.57 (t, JRh-H
)
25 Hz, 1H, Rh-H-Rh). IR (hexane, cm-1): 2510 w (B-Ht). Anal.
Calcd for C20H36B3Rh2Cl: C, 43.66; H, 6.60. Found: C, 44.92; H, 6.59.

1278 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
Table 1. Crystallographic Data and Structure Refinement Details for Compounds 1, 2, 3, and 6
1
2
3
6
empirical formula
formula weight
crystal system
space group
a (Å)
C20H39B3Ru2
514.08
monoclinic
C2/c
11.372(2)
14.061(3)
15.233(3)
90
103.12(3)
90
2372.1(8)
4
1.439
1048
C20H40B4Ru2
525.90
tetragonal
I41/a
34.003(3)
34.003(3)
8.3206(11)
90
C24H36B3CoO4Ru2
682.03
monoclinic
Cc
8.532(3)
38.001(4)
9.327(3)
90
116.68(3)
90
2702.1(13)
4
1.677
1368
0.710 73
1.738
0.35 × 0.25 × 0.04
2.14-25.00
4728
C20H38B4Ru2
523.88
triclinic
P1h
8.743(2)
8.901(2)
16.019(3)
105.446(12)
103.633(14)
90.769(13)
1163.9(3)
2
1.495
532
0.710 73
1.299
0.25 × 0.22 × 0.12
2.38-24.98
4385
b (Å)
c (Å)
R (deg)
â (deg)
90
90
γ (deg)
V (Å3)
9621(2)
Z
16
Dc (g cm-3
F(000)
)
1.452
4288
wavelength (Mo KR (Å))
0.710 73
1.274
0.28 × 0.22 × 0.12
2.34-24.98
2190
0.710 73
µ (mm-1
)
1.258
crystal size (mm)
0.40 × 0.23 × 0.20
2.40-24.97
4372
θ range (deg)
no. of total reflns collected
no. of unique reflns
no of unique reflns [I > 2σ(I)]
linear decay correction
abs. correction
max and min transmission
refinement method
weighting scheme
2085 [R(int) ) 0.1007]
4211 [R(int) ) 0.0134]
4728 [R(int) ) 0.0000]
4095 [R(int) ) 0.0127]
1783
no
3643
no
4659
no
3735
no
ψ scans
ψ scans
DIFABS
ψ scans
1.0000 and 0.7844
full-matrix on F2
σ weight
1.0000 and 0.8977
full-matrix on F2
σ weight
4195/16/270
1.067
1.0000 and 0.4043
full-matrix on F2
σ weight
1.0000 and 0.8003
full-matrix on F2
σ weight
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
2085/30/211
1.093
4728/20/325
1.082
4093/12/267
1.125
R1 ) 0.0657
wR2 ) 0.1804
R1 ) 0.0736
wR2 ) 0.1885
1.215 and -1.073
R1 ) 0.0278
wR2 ) 0.0658
R1 ) 0.0364
wR2 ) 0.0879
0.353 and -0.526
R1 ) 0.0644
wR2 ) 0.1759
R1 ) 0.0653
wR2 ) 0.1768
0.939 and -1.005
R1 ) 0.0321
wR2 ) 0.0836
R1 ) 0.0349
wR2 ) 0.0864
1.105 and -0.591
R indices (all data)
largest diff peak and hole (e Å-3
)
(m) closo-1-Cl-6-{Co(CO)2}-2,3-(Cp*Rh)2(µ3-CO)B3H3, 12. 11
(0.08 g, 0.15 mmol) and Co2(CO)8 (0.08 g, 0.23 mmol) were placed in
a 100 mL Schlenk tube. A 10 mL portion of freshly distilled hexane
was slowly added to the mixture by syringe at -20 °C. The reaction
mixture was allowed to warm to room temperature and stirred for 20
h. Insoluble black solids were removed by filtration through Celite,
and column chromatography was performed at room temperature.
Elution with hexane gave a brown solution (Co4(CO)12 by IR).
Subsequent elution with ether afforded a red-orange solution, which
was dried to yield 0.083 g of red-orange microcrystals of 12 (80%
based on Rh). MS(EI): P+ ) 690, 3 B, 1 Cl, and 2 Rh atoms; frag-
(a) nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1. An orange blocklike crystal
of 1, obtained by keeping a saturated hexane solution at 4 °C for several
days, was mounted on a glass fiber. Most of the non-hydrogen atoms
were located by the direct method, and the remaining non-hydrogen
atoms were found in succeeding difference Fourier syntheses. It was
found that the Cp* group was disordered over two sites with occupancy
coefficients of 0.5. In addition, one of the boron atoms was disordered
over two sites due to the crystallographic 2-fold axis. In the final
refinement, hydrogen atoms for the two sets of disordered Cp* groups
were refined with an idealized riding model, which restrained the C-H
distance to 0.96 Å and the isotropic thermal parameter of a hydrogen
atom to 1.5 times the equivalent isotropic thermal parameter of its
bonded carbon atom. The positions for the rest of the hydrogen atoms
were not located and not included in the final refinement.
(b) nido-1,2-(Cp*Ru)2(µ-H)B4H9, 2. A red-orange blocklike crystal,
obtained by keeping a saturated hexane solution at 4 °C for several
days, was mounted on a glass fiber in a random orientation. Most of
the non-hydrogen atoms were located by the direct method, and the
remaining non-hydrogen atoms were found in succeeding difference
Fourier syntheses. In the final refinement, the Cp* hydrogen atoms
were refined with an idealized riding model, which restrained the C-H
distance to 0.96 Å and the isotropic thermal parameter of a hydrogen
atom to 1.5 times the equivalent isotropic thermal parameter of its
bonded carbon atom. After all non-hydrogen atoms were refined
anisotropically and hydrogen atoms of Cp* groups refined isotropically,
a difference Fourier synthesis located the rest of the hydrogen atoms,
which were refined isotropically with bond length restraints.
ment peaks corresponding to sequential loss of 3 CO; calcd for
12
C
23
1H3316O311B359Co103Rh235Cl, 689.9853; obsd, 689.9866. NMR: 11B
(hexane, 22 °C), δ 62.8 (s, {1H}, s, 1B), 37.4 (d, JB-H ) 150 Hz, {1H},
s, 2B); 1H (C6D6, 22 °C), δ 5.50 (pcq, 2H, B-Ht), 1.67 (s, 30H, C5Me5),
-7.07 (br, 1H, B-H-B). IR (hexane, cm-1): 2513 w (B-H); 2028 s,
1986 m, 1724 w (CO).
(n) closo-1-Cl-6-{Fe(CO)3}-2,3-(Cp*Rh}2(µ3-CO)B3H2, 13. 11
(0.07 g, 0.13 mmol) and Fe2(CO)9 (0.07 g, 0.19 mmol) were placed in
a 100 mL Schlenk tube. A 10 mL portion of freshly distilled hexane
was slowly added to the mixture by syringe, and the reaction mixture
was heated to 55 °C with stirring for 20 h. Insoluble black solids were
removed by filtration through Celite, and chromatography on a silica
gel plate gave 0.04 g of red microcrystals of 13 (43% based on Rh).
MS(FAB): P+ ) 714, 3 B, 1 Cl, and 2 Rh atoms; fragment peaks
corresponding to sequential loss of 4 CO; calcd for 12C24 H3216O411B3
-
1
56
Fe103Rh235Cl, 713.9728; obsd, 713.9745. NMR: 11B (hexane, 22 °C),
δ 75.5 (s, {1H}, s, 1B), 64.9 (d, JB-H ) 150 Hz, {1H}, s, 2B); H
1
(c) nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO)2(µ3-CO)B3H6, 3. Black
platelike crystals suitable for X-ray diffraction were obtained by keeping
a saturated toluene solution at 4 °C for several days, and one was
mounted on a glass fiber in a random orientation. Most of the non-
hydrogen atoms were located by the direct method; the remaining non-
hydrogen atoms were found in succeeding difference Fourier syntheses.
In the final refinement, hydrogen atoms for the Cp* groups were refined
(C6D6, 22 °C), δ 8.16 (pcq, 2H, B-Ht), 1.62 (s, 30H, C5Me5). IR
(hexane, cm-1): 2450 w, 2492 w (B-H); 2028 s, 1981 m, 1965 m,
1735 m (CO).
X-ray Structure Determinations. Crystallographic information for
compounds 1-3, 6, 7, 9, 11, and 12 is given in Tables 1 and 2.
Preliminary examination and data collection were performed with Mo
KR radiation (λ ) 0.710 73 Å) on an Enraf-Nonius CAD4 computer
controlled κ axis diffractometer equipped with a graphite crystal,
incident beam monochromator at room temperature.48 Structure solution
and refinement (based on F2) were performed on a PC by using the
SHELXTL V5 package.49 All reflections, including those with negative
intensities, were included in the refinement.
(48) Frenz, B. A. In The Enraf-Nonius CAD4-A Real-time System for
Concurrent X-ray Data Collection and Crystal Structure Determination;
Schenk, H., Olthof-Hazelkamp, R., von Konigsveld, H., Bassi, G. C., Eds.;
Delft University Press: Delft, Holland, 1978; p 64.
(49) Sheldrick, G. M. SHELXTL V5; Siemens Industrial Automation
Inc.: Madison, WI, 1994.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1279
Table 2. Crystallographic Data and Structure Refinement Details for Compounds 7, 9, 11, and 12
7
9
11
12
empirical formula
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
C24H34B4FeO4Ru2
687.74
monoclinic
P21/n
C20H37B3Rh2
515.75
monoclinic
P21/c
15.2677(14)
8.7342(11)
17.324(2)
90
103.572(9)
90
C20H36B3ClRh2
550.19
monoclinic
P21/c
15.574(4)
8.740(2)
18.038(5)
90
104.76(2)
90
2374.2(10)
4
1.539
1112
C23H33B3ClCoO3Rh2
690.12
orthorhombic
Pna21
20.160(2)
9.113(2)
14.544(2)
90
90
90
2672.1(6)
4
1.715
9.241(3)
15.112(3)
20.047(3)
90
99.944(10)
90
2757.5(11)
4
1.657
R (deg)
â (deg)
γ (deg)
V (Å3)
2245.7(4)
4
1.525
Z
Dc (g cm-3
F(000)
)
1376
1048
1376
wavelength (Mo KR (Å))
0.710 73
1.628
0.42 × 0.30 × 0.25
2.06-24.99
4969
4828 [R(int) ) 0.0383]
4406
linear decay (0.0)
ψ scans
0.710 73
1.470
0.35 × 0.28 × 0.17
2.42-24.97
3937
3937
3619
linear decay (0.0)
DIFABS
0.710 73
1.505
0.29 × 0.18 × 0.01
2.34-24.97
4338
4172 [R(int) ) 0.0340]
2922
linear decay (-2.6%/63.9 h)
ψ scans
0.710 73
1.957
0.18 × 0.11 × 0.06
2.02-24.98
4702
4702
4117
linear decay (0.0)
ψ scans
µ (mm-1
)
crystal size (mm)
θ range (deg)
no. of total reflns collected
no. of unique reflns
no of unique reflns [I > 2σ(I)]
decay correction
abs correction
max and min transmission
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
1.000 and 0.9359
4825/24/420
1.108
R1 ) 0.0323
wR2 ) 0.0866
R1 ) 0.0363
wR2 ) 0.0912
1.266 and -0.546
1.0000 and 0.6752
3937/11/258
1.105
R1 ) 0.0266
wR2 ) 0.0713
R1 ) 0.0298
wR2 ) 0.0739
0.618 and -0.524
0.9983 and 0.8797
4170/10/263
1.110
R1 ) 0.0538
wR2 ) 0.1191
R1 ) 0.0895
wR2 ) 0.1482
0.775 and -0.717
0.9998 and 0.8734
4702/6/311
1.138
R1 ) 0.0378
wR2 ) 0.0803
R1 ) 0.0503
wR2 ) 0.0902
0.575 and -0.341
R indices (all data)
largest diff peak and hole (e Å-3
)
with an idealized riding model, which restrained the C-H distance to
0.96 Å and the isotropic thermal parameter of a hydrogen atom to 1.5
times the equivalent isotropic thermal parameter of its bonded carbon
atom. After all non-hydrogen atoms were refined anisotropically and
hydrogen atoms of Cp* groups refined isotropically, a difference Fourier
synthesis located the rest of the hydrogen atoms, which were refined
isotropically with bond length restraints.
(f) nido-2,3-(Cp*Rh)2B3H7, 9. Dark red blocklike crystals suitable
for X-ray diffraction were obtained by keeping a saturated ether solution
at 4 °C for several days, and one was mounted on a glass fiber in a
random orientation. Most of the non-hydrogen atoms were located by
the direct method; the remaining non-hydrogen atoms were found in
succeeding difference Fourier syntheses. After all non-hydrogen atoms
were refined anisotropically, a difference Fourier synthesis located all
hydrogen atoms. In the final refinement, Cp* hydrogen atoms were
refined with an idealized riding model, which restrained the C-H
distance to 0.93 Å and the isotropic thermal parameter of a hydrogen
atom to 1.2 times the equivalent isotropic thermal parameter of its
bonded carbon atom. The rest of the hydrogen atoms were refined
isotropically with bond length restraints, which used four free variables
in the refinement to confine B-H and Rh-H bond distances around
their mean values.
(d) pileo-2,3-(Cp*Ru)2(µ-H)B4H7, 6. Red-orange blocklike crystals
suitable for X-ray diffraction were obtained by keeping a saturated
hexane solution at 4 °C for several days, and one was mounted on a
glass fiber in a random orientation. Most of the non-hydrogen atoms
were located by the direct method; the remaining non-hydrogen atoms
were found in succeeding difference Fourier syntheses. In the final
refinement, hydrogen atoms for the Cp* groups were refined with an
idealized riding model, which restrained the C-H distance to 0.96 Å
and the isotropic thermal parameter of a hydrogen atom to 1.5 times
the equivalent isotropic thermal parameter of its bonded carbon atom.
After all non-hydrogen atoms were refined anisotropically and hydrogen
atoms of Cp* groups refined isotropically, a difference Fourier synthesis
located the rest of the hydrogen atoms, which were refined isotropically
with bond length restraints.
(g) nido-1-Cl-2,3-(Cp*Rh)2B3H6, 11. Red-orange crystals suitable
for X-ray diffraction were obtained by keeping a saturated ether solution
at -40 °C for 1 week, and one was mounted on a glass fiber in a
random orientation. Most of the non-hydrogen atoms were located by
the direct method, and the remaining non-hydrogen atoms were found
in succeeding difference Fourier syntheses. After all non-hydrogen
atoms were refined anisotropically, a difference Fourier synthesis
located all hydrogen atoms. In the final refinement, cyclopentadienyl
hydrogen atoms were refined with an idealized riding model, which
restrained the C-H distance to 0.93 Å and the isotropic thermal
parameter of a hydrogen atom to 1.2 times the equivalent isotropic
thermal parameter of its bonded carbon atom. The rest of the hydrogen
atoms were refined isotropically with bond length restraints, which used
four free variables in the refinement to confine B-H and Rh-H bond
distances around their mean values.
(e) pileo-6-Fe(CO)3-2,3-(Cp*Ru)2(µ3-CO)B4H4, 7. Red polyhedral
crystals suitable for X-ray diffraction were obtained by keeping a
saturated toluene solution at room temperature for 2 days, and one was
mounted on a glass fiber in a random orientation. Most of the non-
hydrogen atoms were located by the direct method; the remaining non-
hydrogen atoms were found in succeeding difference Fourier syntheses.
It was found that one of the Cp* groups was disordered over two sites
with occupancy coefficients of 0.5. In the final refinement, hydrogen
atoms for the Cp* groups were refined with an idealized riding model,
which restrained the C-H distance to 0.96 Å and the isotropic thermal
parameter of a hydrogen atom to 1.5 times the equivalent isotropic
thermal parameter of its bonded carbon atom. After all non-hydrogen
atoms were refined anisotropically and hydrogen atoms of Cp* groups
refined isotropically, a difference Fourier synthesis located the rest of
the hydrogen atoms, which were refined isotropically with bond length
restraints.
(h) closo-1-Cl-6-{Co(CO)2}-2,3-(Cp*Rh)2(µ3-CO)B3H3, 12. Red-
orange crystals suitable for X-ray diffraction were obtained by keeping
a saturated ether solution at -40 °C for 1 week, and one was mounted
on a glass fiber in a random orientation. Most of the non-hydrogen
atoms were located by the direct method; the remaining non-hydrogen
atoms were found in succeeding difference Fourier syntheses. After
all non-hydrogen atoms were refined anisotropically, a difference

1280 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
Scheme 1
Figure 1. Molecular structure of nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1.
Selected bond lengths (Å) and angles (deg): Ru-Ru′ 2.814(1), Ru-
B2 2.014(17), Ru-B1 2.293(12), B1-Ru1′ 2.246(10), B1-B2 1.82(2),
B2-B1′ 2.02(2), B2-Ru-B1′ 56.3(6), B2-B1-Ru1′ 104.7(8), B2-
B1-Ru 57.3(7), Ru1′-B1-Ru 76.6(3), B1-B2-Ru 73.2(8), B1-B2-
B1′ 86.8(9), Ru-B2-B1′ 67.7(7).
Fourier synthesis located all hydrogen atoms. In the final refinement,
Cp* hydrogen atoms were refined with an idealized riding model, which
restrained the C-H distance to 0.93 Å and the isotropic thermal
parameter of a hydrogen atom to 1.2 times the equivalent isotropic
thermal parameter of its bonded carbon atom. The rest of the hydrogen
atoms were refined isotropically with bond length restraints, which used
four free variables in the refinement to confine B-H and Rh-H bond
distances around their mean values.
ranes lacking halogen. Borane removes chlorides from metals
as BH2Cl18 and adds to yield metallaboranes, but these can then
undergo cluster expansion with additional borane.53 Thus, the
low selectivity of borane relative to borohydride results from
competitive reaction of 1 with borane to give 2. Experimental
confirmation of this point is given in the next section.
Results
Synthesis of nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1. The reaction
of [Cp*RuCl2]2 with [BH4]- in THF gives a single metallabo-
rane, and the spectroscopic data support its formulation as a 7
skeletal electron pair (sep)3,4,50 nido-dimetallapentaborane with
the molecular formula (Cp*Ru)2B3H9. Although the molecules
are disordered in the solid state and the hydrogen atom positions
are not defined by the structure determination, the square
pyramidal core geometry expected on the basis of the electron
count is confirmed and the compound is identified as the 1,2-
isomer (Figure 1). This core structure symmetry is consistent
Although the first step of the reaction of monoboranes with
the metal dimer is uncertain, we do know that reduction can
compete with metallaborane formation.20 Unfortunately, [Cp*Ru-
Cl2]2 is poorly soluble under the reaction conditions, making it
difficult to characterize the initial reaction. Visual observation
shows that the mixed valent dimer, [Cp*2Ru2Cl3], which is very
soluble and highly colored, is not present in large quantities
during the synthetic reaction. On the other hand, preparation of
this species46 in situ using [Et3BH]- followed by reaction with
[BH4]- shows only the formation of 1. As [Cp*RuCl]4 is also
poorly soluble and, in any case, is not expected to give 1 upon
treatment with [BH4]-, we suggest that the reaction proceeds
through the mixed-valent chloride dimer and a ruthenium
borohydride as low concentration intermediates as shown in
Scheme 1. Loss of hydrogen from the ruthenium borohydride
with M-B and B-B bond formation leads to the first observed
product 1. Addition of BH3‚THF to give 2 is facile (see below)
whereas further reaction with Li[BH4] is not.
Syntheses of (Cp*Rh)2B2H6, 8, and nido-2,3-(Cp*Rh)2B3H7,
9. Reaction of [Cp*RhCl2]2 with Li[BH4] at room temperature
produces a single metallaborane (Scheme 2). Although we were
unable to obtain single crystals suitable for a structure deter-
mination, the spectroscopic data unambiguously show the
compound to be (Cp*Rh)2B2H6, 8, a rhodium dimer perpen-
dicularly bridged by an ethane-like B2H6 ligand (Figure 2). That
is, the high-resolution mass data establish the molecular formula,
the 11B NMR spectrum shows equivalent borons, and the proton
NMR spectrum shows signals due to Cp* ligands, BH terminal
protons, and RhHB bridging protons in the ratio of 15:1:2. There
are now several known examples of species such as 8 which
contain formal [B2H6]2- ligands in one of two tautomeric forms,
and four have been crystallographically characterized.20-22,54
1
1
with the solution 11B and H NMR data. In addition, the H
NMR data reveal the presence of a pair of BHB protons, a pair
of BHRu protons, and a pair of RuH protons. The first pair can
only be placed on the two B-B edges of the square face, and
the second logically occupies the remaining B-Ru edges of
this face. This requires the third pair to be associated with the
Ru centers, and we assign them as RuHRu bridging on the basis
of a comparison of the chemical shift with that of the RuHRu
proton of 2. A comparison of 1 with two structurally character-
ized examples of 1,2- dimetallapentaboranes formed from two
51
electron metal fragments, i.e., 1,2-{Fe(CO)3}2B3H7 and
1-Cp*Co-2-{Fe(CO)3}B3H7,52 shows that 1 is distinguished by
the presence of two additional metal-hydride bridges.
Reaction of [Cp*RuCl2]2 with BH3‚THF in THF yields BH2-
Cl and a mixture containing 1 and 2 (see below). In contrast to
the reaction with borohydride, no conditions gave pure 1. Indeed,
above room temperature and in the presence of sufficient BH3‚
THF, the reaction yields product 2 directly (Scheme 1). The
explanation of the differing behavior of the two monoboranes
is straightforward. In our experience, borohydride reacts with
available chlorides to give a simple coordinated borohydride19
or a metallaborane containing the B2H6 ligand, e.g., Cp*2Mo2-
Cl2B2H620 but does not lead to cluster expansion of metallabo-
(50) Wade, K. Electron Deficient Compounds; Nelson: London, 1971.
(51) Andersen, E. L.; Haller, K. J.; Fehlner, T. P. J. Am. Chem. Soc.
1979, 101, 4390.
(53) Aldridge, S.; Hashimoto, H.; Kawamura, K.; Shang, M.; Fehlner,
T. P. Inorg. Chem. 1998, 37, 928.
(54) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. J. Am.
Chem. Soc. 1996, 118, 4830.
(52) Lei, X.; Shang, M.; Fehlner, T. P. Organometallics 1998, 17, 1558.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1281
Scheme 2
different compound which is identified as 7-sep nido-3-Cl-1,2-
(Cp*Rh)2B3H6, 10 (Figure 4, Schemes 2 and 3), on the basis of
a comparison of its spectroscopic data with those of 9 and 11.
Curiously, one of the four cluster bridging hydrogens bridges
the apical and basal Rh atoms rather than an edge of the open
square face as found in B5H9 itself. However, the distinctly
broadened line width of the triplet associated with this proton
shows it to be associated with, if not bonded to, the adjacent
boron atom. At 60 °C, pure 10 cleanly isomerizes to 11, and
this reaction constitutes a rare example of a metallaborane cluster
isomerization. Further, it demonstrates kinetic control of the
reaction system and a greater thermodynamic stability for the
2,3-isomer relative to the 1,2-isomer. As we find no evidence
for thermal rearrangement of 1, the difference in isomer
stabilities is attributed to the accommodation of RuH vs Rh in
a nido-M2B3 cluster.
Reaction of [Cp*RhCl2]2 with BH3‚THF in THF at room
temperature results in a change in color from orange of the
suspension to the characteristic blue of [Cp*RhCl]2,56 showing
that reduction precedes metallaborane formation. The color
change is accompanied by the formation of BH2Cl and a gas,
presumably H2. On continued reaction, the blue color changes
to red-orange, additional BH2Cl is formed, and a metallaborane
product is observed. No similar color change is observed for
the reaction of [Cp*RhCl2]2 with Li[BH4], probably because
the steady-state concentration of [Cp*RhCl]2 never reaches a
high enough level for visual observation. The fact that no
(Cp*RhCl)2B2H6 or (Cp*Rh)2(B2H6)2, analogous to (Cp*Mo-
Cl)2B2H6 or (Cp*Mo)2(B2H6)2 formed in the reaction of
{Cp*MoCl2}2 with 2 Li[BH4] and 4 Li[BH4], respectively,20 is
observed is also consistent with reduction followed by metall-
aborane formation as shown in Scheme 2.
The incorporation of Cl in the borane pathway suggests a
competition between H2 loss with concomitant BB bond
formation in an intermediate chloroborohydride complex and
exchange of the last Cl atom between free borane and the
complex. Note that a mononuclear chlorohydride ruthenium
complex was recently reported.28 Also, chloroborohydride metal
salts are known and have been structurally characterized.57
Exchange between a coordinated chloroborohydride and free
borane could easily proceed via a pathway analogous to that
found for the redistribution reaction of haloboranes with
boranes.58 For rhodium, but thus far for no other metal, hydrogen
elimination to give the chlorine-substituted dirhodaborane
intermediate (Cp*Rh)2B2H5Cl dominates the exhange of the
chlorine with the chloroborohydride to give (Cp*Rh)2B2H6, 8.
The former leads to 10 and then 11, whereas the latter would
give 9, which is not observed. Consistent with this hypothesis,
spectroscopic evidence for small amounts of a dichloro analogue
of 11 was observed.
Cluster Expansion of nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1. The
reaction of pure 1 with BH3‚THF results in a facile and
quantitative conversion to a single compound. On the basis of
spectroscopic data, this compound is an 8-sep nido-dimetalla-
hexaborane and demonstrates that cluster expansion by the
formal insertion of a BH fragment into 1 has taken place. A
solid-state structure determination (Figure 5) shows that the new
metallaborane is nido-1,2-(Cp*Ru)2(µ-H)B4H9, 2, in which the
core geometry and the skeletal hydrogen atom positions are in
accord with the solution NMR data.
Figure 2. Proposed structure of (Cp*Rh)2B2H6, 8.
Compound 8 reacts with BH3‚THF at elevated temperature
to yield one major metallaborane product but in a mixture with
significant amounts of two additional species. Isolated by
fractional crystallization, the major product is 7-sep nido-2,3-
(Cp*Rh)2B3H7, 9 (Scheme 3). Both the spectroscopic data and
solid-state structure (Figure 2) define the structure as a square
pyramidal dimetallaborane, but in contrast to those of 1, both
metal atoms are found in basal positions. Thus, it constitutes a
homometallic example analogous to 2-(Cp*Ir)-3-{(PPh3)2(CO)-
Os}B3H7.55 The route to 9 is not as selective as one would wish,
and the chloro derivative 11, isolated in better yield without
painstaking separation (see below), provides a more convenient
platform for the investigation of reaction chemistry.
Syntheses of nido-3-Cl-1,2-(Cp*Rh)2B3H6, 10, and nido-
1-Cl-2,3-(Cp*Rh)2B3H6, 11. The reaction of [Cp*RhCl2]2 with
BH3‚THF at 60 °C results in the formation of a single
metallaborane which can be isolated in high yield. The new
compound is identified as 7-sep nido-1-Cl-2,3-(Cp*Rh)2B3H6,
11 (Schemes 2 and 3), on the basis of its spectroscopic properties
and solid-state structure (Figure 3).36 It is isostructural with 9
and differs only in the substitution of the terminal hydrogen on
the apical boron atom with a chlorine atom (Figure 2). Cl
substitution for H appears to be a minor structural perturbation.
At room temperature and relatively short reaction times, only
small amounts of 11 are observed and the major product is a
(56) Sharp, P. R.; Hoard, D. W.; Barnes, C. L. J. Am. Chem. Soc. 1990,
112, 2024.
(57) Lawrence, S. H.; Shore, S. G.; Koetzle, T. F.; Huffman, J. C.; Wei,
C.-Y.; Bau, R. Inorg. Chem. 1985, 24, 3171.
(58) Onak, T. Organoborane Chemistry; Academic Press: New York,
1975.
(55) Bould, J.; Pasieka, M.; Braddock-Wilking, J.; Rath, N. P.; Barton,
L.; Gloeckner, C. Organometallics 1995, 14, 5138.

1282 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
Scheme 3
Figure 3. Molecular structure of nido-2,3-(Cp*Rh)2B3H7, 9. Note that
9 is isostructural with nido-1-Cl-2,3-(Cp*Rh)2B3H6, 11.36 Selected bond
lengths (Å) and angles (deg) for 9: Rh1-Rh2 2.8491(4), Rh1-B1
2.112(4), Rh1-B2 2.188(4), Rh2-B1 2.114(4), Rh2-B3 2.198(4),
B1-B2 1.741(6), B1-B3 1.756(6), B2-B3 1.758(6), B1-Rh1-B2
47.7(2), B2-Rh1-Rh2 75.54(11), B2-B1-B3 60.4(2), B3-B1-Rh1
108.0(3), B3-B1-Rh2 68.5(2), Rh1-B1-Rh2 84.79(13), B1-B2-
B3 60.3(2), B3-B2-Rh1 104.8(2), B2-B3-Rh2 104.0(2).
Figure 4. Proposed structure of nido-3-Cl-1,2-(Cp*Rh)2B3H6, 10.
Chart 1
Examples of monometallahexaboranes are known, e.g.,
2-(CO)3FeB5H9 (Chart 1).59 But cluster 2 is the first example
of a structurally characterized dimetallahexaborane analogous
to B6H10 in which the apical BH and a basal BH are subrogated
by Cp*RuH; i.e., in contrast to a hypothetical nido-1,2-
{(CO)3Fe}2B4H8 dimetallahexaborane (Chart 1), 2 possesses two
additional endo hydrogen atoms. Thus, all five edges of the
pentagonal open face are bridged by hydrogen atoms as is the
Ru-Ru apical-basal edge. Note that 2 is closely related to the
organometallic “ferrole” Fe2(CO)6C4H4 and is a metallaborane
analogue of Cp*Cl2Ru(η2:η4-µ2-C4H4)RuCp*.60
The clean conversion of 1 to 2 confirms the cluster expansion
step incorporated into the pathway given in Scheme 1. Although
the reaction of 1 with excess [BH4]- at 60 °C also produces
some 2, the reaction is much slower than that with BH3‚THF
and requires elevated temperatures. Slow formation of borane
from the borohydride would account for the 2 observed.
Compound 1 also reacts with Co2(CO)8 at room temperature
to give a single metallaborane in good yield. Spectroscopic data
suggests addition of the components of Co(CO)4 accompanied
by the loss of hydrogen. Spectroscopic and crystallographic
characterizations combined define the new compound as the
8-sep trimetallic cluster nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co-
(CO)2(µ3-CO)B3H6, 3 (Figure 6). Like 2, 3 can be considered
a metal analogue of B6H10 but one in which an apical and two
adjacent basal BH fragments are subrogated by metal fragments
as in the hypothetical 1,2,3-{(CO)3Fe}3B3H7 (Chart 1).
(59) Shore, S. G.; Raganini, D.; Smith, R. L.; Cottrell, C. E.; Fehlner,
T. P. Inorg. Chem. 1979, 18, 670.
(60) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1990,
9, 1106.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1283
Figure 7. Proposed structure of arachno-(Cp*Ru)(CO)(µ-H)B3H7, 4.
above although in these cases there were no obvious alternatives
for the placement of the extra hydrogens. The observed
distribution of endo hydrogens on 3 can be taken to reflect the
framework charge distribution perturbed by the requirements
of the metal fragments.61
Figure 5. Molecular structure of nido-1,2-(Cp*Ru)2(µ-H)B4H9, 2.
Selected bond lengths (Å) and angles (deg): Ru1-Ru2 2.8527(4),
Ru1-B2 2.121(5), Ru1-B3 2.122(5), Ru1-B1 2.167(5), Ru1-B4
2.169(5), Ru2-B4 2.317(5), Ru2-B1 2.321(5), B1-B2 1.831(8), B2-
B3 1.784(11), B3-B4 1.805(9), B4-Ru2-Ru1 48.26(13), B1-Ru2-
Ru1 48.19(13), B2-Ru1-B1 50.5(2), B3-Ru1-B1 87.4(3), B2-
Ru1-B4 86.9(3), B3-Ru1-B4 49.7(2), B1-Ru1-B4 89.3(2), B4-
Ru2-B1 82.2(2), B2-B1-Ru2 117.1(4), B3-B2-B1 110.1(4), B2-
B3-B4 110.6(4), B3-B4-Ru2 117.8(4).
A
11B NMR study showed that 1 does react with Fe2(CO)9
but yields only small amounts of boron-containing species. It
completely decomposes to intractible material when heated to
80 °C for 20 h.
Degradation of nido-1,2-(Cp*Ru)2(µ-H)2B3H7, 1. The reac-
tion of 1 with excess Co2(CO)8 cleanly results in 3; however,
small amounts of a second product were observed in the 11B
NMR spectra. Addition of 1 equiv of Co2(CO)8 under more
dilute conditions gave an increased yield of this other species,
permitting its isolation even though 3 remained the major
product. The spectroscopic data allow unambiguous identifica-
tion of the new compound as arachno-(Cp*Ru)(CO)(µ-H)B3H7,
4 (Figure 7), a new “borallyl” metal complex related to the many
borallyl complexes already known (Co52 , Ir,62,63 Pd,64,65 and
Pt66-68). This additional reaction pathway is that observed
exclusively in the reaction of nido-2,4-(Cp*Co)2B3H7 with Co2-
(CO)8 to give arachno-(Cp*Co)(CO)B3H7.52 Thus, the sensitiv-
ity of the reaction of 1 with Co2(CO)8 to reaction conditions is
mechanistically significant (see below).
Dehydrogenation of nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co-
(CO)2(µ3-CO)B3H6, 3. The mild pyrolysis of 3 results in the
clean conversion to a new metallaborane. The mass spectro-
metric data show the loss of two hydrogen atoms from 3, and
the IR spectrum confirms the retention of the triply bridging
CO ligand and the Co(CO)2 fragment of 3 as well as the
presence of another bridging CO. The 11B NMR indicates the
existence of a plane of symmetry in the molecule, which is
confirmed by the observation of a single Cp* methyl resonance
in the 1H NMR. Consistent with the molecular formula derived
from the mass data, a single skeletal hydrogen is observed in
the 1H NMR with a peak width and a chemical shift consistent
with association with the quadrupolar Co atom. Thus, the
Figure 6. Molecular structure of nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-
Co(CO)2(µ3-CO)B3H6, 3. Selected bond lengths (Å) and angles (deg):
Ru1-Ru2 2.8155(13), Ru1-Co 2.597(2), Ru2-Co 2.648(2), Ru1-
B1 2.09(2), Ru1-B2 2.18(2), Ru1-B3 2.09(3), Ru1-C21 2.121(13),
Ru2-B1 2.146(13), Ru2-C21 2.178(13), Ru2-C24 2.00(2), Co-B3
2.25(3), Co-C21 1.903(13), Co-C22 1.73(2), Co-C23 1.82(2), Co-
C24 2.42(2), B1-B2 1.76(3), B2-B3 1.62(4), Co-Ru1-Ru2 58.41(6),
B1-Ru1-B3 85.1(8), B1-Ru1-B2 48.6(7), B3-Ru1-B2 44.5(12),
B1-Ru2-Co 92.9(5), B3-Co-Ru2 90.6(7), B2-B3-Co 120.0(13),
B3-B2-B1 114(2), B2-B1-Ru2 116.4(12).
Curiously, 3 contains a Co(CO)2 fragment, a CO ligand triply
bridging the Ru2Co face, and a nearly terminal CO attached to
the basal Ru atom. Selective 1H{11B}NMR experiments verify
the most surprising feature of its solid state structuresone basal
BH fragment is left completely unbridged and the missing
hydrogen ends up triply bridging a RuB2 face. The chemical
shift of this bridging hydrogen is likewise very unusual, being
3.2 ppm downfield of the B-H-B proton. In a normal nido-
borane, the endo hydrogen found bridging the Ru(1)-B(1)-
B(2) face would be expected to be bridging the B(2)-B(3) edge.
As there are alternative positions to place this particular
hydrogen on the cluster, its observed location suggests that each
one-electron Cp*Ru fragment demands an associated H atom.
This consistent with the hydrogen atom locations of 1 and 2
(61) Fehlner, T. P. Polyhedron 1990, 9, 1955.
(62) Greenwood, N. N.; Kennedy, J. D.; Reed, D. J. Chem. Soc.,Dalton
Trans. 1980, 196.
(63) Bould, J.; Greenwood, N. N.; Kennedy, J. D.; McDonald, W. S. J.
Chem. Soc., Dalton Trans. 1985, 1843.
(64) Housecroft, C. E.; Shaykh, B. A. M.; Rheingold, A. L.; Haggerty,
B. S. Inorg. Chem. 1991, 30, 125.
(65) Housecroft, C. E.; Owen, S. M.; Raithby, P. R.; Shaykh, B. A. M.
Organometallics 1990, 9, 1617.
(66) Bould, J.; Kennedy, J. D.; Mcdonald, W. S. Inorg. Chim. Acta 1992,
196, 201.
(67) Guggenberger, L. J.; Kane, A. R.; Muetterties, E. L. J. Am. Chem.
Soc. 1972, 94, 5665.
(68) Haggerty, B. S.; Housecroft, C. E.; Rheingold, A. L.; Shaykh, B.
A. M. J. Chem. Soc., Dalton Trans. 1991, 2175.

1284 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
Figure 9. Molecular structure of pileo-2,3-(Cp*Ru)2(µ-H)B4H7, 6.
Selected bond lengths (Å) and angles (deg): Ru1-Ru2 2.8550(7),
Ru1-B4 2.038(5), Ru1-B2 2.210(5), Ru1-B1 2.235(5), Ru2-B4,
2.047(5), Ru2-B3 2.226(5), Ru2-B1 2.243(5), B1-B3 1.717(7), B1-
B2 1.719(8) B1-B4 1.740(7), B2-B3 1.834(8), B4-Ru1-B2 93.2(2),
B2-Ru1-Ru2 76.76(14), B4-Ru2-B3 92.7(2), B4-Ru2-B1 47.6(2),
B3-Ru2-Ru1 76.63(14), B3-B1-B2 64.5(3), B3-B1-B4 127.0(4),
B2-B1-B4 126.4(4), B3-B1-Ru1 106.8(3), Ru1-B1-Ru2 79.2(2),
B1-B2-B3 57.7(3), B3-B2-Ru1 103.7(3), B2-B3-Ru2 102.9(3),
B1-B4-Ru1 72.0(2), B1-B4-Ru2 72.1(2), Ru1-B4-Ru2 88.7(2).
Figure 8. Proposed structure of closo-6-Co(CO)2-2,3-(Cp*Ru)2(µ-CO)-
(µ3-CO)B3H4, 5.
compound is formulated as a 7-sep M3B3 octahedral cluster,
closo-6-Co(CO)2-2,3-(Cp*Ru)2(µ-CO)(µ3-CO)B3H4, 5, and the
proposed structure is shown in Figure 8. It is a member of a
small class of such compounds,69-72 and there is a close
relationship between the proposed structure of 5 and the
observed structures of compounds 7 and 12, discussed below.
Dehydrogenation of nido-1,2-(Cp*Ru)2(µ-H)B4H9, 2. At-
tempts to react 2 further with BH3‚THF or with either Co2-
(CO)8 or Fe2(CO)9 only gave unreacted 2 despite temperatures
up to 80 °C and long reaction times. However, at 90 °C, a new
compound was observed in the 11B NMR spectra of the reaction
mixture. Subsequently, it was found that this compound could
be generated quantitatively by simply heating a solution of 2 at
95 °C. The spectroscopic data showed that 2 had lost two
hydrogen atoms, presumably as H2, suggesting a six-atom, 7-sep
cluster product. However, these data are not easily accom-
modated by a closo cluster structure and a solid-state structure
determination revealed the product to be pileo-2,3-(Cp*Ru)2-
(µ-H)B4H7, 6, with a BH-capped nido-2,3-(Cp*Ru)2B3H7 struc-
ture (Figure 9). As such, it is the dimetal analogue of pileo-
1,2,3-(Cp*Ru)3(µ-H)B3H7 formed as a minor byproduct in the
synthesis of 1.73
The square pyramidal framework of 6 is similar to that
observed for 9, but 6 possesses an added BH fragment which
caps the M2B face. Again, each Cp*Ru fragment is associated
with a hydrogen atom; however, there is nothing unusual about
the placement of these atoms on the cluster framework. This is
the second example of a capped dimetallapentaborane to be
reported. The first is pileo-2,3-{(PMe3)2HFe}2B4H8 isolated in
1% yield from the reaction of Fe(PMe3)3(η2-CH2PMe2)H with
B5H9.74 The synthetic advantage using monoboranes to build a
metallaborane on a dimetal species in this case corresponds to
a factor of 76 in yield even though the route involves three
separate steps.
Figure 10. Molecular structure of pileo-6-Fe(CO)3-2,3-(Cp*Ru)2(µ3-
CO)B4H4, 7. Selected bond lengths (Å) and angles (deg): Ru1-Ru2
2.6816(5), Ru1-Fe 2.7178(9), Ru2-Fe 2.709(1), Ru1-C(21) 2.045(4),
Ru1-B4 2.068(5), Ru1-B2 2.161(5), Ru1-B1 2.220(5), Ru2-C(21)
2.047(4), Ru2-B4 2.066(5), Ru2-B3 2.162(5), Ru2-B1 2.232(5), Fe-
C22 1.750(6), Fe-C23 1.765(6), Fe-C24 1.810(7), Fe-B3 2.083(6),
Fe-B2 2.086(6), Fe-C(21) 2.492(4), B1-B2 1.712(9), B1-B3
1.715(9), B1-B4 1.773(8), B2-B3 1.772(8), C21-Ru1-B4 85.7(2),
C21-Ru1-B2107.0(2),B4-Ru1-B294.6(2),B2-Ru1-Ru277.90(14),
C21-Ru1-Fe 61.16(13), B4-Ru1-Fe 106.40(14), C(21)-Ru2-B4
85.7(2), C(21)-Ru2-B3 107.0(2), B4-Ru2-B3 94.4(2), B2-B1-
B4 126.2(4), B3-B1-Ru1 101.7(3), B2-B1-Ru2 101.5(3), B1-B2-
B3 59.0(3), B1-B2-Fe 105.0(3), B3-B2-Ru1 102.1(3), B1-B3-
Fe 105.0(3), B2-B3-Ru2 102.2(3), Ru2-B4-Ru1 80.9(2).
with BH3‚THF up to 100 °C gave no reaction. However, 6 reacts
cleanly with Fe2(CO)9 to give a single metallaborane in good
yield. The spectroscopic data showed the addition of the
elements of Fe(CO)4 and the loss of four hydrogen atoms,
suggesting a seven-vertex, 7-sep cluster product. The capped
closo cluster structure suggested by the NMR data was
confirmed by a solid-state structure determination as pileo-6-
Fe(CO)3-2,3-(Cp*Ru)2(µ3-CO)B4H4, 7 (Figure 10). The Fe(CO)3
fragment formally replaces two endo hydrogens, and the CO
ligand triply bridging the trimetal face replaces the other two.
The capping BH fragment retains its position on the Ru2B face
as found in 6. Presumably, the incoming metal fragment interacts
with the square face of 6, leaving the remainder of the cluster
intact. The existing example of a capped trimetal octahedral
Metal Fragment Addition to pileo-2,3-(Cp*Ru)2(µ-H)B4H7,
6. Heating 6 in the solid state under vacuum to higher
temperatures only resulted in sublimation of 6 accompanied by
some decomposition to intractible material. Likewise, treatment
(69) Zimmerman, G. J.; Hall, L. W.; Sneddon, L. G. Inorg. Chem. 1980,
19, 3642.
(70) Miller, V. R.; Weiss, R.; Grimes, R. N. J. Am. Chem. Soc. 1977,
99, 5646.
(71) Pipal, J. R.; Grimes, R. N. Inorg. Chem. 1977, 16, 3255.
(72) Weiss, R.; Bowser, J. R.; Grimes, R. N. Inorg. Chem. 1978, 17,
1522.
(73) Lei, X.; Shang, M.; Fehlner, T. P. Inorg. Chem. 1998, 37, 3900.
(74) Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Leach, J. B.;
Mountford, P. J. Chem. Soc., Chem. Commun. 1990, 1234.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1285
Figure 11. Molecular structure of closo-1-Cl-6-{Co(CO)2}-2,3-
(Cp*Rh)2(µ3-CO)B3H3, 12. Selected bond lengths (Å) and angles
(deg): Rh1-Co 2.5450(12), Rh2-Co 2.5465(11), Rh1-Rh2 2.7070(8),
Rh1-B1 2.114(10), Rh1-B2 2.170(10), Rh2-B1 2.125(9), Rh2-B3
2.152(9), Co-B2 2.240(11), Co-B3 2.225(12), B1-Cl 1.805(11), B1-
B2 1.70(2), B1-B3 1.71(2), B2-B3 1.77(2), Co-Rh1-Rh2 57.91(3),
B1-Rh2-B3 47.2(4), B1-Rh2-Rh1 50.1(3), Co-Rh2-Rh1 57.85(3),
B1-B2-B3 59.2(6), B2-B1-B3 62.4(7), B2-B1-Rh1 68.4(5), B3-
B1-Rh1 106.3(6), Rh1-B1-Rh2 79.4(3), B1-B2-Co 95.7(6).
Figure 12. Proposed structure of closo-1-Cl-6-{Fe(CO)3}-2,3-(Cp*Rh)2-
(µ3-CO)B3H2, 13.
Scheme 4
metallaborane, pileo-1,2,3-(Cp*Co)3B4H4,75,76 displays a BH cap
on the triangular Co3 face. Compound 6 also reacts with Co2-
(CO)8 but gives more than two compounds in low yields on
the basis of 11B NMR spectroscopy.
Metal Fragment Addition to nido-1-Cl-2,3-(Cp*Rh)2B3H6,
11. Reaction of 11 with Co2(CO)8 at room temperature results
in the formation of a single metallaborane product isolated in
good yield. Spectroscopic characterization in solution and a
solid-state structure determination define the cluster product as
7-sep closo-1-Cl-6-{Co(CO)2}-2,3-(Cp*Rh)2(µ3-CO)B3H3, 12
(Figure 11). As noted already, there are examples of homome-
tallic M3B3 octahedral cluster structures known,69-72 but 5 and
12 are the first examples derived from directed syntheses.
In contrast to metal fragment additions to 1 and 6, 11 also
reacts cleanly with Fe2(CO)9 to give a single metallaborane
product. Although a solid-state structure was not obtained, the
spectroscopic data show it to be a close analogue of 12, namely,
7-sep closo-1-Cl-6-{Fe(CO)3}-2,3-(Cp*Rh)2(µ3-CO)B3H2, 13.
Rh behaves like Co whereas Ru yields a M2B3 product directly
from the mixed-valent dimer. Reaction with borane and elimina-
tion of BH2Cl lead directly to a M2B3 cluster (Co) or a M2B5
cluster (Mo) although considerable manipulation of the reaction
system to yield other products is possible for the latter metal.
[Cp*CrCl]2 is an exception in that the formation of a M2B4
cluster is accompanied by the formation of [Cp*CrCl2]2 rather
than BH2Cl. With borane, both Rh and Ru generate BH2Cl and
form M2B3 clusters. For Rh, chlorine removal as BH2Cl is less
efficient and the major M2B3 product contains a chlorine atom.
For Ru, the M2B3 to M2B4 expansion reaction is competitive
with M2B3 formation.
Cluster Expansion. Generally, dimeric monocyclopentadi-
enylmetal chlorides give dimetallaboranes as the principal
product, but the number of borane fragments in the cluster
depends on the metal and the monoborane used. E.g., for
M2Bn: Co, n ) 3;18 Cr, n ) 4;19 Mo, n ) 5.20 A stepwise
buildup of M2Bn from a M2B2H6 intermediate is very appealing.
In the present work, we have established examples of cluster
expansion for M2B2 to M2B3 and M2B3 to M2B4. As we have
previously reported the M2B4 to M2B5 step for M ) Cr,53
prototypes for a stepwise assembly of all the dimetallaboranes
observed now exist (Scheme 4, where the metals on each arrow
refer to the metals for which examples of the cluster expansion
reaction exist). In a synthesis with a given metal, the product
observed depends on the relative barrier heights of the steps as
well as the reaction conditions.
1
The exact mass establishes the molecular formula, the H and
11B NMR spectra reveal a plane of symmetry, and the IR data
suggest the presence of an Fe(CO)3 fragment plus a µ3-CO
ligand. Thus, we propose the structure shown in Figure 12, in
which the iron fragment has simply added to the square base
of 11 in the same manner as the cobalt fragment. This structure
also contains a triangular metal fragment to support the triply
bridging carbonyl ligand. The difference between 12 and 13 is
the presence of a CoH moiety in the former vs Fe(CO) in the
latter.
Discussion
Formation of M-B Bonds. Pathways for the formation of
the M2B3 clusters of Ru and Rh are given in Schemes 1 and 2.
They conforms in general to past experience with Co,18 Cr,19
and Mo;20 however each metal exhibits unique aspects associ-
ated with the competition among formal reduction of the
monocyclopentadienylmetal chloride, formation of M-B and
B-B bonds, and cluster expansion by borane. That is, reaction
of the monocyclopentadienylmetal chloride with borohydride
yields a metal borohydride (Cr) or a metal dimer with a formal
[B2H6]2- ligand bridging the metal-metal bond (Co, Mo). Here,
Cluster Reactivities. Although the structural properties of
the new clusters produced in this work are interesting, all obey
the electron-counting rules. More importantly, the work defines
a set stoichiometric reactions interconnecting stable compounds
of defined structures. At this elementary level, this work, along
with one earlier study,52 establishes for the first time important
aspects of the reactvity of the three isomeric forms of a
dimetallapentaborane.
(75) Venable, T. L.; Grimes, R. N. Inorg. Chem. 1982, 21, 887.
(76) Venable, T. L.; Sinn, E.; Grimes, R. N. Inorg. Chem. 1982, 21,
904.

1286 J. Am. Chem. Soc., Vol. 121, No. 6, 1999
Lei et al.
Scheme 5
We chose to use BH3‚THF, Fe2(CO)9, and Co2(CO)8 as test
reagents. Although each reagent has unique features, all are
commonly used in cage- and cluster-building reactions.7,77 In
addition, thermolysis at modest temperatures (80-100 °C) was
investigated. In the following, only reactions which give a single
metallaborane product in good yield are considered significant
for the purpose of this discussion. The observations are
summarized in Scheme 5, where positive reactions are indicated
by the reagents listed on the reaction arrow. Included in Scheme
5 are the previously published results on the reactions of nido-
2,4-(Cp*Co)2B3H7, 14, with Fe2(CO)9 to give nido-1-(Cp*Co)-
2-{Fe(CO)3}B3H7, 15, and with Co2(CO)8 to give arachno-
(Cp*Co)(CO)B3H7, 16 (Scheme 5d).52
Ruthenaborane 1 displays the most varied chemistry. It
undergoes facile cluster expansion reactions with borane
(Scheme 5a1) and with Co2(CO)8 (Scheme 5b1) to yield
analogous nido six-atom clusters. Both compounds lose hydro-
gen on heating, with the former yielding a capped square
pyramidal cluster (6, Scheme 5a2) and the latter a closo cluster
(5, Scheme 5b2). As the sep of both clusters is the same, the
difference in final structure is attributed to the differing numbers
of endo hydrogens in the two compounds.73 The open square
face left because the capping option is adopted by 6 permits
the addition of an iron fragment to give 7 (Scheme 5a3).
The observation of two distinct steps to reach the condensed
clusters 5 and 6 demonstrates kinetic control of the fragment
addition reactions. A more forceful demonstration of kinetic
control comes from an examination of the reaction of 1 with
differing amounts and concentrations of the reagent Co2(CO)8.
Low levels of Co2(CO)8 give the cluster degradation product
analogous to 16 (Scheme 5e) whereas high levels give metal
fragment addition to give 3. As Co2(CO)8 is well-known to
generate Co(CO)4 radicals78 which participate in cluster reac-
tions,79 a simple explanation of the observations can be offered.
Addition of a Co(CO)4 radical to 1 would give a {(Cp*Ru)2-
(µ-H)2B3H7Co(CO)4} radical cluster intermediate. In the pres-
ence of relatively high concentrations of Co(CO)4 radicals,
bimolecular H atom abstraction to give HCo(CO)4 and molecular-
{(Cp*Ru)2(µ-H)2B3H6Co(CO)4} dominates and loss of H2
generates 3 as the major product. With low concentrations of
Co(CO)4 radicals, unimolecular decomposition of the radical
intermediate to 4 plus coproducts [Cp*Ru(CO)2]2, [Co(CO)3]4,
and H2 (first two coproducts were spectroscopically observed)
becomes competitive with the bimolecular process that forms
3.
In contrast to 1, 11 undergoes the addition of either Co or Fe
fragments to give the closo cluster 12 or 13 in one step (Scheme
5d). It is tempting to suggest that a nido intermediate analogous
to 3 is involved, but this would imply that the barrier to
hydrogen elimination is considerably reduced in the change from
RuH to Rh. More appealing is the addition of the metal fragment
to give a distinct intermediate which reacts by two competitive
(78) Absi-Halabi, M.; Atwood, J. D.; Forbus, N. P.; Brown, T. L. J.
Am. Chem. Soc. 1980, 102, 6248.
(77) The Chemistry of Metal Cluster Complexes; Shriver, D. F., Kaesz,
H. D., Adams, R. D., Eds.; VCH: New York, 1990.
(79) Horwitz, C. P.; Holt, E. M.; Shriver, D. F. Organometallics 1985,
4, 1117.

Chemistry of Dimetallaboranes
J. Am. Chem. Soc., Vol. 121, No. 6, 1999 1287
pathways: insertion to give the nido cluster product vs elimina-
tion of hydrogen to give the closo product. If correct, the
different reactivities of the three isomer types relative to the
metal-based test reagents may be associated with the metal atom
positions in the square pyramidal cluster. Conceptually, addition
of the incoming metal fragment to an edge of the square face
of 1, 11, or 14 leads to different metal arrangements. Why could
this lead to different products?
In all cases when the metals are adjacent, the incoming metal
fragment generates a metal triangle capped with a triply bridging
CO ligand in the product (Scheme 5a3,b1,d). This structural
feature is retained in the thermal conversion of 3 to 5 (Scheme
5b2), suggesting it provides some cluster stability. In the case
of 1 and 14, but not 11, the generation of this feature would
require cluster rearrangement. This additional contribution to
the reaction barrier leading to a closo product permits other paths
to compete effectively, e.g., Scheme 5b1. As suggested previ-
ously,52 addition of the incoming metal moiety to a metal in
the square face is a reasonable way to account for the fact that
1-(Cp*Co)-2-{Fe(CO)3}B3H7, 15, is observed rather than 1-{Fe-
(CO)3}-2-(Cp*Co)B3H7 as the product of the reaction of 14 with
Fe2(CO)9. These observations suggest a connection between the
nature and position of the metal moieties in the dimetallaborane
and the products observed.
Directly or indirectly, the added endo hydrogen leads to a richer
reaction chemistry for Ru vs Rh based on reactions with selected
main group and transition metal fragments. The reactions
observed suggest that fragment addition and hydrogen loss
determine the product found. Metal positions in the nido-M2B3
cluster appear to be important in determining the cluster products
observed under the mild conditions used. The reactions observed
are consistent with known metal cluster chemistry,4,77 but the
fact that rather subtle factors such as cluster geometry, metal
position, number of endo hydrogens, or reagent concentrations
can determine product type for a dimetallaborane shows that
only the barest outline of the reaction chemistry of multimetal
metallaborares has been revealed thus far.
Note Added in Proof: After revision of this manuscript we
learned that compounds 1 and 2 have been synthesized and
characterized by others using a similar approach.80
Acknowledgment. Generous support of the National Science
Foundation is gratefully acknowledged. We thank Mr. D.
Schifferl for aid with the NMR experiments.
Supporting Information Available: X-ray crystallographic
files, in CIF format, and tables giving information on data
collection and reduction and structure solution and refinement,
crystallographic parameters, atomic coordinates and equivalent
isotropic temperature factors, bond distances, bond angles, and
anisotropic temperature factors for 1-3, 6, 7, 9. This material
is available free of charge via the Internet at http://pubs.acs.org.
The crystal data and related metrics for 11 and 12 have been
deposited elsewhere.36
Summary
A rational, useful synthetic route to nido-M2B3 clusters of
Ru and Rh has been developed based on the reactions of
monoboranes with monocyclopentadienylmetal chlorides. Rela-
tive to Cp*Rh in a dimetallaborane cluster, the Cp*Ru fragment
compensates by incorporating an additional endo hydrogen into
the cluster structure. In terms of geometry and hydrogen atom
placement, the appropriate comparison is Cp*Rh with Cp*RuH.
JA982954J
(80) Kawano, Y.; Shimoi, M., personal communication.