Infrared spectra of methylidynes formed in reactions of Re atoms with methane, methyl halides, methylene halides, and ethane: Methylidyne C-H stretching absorptions, bond lengths, and S character

Article abstract of DOI:10.1021/ic701505w

Rhenium carbyne complexes (HC≡ReH₃, HC≡ReH ₂X, HC≡ReHX₂, [X = F, Cl, and Br] and CH ₃C≡ReH₃) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and α-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC≡ReH₃ and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C_s structures, and as a result the HC≡ReH₃ and CH₃C≡ReH₃ complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC≡WH₃ with three equivalent W-H bonds.

Full text of DOI:10.1021/ic701505w

Inorg. Chem. 2008, 47, 1653-1662
Infrared Spectra of Methylidynes Formed in Reactions of Re Atoms with
Methane, Methyl Halides, Methylene Halides, and Ethane: Methylidyne
C-H Stretching Absorptions, Bond Lengths, and s Character
Han-Gook Cho† and Lester Andrews*,‡
Department of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749,
South Korea, and Department of Chemistry, UniVersity of Virginia, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319
Received July 27, 2007
Rhenium carbyne complexes (HC≡ReH3, HC≡ReH2X, HC≡ReHX2, [X ) F, Cl, and Br] and CH3C≡ReH3) are
produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via
oxidative C-H(X) insertion and R-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of
the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic
methylidyne C-H stretching absorptions of HC≡ReH3 and its mono- and dihalo derivatives are observed on the
blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases
in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo
methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium
methylidynes have Cs structures, and as a result the HC≡ReH3 and CH3C≡ReH3 complexes have two equivalent
shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC≡WH3 with three equivalent
W-H bonds.
Introduction
remain largely uninvestigated owing in part to very low
infrared intensity and to interference from other hydrogen
stretching absorptions, whereas the stronger C≡M stretching
bands are observed and frequently used as a probe, but these
often suffer from ligand effects.4,5
The trans-W(≡CH)(PMe3)4Cl complex with deuteriated
methyl groups has been prepared, and the weak methylidyne
C-H stretching band near 2980 cm-1 and its deuterium
counterpart at 2240 cm-1 were assigned by the Hopkins
group.6 Bond lengths in the H-C≡W moiety for a related
complex were later measured by neutron diffraction.7
Recently, small carbene and carbyne complexes (CH2)MH2,
HC≡MH3, and their halide derivatives) have been prepared
The discovery of transition metal complexes with carbon-
metal multiple bonds in the 1970s has opened exciting new
fields in chemistry, including alkene metathesis and alkane
C-H insertion.1 The numerous carbyne complexes have been
introduced to show their versatile chemistry and catalytic
activities,2 but only a few of them contain the simple
M≡C-H or M≡C-CH3 moiety (M ) transition metal).3
Moreover, the methylidyne C-H vibrational characteristics
* To whom correspondence should be addressed. E-mail: lsa@
virginia.edu.
†
University of Incheon.
University of Virginia.
‡
(1) (a) Schrock, R. R. Chem. ReV. 2002, 102, 145. (b) Herndon, J. W.
Coord. Chem. ReV. 2007, 251, 1158. (c) Herndon, J. W. Coord. Chem.
ReV. 2006, 250, 1889. (d) Herndon, J. W. Coord. Chem. ReV. 2005,
249, 999. (e) Herndon, J. W. Coord. Chem. ReV. 2004, 248, 3. (f) Da
Re, R. E.; Hopkins, M. D. Coord. Chem. ReV. 2005, 249, 1396.
(2) (a) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; Müller, J.; Huttner, G.;
Lorenz, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 564 (carbyne
synthesis). (b) McLain, S. J.; Wood, C. D.; Messerle, L. W.; Schrock,
R. R.; Hollander, F. J.; Youngs, W. J.; Churchill, M. R. J. Am. Chem.
Soc. 1978, 100, 5962 (carbyne synthesis).
(4) Barnes, D.; Gillett, D. A.; Merer, A. J.; Metha, G. F. J. Chem. Phys.
1996, 105, 6168 (W≡CH).
(5) (a) Mavridis, A.; Alvarado-Swaisgood, A. E.; Harrison, J. F. J. Phys.
Chem. 1986, 90, 2584 (M≡CH+). (b) Kalemos, A.; Dunning, Jr., T. H.;
Harrison, J. F.; Mavridis, A. J. Chem. Phys. 2003, 119, 3745 (Ti≡CH).
(c) Barnes, M.; Merer, A. J.; Metha, G. F. J. Mol. Spectrosc. 1997,
181, 168 (Ti≡CH). (d) Sari, L.; Yamaguchi, Y.; Schaefer, H. III
J. Chem. Phys. 2001, 115, 5932 (HC≡Ge). (e) Fujitake, M.; Echizenya,
R.; Shirai, T.; Kurita, S.; Ohashi, N. J. Mol. Spectrosc. 2004, 223,
113 (HC≡Si).
(3) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley
and Sons: New York, 1988.
(6) Manna, J.; Dallinger, R. F.; Miskowski, V. M.; Hopkins, M. D. J.
Phys. Chem. B 2000, 104, 10928 ((H-C≡W)(PMe3)4Cl).
10.1021/ic701505w CCC: $40.75
Published on Web 02/08/2008
2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 5, 2008 1653

Cho and Andrews
numerical differentiation15 (with Gaussian 03 keyword “anharmonic”)
were also carried out with B3LYP to compare with experimental values
and to examine the effects of anharmonicity. BPW91,16 MP2,17 and
CCSD18 calculations were also done to confirm the B3LYP results.
All of the vibrational frequencies were calculated analytically. In the
calculation of binding energy of a metal complex, the zero-point energy
is included.
in reactions of Group 3–6 metal atoms with methane and
methyl halides, showing interesting structures and fascinating
photochemistry.8 Particularly, Group 5 and 6 metals have
formed anionic and neutral methylidyne complexes
–
(HC≡MH3 , HC≡MH3, and their halide derivatives), re-
spectively. More recently, rhenium atom reactions with CHX3
and CX4 precursors (X ) F and Cl) have produced the
halogen substituted HC≡ReX3 and XC≡ReX3 complexes,
and calculations show that spin–orbit coupling is not strong
enough to override significant Jahn–Teller distortion.9
Here, we report the formation and IR spectra of HC≡ReH3,
HC≡ReH2F, HC≡ReH2Cl, HC≡ReH2Br, HC≡ReHF2, HC≡
ReHFCl, HC≡ReHCl2 and CH3C≡ReH3 in reactions of
laser-ablated Re atoms with methane, methyl and methylene
halides, and ethane. The elusive methylidyne C-H stretching
absorptions are observed from HC≡ReH3 and its halide
derivatives, and these frequencies show a decreasing trend
with the order of H, F, Cl, and Br. Only the s character in
the C-H bond among the molecular properties varies
consistently with the frequency. In addition, the di- and
trihydrido complexes provide model systems for comparison
with larger ligated monohydrido and dihydrido alkylidyne
compexes.10 A preliminary account of the Re and methane
reaction system has been communicated.11
Results and Discussion
The major product in the reaction of laser-ablated Re atoms
and methane, methyl halides, methylene halides or ethane will
be identified from the effect of isotopic substitution on the
matrix infrared spectra and comparison with frequencies
calculated by density functional theory. Weak absorptions were
detected at 932.3 and 931.7 cm-1 for the very strong antisym-
metric stretching mode of 185ReO2 and 187ReO2 from the reaction
with trace oxygen impurity in the argon matrix gas.19a
Re + CH4. Infrared spectra for the HC≡ReH3 product of
laser-ablated Re atom reaction with CH4, 13CH4, CD4, and
CH2D2 have been reported in our communication.11 The
methylidyne C-H stretching absorption was observed at
3101.8 cm-1 with deuterium and 13C counterparts at 2334.3
and 3090.8 cm-1 (H/D and 12C/13C ratios of 1.329 and 1.004),
respectively. This C-H stretching frequency is substantially
(about 200 cm-1) higher than those of normal saturated
hydrocarbons, because of higher s character in the C-H bond
arising from interaction with the adjacent multiple carbon-
rhenium bond.3,20 The C-H stretching mode is predicted
by DFT in the harmonic approximation at 3264.3 cm-1,
which is 5.2% higher and in accord with expectations.21
Notice, however, that anharmonic frequency calculations
Experimental and Computational Methods
Laser ablated Re atoms (Johnson-Matthey) were reacted with
CH4 (Matheson, UHP grade), (CH3X (Matheson), CD3F (synthe-
sized from CD3Br and HgF2)), 13CH3F, CD3Br, CD2Cl2, 13CH2Cl2
(Cambridge Isotopic Laboratories, 99%), CD3Cl (synthesized from
CD3Br and HgCl2), CH2F2, CH2FCl (Du Pont), CD2FCl (synthesized
from CD2Cl2 and HgF2), and CH2Cl2 (Fisher) in excess argon during
condensation at 8 K. These methods have been described in detail
elsewhere.12 Reagent gas mixtures were typically 0.5% in argon.
After reaction, infrared spectra were recorded at a resolution of
0.5 cm-1 using a Nicolet 550 spectrometer with an MCT-B detector.
Samples were later irradiated for 20 min periods by a mercury arc
lamp (175 W) with the globe removed and a combination of optical
filters and subsequently annealed to allow further reagent diffusion.
Complementary density functional theory (DFT) calculations were
carried out using the Gaussian 03 package,13 B3LYP density functional,
6–311++G(3df,3pd) basis sets for C, H, F, Cl, and Br13 and the SDD
pseudopotential and basis set14 for Re to provide a consistent set of
vibrational frequencies for the reaction products. Geometries were fully
relaxed during optimization, and the optimized geometry was con-
firmed by vibrational analysis. Anharmonic frequency calculations by
(13) Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross,
J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg,
J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck,A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03 Revision
B.04, Gaussian, Inc.: Pittsburgh, Pennsylvania, 2003.
(14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Frisch, M. J.; Pople,
J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (d) Raghavachari,
K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (e) Andrae, D.;
Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta
1990, 77, 123.
(15) Page, M.; Doubleday, C.; McIver, J. W. Jr. J. Chem. Phys. 1990, 93,
5634.
(7) Ménoret, C.; Spasojevic´-de Bire, A.; Dao, N. Q.; Cousson, A.; Kiat,
J.-M.; Manna, J. D.; Hopkins, M. D. J. Chem. Soc., Dalton Trans.
2002, 3731 (BrW≡CH(dmpe-d12)2).
(16) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional
Theory: Recent Progress and New Directions; Dobson, J. F., Vignale,
G., Das, M. P. Eds.; Plenum, 1998.
(8) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040 and
references therein.
(17) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990,
166, 281.
(9) Lyon, J. T.; Cho, H.-G.; Andrews, L.; Hu, H.-S.; Li, J Inorg. Chem.
2007, 46, 8728 (Re + CHX3).
(18) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J.
Quantum Chem. 1978, 14, 545.
(10) (a) Ozerov, O. V.; Watson, L. A.; Pink, M.; Caulton, K. G J. Am.
Chem. Soc. 2004, 126, 6363. (b) Leeaphon, M.; Fanwick, P. E.;
Walton, R. A. J. Am. Chem. Soc. 1992, 114, 1890.
(19) (a) Zhou, M.; Citra, A.; Liang, B.; Andrews, L. J. Phys. Chem A.
2000, 104, 3457 (Re + O2). (b) Wang, X.; Andrews, L J. Phys. Chem.
A 2003, 107, 4081 (Re + H2).
(11) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4098 (HC≡ReH3).
(12) (a) Andrews, L.; Citra, A Chem. ReV. 2002, 102, 885 and references
therein. (b) Andrews, L Chem. Soc. ReV 2004, 33, 123 and references
therein. (c) Andrews, L.; Willner, H.; Prochaska, F. T. J. Fluorine
Chem. 1979, 13, 273.
(20) Pavia, D. L.; Lampman, G. M.; George, S. K. Introduction to
Spectroscopy, 3rd ed.; Brooks Cole: New York, 2000.
(21) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937.
1654 Inorganic Chemistry, Vol. 47, No. 5, 2008

IR Spectra of Methylidynes Formed in Reactions of Re Atoms
a
2
Table 1. Observed and Calculated Fundamental Frequencies (cm-1) of HC≡ReH3 Isotopomers in the Ground A′ Electronic State
HC≡ReH3
DC≡ReD3
H13C≡ReH3
CH2D2
obs
approximate
description
obs
3101.8
anharm
harm
int
obs
harm
int
obs
harm
int
A′ C-H str.
A′ ReH2 str.
A′ ReH str.
3118.0 3264.3
2010.7 2088.8
36 2334.3
76 1399.3
2430.5
1481.4
25 3090.8
40 1946.4
3252.1
2088.8
35 3101.9, 2334.3
76
1946.4
1809.9, 1804.0 1781.8 1855.7 244 1302.6, 1298.1 1318.1 121 1809.9, 1803.9 1855.7 244 1808.5, 1882.1, 1300.2
b
b
A′ C≡Re str.
A′ ReH3 deform
A′ HCRe bend
A′ ReH3 rock
A′ ReH2 scis.
A′′ ReH2 str.
A′′ ReH2 twist
A′′ HCRe bend
A′′ ReH3 rock
a
1048.8
1092.9 1096.8
10
2
56
24
2
1041.2
549.6
447.9
368.6
325.9
1449.0
637.8
501.6
218.3
7
0
24
18
1
1012.9
1061.4
749.5
601.7
489.3
459.5
2040.2
825.1
9
2
56
23
2
732.3
559.6
454.1
447.6
751.4
603.7
492.3
459.5
1910.1
1967.3 2040.3
46 1370.8
0
24 1910.1
46
0
883.6
661.4
272.2
830.3
695.7 109
304.6 23
4
56
11
694.1 108
304.5 23
Observed in an argon matrix. Frequencies and intensities (km/mol) computed with in harmonic approximation using B3LYP/6–311++G(3df, 3pd)/
SDD except where noted anharmonic. HC≡ReH3 has a Csstructure with two equal Re-H bonds, and symmetry notations are based on the Cs
b
structure. Covered by precursor band.
(Table 1) locate the C-H stretching frequency at 3118.0
cm-1, which is only 0.5% higher and demonstrates that the
C-H vibration contains considerable anharmonicity.
The strongest absorption is split by the matrix cage at
1809.9 and 1804.0 cm-1, which shifts upon deuterium
substitution to 1302.5 and 1298.1 cm-1 (H/D ratios of 1.390
for a heavy metal hydride), and the 13C counterparts show
no isotopic shifts. These bands are due to a unique Re-H
stretching absorption, and they indicate that oxidative C-H
insertion by a Re atom clearly occurs. Two weaker product
absorptions at 1946.4 and 1910.1 cm-1 are also appropriate
for Re-H stretching modes. These weaker Re-H stretching
absorptions are consistent with the predicted symmetric and
antisymmetric Re-H stretching frequencies for the ReH2
subunit with equivalent Re-H bonds in HC≡ReH3, as shown
in Table 1 (predicted 6.8 and 6.3% higher). Anharmonic
frequency calculations find values that are only about 3%
higher than observed, which verifies the anharmonic nature
Figure 1. The C-H and C-D stretching regions for the laser-ablated
of these metal hydride modes.
Re atom reaction products with CH3X or CD3X (X ) F, Cl, and Br) in
excess argon at 8 K and their variation on UV (240 < λ < 380 nm)
photolysis. (a) Re + 0.5% CH3F or CD3F in Ar codeposited for 1 h. (b)
After photolysis. (c) Re + 0.5% CH3Cl or CD3Cl in Ar codeposited for
1 h. (d) After photolysis. (e) Re + 0.5% CH3Br or CD3Br in Ar
codeposited for 1 h. (f) After photolysis. (g) Re + 0.5% 13CH3F in Ar
codeposited for 1 h, and (h) After photolysis. The y label indicates the
methylidyne product absorptions.
An important weak absorption at 1048.8 cm-1 has its 13
C
counterpart at 1012.9 cm-1. These bands are due to the C≡Re
stretching mode. The observed 13C isotopic shift (35.9 cm-1)
is almost that computed for a pure C-Re stretching mode
[38.6 cm-1]. This diagnostic mode can appear only for a
methylidyne product.11 The calculated band position (1096.8
cm-1) is slightly higher, as expected for density functional
theory,21 and the calculated 13C shift for the methylidyne
(35.4 cm-1) is in excellent agreement with the observed shift
for the HC≡ReH3 molecule.
13C substitution for CH3F leads to a red shift of 10.8 cm-1
(12C/13C ratio of 1.0035), which is close to the calculated
12.0 cm-1 shift.
On the basis of the frequencies being higher than for C-H
stretching frequencies for saturated hydrocarbons and their
variation upon halide and isotopic substitution, the above
observed absorptions verify that a primary reaction product
containing a single C-H bond with higher s character
(HC≡ReH2X) is also formed in reactions of methyl halides,
parallel to the case of HC≡ReH3. No other strong product
absorptions are observed in the Re + CH3X spectra, in
contrast to the previous results of Group 3–6 metals, which
also reveal Grignard-type insertion (CH3-ReX) and meth-
ylidene (CH2)ReHX) complexes in addition to the meth-
ylidyne (HC≡ReH2X) products.8
Re + CH3X. Figure 1 illustrates isotopic methyl halide
spectra in the C-H stretching regions. The product absorp-
tion marked “y” increases about 30% upon UV photolysis
regardless of the reactant. CH3F and CH3Cl give similar
yields whereas CH3Br shows a relatively lower yield. The
C-H stretching frequencies of 3092.9, 3090.1, and 3089.8
cm-1 in the CH3F, CH3Cl, and CH3Br reaction product
spectra are 8.9, 11.7, and 12.0 cm-1 lower, respectively, than
the C-H mode at 3101.8 cm-1 in the HC≡ReH3 spectrum.
Similarly the C-D stretching frequencies of 2327.0, 2324.7,
and 2324.4 cm-1 in the CD3F, CD3Cl, and CD3Br spectra
(H/D ratios of 1.329) are 7.3, 9.6, and 9.9 cm-1 lower than
that of 2334.3 cm-1 in the CD4 spectra, respectively. The
The high preference for the Ct Re triple bond is predicted
from the low carbyne energy in DFT calculations, in line
Inorganic Chemistry, Vol. 47, No. 5, 2008 1655

Cho and Andrews
a
2
Table 2. Observed and Calculated Fundamental Frequencies (cm-1) of HC≡ReH2F Isotopomers in the Ground A′ Electronic State
HC≡ReH2F
DC≡ReD2F
H13C≡ReH2F
approximate
description
obs
anharmb
harm
int
obs
2327.0
harm
int
obs
3082.1
harm
int
A′ C-H str.
A′ ReH2 str.
A′ C≡Re str.
A′ HCRe bend
A′ ReH2 wag
A′ ReF str.
A′ ReH2 scis.
A′ CReF bend
A′′ ReH2 str.
A′′ HCRe bend
A′′ ReH2 twist
A′′ ReH2 rock
3092.9
3095.7
2091.6
1066.9
782.7
620.8
570.6
373.9
175.9
2059.2
827.4
613.6
178.4
3249.7
2155.3
1081.6
821.1
636.8
574.4
472.4
176.0
2126.4
853.0
639.2
255.5
24
52
7
2418.5
1529.8
1030.7
636.1
430.0
608.7
335.7
167.4
1508.6
671.0
454.0
194.8
18
27
6
13
55
94
1
5
20
6
3237.7
2155.3
1045.8
815.6
23
52
7
c
1480.2
2063.7
3
620.7
2
612.7
573.2, 566.4
24
168
5
5
38
8
612.5
572.6, 565.8
636.2
25
168
5
5
38
8
608.5, 606.6
573.9
472.4
173.1
2126.3
845.4
639.1
255.1
2041.0
632.3
1459.2
2040.4
631.8
50
7
25
4
50
7
a Observed in an argon matrix. Frequencies and intensities are for harmonic DFT calculations except where otherwise noted. HC≡ReH2F has a Cs structure with
c
two equal Re-H bonds. The symmetry notations are based on the Cs structure. b Calculated anharmonic frequencies. Covered by precursor band.
a
2
Table 3. Observed and Calculated Fundamental Frequencies (cm-1) of HC≡ReH2Cl Isotopomers in the Ground A′ Electronic State
HC≡ReH2Cl
DC≡ReD2Cl
approximate
description
obs
3090.1
anharmb
harm
int
obs
harm
int
A′ C-H str.
A′ ReH2 str.
3098.5
2079.6
1063.4
785.5
586.1
381.1
347.2
134.4
2053.3
837.4
607.4
127.7
3245.6
2144.5
1078.3
833.2
618.8
422.1
350.8
140.4
2113.4
852.6
636.6
220.9
27
72
8
3
73
2
63
3
35
5
45
5
2324.7
1479.8
2415.4
1521.9
1027.4
644.8
449.6
356.9
290.4
130.8
1499.4
668.3
452.4
162.9
20
38
7
2047.7
A′ C≡Re str.
A′ HCRe bend
A′ ReH2 wag
A′ ReH2 scis.
A′ ReCl str.
A′ CReCl bend
A′′ ReH2 str.
A′′ HCRe bend
A′′ ReH2 twist
A′′ ReH2 rock
3
593.5, 591.2
21
65
8
3
19
4
2026.1
616.6
1459.0
437.4?
22
3
a Observed in an argon matrix. Frequencies and intensities (km/mol) are from harmonic DFT calculations except where otherwise noted. HC≡ReH2Cl has
b
a Cs structure with two equal Re-H bonds. The symmetry notations are based on the Cs structure. Calculated anharmonic frequencies.
Table 4. Observed and Calculated Fundamental Frequencies (cm-1) of
with the Re + CH4 case. The most likely CH3-ReF (sextet
state), CH2)ReHF (quartet state), and HC≡ReH2F (doublet
state) products in their ground states are 47, 50, and 63
kcal/mol lower in energy than the reactants, respectively.
Furthermore, CH-ReCl (S), CH2)ReHCl (Q), and
HC≡ReH2Cl (D) are 51, 52, and 64 kcal/mol more stable
than the reactants, respectively, and CH3-ReBr (S) and
CH2)ReHBr (Q), and HC≡ReH2Br (D) are 52, 53, and 64
kcal/mol lower than the reactants, respectively. Regardless
of halogen size, the methylidyne complex is the most stable
among the most plausible products. Furthermore, the
observed and calculated methylidyne frequencies are in good
agreement, as shown in Tables 2, 3, and 4. The anharmonic
correction improves the agreement with the observed fre-
quencies for C-H and Re-H stretching modes, but not for
low frequency bending modes with more complicated
potential functions.
a
2
HC≡ReH2Br Isotopomers in the Ground A′ Electronic State
HC≡ReH2Br
DC≡ReD2Br
approximate
description
obs
anharmb harm int
obs harm int
A′ C-H str.
A′ ReH2 str.
A′ C≡Re str.
A′ HCRe bend
A′ ReH2 wag
A′ ReH2 scis.
A′ ReBr str.
A′ CReBr bend
A′′ ReH2 str.
A′′ HCRe bend
A′′ ReH2 twist
A′′ ReH2 rock
a
3089.8 3095.3 3245.2 27 2324.4 2415.1 21
2043.9 2081.5 2138.6 85 1478.1 1517.6 44
1061.9 1077.1
9
4
1026.2
640.7
9
3
819.1
628.1
585.3
246.2
138.3
825.3
592.6
609.5 85
379.2
242.0 30
125.8
438.5 37
274.6
235.2 27
116.3
1494.5 18
665.4
442.5 21
129.4
0
3
3
3
2060.9 2106.5 35
843.7
659.2
482.5
847.6
623.6 43
172.1
6
5
c
5
2
Observed in an argon matrix. Frequencies and intensities (cm-1
)
are for harmonic DFT calculations except where otherwise noted.
HC≡ReH2Br has a Cs structure with two equal Re-H bonds. The
symmetry notations are based on the Cs structure. b Calculated anharmonic
c
frequencies. Covered by precursor band.
Shown in Figure 2 are the methyl halide spectra in the
low frequency region. The strongest Re-F stretching absorp-
tions, split by the matrix at 573.2 and 566.4 cm-1, show very
small 13C shifts of -0.6 cm-1. The Re-F stretching mode
is, in fact, highly mixed with the ReH2 wagging and HCRe
bending modes, and DFT calculations predict that deuterium
substitution leads to a blue shift (34.3 cm-1), as shown in
Table 2 because of changes in mode mixing in this region
upon deuterium substitution. The CD3F spectra (Figure 2
(i-k)) show the strong Re-F stretching absorption at 608.5
cm-1. Observation of this unusual blue shift upon deuteration
further substantiates the formation of HC≡ReH2F because
similar blue shifts are not expected upon deuteration of
CH3-ReF and CH2)ReHF.
The absorptions at 632.3 and 612.7 cm-1 in the CH3F
spectra (Figure 1 (a and b)) have their 13C counterparts at
631.8 and 612.5 cm-1, but the D counterparts are not
observed because of their low frequency. They are assigned
1656 Inorganic Chemistry, Vol. 47, No. 5, 2008

IR Spectra of Methylidynes Formed in Reactions of Re Atoms
Figure 3. Re-H stretching regions for the laser-ablated Re atom reaction
product with CH3X (X ) F, Cl, and Br) in excess argon at 8 K and their
variation on UV (240 < λ < 380 nm) photolysis. (a) Re + 0.5% CH3F in
Ar codeposited for 1 h. (b) After photolysis. (c) Re + 0.5% CH3Cl in Ar
codeposited for 1 h. (d) After photolysis. (e) Re + 0.5% CH3Br in Ar
codeposited for 1 h. (f) After photolysis. (g) Re + 0.5% 13CH3F in
Ar codeposited for 1 h. (h) After photolysis. The labels y and P stand for
the methylidyne product and precursor absorptions, respectively.
at 1480.2 and 1459.2 cm-1 (not shown) (H/D ratios of 1.394
and 1.398).
Figure 2. The ReH2 wagging and twisting and Re-F stretching regions
for the laser-ablated Re atom reaction product with CH3X (X ) F, Cl, and
Br) and CD3F in excess argon at 8 K and their variation on UV (240 < λ
< 380 nm) photolysis. (a) Re + 0.5% CH3F in Ar codeposited for 1 h. (b)
After photolysis. (c) Re + 0.5% CH3Cl in Ar codeposited for 1 h. (d) After
photolysis. (e) Re + 0.5% CH3Br in Ar codeposited for 1 h. (f) After
photolysis. (g) Re + 0.5% 13CH3F in Ar codeposited for 1 h. (h)
After photolysis. (i) Re + 0.5% CD3F in Ar codeposited for 1 h. (j) After
photolysis. (k) After annealing to 28 K. The labels y and P stand for the
methylidyne product and precursor absorptions, respectively.
The absorptions at 2047.7 and 2026.1 cm-1 in the CH3Cl
spectra have their D counterparts at 1479.8 and 1459.0 cm-1
(H/D ratios of 1.384 and 1.389) and are assigned to the ReH2
symmetric and antisymmetric absorptions of HC≡ReH2Cl.
Only a weak absorption is observed at 2043.9 cm-1 in the
CH3Br spectra, and it is assigned to the symmetric ReH2
stretching absorption of HC′ReH2Br. The D counterpart is
found at 1478.1 cm-1 (H/D ratio of 1.383).
to the ReH2 twisting and wagging modes. The absorption at
620.7 cm-1 in the CD3F spectra is attributed to the HCRe
bending mode, whereas the H and 13C counterparts are not
observed because of the low intensities, as shown in Table
2. The observed low frequency absorptions all support
formation of HC≡ReH2F.
Re + CH2X2. Parallel investigations were done with
methylene halides. Figure 4 shows the C-H stretching and
low frequency regions of the IR spectra from reactions of
Re with CH2F2 and CH2FCl isotopomers, and the observed
frequencies are compared with the calculated values in Table
5. The product absorptions remain almost unchanged on
visible photolysis but double upon UV photolysis. Clearly,
fluorine substitution for hydrogen bonded to Re leads to an
increase in the methylidyne C-H stretching frequency, which
is ∼100 cm-1 higher than the precursor C-H stretching
frequency. We find 18.4 and 15.3 cm-1 increases in
frequency from HC≡ReH2F and HC≡ReH2Cl to HC≡ReHF2
and HC≡ReHFCl, respectively. The 13C substitution of
CH2FCl results in a 11.0 cm-1 red shift in the C-H stretching
frequency of the product (12/13 ratio of 1.0036), but
unfortunately, the product C-D stretching absorption in the
CD2FCl spectrum is covered by the CO2 impurity absorp-
tions.
In the CH3Cl spectra, the absorptions at 616.6 and 593.5
cm-1 are attributed to the ReH2 twisting and wagging modes
of HC≡ReH2Cl, and a weak absorption at 437.4 cm-1 is
tentatively assigned to the ReD2 twisting mode. The Re-Cl
and Re-Br stretching bands are too low in frequency to
observe. The only absorption, at 592.6 cm-1, in the CH3Br
spectra is assigned to the ReH2 wagging mode, whereas the
ReH2 twisting absorption is most likely covered by the strong
C-Br stretching band of the precursor.
The ReH2 stretching absorptions of HC≡ReH2X are more
difficult to observe because of their relatively lower intensi-
ties than those of previously studied C-H activation prod-
ucts8 and interfering bands in the region including precursor
overtones and metal cluster absorptions. The antisymmetric
ReH2 stretching absorption of HC≡ReH2F is observed at
2041.0 cm-1 in Figure 3 whereas the symmetric stretching
absorption is probably covered by the precursor C-F
stretching overtone at 2063.4 cm-1. The 13C counterparts are
observed at 2063.7 and 2040.4 cm-1 and the D counterparts
The Re-F stretching absorptions are observed at 628.2,
629.0, 631.4, and 628.6 cm-1 for HC≡ReHF2, HC≡ReHFCl,
H13C≡ReHFCl, and DC≡ReDFCl, respectively, and are
relatively insensitive to isotopic and halogen substitution.
The in- and out-of-plane C-H bending absorptions are also
observed on the blue and red sides of the Re-F stretching
bands, and the C-D counterparts are observed in the further
Inorganic Chemistry, Vol. 47, No. 5, 2008 1657

Cho and Andrews
at 2331.9 and 2324.0 cm-1 (H/D ratios of 1.329 and 1.330).
The in- and out-of-plane C-H bending absorptions are
observed at 623.9 and 602.2 cm-1, the 13C counterparts are
at 623.4 and 598.0 cm-1 (12/13 ratios of 1.0008 and 1.0070),
and the D counterparts are at 446.0 and 469.8 cm-1 (H/D
ratios of 1.399 and 1.282). Weak bands were also observed
at 2103.0 and 1509.6 cm-1 for the Re-H and Re-D
stretching modes (H/D ratio 1.393), which is the highest such
mode we have found for these compounds. The observed
and calculated frequencies and isotopic frequencies are in
very good agreement, as shown in Table 6. Finally, the
observed C-H stretching and bending absorptions with
appropriate isotopic shifts substantiate formation of the
methylidyne product, HC≡ReHCl2 in the reaction of Re with
methylene chloride.
Figure 5 also shows a sharp absorption at 552.5 cm-1
marked “m”. Previous studies indicate that a methylidyne
product is often formed after its methylidene derivative.8 In
this case, the CH2)ReCl2 methylidene is 9.2 kcal/mol higher
in energy than the methylidyne product. It is, therefore,
probable that the absorption originates from the CD2 wagging
mode of CD2)ReCl2(Q), the only strong absorption of the
product (calculated at 610.0 cm-1) in our observation range.
However, unfortunately the CH2 counterpart, predicted at
775.3 cm-1, is covered by the strong precursor C-Cl
stretching absorption.
Figure 4. Re-H and low frequency regions for laser-ablated Re atom
reaction product with CH2F2 and CH2FCl isotopomers in excess argon at 8
K. (a) Re + 0.5% CH2F2 in Ar codeposited for 1 h. (b) After visible (λ >
420 nm) photolysis. (c) After UV (240 < λ < 380 nm) photolysis. (d)
After visible and UV photolysis following codeposition Re with 0.5%
CH2FCl in Ar codeposited for 1 h. (e) After visible and UV photolysis
following deposition of Re with 0.5% 13CH2FCl in Ar for 1 h. (f) After
visible and UV photolysis following deposition of Re with 0.5% CD2FCl
in Ar for 1 h. The labels y and P designate the methylidyne product
absorption and the absorption in the precursor spectrum, respectively.
Re + C2H6. Shown in Figure 6 are spectra from the Re
and ethane reaction product in the Re-H stretching region.
The product absorptions marked y remain unchanged on
visible (λ > 420 nm) irradiation but increase ∼15% upon
UV (240 < λ < 380 nm) irradiation. They increase another
10% upon full arc (λ > 220 nm) irradiation and sharpen in
the early stage of annealing. Similar to those in the CH4
spectra, unique strong absorptions split by the matrix are
observed at 1786.3 and 1780.5 cm-1, which are 23.6 and
23.5 cm-1 lower than the corresponding frequencies in the
CH4 spectrum, respectively. Their D counterparts are ob-
served at 1285.9 and 1281.6 cm-1 (H/D ratios of 1.389),
which are 16.7 and 16.5 cm-1 lower, respectively, than the
corresponding frequencies in the CD4 spectrum. Substitution
with deuterium appropriately lowers the hydrogen stretching
frequencies.
The methylidyne complex (CH3C≡ReH3) is again the most
stable among the plausible products of Re + C2H6. The
monohydrido insertion product (CH3CH2-ReH) in its quartet
ground-state is 0.1 kcal/mol higher in energy than the
reactants, and its Re-H stretching frequency is predicted at
2072.4 cm-1, which is almost 300 cm-1 higher than the
observed values. The methylidene CH3-CH)ReH2 (Q),
another plausible product, is 3.2 kcal/mol lower than the
reactants, and the Re-H stretching bands are predicted at
1927.1 and 1848.9 cm-1 with an intesity ratio of about 1:2.
Cyclic (CH2)2ReH2 (Q) is 12 kcal/mol more stable than the
reactants, and the Re-H stretching frequencies are predicted
at 2013.2 and 1956.6 cm-1 with about 40% more intensity
for the former. On the other hand, the methylidyne complex
(CH3C≡ReH3) is 18 kcal/mol more stable than the reactants,
and the frequency of the strongest Re-H stretching band is
Figure 5. Re-H and D stretching and low frequency regions for laser-
ablated Re atom reaction product with CH2Cl2 isotopomers in excess argon
at 8 K. (a) Re + 0.5% CH2Cl2 in Ar codeposited for 1 h. (b) After visible
(λ > 420 nm) photolysis. (c) After UV (240 < λ < 380 nm) photolysis.
(d) After visible and UV photolysis following codeposition Re with 0.5%
13CH2Cl2 in Ar codeposited for 1 h. (e) After visible and UV photolysis
following deposition of Re with 0.5% CD2Cl2 in Ar for 1 h. The product
absorptions increase only slightly in the process of photolysis. The labels
y and m stand for the methylidyne and methylidene product absorptions,
respectively.
low frequency region. The single C-H stretching absorptions
on the blue sides of the precursor C-H stretching bands and
other product absorptions show a good correlation with the
predicted values, which substantiates the formation of
HC≡ReHX2 methylidynes as the primary product.
The IR spectra of CH2Cl2 and its isotopomers are shown
in Figure 5, and the frequencies are listed in Table 6. The
C-H stretching absorptions are split by the matrix at 3098.6
and 3090.7 cm-1 with 13C counterparts at 3088.2 and 3079.7
cm-1 (12/13 ratios of 1.0034 and 1.0036) and D counterparts
1658 Inorganic Chemistry, Vol. 47, No. 5, 2008

IR Spectra of Methylidynes Formed in Reactions of Re Atoms
Table 5. Observed and Calculated Fundamental Frequencies (cm-1) of HC≡ReHF2 and HC≡ReHFCl Isotopomers in the Doublet Ground Statesa
HC≡ReHF2
HC≡ReHFCl
DC≡ReDFCl
H13C≡ReHFCl
approximate
description
obs
harm
int
obs
harm
int
obs
harm
int
obs
harm
int
C-H str.
3111.3
3269.0
2206.8
1091.5
900.2
664.7
628.5
588.0
631.3
253.7
225.9
184.9
147.7
41
16
11
3
57
135
129
72
6
3105.4
3264.6
2205.2
1089.5
888.9
662.8
627.9
609.9
381.4
335.5
218.4
151.2
121.3
41
13
9
2430.2
1564.5
1038.0
698.2
481.2
627.6
461.5
381.1
242.3
203.4
145.3
119.8
29
7
7
3094.4
3252.4
2205.2
1053.4
881.3
662.1
627.8
606.2
381.3
335.3
213.4
149.7
121.1
40
13
8
Re-H str.
C≡Re str.
CReH bend
ReCH ip bend
Re-F str.
3
3
3
634.4
628.2
564.8
634.9
629.0
582.8
57
100
119
47
7
2
1
2
462.0
631.4
450.7
37
132
40
46
4
2
1
2
634.6
628.6
579.4
57
103
115
48
7
2
1
2
ReCH oop bend
Re-Cl(F) str.
HReFCl deform
CReHF deform
CReFCl deform
ReFCl bend
a
0
3
5
Observed in an argon matrix. Frequencies and intensities (km/mol) are for harmonic DFT calculations except where otherwise noted. HC≡ReHF2 has
a Cs structure with two equal Re-F bonds. The symmetry notations are based on the Cs structure.
a
2
Table 6. Observed and Calculated Fundamental Frequencies (cm-1) of HC≡ReHCl2 Isotopomers in the Ground A′′ Electronic State
HC≡ReHCl2
DC≡ReDCl2
H13C≡ReHCl2
approximate
description
obs
harm
int
obs
harm
int
25
5
5
obs
harm
int
A′ C-H str.
A′ ReH str.
3098.6, 3090.7
2103.0
3261.1
2200.6
1086.0
882.6
657.0
632.5
385.3
370.8
353.1
201.8
128.8
97.6
41
10
6
2331.9, 2324.0
1509.6
2427.4
1561.2
1034.6
692.8
490.1
468.4
385.0
370.4
254.0
184.8
124.0
96.3
3088.2, 3079.7
2103.0
3248.9
2200.6
1049.9
875.1
656.6
628.1
385.2
173.1
2126.3
845.4
639.1
255.1
A′ C≡Re str.
A′ HCRe bend
A′ HReC bend
A′′ HCRe bend
A′′ ReCl2 str.
A′ ReCl2str
A′′ ReH wag
A′′ CReCl bend
A′ ReCl2 bend
A′ ReCl2 def
3
3
623.9
46
90
101
13
6
3
0
0
446.0
52
22
99
14
4
2
0.3
0.3
623.4
602.2, 599.1
469.8, 467.2
598.0, 594.8
a Observed in an argon matrix. Frequencies and intensities (km/mol) are for harmonic DFT calculations except where otherwise noted. HC≡ReHCl2 has
a Cs structure with two equal Re-Cl bonds. The symmetry notations are based on the Cs structure.
Table 7. Methylidyne C-H Stretching Frequencies and Bond Propertiesa
properties
HC≡CH
HC≡ReH3 HC≡ReH2F HC≡ReH2Cl HC≡ReH2Br HC≡ReHF2 HC≡ReHFCl HC≡ReHCl2
CH3CH3
obs.
calc.
3284b
3412.3
0.962
47.82
52.34
0.222
-0.222
3101.8
3264.8
3092.9
3249.7
3090.1
3246.1
3089.8
2145.2
3111.3
3269.0
3105.4
3264.6
3098.6
3261.1
2979.3
3027.3
0.984
23.46
29.75
0.190
-0.570
obs./calc.
0.950
0.952
0.952
0.952
0.952
0.951
0.950
s-character (C-H)c
s-character (C-M)c
q(H)d
46.55
53.20
0.179
-0.236
0.577
1.0795
1.714
179.1
45.86
53.92
0.180
-0.283
0.885
1.0806
1.742
172.1
45.65
54.18
0.181
-0.218
0.608
1.0810
1.723
172.8
45.52
54.30
0.181
-0.212
0.556
1.0811
1.722
173.3
47.99
51.82
0.191
-0.294
1.276
1.0788
1.719
179.2
47.18
52.62
0.189
-0.225
0.962
1.0792
1.718
179.2
46.73
53.08
0.190
-0.175
0.648
1.0796
1.718
179.9
q(C)d
q(Re)d
r(C-H) (Å)
R(C-M) (Å)
HCRe (°)
1.0617
1.196
180.0
1.0908
1.527
111.3
a Frequencies are in cm-1. Harmonic frequencies computed with B3LYP, 6–311++G(3df, 3pd), and the SDD core potential. b Reference 22. c s-Character
of the carbon atom in the bond. Natural charge.
d
predicted at 1829.9 cm-1 as shown in Table 8, which is 2.4%
higher than the observed value and is appropriate for the
B3LYP calculation.21
(CH3)2Re, CH2CHReH3, CHCHReH4, and HC≡CReH5,
but none of them is predicted to be as stable as
CH3C≡ReH3, and none reproduces the observed results
in the Re-H stretching region.
Two more Re-H stretching bands are observed at
1953.5 and 1900.1 cm-1, as shown in Figure 6, and their
D counterparts are observed at 1406.0 and 1359.8 cm-1
(H/D ratios of 1.389 and 1.397, respectively). The
observed three hydrogen stretching absorptions, one with
much lower frequency, support a principal product
containing a ReH3 group with unequal Re-H bonds,
parallel to the case of Re + CH4. The observed Re-H
stretching frequencies reasonably match with the predicted
frequencies for CH3C≡ReH3. Extensive calculations are
also carried out for other plausible products, such as
Another absorption at 701.4 cm-1 (not shown) has its D
counterpart at 517.0 cm-1 (H/D ratio of 1.357) and is
assigned to the ReH3 deformation mode. A weak absorption
observed at 449.7 cm-1 is tentatively assigned to the ReH3
rocking mode without observation of the D counterpart,
which is beyond our observation limit. The observed
absorptions, whose frequencies match with those of the bands
predicted to be strong for CH3C≡ReH3, substantiate forma-
tion of the methylidyne complex, parallel to formation of
HC≡ReH3 and its halide derivatives. No other product
Inorganic Chemistry, Vol. 47, No. 5, 2008 1659

Cho and Andrews
and this trend is consistent with the computed s character
and bond length values.
Whereas most molecular properties change with the
halogen electronegativity, it is interesting that only the
variation of the s character in the C-H bond estimated by
NBO analysis10,23 is in line with the trend of the C-H
stretching frequencies and the calculated bond lengths as
shown in Table 7. The NBO computed s characters of the
carbon contribution to the C-H bonds are 47.82, 46.55,
45.86, 45.65, 45.52, and 23.46% for HC≡CH, HC≡ReH3,
HC≡ReH2F, HC≡ReH2Cl, HC≡ReH2Br, and C2H6, respec-
tively, and they are compared with the C-H stretching
frequencies observed at 3284, 3101.8, 3092.9, 3090.1,
3089.8, and 2979.3 cm-1. Unlike in acetylene, the difference
in electronegativity (2.5, 2.1, and 1.9 for C, H, and Re using
the Pauling scale, respectively) leads to polarization of the
C≡Re bond, which in turn allocates more s character to the
polarized C≡Re bond. As a result, the methylidyne C-H
bond gains p character and weakens accordingly, as com-
pared to acetylene itself. Although the observed C-H
stretching frequency decrease is only 4.1 cm-1 in the
HC≡ReH2X series, it is matched by a 0.0005 Å increase in
the C-H bond length (Table 7).
This trend continues with the HC≡ReHF2, HC≡ReHFCl,
and HC≡ReHCl2 methylidynes, which have higher C-H
stretching frequencies and s characters as given in Table 7.
The frequency decreases from 3111.3 to 3098.6 cm-1 and
the s character from 47.99 to 46.73 in this dihalogen series.
A similar trend was found within the trihalogen series, but
a smaller 3103.8 to 3097.3 cm-1 C-H frequency decrease
was accompanied by a larger 49.22 to 47.80 s character
decrease.9 In contrast, the highest s character HC≡ReF3
methylidyne had a 7.5 cm-1 lower C-H frequency than
HC≡ReHF2, and this reversal is probably due to the greater
inductive effect of three fluorine atoms.
Reaction Mechanism. The methylidyne complexes are
produced through initial C-H or C-X insertion reactions
of Re with methane, methyl halides, methylene halides, and
ethane. A Re atom, electronically excited in the laser ablation
process or by UV irradiation into the strong 326 nm argon
matrix absorption involving the resonance 6sf6p transi-
tion,24,25 executes C-H or C-X insertion, and R-hydrogen
migration follows, as given in reaction 1. The increase in
product absorptions on UV irradiation along with a decrease
in methane overtone absorption further supports the initial
step in reaction 1 for Re atoms and methane molecules in
close proximity in the argon matrix. Furthermore, the
methylidyne complexes are clearly the most stable among
the plausible products in all of the Re systems investigated
in this study. Nevertheless, the high preference of the Ct Re
triple bond is perhaps surprising in consideration of the
relatively small energy lowering relative to the next most
stable methylidene products, namely 7, 17, 13, 11, and 18
Figure 6. The Re-H and Re-D stretching regions for the laser-ablated
Re atom reaction product with ethane in excess argon at 8 K and their
variations. (a) Re + 0.5% C2H6 in Ar codeposited for 1 h. (b) After
photolysis (λ > 420 nm). (c) After photolysis (240 < λ < 380 nm). (d)
After photolysis (λ > 220 nm). (e) Annealing to 28 K. (f) Re + 0.5%
C2D6 in Ar codeposited for 1 h. (g) After photolysis (240 < λ < 380 nm).
(h) After photolysis (λ > 220 nm). (i) Annealing to 28 K. The y and *
labels denote the methylidyne product and Re metal cluster absorptions.
species were observed in the Re + C2H6 spectra, as in the
methane and methyl halide cases.
Methylidyne C-H Stretching Absorptions. The carbon-
hydrogen stretching region is normally congested, prohibiting
observation of important weak absorptions in this diagnostic
region. The simple precursor spectrum and the methylidyne
product with no interfering ligand chromophores allow for
observation of the elusive C-H stretching band of the
H-C≡M moiety. Although in principle, the C-H stretching
mode interacts with the C≡M stretching mode, these modes
are separated by some 2000 cm-1, and this interaction is
minimal. In addition, it is well-known that the hydrogen
stretching frequency generally increases with the amount of
s character in the C-H bond, and a C-H stretching
frequency above 3000 cm-1 is normally regarded as evidence
for the presence of an adjacent multiple bond.20 However,
the high observed C-H stretching frequency of 3101.8 cm-1
is substantially lower than those of acetylene (the antisym-
metric stretching band at 3284 cm-1 in an argon matrix and
the IR-inactive symmetric stretching mode is some 90 cm-1
higher).22 The methylidyne C-H stretching frequencies and
related bond properties are summarized in Table 7. The
observed C-H stretching frequency decreases in the order
of HC≡ReH3, HC≡ReH2F, HC≡ReH2Cl, and HC≡ReH2Br,
(23) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(24) Klotzbucher, W. E.; Ozin, G. A. Inorg. Chem. 1980, 19, 376.
(25) Carstens, D. H. W.; Brashear, H.; Eslinger, D. R.; Gruen, D. M. Appl.
Spectrosc. 1972, 26, 184.
(22) Herzberg, G. Infrared and Raman Spectra; D. Van Nostrand: Princeton,
New Jersey, 1945.
1660 Inorganic Chemistry, Vol. 47, No. 5, 2008

IR Spectra of Methylidynes Formed in Reactions of Re Atoms
a
2
Table 8. Observed and Calculated Fundamental Frequencies (cm-1) of CH3C≡ReH3 Isotopomers in the Ground A′ Electronic State
CH3C≡ReH3
CD3C≡ReD3
approximate
description
obs
1953.5
1786.3, 1780.5
449.7
1900.1
1425.6
anharmb
harm
int
obs
1406.0
harm
int
A′ ReH2 str.
A′ ReH str.
A′ ReH3 rock
A′′ ReH2 str.
A′′ CH3 scis.
A′′ ReH3 deform
1985.5
1758.4
451.4
1936.6
1418.2
706.1
2073.3
1829.9
477.3
2021.5
1460.5
763.8
101
282
26
55
13
1470.9
1299.9
338.7
1435.8
1051.4
549.2
55
138
1
29
6
1285.9, 1281.6
1359.8
1030.6
517.0
701.4
31
12
a
Observed in an argon matrix. Frequencies and intensities (km/mol) are from harmonic DFT calculations except where otherwise noted. CH3C≡ReH3
b
has a Cs structure with two equal Re-H bonds. The symmetry notations are based on the Cs structure. Calculated anharmonic frequencies.
Figure 7. Energies of the insertion, methylidene, and methylidyne
complexes relative to the Re atom and halomethane reactants. Notice that
the methylidyne product becomes more stable than other complexes as the
number of chlorine atoms increases. For the methane reaction the products
are quartet, doublet, and doublet states, respectively, and for the halogen
derivatives, the products are sextet, quartet, and doublet states, respectively.
kcal/mol in the CH4, CH3F, CH3Cl, CH3Br, and C2H6reactions.
Reaction products some 20 kcal/mol higher than the most
stable have often been formed during reactions of transition
metal atoms with CH4 and methyl halides or photolysis
afterward and trapped in the solid argon matrix.8 However,
in these Re complexes the dynamics are different, and
R-hydrogen migration appears to be faster than relaxation
of the energized intermediates in the matrix cage, regardless
of the reactant. Also, reaction 1 goes to the lowest energy
final methylidyne product. However, for the dihalogen
Figure 8. The structures of HC≡ReH3, HC≡ReH2F, HC≡ReH2Cl,
HC≡ReH2Br, HC≡ReHF2, HC≡ReHFCl, and HC≡ReHCl2 optimized at
the level of B3LYP/6–311++G(3df,3pd)/SDD. The bond lengths and angles
substituted precursors, the methylidene complex is only 9.2
kcal/mol higher in energy than the methylidyne final product,
and a small amount of the methylidene intermediate is
trapped in the matrix.
2
2
are in Å and °. These methylidynes have Cs structures in their A′ or A′′
ground states (except for HC≡ReHFCl). The HC≡ReH3 and CH3C≡ReH3
complexes have the two equal shorter Re-H bonds and one longer Re-H
bond.
Re* + CH3X f [CH3-ReX]* f
[CH2)ReHX]* f HC ≡ ReH2X (1)
Structures. The methylidyne complexes in their doublet
ground states, except for the mixed halogen species, all have
Cs structures as illustrated in Figure 7. One of the hydrogen
atoms bonded to the Re atom in the structures of HC≡ReH3
and CH3C≡ReH3 is tilted more with a longer Re-H bond
(lower stretching frequency, higher electron density, and
higher absorption intensity) than the other two. It is important
to note that the singly occupied HOMO is antibonding in
this longer Re-H bond and is bonding in the shorter Re-H
bonds. In addition, calculations for HC≡ReH3 and
Re* + CH2X2 f [CH2X-ReX]* f
[CH2)ReX2]* f HC ≡ ReHX2 (2)
Figure 8 summarizes the energies of the three steps in these
reaction mechanisms as a function of the number of chlorine
substituents. The thermodynamic driving force for R-halogen
transfer is obvious, and the preference for the methylidyne
product is enhanced in the haloform and tetrahalide systems.9
Inorganic Chemistry, Vol. 47, No. 5, 2008 1661

Cho and Andrews
CH3C≡ReH3 with BPW91/6–311++G(3df,3pd)/SDD, MP2/
6–311++G(2d,p)/SDD, and the more rigorous CCSD/
6–311++G(3df,3pd)/SDD for Re and (CCSD/6–311++G
(2d,p)/SDD for CH3C≡ReH3) also give similar Cs structures
with a longer Re-H bond (1.735, 1.702, and 1.719 Å,
respectively, for HC≡ReH3 and 1.741, 1.740, and 1.742 Å,
respectively, for CH3C≡ReH3). These rhenium methylidyne
complexes are particularly interesting because of the unique
C≡Re triple bonding and the distorted structures about the
metal center, which is due to the Jahn–Teller effect,26 as
has been described in detail for the XC≡ReX3 and
HC≡ReX3 complexes.9
C5Me5)(Br)3Re≡CC(CH3)3,28 H-C≡W,4 and BrW≡CH(dmpe-
d12)2 compounds,7respectively.
Conclusions
2
The A′ ground-state rhenium methylidyne complexes,
HC≡ReH3, HC≡ReH2F, HC≡ReH2Cl, HC≡ReH2Br, and
CH3C≡ReH3 are exclusively produced in reactions of laser-
ablated Re atoms with the corresponding alkane and methyl
halide precursors. Analogous 2A′′ methylidynes HC≡ReHF2,
HC≡ReHFCl, and HC≡ReHCl2 are observed in the reaction
with methylene halides. The relative stabilities of the
methylidyne complexes among the plausible reaction prod-
ucts are reproduced by DFT calculations. The observed
vibrational characteristics are in very good agreement with
the calculated values for the methylidyne complexes within
the limits of the harmonic approximation. Particularly, the
uniquely strong Re-H absorptions with relatively low
frequencies observed in CH4 and C2H6 spectra are mostly
due to stretching of the hydrogen atom with the longer Re-H
bond length where Jahn–Teller distortion26 is responsible for
reduction of 3-fold symmetry in the HC≡ReH3 and
CH3C≡ReH3 complexes.
2
The nondegenerate A′ ground states and asymmetric
molecular structures of HC≡ReH3 and CH3C≡ReH3 at the
Re centers are due to the lifting of 3-fold symmetry by
Jahn–Teller distortion driven by an open shell e-symmetry
orbital,26 which is not present in the symmetrical singlet state
HC≡WH3 complex.27 We have shown that the spin–orbit
coupling effects are significant for the XC≡ReX3 and
HC≡ReX3 complexes but are not large enough to quench
the large Jahn–Teller distortion.9
In the monohalogen derivatives, the halogen atoms replace
the hydrogen atom with the longer Re-H bond, and the
equivalent Re-H bonds are even shorter (1.654 Å) as shown
in Figure 8. The singly occupied HOMO is antibonding for
this longer bond, as shown in the TOC graphic. In the present
series of methylidyne molecules, the shortest Re-H bond
is found for HC≡ReHF2, with the highest Re-H stretching
frequency (2103.0 cm-1) where the inductive effect of
fluorine on the Re orbitals is again apparent. The C≡Re bond
and the Re-F bonds become progressively shorter as the
number of fluorine ligands at the Re center is increased from
1 to 3.9
The methylidyne H-C bond lengths of 1.079–1.081 Å
are compared to those of 1.076 and 1.081 Å for H-C≡W4
and HC≡WH3,27 respectively, and the C-H stretching
frequency for the latter is calculated to be 29 cm-1 lower
with half of the infrared intensity than that of HC≡ReH3.
The C≡Re bond length increases substantially upon substitu-
tion of F (from 1.714 to 1.742 Å) due to the reduced electron
density in the bond or alternatively increased positive charge
and contraction of 5d valence orbitals, and the C≡Re bond
then decreases with increasing halogen size, indicating that
the C≡Re bond becomes stronger with a lower ligand
electronegativity. The calculated C≡Re bond lengths of
1.714–1.742 Å are compared with the C≡M bond lengths
of 1.755, 1.737, and 1.77(1) Å measured for the (η5-
The methylidyne C-H stretching absorptions, which are
seldom observed because of low intensity and interference
from other hydrogen stretching absorptions, are clearly
observed with isotopic shifts in the matrix-isolated product
spectra. The C-H stretching frequency is ∼100 cm-1 higher
than those of normal saturated hydrocarbons but is substan-
tially lower than that of acetylene. The observed frequency
decreases in the order of HC≡ReH3, HC≡ReH2F,
HC≡ReH2Cl, and HC≡ReH2Br, which matches with the
calculated frequencies and bond lengths for these species.
Natural bond orbital (NBO) analyses reveal that the meth-
ylidyne C-H stretching frequency increases steadily with
the amount of s character in the C-H bond. Polarization in
the C≡Re bond apparently increases the s character in the
triple bond, which in turn decreases the s character in the
C-H bond instead and lowers the methylidyne C-H
stretching frequency. The C-W bond is polarized even more
with calculated natural charges of -0.34 and +0.75, and
the s character for the H-C bond is reduced to 45.6%, which
results in the longer and lower frequency C-H bond in the
HC≡WH3 molecule.27
Trihydrido complexes of Re have been proposed as
intermediates in the synthesis of dihydrido complexes.10b We
have prepared the simple, trihydrido (HC≡ReH3 and
CH3C≡ReH3) as well as dihydrido halide (HC≡ReH2X) and
monohydrido dihalide (HC≡ReHX2) methylidyne complexes
in reactions of Re with the simple halomethanes and ethane
and have trapped them in solid argon for spectroscopic
analysis.
(26) Herzberg, G., Electronic Spectra of Polyatomic Molecules; D. Van
Nostrand: Princeton, New Jersey, 1966.
(27) Cho, H.-G.; Andrews, L.; Marsden, C. Inorg. Chem. 2005, 44, 7634
and references therein (W + CH4).
Acknowledgment. We gratefully acknowledge financial
support from NSF Grant CHE 03-52487 to L. A.
(28) (a) Felixberger, J. K.; Kiprof, P.; Herdtweck, E.; Herrmann, W. A.;
Jakobi, R.; Gutlich, P Angew. Chem., Int. Ed. Engl. 1989, 28, 334.
(b) Herrmann, W. A.; Felixberger, J. K.; Anwander, R.; Herdtweck,
E.; Kiprof, P.; Riede, J. Organometallics 1990, 9, 1434.
IC701505W
1662 Inorganic Chemistry, Vol. 47, No. 5, 2008