Titanium, zirconium, and hafnium metal atom reactions with CF4, CCl4, and CF2Cl2: A matrix isolation spectroscopic and DFT investigation of triplet XC÷MX3 complexes

Article abstract of DOI:10.1021/om070120g

Laser-ablated group 4 transition metal atoms react with CF₄ to form triplet state electron-deficient FC÷MFa methylidyne complexes, which are identified by their infrared spectra and comparison to density functional vibrational frequency calculations of stable possible products. Of particular interest in these complexes are the strong C-X bonds and carbon-metal n bonding. The two unpaired electrons on carbon are drawn to the electron-deficient transition metal center, forming a partially filled triple bond, which is approximately equal in length to a classical C=M double bond. Reactions with carbon tetrachloride form the analogous ClC÷MCl₃ complexes, whereas reactions with CF₂Cl₂ form a mixture of FC÷MFCl₂ and ClC÷MF₂Cl species. The FC÷MFCl₂ complexes involving more α-Cl transfer are favored in the reaction of excited metal atoms during sample deposition, but UV irradiation photoisomerizes FC÷MFCl₂ to the lower energy ClC÷MF₂Cl complexes with more α-F transfer to the metal center.

Full text of DOI:10.1021/om070120g

Organometallics 2007, 26, 2519-2527
2519
Titanium, Zirconium, and Hafnium Metal Atom Reactions with
CF4, CCl4, and CF2Cl2: A Matrix Isolation Spectroscopic and DFT
Investigation of Triplet XC÷MX3 Complexes
Jonathan T. Lyon and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319
ReceiVed February 6, 2007
Laser-ablated group 4 transition metal atoms react with CF4 to form triplet state electron-deficient
FC÷MF3 methylidyne complexes, which are identified by their infrared spectra and comparison to
density functional vibrational frequency calculations of stable possible products. Of particular interest
in these complexes are the strong C-X bonds and carbon-metal π bonding. The two unpaired electrons
on carbon are drawn to the electron-deficient transition metal center, forming a partially filled triple
bond, which is approximately equal in length to a classical CdM double bond. Reactions with carbon
tetrachloride form the analogous ClC÷MCl3 complexes, whereas reactions with CF2Cl2 form a
mixture of FC÷MFCl2 and ClC÷MF2Cl species. The FC÷MFCl2 complexes involving more R-Cl transfer
are favored in the reaction of excited metal atoms during sample deposition, but UV irradiation
photoisomerizes FC÷MFCl2 to the lower energy ClC÷MF2Cl complexes with more R-F transfer to the
metal center.
but reactions with CHX3 (X ) F and Cl) yielded triplet
HC÷MX3 species (except for Ti + CHF3, where the reaction
stopped at the CHFdTiF2 intermediate).11,12 These complexes
are unique in that the two unpaired electrons on carbon are
shared partially with the transition metal center. The formation
of this novel electron-deficient C÷M triple bond is more
prevalent in the FC÷TiF3 and ClC÷TiCl3 complexes in the
reaction between laser-ablated titanium atoms and CX4 (X ) F
and Cl).13 We now report on reactions of the entire group 4
transition metal series with CF4 and CCl4, and the mixed
chlorofluorocarbon CF2Cl2 for comparison. A particular question
we want to answer in this investigation is whether FC÷MFCl2,
ClC÷MF2Cl, neither complex, or both complexes are formed
and how this distortion of symmetry effects the partially filled
C÷M triple bonds.
Introduction
Transition metal compounds are important for their roles in
catalysis. In particular, the study of organotitanium compounds
has been an active field of research.1-3 Recently, nanostructural
carbon has been synthesized by the reaction of titanium
carbide with chlorine gas, with TiCl4 as a byproduct of the
reaction.4-6 Freon compounds are hazardous to the ozone
layer, and the activation of C-X bonds is a necessary process
in their remediation.7 However, C-F bonds are the strongest
known carbon single bond,8 and their activation is not always
trivial.
We have recently reacted group 4 transition metal atoms with
CH3X precursors (X ) H, F, Cl, and Br).9 In these experiments,
the primary reaction products are the double-bonded CH2dMHX
methylidene complexes, which show considerable agostic
interactions between the transition metal center and one set of
C-H-bonding electrons. Titanium, zirconium, and hafnium all
combined with CH2X2 (X ) F and Cl) to yield the very stable
CH2dMX2methylidenes,whichshowednoagosticdistortions,10-12
Experimental and Theoretical Methods
Our experimental design has been described in detail previ-
ously.14 In brief, metal atoms, produced by laser ablation with a
Nd:YAG laser, were co-deposited with a dilute mixture (0.25-
1.0%) of reagent vapor (CF4, CCl4, CF2Cl2, 13CCl4, or 13CF2Cl2)15
in argon onto a CsI window cooled to 8 K. The resulting reaction
products were frozen in the inert matrix, and the infrared spectrum
was recorded on a Nicolet Magna 550 spectrometer. Matrix samples
were irradiated for 10 min periods by a medium-pressure mercury
arc lamp with the globe removed (λ > 220 nm) with or without a
Pyrex (λ > 290 nm) filter and subsequently annealed to various
temperatures. Additional infrared spectra were recorded following
each procedure.
* Corresponding author. E-mail: lsa@virginia.edu.
(1) Bini, F.; Rosier, C.; Saint-Arroman, R. P.; Neumann, E.; Dadlemont,
C.; de Mallmann, A.; Lefebvre, F.; Niccolai, G. P.; Basset, J.-M.; Crocker,
M.; Buijink, J.-K. Organometallics 2006, 25, 3743.
(2) Zhang, Y.; Mu, Y. Organometallics 2006, 25, 631.
(3) Anthis, J. W.; Filippov, I.; Wigley, D. E. Inorg. Chem. 2004, 43,
716.
(4) Leis, J.; Perkson, A.; Arulepp, M.; Nigu, P.; Svensson, G. Carbon
2002, 40, 1559.
(5) Zetterstro¨m, P.; Urbonaite, S.; Lindberg, F.; Delaplane, R. G.; Leis,
J.; Svensson, G. J. Phys.: Condens. Matter 2005, 17, 3509.
(6) Permann, L.; La¨tt, M.; Leis, J.; Arulepp, M. Electrochim. Acta 2006,
51, 1274.
(12) Lyon, J. T.; Andrews, L. Organometallics 2007, 26, 332 (Ti, Zr,
and Hf + CH2Cl2 and CHCl3).
(7) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810.
(8) Strauss, S. H. Chem. ReV. 1993, 93, 927.
(9) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and
references therein.
(13) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 9858 (Ti + CF4
and CCl4).
(14) (a) Andrews, L. Chem. Soc. ReV. 2004, 33, 123. (b) Andrews, L.;
Citra, A. Chem. ReV. 2002, 102, 885, and references therein.
(15) (a) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1978, 68, 5568.
(b) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1978, 68, 5577. (c)
Milligan, D. E.; Jacox, M. E.; McAuley, J. H.; Smith, C. E. J. Mol.
Spectrosc. 1973, 45, 377.
(10) Lyon, J. T.; Andrews, L. Organometallics 2006, 25, 1341 (Ti +
CH2F2).
(11) Lyon, J. T.; Andrews, L. Inorg. Chem. 2007, 46, ASAP 12/19/06
(Ti, Zr, and Hf + CH2F2 and CHF3).
10.1021/om070120g CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/06/2007

2520 Organometallics, Vol. 26, No. 10, 2007
Lyon and Andrews
Figure 1. Infrared spectra in the 1480-1400, 800-680, and 500-440 cm-1 regions taken after (a) laser-ablated titanium atoms
were reacted with 1.0% CF4/Ar during condensation at 8 K for 1 h, and the resulting matrix was subjected to (b) irradiation with
light λ > 290 nm, (c) irradiation with λ > 220 nm, (d) annealing to 30 K, and (e) irradiation with λ > 220 nm. Arrows indicate product
absorptions.
have been reported by earlier workers.15 Some of these are
Theoretical computations on possible reaction products and
intermediates were performed using the Gaussian 98 program.16
In all instances, the B3LYP hybrid density functional was em-
ployed.17 All atoms were given a moderate 6-311+G(2d) basis set
labeled in the chlorofluoromethane figures.
Ti + CF4 and CCl4. Titanium reactions with carbon
tetrafluoride and carbon tetrachloride have been reported
except for the transition metal center, where the SDD pseudopo-
recently.13 Here we discuss these assignments to provide a
tential was utilized.18,19 Such DFT calculations predict vibrational
foundation for this article and report computed parameters for
frequencies with reasonable accuracy for transition metal, halogen
these complexes at the same level of theory that will be used
compounds. For example, the strong t2 mode for TiF4 is computed
for the zirconium and hafnium analogues in order to allow for
as 805 cm-1, which is slightly higher than the 800 cm-1 observed
comparisons and analysis of group trends.
value.20 The strong antisymmetric Ti-Cl stretching frequency for
The reaction between titanium and CF4 produced four
absorptions at 481.7, 686.1, 752.8, and 1453.1 cm-1 (Figure
1), which increased together on UV irradiation. These product
peaks are assigned to the symmetric (FC)-Ti stretching mode,
two Ti-F stretching frequencies, and a very high C-F
stretching mode, respectively.13 The characterization of these
functional group vibrations leads to two possible products.
Analogous to titanium reactions with CH4, CH3F, CH2F2, and
CHF3,10,11,23-25 the singlet CF2dTiF2 methylidene complex
was considered. However, this possible product is predicted
to have strong infrared active modes at 688.8, 770.5, 1087.7,
and 1237.9 cm-1. Although the two Ti-F stretching modes
are in fairly good agreement with the observed product
peaks, the C-F stretching modes are predicted too far below
the observed frequency to warrant this assignment. Triplet
FC÷TiF3 is calculated to be 19 kcal/mol lower in energy
than the possible singlet methylidene and 146 kcal/mol below
Ti and CF4. This very stable product is computed to have a
weak band at 480.3 cm-1 and strong infrared absorptions at
705.6, 766.2, and 1470.2 cm-1, which reproduce the observed
vibrations (Table 1) within the accuracy expected for DFT.23-26
We must point out the very high-frequency diagnostic C-F
stretching mode. Hence, the observed spectrum is assigned to
the triplet FC÷TiF3 species with a unique very high C-F
stretching mode.13 In addition, a weak 792.5 cm-1 band
increased slightly on UV irradiation, which is appropriate for
TiF3 in solid argon.27
TiCl4 is calculated at 500 cm-1, which compares to the 503 cm-1
argon matrix observation.21 All energy values reported include zero-
point vibrational corrections. Our electronic energy calculations find
TiCl4 to be 285 kcal/mol lower in energy than the Ti atom and
two Cl2 molecules, and gas phase thermodynamic values give 295
( 2 kcal/mol.22 Although we cannot determine the absolute
accuracy of the new product energies, we believe that the relative
stabilities are correct. We employ calculated product energies and
vibrational frequencies to help identify the new product molecules
in these reactions.
Results and Discussion
Products formed in the reactions of group 4 transition metals
with CF4, CCl4, and CF2Cl2 will be reported in turn. Irradiation
of these precursors by the laser-ablation plume gave weak metal-
independent absorptions for radical and intermediate species that
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega,
N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A.
G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; 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.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.11.4; Gaussian, Inc.:
Pittsburgh, PA, 2002.
(17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(18) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
(19) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(20) Beattie, I. R.; Jones, P. J. J. Chem. Phys. 1989, 90, 5209.
(21) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1971,
3, 257.
Similarly, titanium reactions with carbon tetrachloride pro-
duced two product absorptions at 487.0 and 1151.5 cm-1. In
(23) Andrews, L.; Cho, H.-G.; Wang, X. Inorg. Chem. 2005, 44, 4834
(Ti + CH4).
(24) Cho, H.-G.; Andrews, L. Inorg. Chem. 2004, 43, 5253 (Ti + CH3F).
(25) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2004, 108, 6294 (Ti +
CH3F).
(26) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Bytheway, I.; Wong, M. W. Chem. Phys. Lett. 1998, 282, 219.
(27) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. J. Chem. Phys. 1969,
51, 2648.
(22) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values
for Thermodynamics; Hemisphere Publishing Corp.: New York, 1989.

Triplet XC÷MX3 Complexes
Organometallics, Vol. 26, No. 10, 2007 2521
Table 1. Observed and Calculated Fundamental Frequencies
a
of FC÷TiF3
FC÷TiF3
F13C÷TiF3
calcd (int)
approximate modeb
obsdc
calcd (int)
CTiF bend (e)
FTiF bend (e)
76.4 (0)
182.3 (4)
75.9 (0)
180.5 (2)
TiF3 umbrella (a1)
FCTi def (e)
(FC)÷Ti stretch (a1)
Ti-F stretch (a1)
Ti-F stretch (e)
C-F stretch (a1)
190.4 (9)
189.8 (9)
216.1 (21)
480.3 (52)
705.6 (210)
766.2 (462)
1470.2 (274)
212.7 (22)
478.0 (51)
705.6 (211)
766.2 (462)
1429.3 (262)
481.7
686.1
752.8
1453.1
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
b
scaled and in cm-1, and computed infrared intensities are in km/mol. Mode
c
symmetries for C3V molecule. Argon matrix.
Figure 2. Infrared spectra in the 1440-1390 and 665-635 cm-1
regions taken after (a) laser-ablated zirconium atoms were
reacted with 0.5% CF4/Ar during condensation at 8 K for 1 h,
and the resulting matrix was subjected to (b) irradiation with
light λ > 290 nm and (c) irradiation with λ > 220 nm, and
when (d) laser-ablated hafnium atoms were reacted with 0.5%
CF4/Ar during condensation at 8 K for 1 h, and the resulting
matrix was subjected (e) irradiation with light λ > 290 nm
and (f) irradiation with λ > 220 nm. Arrows indicate product
absorptions.
Table 2. Observed and Calculated Fundamental Frequencies
a
of ClC÷TiCl3
ClC÷TiCl3
Cl13C÷TiCl3
approximate modeb
obsdc
calcd (int)
obsdc
calcd (int)
CTiCl bend (e)
51.1 (0)
111.7 (0)
112.6 (1)
218.5 (2)
346.8 (0)
435.8 (125)
481.6 (288)
51.0 (0)
111.5 (0)
112.4 (1)
211.4 (2)
346.5 (0)
435.8 (125)
481.6 (287)
ClTiCl bend (e)
TiCl3 umbrella (a1)
ClCTi def (e)
(ClC)÷Ti stretch (a1)
Ti-Cl stretch (a1)
Ti-Cl stretch (e)
C-Cl stretch (a1)
d
d
Table 3. Observed and Calculated Fundamental Frequencies
487.0
487.0
a
of FC÷ZrF3 and FC÷HfF3
1151.5 1133.6 (74)
1114.1 1095.9 (69)
FC÷ZrF3
FC÷HfF3
calcd (int)
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
approximate modeb
obsdc
calcd (int)
obsdc
scaled and in cm-1, and computed infrared intensities are in km/mol. Mode
b
c
d
symmetries for C3V molecule. Argon matrix. Absorption below our spectral
CMF bend (e)
91.9 (2)
92.1 (4)
limits.
FMF bend (e)
MF3 umbrella (a1)
FCM def (e)
(FC)÷M stretch (a1)
M-F stretch (a1)
M-F stretch (e)
C-F stretch (a1)
161.4 (20)
163.0 (14)
204.3 (23)
420.8 (81)
637.0 (120)
655.5 (374)
163.5 (14)
156.7 (20)
191.8 (24)
397.0 (72)
628.9 (282)
635.4 (80)
this instance, carbon-13 isotopic substitution and natural abun-
dance chlorine isotopes were employed to confirm the mode
assignments as Ti-Cl and C-Cl stretching modes, respec-
tively.13 Two possible products are again likely candidates. The
CCl2dTiCl2 species must be considered, as analogous CH2d
TiHCl and CH2dTiCl2 complexes have been formed previ-
ously.12,28 However, the possible singlet CCl2dTiCl2 meth-
ylidene complex was predicted to have strong infrared absorptions
at 435.4, 506.0, 739.4, and 939.3 cm-1. Since the product C-Cl
stretching mode is at 1151.5 cm-1, the spectrum cannot be
assigned to the CCl2dTiCl2 species. The predicted absorptions
of the other possible triplet ClC÷TiCl3 species (Table 2) are in
good agreement with experiment. Hence, the observed spectrum
is assigned to this electron-deficient methylidyne complex. In
addition, we observe small amounts of TiCl4 in our spectra at
503 cm-1 and TiCl3 at 497 cm-1, indicating that intermediates
along the Ti + CCl4 f TiCl4 + C reaction pathway are possible
complexes frozen in our inert matrix.21 We also observe
chlorocarbon transient species such as the CCl3 radical from
laser-plume photochemistry of the precursor.15a,c
d
d
648.4
656.8
641.9
658.2
1413.7 1433.9 (192) 1426.5 1438.1 (162)
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are in
b
cm-1
symmetries for C
,
and computed infrared intensities are in km/mol. Mode
c
d
3V molecule. Argon matrix. Absorption below our spectral
limits.
The possible singlet CF2dZrF2 complex is predicted to be
150 kcal/mol more stable than the sum of the individual
zirconium and CF4 precursors. However, this complex is
computed to have two strong C-F stretching modes at 1041.2
and 1207.7 cm-1, and no appropriate absorptions were observed.
Conserving the singlet multiplicity, a third fluorine atom can
transfer to the zirconium center, forming a bent FCsZrF3
complex, which is 13 kcal/mol lower in energy than the
methylidene. This species is predicted to have strong infrared
absorptions at 642.2, 648.8, and 1388.6 cm-1. However, the
triplet FC÷ZrF3 analogue is an additional 24 kcal/mol lower in
energy, and the predicted spectrum (637.0, 656.8, and 1433.9
cm-1) matches the observed absorptions even better (Table 3)
particularly since the calculated C-F mode is expected to fall
above the observed value.13,26 Hence the observed spectrum is
assigned to the FC÷ZrF3 complex.
Zr + CF4. Laser-ablated zirconium atoms react with tet-
rafluoromethane to produce a single reaction product with
infrared absorptions at 648.4, 656.8, and 1413.7 cm-1. All three
absorptions increased in unison on UV irradiation (Figure 2)
and can be assigned to a single reaction product. The upper
absorption at 1413.7 cm-1 is almost 40 cm-1 below the C-F
stretching mode of the FC÷TiF3 complex. The two lower
absorptions are between the strong Zr-F stretching modes of
Hf + CF4. Carbon tetrafluoride reacts with hafnium atoms
to produce a single reaction product with infrared absorptions
at 641.9, 658.2, and 1426.5 cm-1 (Figure 2). The upper
absorption falls between the C-F stretching modes of the
FC÷TiF3 and FC÷ZrF3 complexes, and the two lower absorp-
tions are appropriate for Hf-F stretching modes, as HfF4 has
ZrF4 at 668 cm-1 and ZrF2 at 633.5 cm-1 29,30
.
(28) Cho, H.-G.; Andrews, L. Inorg. Chem. 2005, 44, 979 (Ti +
CH3Cl).
(29) Bu¨chler, A.; Berkowitz-Mattuck, J. B.; Dugre, D. H. J. Chem. Phys.
1961, 34, 2202.
been observed at 645 cm-1 29
.
The singlet CF2dHfF2 species is calculated to be 144 kcal/
(30) Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1973, 5, 89.
mol lower in energy than the sum of the precursors. However,

2522 Organometallics, Vol. 26, No. 10, 2007
Lyon and Andrews
Figure 4. IR spectra in the 1140-1080 and 440-400 cm-1 regions
of the spectra taken after (a) laser-ablated zirconium atoms were
reacted with 0.5% CCl4/Ar, (b) the matrix sample was irradiated
at λ > 290 nm and (c) at λ > 220 nm, and after (d) zirconium
atoms were reacted with 0.5% 13CCl4/Ar (90% 13C enriched), (e)
the matrix sample was irradiated at λ > 290 nm and (f) at λ > 220
nm. The product absorptions are marked with arrows.
exception of the one C-Cl stretching peak at 1119.2 cm-1 (with
IR intensity 59 km/mol). For the triplet complex, a Zr-Cl
stretching mode calculated at 408.7 cm-1 is in reasonable
agreement with experiment (Table 4) and the C-Cl stretching
mode is computed at 1100.9 cm-1 (28 km/mol). Hence it is
most likely that the reaction product observed at 1131.8 and
419.2 cm-1 is the lower energy triplet ClC÷ZrCl3 complex.
Our calculation overestimates the C-F stretching mode and
underestimates the C-Cl stretching mode for the titanium
product species.13 We note that the ZrCl4 antisymmetric
stretching fundamental has been observed in the 250 °C gas-
Figure 3. Optimized geometries of the FC÷MF3 and ClC÷MCl3
complexes computed with C3V symmetry at the B3LYP//6-311+G-
(2d)/SDD level of theory. Bond lengths are in angstroms and angles
are in degrees.
the infrared spectrum is predicted to have two C-F stretching
absorptions at 1041.4 and 1200.8 cm-1, and these absorptions
are not observed. The next intermediate is the singlet FCsHfF3
species, which is 24 kcal/mol lower in energy. This complex is
expected to relax to triplet FC÷HfF3, which has C3V symmetry
and is 23 kcal/mol lower in energy than the singlet analogue.
The predicted infrared absorptions for this species (strong
infrared absorptions at 628.9, 635.4, and 1428.1 cm-1) match
those observed within the limits expected for DFT (Table 3).
Hence, the observed spectrum is assigned to the triplet FC÷HfF3
complex.
phase IR spectrum at 423 cm-1 29 Our weak 427 cm-1 argon
.
matrix band is probably due to ZrCl4.
CCl2dHfCl2 is predicted by our theoretical computations to
be 174 kcal/mol more stable than the CCl4 and Hf atom
reactants. For this complex, the strongest infrared active mode
(C-Cl stretching; 129 km/mol) is predicted at 705.4 cm-1
.
Again this is a clean region of the spectrum, and no band was
observed. The singlet ClCsHfCl3 species is 22 kcal/mol lower
in energy than the double-bonded CCl2dHfCl2 complex. For
this species, the only absorption that is within our spectral range
is the C-Cl stretching mode, which is predicted at 1120.3 cm-1
(46 km/mol). The last complex that needs to be considered is
the triplet ClC÷HfCl3, which is an additional 22 kcal/mol lower
in energy. For this species, the C-Cl stretching mode is
predicted (Table 4) at 1107.5 cm-1 with lower intensity (19
km/mol). Although no absorptions were detected, it is most
likely that the lowest energy triplet complex is formed in low
yield but not observed here. Our prediction for the strongest
mode of ClC÷HfCl3 (375 cm-1) is below the 393 cm-1
absorption29 reported for HfCl4, and we cannot observe these
low-frequency absorptions. For comparison, we display the
structures of the lowest energy triplet complexes in Figure 3.
The structures calculated for the FC÷MF3 product species
are illustrated in Figure 3, where bonding trends, particularly
the common C-F bond length, can be compared.
Zr and Hf + CCl4. Zirconium reacted with carbon tetra-
chloride to produce a product with a weak band at 1131.8 cm-1
and a strong infrared absorption at 419.2 cm-1, which increased
together on UV irradiation, as shown in Figure 4. The weak
band shifted to 1195.0 cm-1 with carbon-13, but the strong
absorption showed no isotopic shift, which identifies C-Cl and
Zr-Cl stretching modes, respectively. Hafnium is more difficult
to evaporate, and no product absorptions were observed with
hafnium. We will discuss possible product species and base our
assignment in part on relative calculated product energies.
The CCl2dZrCl2 complex lies 180 kcal/mol lower in energy
than the sum of the reactant energies. Our theoretical computa-
tions on this species find only one Zr-Cl stretching mode to
be within our spectral range at 418.7 cm-1, which agrees fairly
well with the observed absorption at 419.2 cm-1. However, this
species is also predicted to have two C-Cl stretching modes,
with the strongest (139 km/mol) at 707.0 cm-1. This is a clean
region in the infrared spectrum where no product absorption
was observed. The bent singlet ClCsZrCl3 complex is 14 kcal/
mol lower in energy than the methylidene complex, and the
symmetrical triplet ClC÷ZrCl3 complex is an additional 21 kcal/
mol lower in energy. For the singlet complex, all vibrational
modes are predicted to fall below our spectral limits with the
Ti + CF2Cl2. Titanium atoms react with CF2Cl2 to produce
four sets of new absorptions. Product peaks at 486.9, 730.7,
and 1453.1 cm-1 decreased dramatically after radiation with λ
> 290 nm and are labeled A in Figure 5. Absorptions at 466.9,
705.9, and 755.2 cm-1 decreased slightly on exposure to light
with λ > 290 nm, but increased on full-arc photolysis (λ >
220 nm), and are denoted B in Figure 5. Two absorptions at
646.9 and 743.6 cm-1 increased markedly on photolysis with a
Pyrex filter (λ > 290 nm), decreased slightly on full-arc
irradiation, and are labeled C. Finally, stable absorptions at 508.9

Triplet XC÷MX3 Complexes
Organometallics, Vol. 26, No. 10, 2007 2523
a
Table 4. Observed and Calculated Fundamental Frequencies of ClC÷ZrCl3 and ClC÷HfCl3
ClC÷ZrCl3
calcd (int)
Cl13C÷ZrCl3
ClC÷HfCl3
calcd (int)
approximate modeb
obsdc
obsdc
calcd (int)
obsdc
CMCl bend (e)
53.2 (0)
94.2 (3)
94.9 (2)
199.6 (6)
321.7 (22)
374.0 (87)
408.7 (238)
1100.9 (28)
53.2 (0)
94.1 (3)
94.9 (2)
192.7 (6)
321.1 (22)
373.9 (87)
408.7 (238)
1063.9 (27)
52.4 (0)
90.9 (4)
94.1 (2)
193.4 (8)
302.8 (41)
363.3 (40)
375.0 (176)
1107.5 (19)
MCl3 umbrella (a1)
ClMCl bend (e)
ClCM def (e)
(ClC)÷M stretch (a1)
M-Cl stretch (a1)
M-Cl stretch (e)
C-Cl stretch (a1)
d
d
d
d
419.2
1131.8
419.2
1095.0
b
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are unscaled and in cm-1, and computed infrared intensities are in km/mol. Mode symmetries
c
d
for C3V molecule. Argon matrix. Absorption below our spectral limit.
Table 5. Observed and Calculated Fundamental Frequencies
a
of FC÷TiFCl2
FC÷TiFCl2
F13C÷TiFCl2
approximate mode
obsdb
calcd (int)
obsdb
calcd (int)
CTiCl bend
CTiF bend
62.9 (0)
68.0 (0)
62.9 (0)
67.8 (0)
ClTiCl bend
FTiCl bend
TiFCl2 umbrella
FCTi def
113.3 (1)
146.2 (2)
151.3 (3)
212.2 (2)
235.4 (0)
399.3 (19)
482.2 (165)
503.0 (102)
744.5 (190)
113.1 (1)
146.0 (2)
150.7 (3)
206.5 (2)
228.0 (1)
398.3 (18)
482.1 (165)
501.5 (102)
744.5 (190)
FCTi def
Ti-Cl stretch
Ti-Cl stretch
(FC)÷Ti stretch
Ti-F stretch
C-F stretch
c
c
486.6
d
486.9
(495)
730.7
730.7
Figure 5. Infrared spectra in the 1480-1425 and 800-450 cm-1
regions taken after (a) laser-ablated titanium atoms were reacted
with 1.0% CF2Cl2/Ar during condensation at 8 K for 1 h, and the
resulting matrix was subjected to (b) irradiation with light λ > 290
nm, (c) irradiation with λ > 220 nm, (d) annealing to 30 K, and
(e) irradiation with λ > 470 nm. Product absorptions labeled A
belong to the FC÷TiFCl2 complex, B peaks are assigned to the
ClC÷TiF2Cl species, and the identity of C peaks is not resolved.
Common precursor products CF2Cl radical and parent anion (P-)
are labeled in the figure; see ref 15.
1453.1 1472.4 (439) 1413.8 1431.5 (420)
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
b
scaled and in cm-1, and computed infrared intensities are in km/mol. Argon
c
d
matrix. Absorption below our spectral limit. Absorption in the 490-495
cm-1 region shows the photochemical behavior of this species.
only a 0.3 cm-1 13C red shift. These two product absorptions
are in the regions of Ti-F and Ti-Cl stretching modes,
respectively. (The observed Ti-F stretching mode is halfway
between the two Ti-F stretching modes of the FC÷TiF3 species,
and the Ti-Cl mode is almost identical to the antisymmetric
Ti-Cl stretching mode of the ClC÷TiCl3 complex.) These three
product peaks identify the reaction product as either CFCld
TiFCl or FC÷TiFCl2. The singlet CFCldTiFCl methylidene is
136 kcal/mol lower in energy than the sum of the CF2Cl2 and
titanium atom reactants, and the triplet analogue has the same
energy. However, for the singlet complex the C-F stretching
mode is predicted to be at 1168.9 cm-1, nearly 285 cm-1 lower
than the observed peak. The triplet CFClsTiFCl complex has
the C-F stretching mode at 1200.3 cm-1, which is still too far
below the observed peak to warrant assigning the A absorptions
to either of these complexes. The bent singlet FCsTiFCl2
complex is predicted to be only 1 kcal/mol higher in energy
than the methylidene complex. Here the C-F stretching mode
is predicted higher at 1306.4 cm-1. Once the third halogen
transfers to the titanium center, spin relaxation to the lower
energy triplet FC÷TiFCl2 complex occurs. The triplet complex
is 17 kcal/mol lower in energy than the singlet analogue (152
kcal/mol below Ti + CF2Cl2). Here the C-F stretching mode
is predicted at 1472.4 cm-1 with a 40.9 cm-1 13C isotopic shift,
in excellent agreement with the 1453.1 cm-1 observed band
and 39.3 cm-1 shift. The Ti-F and Ti-Cl stretches are predicted
at 744.5 and 482.2 cm-1, respectively, also in good agreement
with the observed peaks (Table 5). The broad 495 cm-1
absorption is probably due to the symmetric (FC)-Ti stretch
calculated at 503.0 cm-1. The second Ti-Cl stretching mode
Figure 6. IR spectra in the 1470-1390 and 780-450 cm-1 regions
of the spectra taken after (a) laser-ablated titanium atoms were
reacted with 1.0% CF2Cl2/Ar, and (b) the matrix sample was
photolyzed with λ > 220 nm, and after (c) titanium atoms were
reacted with 1.0% 13CF2Cl2/Ar (90% 13C enriched), and (d) the
matrix sample was photolyzed with λ > 220 nm. Common precursor
products CF2Cl radical and parent anion (P-) are labeled in the
figure; see ref 15.
and 785.4 cm-1 increased on full-arc irradiation and are labeled
TiF2Cl2 in Figure 3.
The upper A absorption at 1453.1 cm-1 shifts to 1413.8 cm-1
on 13C isotopic substitution, indicative of a C-F stretching mode
(Figure 6). The next absorption at 730.7 cm-1 does not shift on
carbon-13 substitution, and the last A peak at 486.9 cm-1 shows
is predicted at 399.3 cm-1
, which is too low for us to observe
here. Hence the observed A absorptions are assigned to the

2524 Organometallics, Vol. 26, No. 10, 2007
Lyon and Andrews
Table 6. Observed and Calculated Fundamental Frequencies
be assigned to mostly Ti-F stretching modes. Hence they could
arise from either triplet CCl2sTiF2, singlet CCl2dTiF2, or
singlet ClCsTiF2Cl intermediate species. Our calculations find
these complexes 143, 144, and 145 kcal/mol lower in energy
than the reactants, respectively. The two Ti-F stretching modes
of the triplet CCl2sTiF2 complex are predicted to be strong
infrared absorbers at 675.2 and 760.6 cm-1 and to show 0.9
and 0.0 cm-1 13C isotopic shifts, respectively. However, this
complex also has a strong antisymmetric C-Cl stretching mode
calculated at 771.6 cm-1, which is not observed. Next, the
singlet CCl2dTiF2 methylidene complex is predicted to have
two strong Ti-F stretching modes at 668.5 and 776.9 cm-1
(1.2 and 1.0 cm-1 13C shift, respectively), which are again in
good agreement with the two observed peaks. Again a stronger
antisymmetric C-Cl stretching mode is predicted lower at 737.2
cm-1. This peak again is not observed, but it could be hidden
behind the CF2Cl2 precursor absorption at 660-670 cm-1. As
all other absorptions for this species are predicted to be below
our spectral limits or too weak to be observed, it is possible
that the group C absorptions correspond to the singlet CCl2d
TiF2 complex. The third possible product is the bent singlet
ClCsTiF2Cl complex. Here the two Ti-F stretching modes are
predicted at 721.7 and 768.6 cm-1 (0.1 and 0.0 cm-1 13C isotopic
shifts, respectively), which again are in fairly good agreement
with experiment. The next two strongest bands are calculated
at 452.8 (Ti-Cl stretch) and 987.7 cm-1 (C-Cl stretch).
Although the Ti-Cl stretching mode could fall below our
spectral limits, the C-Cl stretching mode predicted at 987.7
cm-1 should appear in a clean region of the spectrum. Hence,
the C absorptions cannot belong to the singlet ClCsTiF2Cl
complex. In addition, we believe it is more likely that the rigid
argon matrix would prevent R-Cl transfer rather than singlet to
triplet spin relaxation in the formation of triplet ClC÷TiF2Cl.
It is also quite likely that the group C absorptions correspond
to an intermediate species between the conversion of the triplet
FC÷TiFCl2 and ClC÷TiF2Cl complexes, which we did not
locate theoretically. Hence, the weak group C absorptions remain
unidentified.
of ClC÷TiF2Cla
ClC÷TiF2Cl
Cl13C÷TiF2Cl
approximate mode
obsdb
calcd (int)
obsdb
calcd (int)
CTiCl bend
CTiF bend
FTiCl bend
TiF2Cl umbrella
FTiF bend
56.5 (0)
61.8 (0)
149.4 (2)
155.7 (2)
179.8 (4)
56.3 (0)
61.3 (0)
149.4 (2)
154.7 (2)
179.4 (4)
ClCTi def
ClCTi def
217.5 (5)
223.1 (4)
212.0 (5)
217.3 (4)
(ClC)÷Ti stretch
Ti-Cl stretch
Ti-F stretch
Ti-F stretch
C-Cl stretch
c
383.9 (29)
467.0 (178)
724.7 (231)
768.3 (203)
1133.9 (29)
c
383.6 (29)
466.9 (178)
724.6 (231)
768.3 (202)
1096.3 (26)
466.9
705.9
755.2
d
466.0
705.9
755.2
d
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
b
scaled and in cm-1, and computed infrared intensities are in km/mol. Argon
c
d
matrix. Absorption below our spectral limit. Peak is hidden behind
precursor absorption.
Scheme 1
triplet FC÷TiFCl2 complex, which is the only complex we found
with such a high frequency and very intense C-F stretching
mode.
The three B absorptions all show practically no carbon-13
isotopic shift (Figure 6), and hence these vibrational modes have
no carbon character. The two upper absorptions at 755.2 and
705.9 cm-1 are slightly above the Ti-F stretching modes of
the FC÷TiF3 complex. Hence they are assigned to Ti-F
stretching modes. The lower absorption at 466.9 cm-1 is near
the observed Ti-Cl stretching mode of the ClC÷TiCl3 complex,
and hence it is assigned to a Ti-Cl mode. The observation of
these three vibrational modes (two Ti-F and one Ti-Cl
stretching) leads to the assignment of the triplet ClC÷TiF2Cl
complex with confidence. Computations led to a stable triplet
complex 163 kcal/mol lower in energy than the sum of the
titanium atom and CF2Cl2 reactants. The three observed
peaks agree well with the predicted infrared spectrum
(Table 6). Note that the only other absorption calculated to be
above the lower limit of our spectrometer is the C-Cl stretching
mode, which is predicted to fall behind a CF2Cl2 precursor
absorption. It is of interest to note here that the FC÷TiFCl2
complex formed on deposition is converted into the ClC÷TiF2-
Cl species on UV irradiation, as summarized in Scheme 1.
Surely this transformation must occur through a bridge-bonded
transition state. The scheme also serves to illustrate the triplet
state product structures.
Zr + CF2Cl2. Zirconium atoms reacted with CF2Cl2 to
produce product absorptions that can be separated into three
groups based on product behavior. Two group A absorptions
at 656.8 and 1428.4 cm-1 increased in intensity on photolysis
with λ > 470 nm and with λ > 290 nm, decreased in intensity
after exposure to radiation of λ > 220 nm, and remained
unchanged after annealing to 30 K (Figure S1 in Supporting
Information). This absorption red shifts 39.2 cm-1 upon carbon-
13 isotopic substitution, and it can be assigned to a C-F
stretching mode (Figure 7). Notice that this mode is 14.7 cm-1
higher than the analogous mode of the triplet FC÷ZrF3 complex.
This high C-F stretching mode was diagnostic for the formation
of triplet FC÷TiF3, as no other molecule was found to have
such a high C-F vibrational frequency.13 Hence, it is probable
that this group A absorption belongs to triplet FC÷ZrFCl2.
Computations on this complex led to a structure 198 kcal/mol
lower in energy that the sum of zirconium atom and CF2Cl2
precursors, which is the lowest energy structure we could find
with a C-F bond. The C-F stretching mode for this FC÷ZrFCl2
complex is predicted at 1439.3 cm-1 (40.1 cm-1 13C shift),
which accurately reproduces the 1428.4 cm-1 (39.2 cm-1
carbon-13 shift) value observed (Table 7). The next strongest
Two other absorptions at 508.9 and 785.4 cm-1 increased
on irradiation like TiF4 and TiCl4 in our experiments with CF4
and CCl4. The above bands correlate with the calculated
spectra31 for TiF2Cl2 and are assigned accordingly.
The two weaker C absorptions at 646.9 and 743.6 cm-1 show
0.7 and 3.0 cm-1 13C isotopic shifts, respectively, which can
predicted mode for this complex (Zr-F stretching, 650.2 cm-1
)
is near the 656.8 cm-1 band. All other modes computed for the
(31) Bauschlicher Jr., C. W.; Taylor, P. R.; Komornicki, A. J. Chem.
Phys. 1990, 92, 3982.
FC÷ZrFCl2 complex are calculated at or below our spectral

Triplet XC÷MX3 Complexes
Organometallics, Vol. 26, No. 10, 2007 2525
Table 8. Observed and Calculated Fundamental Frequencies
of ClC÷ZrF2Cla
ClC÷ZrF2Cl
Cl13C÷ZrF2Cl
approximate mode
obsdb
calcd (int)
obsdb
calcd (int)
CZrCl bend
CZrF bend
58.1 (0)
64.1 (0)
58.0 (0)
63.8 (0)
FZrCl bend
ZrF2Cl umbrella
FZrF bend
ClCZr def
ClCZr def
(ClC)÷Zr stretch
Zr-Cl stretch
Zr-F stretch
Zr-F stretch
C-Cl stretch
124.8 (4)
134.2 (7)
152.7 (8)
193.9 (9)
197.1 (8)
331.0 (58)
398.7 (122)
642.1 (161)
657.3 (170)
1099.1 (9)
124.8 (4)
133.7 (7)
152.6 (8)
188.1 (9)
191.0 (8)
330.3 (58)
398.6 (122)
642.1 (162)
657.3 (170)
1062.2 (9)
c
c
649.7
d
649.7
660.1
Figure 7. IR spectra in the 1440-1380 and 700-630 cm-1
regions of the spectra taken after (a) laser-ablated zirconium
atoms were reacted with 1.0% CF2Cl2/Ar, and (b) the matrix
sample was photolyzed with λ > 220 nm, and after (c)
zirconium atoms were reacted with 1.0% 13CF2Cl2/Ar (90% 13C
enriched), and (d) the matrix sample was photolyzed with λ
> 220 nm.
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
b
scaled and in cm-1, and computed infrared intensities are in km/mol. Argon
c
d
matrix. Absorption below our spectral limit. Absorption hidden by strong
CF2Cl2 precursor.
the Zr-F stretching modes), and it was not observed. Hence,
we assign the group B absorptions to the triplet ClC÷ZrF2Cl
complex.
Table 7. Observed and Calculated Fundamental Frequencies
a
of FC÷ZrFCl2
The two group C absorptions at 694.6 and 697.7 cm-1
increased slightly with λ > 290 nm and more upon full-arc
photolysis (λ > 220 nm) (Figure 5). They show 0.0 and 0.1
cm-1 13C isotopic shifts, respectively, and can be assigned to
Zr-F stretching modes. Hence three higher energy species are
possible. The triplet CCl2sZrF2, singlet CCl2dZrF2, and singlet
ClCsZrF2Cl complexes lie 166, 172, and 186 kcal/mol lower
in energy than the sum of the zirconium atom and CF2Cl2
precursors, respectively. The triplet CCl2sZrF2 complex is
computed to have Zr-F stretching modes at 623.9 and 656.4
cm-1, which are significantly lower than the observed peaks.
In addition, two C-Cl stretching modes are predicted at 758.0
and 887.1 cm-1. The lower C-Cl mode is quite intense (164
km/mol), but was not observed in experiment.
FC÷ZrFCl2
F13C÷ZrFCl2
approximate mode
obsdb
calcd (int)
obsdb
calcd (int)
CZrCl bend
CZrF bend
68.8 (0)
79.7 (1)
68.7 (0)
79.7 (1)
ClZrCl bend
FZrCl bend
ZrFCl2 umbrella
FCZr def
96.8 (2)
96.7 (2)
122.8 (4)
137.3 (6)
211.2 (4)
224.4 (2)
374.8 (22)
409.8 (131)
426.9 (101)
650.2 (151)
122.8 (4)
137.0 (6)
204.5 (4)
217.0 (2)
374.0 (20)
409.7 (130)
424.5 (101)
650.2 (151)
FCZr def
Zr-Cl stretch
Zr-Cl stretch
(FC)÷Zr stretch
Zr-F stretch
C-F stretch
c
c
656.8
656.8
1428.4 1439.3 (294) 1389.2 1399.2 (284)
a B3LYP//6-311+G(2d)/SDD level of theory. All frequencies are un-
b
scaled and in cm-1, and computed infrared intensities are in km/mol. Argon
The possible singlet CCl2dZrF2 species is predicted to have
Zr-F stretching modes at 613.2 and 650.8 cm-1, which again
are considerably lower than the observed peaks. Also, a strong
C-Cl stretching mode is predicted at 704.9 cm-1, which is not
observed. The last possible high-energy complex we considered
was the singlet ClCsZrF2Cl complex. Here the two Zr-F
stretching modes are predicted at 635.1 and 647.6 cm-1, which
are still lower than the observed peaks. However, no other
absorptions are predicted to be strong infrared absorbers and to
fall within our spectral limits. Hence, we are unable to assign
the weak group C absorptions.
c
matrix. Absorption below our spectral limit.
lower limit. Hence, the group A absorptions are assigned to
the triplet FC÷ZrFCl2 complex.
Two group B absorptions were observed at 649.7 and 660.1
cm-1. These absorptions increased slightly on photolysis with
λ > 290 nm, increased by more than a factor of 4 when exposed
to light with λ > 220 nm, and remained unchanged in intensity
after annealing to 30 K. The lower absorption showed no carbon-
13 isotopic shift, the upper B peak probably shows no isotopic
shift (the carbon-13 counterpart is hidden behind the 13CF2Cl2
precursor peak), and both absorptions can be assigned to
Zr-F stretching modes. The presence of two Zr-F stretching
modes and the intensity of the absorptions indicate that the
reaction product is either the singlet CCl2dZrF2 methylidene
complex or the triplet ClC÷ZrF2Cl complex. These two
molecules are predicted to be 172 and 207 kcal/mol lower in
energy than the sum of the individual reactants, respectively.
The singlet CCl2dZrF2 has two Zr-F stretching modes at 613.2
and 650.8 cm-1, which are below the observed bands. In
addition, two C-Cl stretching modes are predicted at 704.9 and
932.1 cm-1. The lower C-Cl stretching mode should be quite
intense, but it was not observed in our experiment. The triplet
ClC÷ZrF2Cl complex is predicted to have two Zr-F stretching
modes at 642.1 and 657.3 cm-1 (Table 8), accurately reproduc-
ing the observed bands. The only other absorption above our
spectral limit (C-Cl stretching) is predicted to be weak (5% of
Hf + CF2Cl2. Laser-ablated hafnium atoms react with CF2-
Cl2 to produce product absorptions, which can be separated into
three categories. Two group A absorptions at 650.6 and 1439.2
cm-1 increased in intensity after photolysis with λ > 290 nm
and λ > 220 nm, remained nearly unchanged after annealing
to 30 K, and decreased on a final full-arc photolysis with λ >
220 nm (Figure S2 in Supporting Information). The upper
absorption shifts to 1399.4 cm-1 upon carbon-13 isotopic
substitution (Figure 8) and can be assigned to the C-F stretching
mode. Notice that this absorption is 12.7 cm-1 higher than the
C-F stretching mode of the triplet FC÷HfF3 complex. Com-
putations on the possible FC÷HfFCl2 complex led to a
structure 201 kcal/mol lower in energy than the sum of
Hf atom and CF2Cl2 precursors (the lowest energy structure
we found with a C-F bond). This complex was predicted to
have one C-F stretching mode at 1442.5 cm-1 (40.5 cm-1 13
isotopic shift), in excellent agreement with the observed
C

2526 Organometallics, Vol. 26, No. 10, 2007
Lyon and Andrews
with considerable infrared intensity. Again, however, only one
absorption was observed. The singlet ClCsHfF2Cl complex
(189 kcal/mol lower in energy than the reactants) is predicted
to have two observable Hf-F stretching modes, and the singlet
FCsHfFCl2 complex (177 kcal/mol lower in energy) has only
one Hf-F stretching mode, but also possesses a strong C-Cl
stretching mode, which is not observed. Hence, we are unable
to assign this group C absorption.
Group Trends and Bonding Considerations. The geom-
etries of the FC÷MF3 and ClC÷MCl3 complexes are shown in
Figure 3, and the calculated parameters are listed in Table S3
(Supporting Information). A few interesting observations merit
comment. First, notice the elongation of the terminal C-X bonds
as the metal atom becomes heavier, which suggests decreasing
halogen conjugation with the C÷M π-bonding system. The
C÷M bond length is shorter in the chlorine complexes than in
the fluorine analogues. Also note that the Mulliken charges on
the carbon-bonded halogen atoms switch between the fluorine
and chlorine complexes, as Cl conjugates more effectively with
carbon and the C÷M π system than F. Hence, it appears that
decreasing the electronegativity of the halogen atom increases
the C÷M bond strength. This probably arises from contraction
of the metal valence d orbitals due to the increased positive
charge on the metal centers induced by the more electronegative
substituent.
The XC÷MX3 complexes have expectation values s2 in the
2.007 to 2.010 range, which are in excellent agreement with
the 2.000 value expected for a pure triplet state with two
unpaired electrons.13,16 These triplet complexes contain a σz
C-M single bond, and the two unpaired electrons in Cpx and
Cpy orbitals are shared with metal d orbitals to form degenerate
electron-deficient π bonds.13 Notice the increasing spin density
on carbon and decreasing spin density on the metal center for
both fluoride and chloride species as the metal size increases.
The C(2p) overlap with M(nd) clearly decreases with increasing
metal size. This decrease in C÷M π bonding accompanies a
decrease in halogen-carbon conjugation. The lowest spin on
carbon for ClC÷TiCl3 goes hand in hand with the shortest C÷Ti
bond, which has slightly more π-bonding character than the
FC÷TiF3 analogue. The corresponding chlorofluorocarbon
complexes also show shorter C÷M bonds for more R-Cl
substituents.
The computed parameters for the triplet mixed halogen
complexes are listed in Table S4 (Supporting Information). Here,
the C÷M bond length is shorter and stronger when two chlorine
atoms (rather that two fluorine atoms) are attached to the
transition metal center, which is in line with this effect for the
pure halogen species. Again the Mulliken charge on the terminal
halogen attached to the carbon switches charge sign when
changed from fluorine to chlorine, whereas all halogens attached
to the metal center remain negatively charged. Notice that the
FC÷MFCl2 complexes, which are the major product formed
on deposition, are roughly 10 kcal/mol higher in energy than
the corresponding ClC÷MF2Cl species. The latter increase at
the expense of the former complexes on UV irradiation. This
suggests that R-Cl transfer may be faster during reaction in the
condensing sample, but the more stable R-F transfer product
dominates after long-term exposure of the solid matrix sample
to UV light.
Figure 8. IR spectra in the 1450-1390 and 700-630 cm-1 regions
of the spectra taken after (a) laser-ablated hafnium atoms were
reacted with 1.0% CF2Cl2/Ar, and (b) the matrix sample was
photolyzed with λ > 220 nm, and after (c) hafnium atoms were
reacted with 0.25% 13CF2Cl2/Ar (90% 13C enriched), and (d) the
matrix sample was photolyzed with λ > 220 nm.
absorption (Table S1 in Supporting Information). The only other
absorption calculated to be in our experimental range (Hf-F
stretching mode at 632.5 cm-1) is observed at 650.6 cm-1 on
the side of the stronger group B absorptions (Figure 8b). Hence,
the group A absorptions are assigned to the triplet FC÷HfFCl2
complex.
Group B absorptions at 644.6 and 652.2 cm-1 increased
slightly in intensity after photolysis with a Pyrex filter (λ >
290 nm), doubled in intensity after exposure to radiation with
λ > 220 nm, remained nearly unchanged on annealing to 30 K,
and increased further after an additional photolysis with λ >
220 nm. Both absorptions show nearly no carbon-13 isotopic
shift (Figure 8) and can be assigned to two Hf-F stretching
modes. Hence, the group B absorptions probably correspond
to either the singlet CCl2dHfF2 or the triplet ClC÷HfF2Cl,
which are predicted to be 166 and 212 kcal/mol lower in energy
than the sum of the hafnium atom and CF2Cl2 precursors,
respectively. The singlet CCl2dHfF2 complex is predicted to
have two Hf-F stretching modes at 620.0 and 622.4 cm-1
.
However, both C-Cl modes of this complex are computed to
be quite intense at 709.6 and 916.9 cm-1. The upper absorption
would be covered by the CF2Cl2 precursor, but the lower C-Cl
stretching mode should be observed. As it was not seen in
experiment, group B absorptions probably do not correspond
to this complex. The triplet ClC÷HfF2Cl complex is computed
to have two Hf-F stretching modes at 629.7 and 632.1 cm-1
,
closer to the observed bands than those predicted for the
methylidene complex. The only other absorption that is calcu-
lated to fall within our spectral limits (C-Cl stretching) is
predicted to be too weak to observe (Table S2 in Supporting
Information). Hence, the two group B absorptions are assigned
to the triplet ClC÷HfF2Cl complex.
One group C absorption observed at 683.0 cm-1 (0.4 cm-1
13C isotopic shift) increased in intensity on exposure to UV
radiation. Hence, we are in search of a third reaction product
with a Hf-F stretching mode. The singlet CClFdHfClF and
triplet CClFsHfClF complexes were computed to be 159 and
161 kcal/mol lower in energy than the reactants and to have
strong C-F and C-Cl stretching modes, neither of which were
observed. The possible singlet CCl2dHfF2 and triplet CCl2s
HfF2 complexes lie 166 and 168 kcal/mol lower in energy than
the sum of the reactants. However, here both Hf-F stretching
and at least one of the C-Cl stretching modes were predicted
Conclusions
Laser-ablated Ti, Zr, and Hf atoms react with CF4 to form
very stable low-energy triplet FC÷MF3 complexes. These
species possess strong C-F bonds, characterized by very high

Triplet XC÷MX3 Complexes
Organometallics, Vol. 26, No. 10, 2007 2527
C-F stretching frequencies, and a unique C÷M electron-
deficient triple bond. Reactions with CCl4 formed the analogous
ClC÷MCl3 complexes, which have an even shorter computed
C÷M bond length, owing to less inductive contraction of the
metal valence d orbitals. Reactions with CF2Cl2 formed both
FC÷MFCl2 and ClC÷MF2Cl complexes. The latter are lower
in energy and increase at the expense of the former complexes
on UV irradiation. The transfer of spin density from carbon to
metal verifies the π-bonding interaction, which is favored by
R-Cl over R-F substitution.
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for support of this
research.
Supporting Information Available: Figures S1 and S2 for Zr,
Hf, and CF2Cl2 infrared spectra and Tables S1, S2, S3, and S4 for
calculated frequencies and structural parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM070120G