Coordination of carbon monoxide and isocyanides to bis(pentamethylcyclopentadienyl)ytterbium and related bivalent ytterbocenes

Article abstract of DOI:10.1021/om0106579

Changes in the infrared spectra of methylcyclohexane solutions of the base-free ytterbocenes (Me₅C₅)₂Yb, [1,3-(Me₃C)₂C₅H₃]₂Yb, and [1,3-(Me₃Si)₂C₅H₃]₂Yb in the presence of CO show that carbonyl adducts are formed at room temperature. The adducts are formed reversibly, and they have not been isolated. The v_CO values are lower than that of free CO for the adducts of (Me₅C₅)₂Yb and [1,3-(Me₃C)₂C₅H₃]₂Yb, and in the case of (Me₅C₅)₂Yb the ¹³C NMR resonance of the carbonyl is deshielded from that of free CO, indicating that CO is acting as a net π-acceptor ligand in those molecules. The v_CO value for the adduct of [1,3-(Me₃Si)₂C₅H₃]₂Yb is slightly higher than that of free CO, which demonstrates that changing the substituents on the cyclopentadienyl rings can alter the electron richness of the ytterbocene. Isocyanide complexes of these ytterbocenes and of (Me₄C₅H)₂Yb have been isolated and fully characterized, including by X-ray crystallography for (Me₅C₅)₂Yb(2,6-Me₂C₆ H₃NC)₂, [1,3-(Me₃C)₂C₅H₃]₂ Yb(2,6-Me₂C₆H₃NC)₂, and [1,3-(Me₃Si)₂C₅H₃]₂ Yb(2,6-Me₂C₆H₃NC)₂. The solution electronic spectra (350-1000 nm) for the base-free ytterbocenes and their carbonyl and isocyanide adducts have been measured and interpreted according to a molecular orbital model. All of the data are consistent with the notion that ytterbium(II) metallocenes can act as net π-donor fragments.

Full text of DOI:10.1021/om0106579

5690
Organometallics 2001, 20, 5690-5699
Coor d in a tion of Ca r bon Mon oxid e a n d Isocya n id es to
Bis(p en ta m eth ylcyclop en ta d ien yl)ytter biu m a n d Rela ted
Biva len t Ytter bocen es
Madeleine Schultz, Carol J . Burns, David J . Schwartz, and
Richard A. Andersen*,1
Chemistry Department and Chemical Sciences Division of Lawrence Berkeley National
Laboratory, University of California, Berkeley, California 94720
Received J uly 24, 2001
Changes in the infrared spectra of methylcyclohexane solutions of the base-free ytter-
bocenes (Me5C5)2Yb, [1,3-(Me3C)2C5H3]2Yb, and [1,3-(Me3Si)2C5H3]2Yb in the presence of CO
show that carbonyl adducts are formed at room temperature. The adducts are formed
reversibly, and they have not been isolated. The νCO values are lower than that of free CO
for the adducts of (Me5C5)2Yb and [1,3-(Me3C)2C5H3]2Yb, and in the case of (Me5C5)2Yb the
13C NMR resonance of the carbonyl is deshielded from that of free CO, indicating that CO
is acting as a net π-acceptor ligand in those molecules. The νCO value for the adduct of [1,3-
(Me3Si)2C5H3]2Yb is slightly higher than that of free CO, which demonstrates that changing
the substituents on the cyclopentadienyl rings can alter the electron richness of the
ytterbocene. Isocyanide complexes of these ytterbocenes and of (Me4C5H)2Yb have been
isolated and fully characterized, including by X-ray crystallography for (Me5C5)2Yb(2,6-
Me2C6H3NC)2, [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2, and [1,3-(Me3Si)2C5H3]2Yb(2,6-
Me2C6H3NC)2. The solution electronic spectra (350-1000 nm) for the base-free ytterbocenes
and their carbonyl and isocyanide adducts have been measured and interpreted according
to a molecular orbital model. All of the data are consistent with the notion that ytterbium-
(II) metallocenes can act as net π-donor fragments.
In tr od u ction
1976 and 1900 cm-1 for (Me3SiC5H4)3UCO and (Me4-
C5H)3UCO, respectively, are much lower than in free
CO (2136 cm-1 in methylcyclohexane solution), consis-
tent with the view that tris(cyclopentadienyl)uranium
fragments can be excellent π-donors. The lanthanide
elements do not interact with ligands such as CO as well
as their actinide congeners, since the valence 4f orbitals
are more spatially compact and better shielded by the
core electrons relative to the 5f orbitals of actinide
metals. This is illustrated by the greatly reduced
stretching frequency of the carbonyl adducts just de-
scribed, compared with the lack of adduct formation
with CO of the Cp′3Ce and Cp′3Nd molecules having
identical cyclopentadienyl ligands.11 This difference may
be rationalized by comparing the relative polarizabilities
of the 4f and 5f electrons; the greater radial extent of
the 5f orbitals makes the electrons in those orbitals
more polarizable. The bivalent lanthanides are more
polarizable than the trivalent lanthanides, however, and
their ionization energies are lower than those of the
trivalent lanthanides, due to the lower effective nuclear
charge. These concepts can be illustrated by examina-
tion of the gas-phase photoelectron spectroscopy of the
cyclopentadienyl derivatives of these metals, which
gives experimental values for the relative f orbital
ionization energies. The ionization of electrons in f
orbitals is not observed in (MeC5H4)3M (M being a
There are no reported examples of isolable lanthanide
carbonyl complexes.2 In an argon matrix, lanthanide
and actinide carbonyls have been observed in which νCO
is reduced from that of free CO.3-7 Evans has reported
that (Me5C5)2Sm and CO react in THF to form a
bimetallic Sm(III) species with the metal atoms bridged
by an O2C3O ligand which is presumed to arise from
the CO.8 Two molecular carbonyl adducts of tris-
(cyclopentadienyl)uranium (Cp′3U) compounds are
known; in both cases adduct formation is reversible, and
only one has been isolated as a crystalline solid,
(Me4C5H)3UCO.9-11 The values of νCO in these adducts,
(1) To whom correspondence should be addressed at the Chemistry
Department, University of California, Berkeley, CA 94720. E-mail:
raandersen@lbl.gov.
(2) Ellis, J . E.; Beck, W. Angew. Chem., Int. Ed. Engl. 1995, 34,
2489-2491.
(3) Sheline, R. K.; Slater, J . L. Angew. Chem., Int. Ed. Engl. 1975,
14, 309-313.
(4) Slater, J . L.; DeVore, T. C.; Calder, V. Inorg. Chem. 1973, 12,
1918-1921.
(5) Slater, J . L.; DeVore, T. C.; Calder, V. Inorg. Chem. 1974, 13,
1808-1812.
(6) Zhou, M.; Andrews, L.; Li, J .; Bursten, B. E. J . Am. Chem. Soc.
1999, 121, 9712-9721.
(7) Zhou, M.; Andrews, L.; Li, J .; Bursten, B. E. J . Am. Chem. Soc.
1999, 121, 12188-12189.
(8) Evans, W. J .; Grate, J . W.; Hughes, L. A.; Zhang, H.; Atwood, J .
L. J . Am. Chem. Soc. 1985, 107, 3728-3730.
(9) Brennan, J . G.; Andersen, R. A.; Robbins, J . L. J . Am. Chem.
Soc. 1986, 108, 335-336.
(11) del Mar Conejo, M.; Parry, J . S.; Carmona, E.; Schultz, M.;
Brennan, J . G.; Beshouri, S. M.; Andersen, R. A.; Rogers, R. D.; Coles,
S.; Hursthouse, M. Chem. Eur. J . 1999, 5, 3000-3009.
(10) Parry, J .; Carmona, E.; Coles, S.; Hursthouse, M. J . Am. Chem.
Soc. 1995, 117, 2649-2650.
10.1021/om0106579 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/27/2001

Coordination of CO and Isocyanides to Ytterbocenes
Organometallics, Vol. 20, No. 26, 2001 5691
lanthanide metal) prior to the onset of the ring Cp(π)
ionizations at about 7 eV. In comparison, the f ionization
in Cp′3U(THF) occurs at about 6.4 eV, indicating that
the 5f electrons are easier to ionize, and presumably also
to polarize, than the 4f electrons of trivalent metal-
locenes.12 The f orbital ionizations in the bivalent
lanthanide metallocenes (Me5C5)2M are observed in the
range 6-6.5 eV, and the ionization energy of the
ytterbium complex is lower than that of the europium
complex by 0.2 eV.13 Thus, ytterbium(II) in a metal-
locene such as (Me5C5)2Yb is the most likely lanthanide
metal to engage in back-donation with appropriate
π-acceptors such as acetylenes, olefins, and CO.
The structure of (Me5C5)2Yb is bent in both the solid
state and the gas phase.14,15 Little reorganization is
required to coordinate another ligand; therefore, the
enthalpy of coordination does not have to compensate
a large reorganization enthalpy. Therefore, both geo-
metric and electronic factors suggest that (Me5C5)2Yb
is likely to bind to weakly Lewis basic ligands. The two
other soluble base-free substituted ytterbocenes, [1,3-
(Me3C)2C5H3]2Yb15 and [1,3-(Me3Si)2C5H3]2Yb,16 are also
bent in the solid state and may be expected to form
similar adducts.
F igu r e 1. Infrared spectrum of (Me5C5)2Yb in methyl-
cyclohexane under 1 atm of CO. This spectrum is back-
ground-corrected for solvent and CO.
with several isocyanides were studied, since isocyanides
and CO are structurally and electronically related.
Bis(pentamethylcyclopentadienyl)ytterbium forms isol-
able 1:1 complexes with disubstituted acetylenes, which
are the only reported examples of lanthanide-acetylene
complexes.17 The electron-rich ethylene (PPh3)2Pt(C2H4)
coordinates to (Me5C5)2Yb, giving a molecule in which
ethylene bridges the two metal centers.18 The platinum-
ethylene complex was used, since ethylene itself (10
atm) reacts with (Me5C5)2Yb in hexane solution at room
temperature to form polyethylene.19 This is similar to
the behavior of (Me5C5)2Sm, which also polymerizes
ethylene.20 In the course of the ethylene binding and
polymerization studies, it was shown that carbon mon-
oxide inhibits ethylene polymerization. Thus, addition
of ethylene (to total pressure of 10 atm) to a solution of
(Me5C5)2Yb in hexane under CO pressure (5 atm) does
not produce polyethylene. Instead, the solution remains
homogeneous and perceptibly green. Removal of the
gases followed by addition of ethylene yields polyeth-
ylene instantaneously.19 This observation suggests that
CO is a better ligand for (Me5C5)2Yb than ethylene, but
the extent of the interaction is difficult to quantify.
In this paper, we report the formation of carbonyl
adducts of the ytterbocene complexes (Me5C5)2Yb, [1,3-
(Me3C)2C5H3]2Yb, and [1,3-(Me3Si)2C5H3]2Yb in meth-
ylcyclohexane solution. No structural information is
available, since the carbonyl adducts are labile. To
model this reaction, the reactions of the ytterbocenes
Resu lts
Ca r bon yl Com p lexes. A dark brown methylcyclo-
hexane solution of base-free (Me5C5)2Yb turns deep
brown-green on exposure to 1 atm of carbon monoxide
with stirring at 25 °C. The infrared spectrum of the
solution under a CO atmosphere (see Experimental
Section for details) contains two absorption peaks at
2114 cm-1 (ν1/2 ) 10 cm-1) and 2072 cm-1 (ν1/2 ) 35
cm-1) (Figure 1). Exposure of the solution to vacuum
followed by dinitrogen results in the disappearance of
the peaks; they reappear when the solution is again
exposed to carbon monoxide. This cycle can be repeated
at least four times before decomposition of the air-
sensitive starting material occurs. The (Me5C5)2Yb
starting material can be recrystallized unchanged after
an exposure to CO, showing that the reaction is revers-
ible. The two peaks in the infrared spectrum appear to
correspond to two discrete carbonyl complexes that are
in equilibrium with each other. Extensive pressure- and
temperature-dependence studies have been carried out
as part of a quantitative study of the equilibrium.21 The
results of those studies indicate that the peak at 2114
cm-1 corresponds to a 1:1 complex that forms at lower
pressures, which is in equilibrium with a 1:2 complex
giving rise to the peak at 2072 cm-1. Both of these
carbonyl complexes have CO stretching frequencies
lower than that of free carbon monoxide in methyl-
cyclohexane (2136 cm-1) (Table 1). The observation that
νCO for the apparent dicarbonyl species is lower and
broader than that for the monocarbonyl has precedent
in matrix-isolated atom (Th, U) carbonyl species where
(12) Green, J . C. Struct. Bonding 1981, 43, 37-112.
(13) Green, J . C.; Hohl, D.; Ro¨sch, N. Organometallics 1987, 6, 712-
720.
(14) Andersen, R. A.; Blom, R.; Boncella, J . M.; Burns, C. J .; Volden,
H. V. Acta Chem. Scand., Ser. A 1987, A41, 24-35.
(15) Schultz, M.; Burns, C. J .; Schwartz, D. J .; Andersen, R. A.
Organometallics 2000, 19, 781-789.
(16) Hitchcock, P. B.; Howard, J . A. K.; Lappert, M. F.; Prashar, S.
J . Organomet. Chem. 1992, 437, 177-189.
(17) Burns, C. J .; Andersen, R. A. J . Am. Chem. Soc. 1987, 109, 941-
942.
ν
CO[M(CO)2] < νCO[M(CO)1].6,7 A similar observation was
made for zirconium and hafnium carbonyls in a cryo-
genic matrix.22
It is of interest to compare this result with the
corresponding experiment on (Me5C5)2Ca.23 In this case,
(18) Burns, C. J .; Andersen, R. A. J . Am. Chem. Soc. 1987, 109, 915-
917.
(19) Burns, C. J . Ph.D. Thesis, University of California, Berkeley,
CA, 1987.
(20) Evans, W. J .; Decoster, D. M.; Greaves, J . Macromolecules 1995,
28, 7929-7936.
(21) Selg, P.; Brintzinger, H. H.; Andersen, R. A. Manuscript in
preparation.
(22) Zhou, M.; Andrews, L. J . Am. Chem. Soc. 2000, 122, 1531-
1539.

5692 Organometallics, Vol. 20, No. 26, 2001
Schultz et al.
Ta ble 1. In fr a r ed Da ta for Ca r bon yl a n d Isocya n id e Com p lexes of Ba se-F r ee Ytter bocen es
ν
CO(complex) - νCO(free CO)
complex
(Me5C5)2Yb(CO)n
[1,3-(Me3C)2C5H3]2Yb(CO)
[1,3-(Me3Si)2C5H3]2Yb(CO)
(Me5C5)2Yb(2,6-Me2C6H3NC)2
(Me5C5)2Yb(Me3CNC)2
(Me5C5)2Yb(C6H11NC)2
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2
[1,3-(Me3C)2C5H3]2Yb(Me3CNC)2
[1,3-(Me3C)2C5H3]2Yb(C6H11NC)2
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2
[1,3-(Me3Si)2C5H3]2Yb(C6H11NC)2
(Me4C5H)2Yb(2,6-Me2C6H3NC)2
(Me4C5H)2Yb(Me3CNC)2
ν
CO or νCN (cm-1
)
or νCN(complex) - νCN(free CNR)
2114, 2072
2126
2138
2131
2160
2164
2129
2161
2166
2142
2168
2132
2162
2173
-22, -64
-10
+2
+13
+26
+26
+11
+27
+28
+24
+30
+14
+28
+35
(Me4C5H)2Yb(C6H11NC)2
Ta ble 2. Solu tion Op tica l Da ta for Ba se-F r ee Ytter bocen es a n d Th eir Ca r bon yl a n d Isocya n id e Com p lexes
complex
max, nm (ꢀ, L mol-1 cm-1
λ
)
(Me5C5)2Yba
751 (173), 515 (277), 474 (374), 429 (610)
699 (256), 520 (436), 472 (540), 425 (831)
736 (221), 547 (277), 429 (665)
733 (249), 425 (754)
655 (228), 453 (537), 396 (540)
653 (171), 451 (440), 396 (483)
648 (244), 468 (404), 392 (584)
645 (168), 463 (320), 391 (500)
776 (610), 434 (1114)
(Me5C5)2Yb + COa
(Me5C5)2Yb
(Me5C5)2Yb + CO
[1,3-(Me3C)2C5H3]2Yba
[1,3-(Me3C)2C5H3]2Yb + COa
[1,3-(Me3Si)2C5H3]2Yba
[1,3-(Me3Si)2C5H3]2Yb + COa
(Me5C5)2Yb(2,6-Me2C6H3NC)2
(Me5C5)2Yb(Me3CNC)2
955 (137), 450 (676)
(Me5C5)2Yb(C6H11NC)2
942 (234), 453 (963)
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2
[1,3-(Me3C)2C5H3]2Yb(Me3CNC)2
[1,3-(Me3C)2C5H3]2Yb(C6H11NC)2
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2
[1,3-(Me3Si)2C5H3]2Yb(C6H11NC)2
(Me4C5H)2Yb(2,6-Me2C6H3NC)2
(Me4C5H)2Yb(C6H11NC)2
586 (755), 390 (1086)
648 (197), 397 (692)
606 (260), 399 (916)
617 (533), 389 (1111)
830 (234), 431 (1059)
776 (952), 448 (1251)
950 (252), 458 (968)
a
In methylcyclohexane solution; for all other cases, toluene is the solvent.
exposure of a solution of (Me5C5)2Ca in toluene to CO
pressures of 2.5-70 bar gives rise to a single νCO band
at 2158 cm-1, which is 22 cm-1 higher than the value
for free CO. This process is reversible, and the adduct
has a 1:1 stoichiometry. The radii of Ca(II) and Yb(II)
are similar,24 and the structures of their decamethyl-
metallocenes in the solid state and the gas phase are
also similar.14,15,25 However, changing from the 3d0
metal calcium to the 4f 14 5d0 metal ytterbium alters the
behavior of these two decamethylmetallocenes toward
carbon monoxide.
Since the color of a solution of (Me5C5)2Yb in meth-
ylcyclohexane changes upon exposure to CO, the optical
spectrum of the (Me5C5)2Yb solution was measured in
a separate experiment. Base-free (Me5C5)2Yb has four
absorbances at 751, 515, 474, and 429 nm, all of which
have similar extinction coefficients (Table 2). The high-
est wavelength absorption is thought to correspond to
the HOMO-LUMO transition, and the three lower
wavelength features are due to higher energy ligand
field transitions.13 Upon brief exposure of the cuvette
to vacuum followed by back-filling with carbon monox-
ide, the HOMO-LUMO transition is blue-shifted to 699
nm (Figure 2). This procedure is also reversible, al-
though the peaks become broad and some resolution is
F igu r e 2. UV-visible spectra of (Me5C5)2Yb in toluene
and methylcyclohexane under N2 and in methylcyclohexane
under 1 atm of CO.
lost, probably due to decomposition of the sample, since
the solute concentration is small. In toluene solution,
the HOMO-LUMO transition of (Me5C5)2Yb is observed
at 736 nm, and the spectrum does not change signifi-
cantly when carbon monoxide is added (Figure 2). This
indicates that toluene may be competing with, or
inhibiting, carbon monoxide binding.
(23) Selg, P.; Brintzinger, H. H.; Andersen, R. A.; Horvath, I. T.
Angew. Chem., Int. Ed. Engl. 1995, 34, 791-793.
(24) Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32, 751-767.
(25) Williams, R. A.; Hanusa, T. P.; Huffman, J . C. Organometallics
1990, 9, 1128-1134.

Coordination of CO and Isocyanides to Ytterbocenes
Organometallics, Vol. 20, No. 26, 2001 5693
The 13C{1H} NMR chemical shift of a saturated
solution of 13CO in methylcyclohexane-d14 is 185.0 ppm
(ν1/2 ) 4 Hz). When a methylcyclohexane-d14 solution
of (Me5C5)2Yb is exposed to 1 atm of 13CO, the new
chemical shift is δ 203 ppm (ν1/2 ) 425 Hz), a significant
downfield shift. The broadness of the resonance indi-
cates that some exchange process is occurring with a
rate that is on the order of the NMR time scale at 25
°C. Cooling the sample leads to a downfield shift and
broadening of the CO resonance; at -30 °C, δCO 215 ppm
and ν1/2 ) 2200 Hz. In the temperature range -70 to
-88 °C, the CO resonance decoalesces into two reso-
nances; at -73 °C, δCO 225 ppm and ν1/2 ) 650 Hz, and
at -88 °C, δCO 239 and 198 ppm, both with ν1/2 ) 580
Hz. As the sample is cooled further, the lower field CO
resonance broadens considerably, while the higher field
CO resonance sharpens. In addition, the Me5C5 methyl
resonance decoalesces into two resonances; the ring
carbon resonance may also decoalesce, but the resolu-
tion of the spectrum is insufficient to state this un-
equivocally. Finally, at -107 °C the 13C{1H} spectrum
contains two sets of resonances for the CO and Me5C5
methyl groups: δ 243 ppm (ν1/2 ) 2800 Hz), δ 199 ppm
(ν1/2 ) 100 Hz); δ 11.2 ppm (ν1/2 ) 90 Hz), δ 9.0 ppm
(ν1/2 ) 50 Hz), while a Me5C5 ring resonance appears at
δ 113 ppm (ν1/2 ) 50 Hz). These changes in the spectra
are fully reversible and do not occur when a sample of
(Me5C5)2Yb under a nitrogen atmosphere is cooled. The
NMR data are consistent with the formation of two
discrete adducts of (Me5C5)2Yb with CO, as was deduced
from infrared spectroscopy.
The optical spectra of the other base-free metallocenes
have been examined. The optical spectrum of [1,3-
(Me3C)2C5H3]2Yb in methylcyclohexane has a HOMO-
LUMO transition at 655 nm, while that of [1,3-(Me3-
Si)2C5H3]2Yb is observed at 648 nm. Exposure of the
cuvette to carbon monoxide does not alter the spectra
of either of these ytterbocenes (Table 2). The molecule
[1,3-(Me3Si)2C5H3]2Yb is purple in the solid state, al-
though in solution it is olive brown. Optical data for the
three metallocenes as well as their CO adducts are
shown in Table 2.
Isocya n id e Com p lexes. Isocyanide complexes of
(Me5C5)2Yb, [1,3-(Me3C)2C5H3]2Yb, and [1,3-(Me3Si)2-
C5H3]2Yb as well as (Me4C5H)2Yb are prepared from the
corresponding etherates, Cp′2Yb(OEt2), in toluene (eq
1). The 1:2 adducts are relatively insoluble in aromatic
Cp′2Yb(OEt2) + 2CNR toluene8 Cp′2Yb(CNR)2 (1)
Cp′ ) Me5C5, 1,3-(Me3C)2C5H3,
1,3-(Me3Si)2C5H3, Me4C5H
R ) 2,6-Me2C6H3, Me3C, C6H11
and aliphatic solvents and can be crystallized from
toluene as dark red (Cp′ ) Me5C5, 1,3-(Me3C)2C5H3, 1,3-
(Me3Si)2C5H3, Me4C5H; R ) C6H11, Me3C), blue (Cp′ )
1,3-(Me3C)2C5H3, 1,3-(Me3Si)2C5H3; R ) 2,6-Me2C6H3),
or green (Cp′ ) Me5C5, Me4C5H; R ) 2,6-Me2C6H3)
blocks.
Infrared data for the isocyanide complexes are pre-
sented in Table 1. The infrared spectra contain a single,
sharp CN stretch (ν1/2 between 12 and 20 cm-1) in both
the solid state and toluene solution for each adduct. This
contrasts with the expected result for a metal center
bound to two isocyanide ligands, as both a symmetric
stretch and an asymmetric stretch are infrared active
in idealized C2v symmetry. One explanation for this
observation is that the two isocyanide ligands are not
electronically or mechanically coupled. The observed
values of νCN for the isocyanide adducts are higher in
frequency than those of the free isocyanides, but the
changes are much less than for the previously reported
isocyanide complexes (C5H5)3LnC6H11NC (Ln ) La-Lu)
of trivalent lanthanide metals, for which νCN increases
between 60 and 74 cm-1 on coordination.27,28 For a given
ytterbocene fragment, the complex with 2,6-Me2C6H3-
NC as the isocyanide ligand has its CN stretching
frequency increased from that of the free ligand to a
lesser extent than the complexes derived from aliphatic
isocyanide ligands. This is most reasonably associated
with the electron-withdrawing nature of the aryl group,
making 2,6-Me2C6H3NC a weaker electron donor. Iso-
cyanide complexes with a 1:1 stoichiometry cannot be
isolated; mixing an ytterbocene with 1 equiv of an
isocyanide ligand results in the isolation of the 1:2
isocyanide complex and the ytterbocene starting com-
plex.
The 13C{1H} NMR spectrum of a saturated methyl-
cyclohexane-d14 solution of (Me5C5)2Yb under natural
isotopic abundance CO varies with pressure. The changes
in chemical shifts of both the ring and methyl carbon
atoms are nonlinear with pressure, although saturation
is not reached up to 220 psi. The chemical shift changes
are reversible; exposure of the NMR tube to vacuum
followed by dinitrogen results in return of the chemical
shifts to their original values.
To probe the effect of ring substitution on the metal-
ligand interaction, the infrared spectroscopic experi-
ments have been repeated with other ytterbocene
complexes. A green-brown methylcyclohexane solution
of [1,3-(Me3C)2C5H3]2Yb, when exposed to 1 atm of
carbon monoxide, produces a single peak at 2128 cm-1
(ν1/2 ) 10 cm-1) in the infrared spectrum. The peak
disappears upon exposure of the solution to vacuum
followed by dinitrogen. Similarly, when a solution of
[1,3-(Me3Si)2C5H3]2Yb in methylcyclohexane is stirred
under carbon monoxide, a peak is observed in the
infrared spectrum at 2138 cm-1 (ν1/2 ) 8 cm-1). Again,
the reaction is reversible and only a single absorption
band is observed for the carbonyl adducts of both of
these ytterbocenes.
The infrared data for the CO adducts of the three
ytterbocenes are collected in Table 1. It can be seen that
the CO stretch is reduced from that of free CO in the
cases of (Me5C5)2Yb and [1,3-(Me3C)2C5H3]2Yb, while in
the case of [1,3-(Me3Si)2C5H3]2Yb, the CO adduct has a
stretching frequency only 2 cm-1 higher than the free
ligand. The only difference between these molecules is
It is interesting to compare (Me5C5)2Yb(2,6-Me2C6H3-
NC)2 with the analogous calcium complex, (Me5C5)2Ca-
(26) Hays, M. L.; Hanusa, T. P. Adv. Organomet. Chem. 1997, 40,
117-170.
(27) Fischer, E. O.; Fischer, H. J . Organomet. Chem. 1966, 6, 141-
148.
the substitution of the cyclopentadienide rings; the
(28) von Ammon, R.; Kanellakopulos, B. Ber. Bunsen-Ges. Phys.
Chem. 1972, 76, 995-999.
26
better donor substituents on the rings lower νCO
.

5694 Organometallics, Vol. 20, No. 26, 2001
Schultz et al.
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for (Me5C5)2Yb(2,6-Me2C6H3NC)2,
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2, a n d
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2
(Me5C5)2Yb- [1,3-(Me3C)2- [1,3-(Me3Si)2-
(2,6-Me2C6- C5H3]2Yb(2,6- C5H3]2Yb(2,6-
H3NC)2
Me2C6H3NC)2 Me2C6H3NC)2
Yb-C(ring) (mean)
Yb-Cp(centroid)
Cp-Yb-Cp
2.69
2.41
145
8.7
2.538(4)
1.165(5)
1.403(4)
91.7(2)
167.3(3)
2.72
2.44
133
2.70
2.42
137
34.0
2.56
1.16
1.42
90.5(2)
176.4
twist angle
1.0
Yb-CCNR (mean)
CCNR-N (mean)
N - CR (mean)
CCNR-Yb-CCNR
Yb-CCN-N (mean)
2.61
1.15
1.40
79.9(1)
171.7
Ta ble 4. Selected Cr ysta l Da ta a n d Da ta
Collection P a r a m eter s for
(Me5C5)2Yb(2,6-Me2C6H3NC)2,
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2, a n d
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2
F igu r e 3. UV-visible spectra of (Me5C5)2Yb2Yb(2,6-
Me2C6H3NC)2 and [1,3-(Me3C)2C5H3](C6H11NC)2 in toluene.
(Me5C5)2Yb-
(2,6-Me2C6-
H3NC)2
[1,3-(Me3C)2-
C5H3]2Yb(2,6-
Me2C6H3NC)2
[1,3-(Me3Si)2-
C5H3]2Yb(2,6-
Me2C6H3NC)2
(2,6-Me2C6H3NC)2.29 In the calcium adduct, the CN
stretching frequency of the complex is increased by 30
cm-1 relative to the free ligand, while the CN stretching
frequency increases by only 13 cm-1 in the ytterbium
adduct. A similar trend was found in the behavior of
these metallocenes with CO as described above.
The optical spectra of the isocyanide complexes have
been measured to investigate the dramatically different
colors observed. Data are presented in Table 2. The
position of the band assigned as the HOMO-LUMO
transition depends on the substituents on the cyclopen-
tadienide rings and on the isocyanide ligand. The bands
for the isocyanide complexes are all very broad with ν1/2
) 200 nm or more (Figure 3).
formula
fw
color
space group
a (Å)
b (Å)
YbC38H48N2
705.85
green
Pbcn (No. 60)
14.7091(1)
15.5227(2)
14.6848(1)
90
YbC44H60N2
790.01
blue
YbC40Si4H60N2
854.31
blue
Pbca (No. 61)
19.9719(9)
20.6253(9)
22.099(1)
90
90
90
9103(1)
8
174
P1h (No. 2)
11.4729(6)
11.6642(6)
16.0596(8)
76.607(1)
86.592(1)
72.470(1)
1993.5(2)
2
113
0.029
0.036
0.039
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
3352.91(4)
4
168
0.018
0.026
temp (K)
Ra
0.028
0.029
0.095
Rw
Rall
1
The H NMR spectrum of the adduct (Me5C5)2Yb(2,6-
0.032
GOF
max ∆/σ in
final cycle
1.11
0.11
1.36
0.00
0.91
0.00
Me2C6H3NC)2 has broad peaks at room temperature
(Me5C5, ν1/2 ) 5 Hz; Me2C6, ν1/2 ) 3 Hz). Solid-state
magnetic susceptibility measurements on this molecule
show that it is diamagnetic, indicating that the broad-
ened signals are probably due to chemical exchange.
a
R ) ∑|Fo| - |Fc||/∑|Fo|.
1
Variable-temperature H NMR experiments have been
carried out on this adduct both with and without added
isocyanide ligand. Adding an excess of isocyanide causes
the room-temperature resonances to broaden further;
warming this sample to +70 °C causes the resonances
to sharpen, indicating that the rate of exchange is
increasing. When the sample is cooled, the resonances
remain unchanged to -50 °C; below that temperature
the signals begin to sharpen as the exchange between
free and bound ligand is slowed. The stopped exchange
1
region cannot be reached. The H NMR spectra of all
the other isocyanide complexes of the ytterbocenes also
have broad peaks at room temperature, indicating that
intermolecular exchange is occurring; in no case can the
exchange be stopped by lowering the temperature.
The structures of three of the 2,6-Me2C6H3NC com-
plexes have been determined by X-ray crystallography.
Important structural data are presented in Table 3, and
the data collection parameters are shown in Table 4.
Each molecule crystallizes as a monomer, and the
overall structures are unsurprising, yet there are im-
portant differences in how the isocyanide ligands are
F igu r e 4. ORTEP diagram of (Me5C5)2Yb(2,6-Me2C6H3-
NC)2 (50% probability ellipsoids).
arranged relative to the two cyclopentadienide rings.
ORTEP diagrams of the three molecules are shown in
Figures 4-6.
For the two molecules with disubstituted cyclopen-
tadienide rings, the centroid-metal-centroid angles are
similar, yet the twist angle of the two cyclopentadienide
rings changes from eclipsed in the Me3C example to
(29) Burns, C. J .; Andersen, R. A. J . Organomet. Chem. 1987, 325,
31-37.

Coordination of CO and Isocyanides to Ytterbocenes
Organometallics, Vol. 20, No. 26, 2001 5695
F igu r e 5. ORTEP diagram of [1,3-(Me3C)2C5H3]2Yb(2,6-
Me2C6H3NC)2 (50% probability ellipsoids).
F igu r e 7. Schematic energy diagram showing the effect
of bending on the energies of the molecular orbitals and
the optical transitions of an ytterbocene. The diagram is
constructed from the data in ref 13 but is not to scale. The
transitions depicted are as follows: (a) HOMO f LUMO,
4e1u f 4a1g; (b) f f d, 4e2u f 3e2g; (c) LMCT, 3e1u f 4a1g
(d) HOMO f LUMO, f f a1.
;
Discu ssion
UV Sp ectr oscop y. The absorptions observed in the
optical spectra of the base-free ytterbocenes and their
adducts from 1000 to 350 nm are listed in Table 2. The
assignment of the transitions for (Me5C5)2Yb is based
upon the calculated transition energies of (C5H5)2Yb in
D5d symmetry, since the ground-state energy does not
change much on bending to C2v symmetry.13,32 Three
transitions in this energy range are predicted. The
highest wavelength transition is the HOMO f LUMO
(e1u f a1g orbitals in D5d symmetry) (Figure 7). The e1u
symmetry orbital is exclusively an f orbital in D5d
symmetry, and this transition, which is predicted to
occur at about 750 nm, is responsible for the color of
the ytterbocene. The next lower wavelength transitions
are predicted to occur at about 460 and 390 nm, due to
the Cp(π) f LUMO (e1u f a1g, LMCT) and the f f d
(e2u f e2g) transition, respectively.13 All of these transi-
tions, in D5d symmetry, are electric dipole and Laporte
allowed; therefore, they are expected to have large
absorptivities, as observed. The first- and third-men-
tioned absorptions involve transitions between f and d
orbitals, which do not mix in D5d symmetry. As the
metallocenes bend and the symmetry is lowered to C2v,
mixing of the f and d orbitals occurs; however, the
transitions are still electric dipole allowed (Figure 7).
In methylcyclohexane, the HOMO-LUMO transition
of (Me5C5)2Yb is assigned to the feature at 751 nm. The
F igu r e 6. ORTEP diagram of [1,3-(Me3Si)2C5H3]2Yb(2,6-
Me2C6H3NC)2 (50% probability ellipsoids).
staggered in the Me3Si case. In addition, the xylyl rings
of the isocyanide ligands lie in different orientations for
these two molecules; in the Me3C case, they are per-
pendicular to the plane bisecting the metallocene wedge,
while in the Me3Si case they lie in that plane. This is
presumably due to the steric demands of the Me3C
groups on the cyclopentadienide rings, which avoid each
other to the greatest possible degree when the rings are
eclipsed. The two xylyl rings tilt to minimize steric
repulsions with the Me3C groups, and the resulting
isocyanide CCNR-Yb-CCNR angle is 80°. In the Me3Si
derivative, the isocyanide CCNR-Yb-CCNR angle is 10°
larger than in the Me3C derivative, and the xylyl rings
lie in the same plane. All of the CCNR-N distances are
very close to that of the free ligand 2,6-Me2C6H3NC (1.16
Å). The centroid-metal-centroid angles of the three
complexes are smaller than in the base-free com-
plexes.15,16 This is an expected consequence of the
addition of two ligands to the coordination sphere of the
metal; the distances and angles are similar to those
found for other complexes of the type Cp′2YbL2.30,31
(30) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Zalkin, A. Inorg.
Chem. 1982, 21, 2647-2649.
(31) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin,
A.; Templeton, D. H. Inorg. Chem. 1980, 19, 2999-3003.
(32) DeKock, R. L.; Peterson, M. A.; Timmer, L. K.; Baerends, E.
J .; Vernooijs, P. Polyhedron 1990, 9, 1919-1934.

5696 Organometallics, Vol. 20, No. 26, 2001
Schultz et al.
lower wavelength transitions at 515, 474, and 429 nm
are assigned to transitions from the lower lying filled
molecular orbitals. Three rather than two absorptions
are observed, possibly due to the reduction in symmetry
from D5d to C2v, which removes the orbital degeneracies.
The average observed energy of these three bands is 473
nm, close to the average calculated value of 425 nm for
(C5H5)2Yb; therefore, the predicted transition energies
are in good agreement with the observed values.
antibonding ones. The net result of adduct formation is
therefore to raise the LUMO orbital in energy. However,
the filled orbitals, including the HOMO, are also in-
creased in energy, since adding electron density raises
their energy. This destabilization can be thought of as
arising from the electron-electron repulsion within the
f orbitals in the closed-shell 4f 14 Yb(II). Thus, the net
effect on the HOMO-LUMO transition of adduct for-
mation could be either a blue or a red shift, depending
on the extent of bending and the extent of electron
donation from the ligands. In the (Me5C5)2Yb and CO
system, the blue shift of 52 nm (1000 cm-1) in methyl-
cyclohexane is presumably due mainly to bending,
which raises the LUMO more than the HOMO, as the
interaction with the ligand is expected to be small. In
the adduct (Me5C5)2Yb(OEt2) in toluene, the HOMO-
LUMO transition is similarly blue shifted by 66 nm
relative to base-free (Me5C5)2Yb in toluene.33,34
The model developed above can be extended to the
isocyanide adducts. In those complexes, the low-energy
band that is a measure of the HOMO-LUMO gap
changes significantly, depending on the substituents on
the cyclopentadienide ring and on the isocyanide ligand.
In crystalline (Me5C5)2Yb(2,6-Me2C6H3NC)2, the bend
angle of 145° is similar to that found in the 1:1 THF
adduct,31 and the HOMO-LUMO transition in solution
(776 nm) is also similar to that of the THF adduct (790
nm).34 Replacing the xylyl group with the more strongly
donating alkyl groups on the isocyanides results in a
red shift of the HOMO-LUMO transition (Table 2),
which indicates that the HOMO-LUMO gap is decreas-
ing. This effect is presumably the result of the higher
electron density of the alkyl isocyanide ligands increas-
ing the energy of the HOMO by electron-electron
repulsion. Changing the substituents on the cyclopen-
tadienide ring to (Me3C)2 and (Me3Si)2 decreases the
centroid-metal-centroid angles of the isocyanide com-
plexes in the solid state relative to (Me5C5)2Yb(2,6-
Me2C6H3NC)2, and the HOMO-LUMO transitions for
the xylyl isocyanide adducts in toluene solution are blue-
shifted to 586 and 617 nm, respectively, consistent with
a greater degree of bending in solution. For those
ytterbocenes, the alkyl isocyanide complexes are also
red-shifted from the xylyl isocyanide complexes. The
ytterbocene (Me4C5H)2Yb appears to be very similar to
(Me5C5)2Yb, on the basis of the optical spectra of its
isocyanide complexes.
The principal change that is predicted to occur on
bending (C5H5)2Yb, and presumably (Me5C5)2Yb, from
D5d to C2v symmetry is an increase in the HOMO-
LUMO gap.13 Thus, bending is predicted to result in a
blue shift of the HOMO-LUMO transition. The in-
creased HOMO-LUMO gap that results from bending
is largely due to destabilization of the lowest energy,
empty a1 symmetry orbital (in C2v symmetry; a1g in D5d),
while the next two higher energy orbitals (a1 and b2 in
C2v, e2g in D5d) change little on bending (Figure 7). It is
therefore tempting to offer this as an explanation for
the blue shift of 14 nm in the HOMO-LUMO transition
on changing the solvent from methylcyclohexane to
toluene. If correct, this implies that the average geom-
etry of the ytterbocene is more bent in aromatic solvents
than in aliphatic ones, which seems reasonable, as
toluene is a better electron donor than its saturated
analogue.
The electronic transitions of [1,3-(Me3C)2C5H3]2Yb and
[1,3-(Me3Si)2C5H3]2Yb in methylcyclohexane are given
in Table 2. The highest wavelength transitions in these
spectra, at 655 and 648 nm, respectively, can be as-
signed to the HOMO-LUMO transition using the
Ro¨sch-Green model.13 The transitions in these two
metallocenes are blue shifted by about 100 nm (2000
cm-1) relative to (Me5C5)2Yb. In addition, the average
value of the lower wavelength absorptions is nearly
identical, 430 nm, with the average in (Me5C5)2Yb, 473
nm. The similarity of the transitions in the two disub-
stituted ytterbocenes is surprising, since the steric and
electronic effects of Me3C and Me3Si are quite different;
Me3C has a larger cone angle and is a better donor than
Me3Si. Also, the values of νCO for the carbonyl complexes
previously discussed lie in the order Me3Si > Me3C,
which supports the idea that Me3C is more donating
(Table 1). One explanation is that the molecules have
similar sandwich structures in methylcyclohexane, which
are more bent than (Me5C5)2Yb. This accounts for the
nearly identical energies of their HOMO-LUMO tran-
sitions in solution. It also accounts for the purple solid
[1,3-(Me3Si)2C5H3]2Yb (centroid-metal-centroid angle
138°16) becoming olive brown when it is dissolved in
methylcyclohexane. The red shift results when the bend
angle becomes larger in solution, as the molecule is no
longer constrained by the intermolecular interactions
of the solid state.
In fr a r ed a n d NMR Sp ectr oscop y. The infrared
spectrum of a methylcyclohexane solution of (Me5C5)2-
Yb under CO clearly shows two CO stretches (Figure
1). Both νCO values are lower than that of free CO,
consistent with population of the C-O π-antibonding
orbital by metal f ligand donation. In a detailed study
of the pressure and temperature dependence of these
two absorptions, the 2114 cm-1 band is attributed to a
monocarbonyl species and the 2072 cm-1 band to a
dicarbonyl species.21 The latter band is clearly broader
and less intense than the former (Figure 1), possibly
due to unresolved, small coupling between the two
carbonyl groups in C2v symmetry.35 The explanation
The HOMO-LUMO transition of (Me5C5)2Yb in me-
thylcyclohexane shifts to 699 nm when it is exposed to
CO, a blue shift of 52 nm. The lower wavelength
transitions shift less than 5 nm. When a 1:1 or 1:2
adduct of a metallocene forms, the resulting angle of
the bent sandwich is smaller than in the base-free
complex. Electronically, the donor ligands interact with
the empty a1 and b2 (C2v symmetry) orbitals, stabilizing
the filled bonding orbitals and destabilizing the empty
(33) Thomas, A. C.; Ellis, A. B. Organometallics 1985, 4, 2223-2225.
(34) Thomas, A. C. Ph.D. Thesis, University of WisconsinsMadison,
Madison, WI, 1985.
(35) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. 1997, 36, 2402-
2425.

Coordination of CO and Isocyanides to Ytterbocenes
Organometallics, Vol. 20, No. 26, 2001 5697
that two carbonyl species are in equilibrium is sup-
ported by the variable-temperature 13C NMR spectra
of a (Me5C5)2Yb solution under 1 atm of 13CO. At room
temperature, only a single 13CO resonance is observed
at δ 203 ppm (ν1/2 ) 425 Hz), which is deshielded by 18
ppm relative to free 13CO at the same temperature and
pressure. The single averaged resonance decoalesces
into two at -107 °C at δ 243 ppm (ν1/2 ) 2800 Hz) and
δ 199 ppm (ν1/2 ) 100 Hz), both of which are deshielded
relative to free 13CO. The corresponding proton NMR
spectra are unchanged from that of a (Me5C5)2Yb
solution in the absence of CO.
to that of the oxygen atom, making the bond more
covalent. Hence, the force constant of the C-O bond
increases, νCO increases, and δCO is shielded. In cases
where νCO is reduced, π-back-bonding from the metal
fragment competes with or dominates the electrostatic
interaction and δCO is deshielded. The resulting charge
distribution is illustrated by the valence bond picture
B. Thus, the variation of νCO observed in the three
ytterbocene molecules examined here, from slightly
higher than that of free CO to somewhat lower, is an
indication of the balance between the resonance forms
A and B.
The 13C chemical shift in a carbonyl complex, δCO, is
directly related to the nuclear magnetic shielding, -σ.
Thus, as -σ decreases the nucleus is deshielded, giving
a resonance at higher frequency corresponding to a
downfield chemical shift. Conversely, a less negative
value of -σ results in an upfield shift.36,37 The origin of
13C chemical shifts in diamagnetic molecules using
Ramsey’s model is σtotal ) σdiamagnetic + σparamagnetic, and
the paramagnetic component is the dominant term. This
term is proportional to the reciprocal of the excitation
The isocyanide complexes of the ytterbocenes are
isolable 1:2 adducts which are therefore structural
models for the postulated dicarbonyl adduct of (Me5C5)2-
Yb. Structurally, the only notable difference between
the three ytterbocene xylyl isocyanide complexes is the
orientation of the disubstituted rings and the isocyanide
ligands of those complexes as described above. The solid-
state structure is not maintained in solution, since the
adducts undergo fast intermolecular exchange on the
NMR time scale, as expected for ytterbocene ad-
ducts.17,18
Even though 1:2 isocyanide adducts are isolated in
the solid state, only a single, sharp CN stretching
frequency is observed in the solid state (Nujol) or
solution infrared spectra. In addition, the values of νCN
in all of the adducts is greater than those found in the
free isocyanides by 11-35 cm-1. The electrostatic model,
persuasively argued by Berke for borane adducts of
isocyanides,43,44 can be applied to these isocyanide
complexes in a manner analogous to that for the
carbonyl complexes just described. When an isocyanide
is placed near a positive charge, the negative charge on
the carbon atom is reduced. This lowers the atomic
orbital energy of the carbon atom, bringing it closer in
energy to the NR fragment. The net effect is to shorten
and strengthen the C-N bond, resulting in an increase
in νCNR. This is illustrated by the valence bond structure
C, in which the charge distribution is similar to that in
the CO complexes described above.
energy multiplied by the distance cubed, ∆E-1r-3
.
Accordingly, the closer the 13C nucleus in question is to
the electrons around the metal, the greater the para-
magnetic term will be, and this larger negative shielding
results in a downfield chemical shift. It is therefore
axiomatic that the 13C chemical shift is correlated with
the CO stretching frequency in the infrared spectrum;
π-back-bonding increases the electron density in the
metal-carbon bond, shortening it and deshielding the
13C resonance, while simultaneously lengthening the
C-O bond and reducing the CO stretching frequency.
The converse is also true; therefore, a shielded 13C
resonance is expected when νCO moves to a higher
frequency. This correlation has been graphically il-
lustrated for d0 metallocene carbonyl complexes by
Parkin.38 Thus, (Me5C5)2HfH2(CO) at room temperature
has δCO at 224 ppm and νCO at 2036 cm-1 39
Conversely,
.
for (Me5C5)2Ca(CO) in toluene, the averaged chemical
shift is shielded (δCO 180.4 ppm) and νCO is raised (2158
cm-1) with respect to free CO.23
An electrostatic bonding model has been advanced to
explain the increase in CO stretching frequency above
that of free CO in some metal carbonyl compounds.35,40-42
In this model, when the carbon atom of CO is located
near an electropositive metal atom, the polarization in
the CO ligand changes as shown in the valence diagram
A. The orbital energy of the carbon atom becomes closer
This model should be particularly applicable to the
isocyanide adducts described here, since the bonding in
lanthanide compounds is largely electrostatic in nature.
The interaction of an isocyanide ligand with a bivalent
metal is expected to be weaker than with a trivalent
metal in the electrostatic bonding model because of the
smaller charge in the former. Consistent with this
argument, the increase in νCN in the trivalent complexes
(C5H5)3Ln(C6H11NC) of 60-74 cm-1 is greater than that
observed for the bivalent complexes described here of
(36) Mason, J . Multinuclear NMR; Plenum Press: New York, 1987;
Chapter 3.
(37) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J .; Ziegler, T.
Inorg. Chem. 1997, 36, 5031-5036.
(38) Howard, W. A.; Parkin, G. F. R.; Rheingold, A. L. Polyhedron
1995, 14, 25-44.
11-35 cm-1 27,28
In addition, the electrostatic model
.
implies that when two isocyanide ligands are present
in the same molecule they will behave as independent
dipoles, giving rise to a single νCN stretch, in accord with
the observed data (Table 1).
(39) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.;
Bercaw, J . E. Organometallics 1985, 4, 97-104.
(40) Goldman, A. S.; Krogh-J esperson, K. J . Am. Chem. Soc. 1996,
118, 12159-12166.
(43) J acobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.;
Fro¨hlich, R.; Meyer, O. Organometallics 1999, 18, 1724-1735.
(44) Stauch, H. C.; Wibbeling, B.; Fro¨hlich, R.; Erker, G.; J acobsen,
H.; Berke, H. Organometallics 1999, 18, 3802-3812.
(41) Hush, N. S.; Williams, M. L. J . Mol. Spectrosc. 1974, 50, 349-
368.
(42) Strauss, S. H. Chemtracts-Inorg. Chem. 1997, 10, 77-103.

5698 Organometallics, Vol. 20, No. 26, 2001
Schultz et al.
was then exposed briefly to vacuum and back-filled with
carbon monoxide (99.99% purity), and a third spectrum was
recorded. The spectrum of the carbonyl complex was obtained
by subtracting the spectrum of the solute from the spectrum
of the solute in the presence of the gas. Only the νCO or νCN
frequencies are reported in Table 1.
Ep ilogu e
Bis(pentamethylcyclopentadienyl)ytterbium is a rather
special metallocene. In solution, it coordinates up to two
carbon monoxide ligands, and the infrared stretching
frequency and 13C NMR chemical shifts of the carbonyl
complexes indicate that the bivalent lanthanide metal-
locene is acting as a π-base. Thus, it has certain
similarities with the lower valent d-transition-metal
metallocenes. As it is a bent sandwich, the electrons in
the 5d orbitals can mix with the 4f electrons, and it is
our view that this mixing is the reason for the π-basicity
toward CO, since the ligand (crystal) field effects are
greater for d than for f electrons, increasing the impor-
tance of their role in π-back-bonding. The other two
ytterbocenes described here each have a single CO
stretch in their infrared spectra upon exposure to CO,
the values of which show that (Me5C5)2Yb is a substan-
tially better donor fragment, as is expected from the
more electron-rich cyclopentadienide rings in (Me5C5)2-
Yb.
One important message from our results is that, in
contrast to the generalization commonly found in text-
books, the physical and chemical properties of bivalent
lanthanide metals and alkaline-earth metals are not the
same. When (Me5C5)2Yb is compared with (Me5C5)2Ca,
the metal radii are nearly identical,24 as are the
structures of the metallocenes in the solid state and gas
phase.15,45 Both coordinate carbon monoxide in solution,
but the values of νCO and δCO for the ytterbium carbon-
yls, with electronic structure 5d0 4f14, show that the
C-O bond force constant is lowered relative to free CO.
In contrast, the values of νCO and δCO for the calcium
fragment, with electronic structure 3d0 3p6, show that
the C-O force constant increases in that case. Thus,
this bivalent ytterbocene does participate in back-
donation, whereas the calcium analogue does not.
(Me5C5)2Yb(2,6-Me2C6H3NC)2. A toluene solution (10 mL)
of (Me5C5)2Yb(OEt2) (0.14 g, 0.32 mmol) was added with
stirring to a toluene solution (5 mL) of 2,6-Me2C6H3NC (0.080
g, 0.61 mmol). Reduction of the volume of solvent to 5 mL and
cooling to -25 °C led to the formation of brown-green blocks.
A second batch of crystals was isolated from the mother liquors
by concentrating and cooling. Yield: 0.18 g (84%). Mp: 199-
202 °C. Anal. Calcd for C38H48N2Yb: C, 64.7; H, 6.85; N, 3.97.
1
Found: C, 65.0; H, 6.71; N, 3.98. H NMR (C6D6): δ 6.90 (t, J
) 8 Hz, 2H, C6H3), 6.55 (d, J ) 8 Hz, 4H, C6H3), 2.36 (s, 30H,
Me5), 2.07 (s, 12 H, Me2) ppm. 13C NMR (C6D6): δ 134.0 (N-
C), 129.2, 128.8, 128.5 (phenyl C’s), 111.3 (C5), 19.0 (Me2), 11.7
(Me5) ppm (CtN not observed).
(Me5C5)2Yb(Me3CNC)2‚(tolu en e). A procedure similar to
that used to prepare (Me5C5)2Yb(2,6-Me2C6H3NC)2 was fol-
lowed, yielding bright red crystals which contain one molecule
of toluene of crystallization per metal atom. Yield: 80%. Mp:
105-109 °C. Anal. Calcd for C37H56N2Yb: C, 63.3; H, 8.04; N,
3.99. Found: C, 64.1; H, 8.16; N, 3.96. 1H NMR (C6D6): δ 7.05
(m, 5H, tol), 2.33 (s, 30H, Me5), 2.11 (s, 3H, tol), 1.01 (br s, 18
H, Me3) ppm.
(Me5C5)2Yb(C6H11NC)2. A procedure similar to that used
to prepare (Me5C5)2Yb(2,6-Me2C6H3NC)2 was followed, yielding
bright red crystals. Yield: 70%. Mp: 175-178 °C. Anal. Calcd
for C34H52N2Yb: C, 61.7; H, 7.92; N, 4.23. Found: C, 62.1; H,
1
7.97; N, 4.25. H NMR (C6D6): δ 2.55 (br s, 2H, C6H11), 2.39
(s, 30H, Me5), 1.35 (br s, 12H, C6H11), 0.81 (br s, 8H, C6H11
)
ppm.
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2. In a nitrogen-
containing glovebox, [1,3-(Me3C)2C5H3]2Yb(OEt2) (0.58 g, 0.97
mmol) and 2,6-Me2C6H3NC (0.25 g, 1.94 mmol) were placed
in a Schlenk flask. Toluene (30 mL) was added, the dark
solution was briefly stirred, and the volume of solvent was
reduced to 10 mL. Cooling to -40 °C resulted in the formation
of deep blue crystals. Yield: 0.6 g (85%). Mp: 163-166 °C.
Anal. Calcd for C44H60N2Yb: C, 66.9; H, 7.66; N, 3.55. Found:
Exp er im en ta l Section
1
C, 66.7; H, 7.76; N, 3.40. H NMR (tol-d8): δ 6.77 (t, J ) 7.7
Hz, 2H, C6H3), 6.56 (d, J ) 7.6 Hz, 4H, C6H3), 6.14 (d, J ) 2.5
Hz, 4H, C5H3), 6.10 (t, J ) 2.4 Hz, 2H, C5H3), 2.12 (s, 12H,
Me2), 1.42 (s, 36H, Me3C) ppm.
Gen er a l Com m en ts. All reactions and product manipula-
tions were carried out under dry nitrogen using standard
Schlenk and drybox techniques. Dry, oxygen-free solvents were
employed throughout. The elemental analyses were performed
by the analytical facility at the University of California at
Berkeley. The 13C NMR spectra were collected at 75 MHz for
the isotopically labeled 13CO experiments. The following
ytterbocene compounds were prepared as previously de-
scribed: (Me5C5)2Yb(OEt2),,46 (Me5C5)2Yb,15 [1,3-(Me3C)2C5H3]2-
Yb(OEt2),15 [1,3-(Me3C)2C5H3]2Yb,15 [1,3-(Me3Si)2C5H3]2Yb-
(OEt2),16[1,3-(Me3Si)2C5H3]2Yb,16(Me4C5H)2Yb(OEt2),15(Me4C5H)2Yb.15
The isocyanide ligands were purified by sublimation (2,6-
Me2C6H3NC) or distillation (Me3CNC, C6H11NC).
[1,3-(Me3C)2C5H3]2Yb(Me3CNC)2. Under a flow of nitro-
gen, [1,3-(Me3C)2C5H3]2Yb(OEt2) (0.73 g, 1.2 mmol) was weighed
into a Schlenk flask. Toluene (30 mL) was added, followed by
Me3CNC (0.20 g, 0.275 mL, 2.4 mmol), and the solution was
stirred briefly. The volume of the blood red solution was
reduced to 25 mL, and then it was cooled to -40 °C to give
deep red crystals. Yield: 0.7 g (85%). Mp: 165-169 °C. Anal.
Calcd for C36H60N2Yb: C, 62.3; H, 8.72; N, 4.04. Found: C,
1
63.4; H, 9.2; N, 4.08. H NMR (tol-d8): δ 6.03 (br, 4H, C5H3),
5.98 (br, 2H, C5H3), 1.46 (s, 36H, ring Me3C), 0.98 (s, 18H,
isocyanide Me3C) ppm.
In fr a r ed Sp ectr oscop y. The infrared spectra for solid
samples were recorded as Nujol mulls on a Mattson Nicolet
instrument. Solution infrared spectra were recorded using an
ASI ReactIR instrument with a Cajon adapter to a modified
Schlenk flask. A background spectrum was collected on the
contents of the flask under a nitrogen atmosphere. Spectra of
the solvent and solvent with solute were then collected. The
last two spectra were subtracted using an interactive proce-
dure to achieve as flat a baseline as possible to give the
spectrum of the compound. For carbonyl complexes, the flask
[1,3-(Me3C)2C5H3]2Yb(C6H11NC)2. A procedure similar to
that used to prepare [1,3-(Me3C)2C5H3]2Yb(Me3CNC)2 was
followed, yielding brick red crystals. Yield: 89%. Mp: 148-
150 °C. Anal. Calcd for C40H62N2Yb: C, 64.4; H, 8.65; N, 3.76.
1
Found: C, 63.0; H, 8.64; N, 3.56. H NMR (tol-d8): δ 6.18 (br,
4H, C5H3), 6.14 (br, 2H, C5H3), 2.79 (br, 2H, C6H11), 1.56 (s,
36 H, Me3C), 1.36 (br, 12H, C6H11), 1.00 (br m, 8H, C6H11) ppm.
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2. A procedure simi-
lar to that used to prepare [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3-
NC)2 gave dark blue crystals. Yield: 90%. Mp: 155-158 °C.
Anal. Calcd for C40H60Si4N2Yb: C, 56.2; H, 7.08; N, 3.28.
(45) Andersen, R. A.; Boncella, J . M.; Burns, C. J .; Blom, R.;
Haaland, A.; Volden, H. V. J . Organomet. Chem. 1986, 312, C49-C52.
(46) Tilley, T. D.; Boncella, J . M.; Berg, D. J .; Burns, C. J .; Andersen,
R. A. Inorg. Synth. 1990, 27, 146-149.
1
Found: C, 54.6; H, 6.64; N, 2.74. H NMR (C6D6): δ 6.80 (br
m, 12H, C5H3 and C6H3), 2.45 (br s, 12H, Me2), 0.47 (s, 36H,
Me3Si) ppm.

Coordination of CO and Isocyanides to Ytterbocenes
Organometallics, Vol. 20, No. 26, 2001 5699
[1,3-(Me3Si)2C5H3]2Yb(C6H11NC)2. A procedure similar to
that used to prepare [1,3-(Me3C)2C5H3]2Yb(Me3CNC)2 was
followed, yielding brick red crystals. Yield: 85%. Mp: 142-
144 °C. Anal. Calcd for C36H64Si4N2Yb: C, 53.4; H, 7.96; N,
3.46. Found: C, 53.0; H, 8.13; N, 3.40. 1H NMR (C6D6): δ 6.72
(br, 4H, C5H3), 6.69 (br, 2H, C5H3), 2.90 (br s, 2H, C6H11), 1.48
(br s, 4H, C6H11), 1.37 (br s, 8H, C6H11), 0.93 (br m, 8H, C6H11),
0.50 (s, 36H, Me3Si) ppm.
(Me4C5H)2Yb(2,6-Me2C6H3NC)2. A procedure similar to
that used to prepare [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2
was followed, and dark blue-green crystals were isolated in
70% yield. Mp: 180-182 °C. Anal. Calcd for C36H44N2Yb: C,
63.8; H, 6.54; N, 4.13. Found: C, 63.5; H, 6.34; N, 3.88. 1H
NMR (C6D6): δ 6.93 (t, J ) 7 Hz, 2H, C6H3), 6.52 (d, J ) 7 Hz,
4H, C6H3), 5.88 (s, 2H, C5H), 2.44 (s, 12H, Me4C5), 2.39 (s, 12H,
Me4C5), 2.03 (s, 12H, Me2) ppm.
The weighting scheme was based on counting statistics and
included a factor (p ) 0.030) to downweight intense reflections.
Plots of ∑w(|Fo| - |Fc|)2 vs |Fo|, reflection order in data
collection, (sin θ)/λ, and various classes of indices showed no
unusual trends. Neutral atom scattering factors were taken
from ref 50. Anomalous dispersion effects were included in Fc;
the values for ∆f ′ and ∆f ′′ were those of Creagh and
McAuley.51 The values for the mass attenuation coefficients
are those of Creagh and Hubbell.52 All calculations were
performed using the teXsan crystallographic software package
of Molecular Structure Corp.53
(Me5C5)2Yb(2,6-Me2C6H3NC)2. An empirical absorption
correction was applied using XPREP54 with µR ) 0.06. A
secondary extinction coefficient was applied (1.4 × 10-7). The
quality of the data indicated that it was reasonable to locate
and refine hydrogen atoms.
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2. On the basis of
a statistical analysis of intensity distribution, and the suc-
cessful solution and refinement of the structure, the space
group was determined to be P1h (No. 2). An empirical absorp-
tion correction was applied using XPREP54 with µR ) 0.03.
Analysis of the data indicated no need for
extinction correction.
[1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2. An empirical ab-
sorption correction was applied using SADABS.55 No secondary
extinction correction was applied.
(Me4C5H)2Yb(Me3CNC)2. A procedure similar to that used
to prepare [1,3-(Me3C)2C5H3]2Yb(Me3CNC)2 was followed, and
the product was isolated as red crystals in 80% yield. Mp:
105-108 °C. Anal. Calcd for C28H44N2Yb: C, 57.8; H, 7.62; N,
1
4.82. Found: C, 58.7; H, 7.40; N, 4.29. H NMR (C6D6): δ 5.9
a secondary
(s, 2H, C5H), 1.79 (s, 12H, Me4C5), 1.75 (s, 12H, Me4C5) ppm
(Me3CNC resonances not observed).
(Me4C5H)2Yb(C6H11NC)2. A procedure similar to that used
to prepare [1,3-(Me3C)2C5H3]2Yb(Me3CNC)2 was followed, and
the product was isolated as red crystals in 90% yield. Mp:
136-139 °C. Anal. Calcd for C32H48N2Yb: C, 60.6; H, 7.63; N,
4.42. Found: C, 60.6; H, 7.43; N, 4.16. 1H NMR (C6D6): δ 5.89
(s, 2H, C5H), 2.63 (s, 2H, C6H11), 2.47 (s, 12H, Me4C5), 2.40 (s,
Ack n ow led gm en t. This work was partially sup-
ported by the Director, Office of Energy Research, Office
of Basic Energy Sciences, Chemical Sciences Division
of the U.S. Department of Energy under Contract No.
DE-AC03-76SF00098. We thank the Fannie and J ohn
Hertz Foundation for a fellowship (C.J .B.), the NSF for
a fellowship (D.J .S.), and Dr. Fred Hollander for as-
sistance with the crystallography. We also thank Pro-
fessor Art Ellis for making A. C. Thomas’ Ph.D. thesis
available and Dr. Wayne Lukens and Professor Hans
Brintzinger for helpful discussion.
12H, Me4C5), 1.3 (br m, 12 H, C6H11), 0.8 (br m, 8 H, C6H11
ppm.
)
Cr ysta llogr a p h ic Stu d ies. Gen er a l P r oced u r e. All rel-
evant crystallographic data are shown in Table 4. For each
complex studied by X-ray diffraction, crystals were grown by
slow cooling of a concentrated toluene solution of the com-
pound. A crystal was mounted on a glass fiber with Paratone
N hydrocarbon oil. All measurements were made on a Siemens
SMART47 diffractometer with graphite-monochromated Mo KR
radiation using a CCD detector. Cell constants and an orienta-
tion matrix for data collection were obtained from a least-
squares refinement of reflections from 60 frames collected at
10 s each. Frames corresponding to a hemisphere of data were
then collected using the ω-scan technique; in each case 10 s
exposures of 0.3° in ω were taken. The reflections were
integrated using SAINT,48 and equivalent reflections were
merged. No decay correction was applied. An empirical ab-
sorption correction was applied to each structure as described
below, and the data were corrected for Lorentz and polarization
effects. The space groups were determined by the systematic
absences of hkl values, unless otherwise noted. The structures
were solved by direct methods49 and expanded using Fourier
techniques. All non-hydrogen atoms were refined anisotropi-
cally; hydrogen atoms were included but not refined unless
otherwise noted. The final cycle of full-matrix least-squares
refinement was based on observed reflections (I > 3.00σ(I))
and converged. The function minimized was ∑w(|Fo| - |Fc|)2.
Note Ad d ed in P r oof. DFT calculations on the
carbon monoxide adducts are underway, O. Eisenstein
and L. Maron, personal communication.
Su ppor tin g In for m ation Available: Tables giving atomic
positions, anisotropic thermal parameters, bond lengths and
angles, and least-squares planes for each structure. This
material is available free of charge via the Internet at http://
pubs.acs.org. Structure factor tables are available from the
authors.
OM0106579
(50) Cromer, D. T.; Waber, J . T. In International Tables for X-ray
Crystallography; Ibers, J . A., Hamilton, W. C., Eds.; Kynoch Press:
Birmingham, England, 1974; Vol. IV, Table 2.2A, pp 71-98.
(51) Creagh, D. C.; McAuley, W. J . In International Tables for
Crystallography; Wilson, A. J . C., Ed.; Kluwer Academic: Boston, MA,
1992; Vol. C, Table 4.2.6.8, pp 219-222.
(52) Creagh, D. C.; Hubbell, J . H. In International Tables for
Crystallography; Wilson, A. J . C., Ed.; Kluwer Academic: Boston, MA,
1992; Vol. C, Table 4.2.4.3, pp 200-206.
(53) teXsan Crystal Structure Analysis Package; Molecular Struc-
ture Corp., The Woodlands, TX, 1992.
(47) SMART Area-Detection Software Package; Siemens Industrial
Automation, Inc., Madison, WI, 1995.
(48) SAINT, version 4.024; Siemens Industrial Automation, Inc.,
Madison, WI, 1995.
(49) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A. J .
Appl. Crystallogr. 1993, 26, 343-350.
(54) Sheldrick, G. M. XPREP, version 5.03; Siemens Industrial
Automation, Inc., Madison, WI, 1995.
(55) Sheldrick, G. M. SADABS; Siemens Industrial Automation, Inc.,
Madison, WI, 1996.