Formation and calculations of the simple terminal triplet pnictinidene molecules n÷MF3, P÷MF3, and As÷MF3 (M = Ti, Zr, Hf)

Article abstract of DOI:10.1021/ic900633k

Laser-ablated Ti, Zr, and Hf atoms react with NF₃, PF ₃, or AsF₃ to produce triplet state terminal pnictinidene N÷MF₃, P÷MF₃, or As÷MF₃ molecules, which are trapped in an argon matrix. Prod

Full text of DOI:10.1021/ic900633k

Inorg. Chem. 2009, 48, 6297–6302 6297
DOI: 10.1021/ic900633k
Formation and Calculations of the Simple Terminal Triplet Pnictinidene Molecules
N÷MF3, P÷MF3, and As÷MF3 (M = Ti, Zr, Hf)
Xuefeng Wang, Jonathan T. Lyon, and Lester Andrews*
Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319
Received March 31, 2009
Laser-ablated Ti, Zr, and Hf atoms react with NF3, PF3, or AsF3 to produce triplet state terminal pnictinidene N÷MF3,
P÷MF3, or As÷MF3 molecules, which are trapped in an argon matrix. Products are identified from infrared spectra and
comparison to theoretically predicted vibrations. Density functional theory calculations converge to C3v symmetry
structures for these lowest energy products. The two unpaired electrons in nitrogen 2p, phosphorus 3p, or arsenic 4p
orbitals are shared in different small amounts with empty metal nd orbitals leading to very weak degenerate πR
molecular orbitals based on bonding orbital analysis and spin density calculations. This weak π bonding interaction
with early transition metal group 4 nd orbitals is optimum for Zr with phosphorus 3p orbitals.
Introduction
isolated and characterized. Examples of related molecules
include the phospha- and arsaalkenes CF3PdCF2 and
CF3AsdCF2, REdC(NMe2)2, (E = P, As) and phosphini-
dene complexes.10,11
Nitrenes, compounds of the general formula R-N, are
analogues of carbenes, and both generally function as reac-
tive intermediates. The common source of nitrenes is the
photolysis of azides.1 Most nitrenes have triplet ground
states, and CH3-N is a simple example.2,3 A characteristic
reaction of alkyl nitrenes is substituent rearrangement to give
the imine.1 Methylene imine CH2dNH is a well-known more
stable isomer of triplet CH3-N.4,5 The more stable perfluoro
analogue CF2dNF is easier to investigate by experiment,6,7
and the CF3-N isomer has recently been observed in the
triplet ground-state by matrix isolation electron spin reso-
nance methods.8 These two isomers have approximately the
same energy within the error of calculations. We have been
unable to find experimental evidence for phosphorus or
arsenic analogues of the simple above reactive intermediates.
Such terminal pnictinidenes tend to coordinate with formal
double bonding to organometallic fragments,9 but under
matrix isolation conditions the terminal pnictinidene can be
Reactions of laser ablated group 4 transition metal atoms
with ammonia have prepared transition metal imine analo-
gues MH2dNH, which were characterized by matrix infrared
spectra and density functional calculations.12,13 The analo-
gous reactions of ammonia with Mo and W metal atoms
produced instead the lower energy MH3tN terminal metal
nitrides. However, the greater stability of metal fluoride
bonds supported the formation of the Cr, Mo, and W
trifluoro derivatives MF3tN and the phosphide species for
Mo and W in the analogous reactions with NF3 and PF3.14,15
Reactions with arsenic trifluoride followed suit with terminal
arsenides for Mo and W.16 Accordingly, group 4 metal atom
reactions with NF3, PF3, and AsF3 were performed to seek
the novel trifluorometal triplet state pnictinidenes MF3-N,
MF3-P, and MF3-As, and a combined experimental and
theoretical investigation is reported here. Hereafter, we will
use the E÷MF3 notation to indicate these triplet ground-
state product group 15 non-metal and group 4 transition
metal containing molecules.
* To whom correspondence should be addressed. E-mail: lsa@virginia.edu.
(1) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part B:
Reactions and Synthesis, 4th ed.; Kluwer Academic/Plenum Publishers: New
York, 2001.
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1986, 25, 3177. (b) Weber, L. Eur. J. Inorg. Chem. 2007, 4095.
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(12) Chen, M.; Lu, H.; Dong, J.; Miao, L.; Zhou, M. J. Phys. Chem. A
2002, 106, 11456.
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r
2009 American Chemical Society
Published on Web 04/29/2009
pubs.acs.org/IC

6298 Inorganic Chemistry, Vol. 48, No. 13, 2009
Wang et al.
Experimental and Computational Methods
Laser ablated Ti (Goodfellow), Zr, and Hf (Johnson-
Matthey) atoms were reacted with NF3 (Matheson), PF3
(PCR Research), or AsF3 (Ozark-Mahoning, vacuum dis-
tilled from dry NaF) in excess argon during condensation at
5-8 K using closed-cycle refrigerators described elsewhere.17,18
Reagent gas mixtures were typically 0.5% in argon and the
laser ablated metal atom concentrations were much lower.
After reaction, infrared spectra were recorded at a resolution
of 0.5 cm-1 using Nicolet 550 or 750 spectrometers with
Hg-Cd-Te B range detectors. Samples were later irradiated
for 15 min periods by a mercury arc street lamp (175 W) with
the globe removed using a combination of optical filters, and
then samples were annealed to allow reagent diffusion and
further reaction.
Following our work on reactions with the isoelectronic
fluoroform molecule, theoretical computations were per-
formed using the Gaussian 03 program with the B3LYP
hybrid density functional and some comparisons with the
BPW91 functional.19-21 The 6-311+G(2d) basis was used to
represent the electronic density of nitrogen, fluorine, phos-
phorus, and arsenic atoms, and SDD pseudopotentials were
used for the metal atoms.22,23 Frequencies were computed
analytically, and allenergyvaluesreportedincludezero-point
vibrational corrections. The calculation of vibrational fre-
quenciesis not an exact science, and density functional theory
(DFT) provides a very good approximation for observed
frequencies. Calculated frequencies are usually a few percent
higher than observed values,24,25 but that is not always the
case. Finally, bonding analysis was done using the natural
bond orbital method in Gaussian 03.19,26
Figure 1. Infrared spectra for group 4 metal atom reaction products
with NF3 in excess argon in the 810-530 cm-1 region. (a) Spectrum after
co-deposition of laser-ablated Ti and NF3 at 0.5% in argon at 8 K for 60
min, (b) after >220 nm irradiation for 20 min, (c) after annealing to 30 K,
and (d) spectrum after co-deposition of laser-ablated Zr and NF3 at 0.5%
in argon at 8 K for 60 min, (e) after >290 nm irradiation, and (f) after
>220 nm irradiation, and (g) spectrum after co-deposition of laser-
ablated Hf and NF3 at 0.5% in argon at 8 K for 60 min, (h) after
annealing to 20 K, and (i) after >220 nm irradiation.
binary titanium fluoride species as have the weaker bands
.
at 677.7, 674.0, and 643.6 cm-1 29,30
The latter assign-
ments were made to these products from the thermal Ti
atom reaction with argon/fluorine samples. New bands at
782.1, 705.1, and 596.7 cm-1 increase 30% on full arc
(>220 nm) irradiation, and sharpen on annealing to 30
K, spectra (b) and (c). The major Zr reaction product at
667.4 cm-1 is joined by weaker 658.2 and 553.1 cm-1
bands, which increase in concert 10% on >290 nm
irradiation and another 30% on >220 nm irradiation,
scans (d,e,f). The shoulder absorption at 668.6 cm-1 is in
agreement with the major product from an experiment
with Zr and F2 in excess argon performed in this labora-
tory and is near the 668.0 cm-1 band observed earlier for
ZrF4.31 The strongest Hf product band shifts lower to
Results and Discussion
Infrared spectra of products formed in the reactions of
laser-ablated titanium, zirconium, and hafnium atoms with
NF3, PF3, and AsF3 in excess argon during condensation at
5-8 K will be presented in turn. Density functional calcula-
tions were performed to support the identifications of new
reaction products. Bands common to experiments using
different laser ablated metals with NF3 [such as NF2 and
NF2-], with PF3 [PF2, PF5, and PF2-], and with AsF3 [AsF2
and AsF5] have been identified previously and will not be
mentioned again here.15,27,28
650.8 cm-1 just above the NF3 absorption at 648.9 cm-1
,
but this is in precise agreement with our major band for
the Hf and F2 reaction and appropriate for HfF4.31
Comparison with the zirconium experiment suggests that
there is a major product absorption at 643.9 cm-1 with
weaker associated 666.7 and 548.1 cm-1 bands that
increase together 20% on >20 K annealing and another
20% on >220 nm irradiation, spectra (g,h,i).
Infrared Spectra. Sets of infrared spectra for Ti, Zr, and
Hf reactions with NF3 are compared in Figure 1. The
strong bands at 791.8 and 740.2 cm-1 in the Ti reaction,
trace (a), have been reported previously and assigned to
(17) (a) Andrews, L. Chem. Soc. Rev. 2004, 33, 123. (b) Andrews, L.;
Citra, A. Chem. Rev. 2002, 102, 885.
(18) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and
references therein (Review article).
(19) Frisch, M. J., et al. Gaussian 03, Revision D.01; Gaussian, Inc.:
Pittsburgh, PA, 2004.
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.;
Parr, R. G. Phys. Rev. B 1988, 37, 785.
(21) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533, and
references therein.
(22) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265.
(23) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
Infrared spectra for the corresponding group 4 metal
atom reactions with PF3 are illustrated in Figure 2. The
TiF3 and TiF2 bands were barely detected as PF3 is a more
stable compound than NF3. The major Ti reaction pro-
duct at 769.0 cm-1 is joined by a weaker band at 688.2
cm-1, and these two bands increase 30% on >290 nm
irradiation and reduce a like amount on subsequent >220
nm irradiation, traces (a,b,c). With Zr the strong product
(24) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(25) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 2937.
(26) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(29) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. J. Chem. Phys. 1969, 51,
2648.
(30) Wilson, A. V.; Roberts, A. J.; Young, N. A. Angew. Chem., Int. Ed.
2008, 47, 1774.
::
(31) (a) Buchler, A.; Berkowitz-Mattuck, J. B.; Dugre, D. H. J. Chem.
(27) Jacox, M. E. J. Phys. Chem. Ref. Data 1994, Monograph 3; 1998, 27
Phys. 1961, 34, 2202. (b) Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1973,
5, 89. (c) unpublished infrared spectra of Zr and Hf reaction products with F2
in excess argon at 5 K.
(2), 115.
(28) (a) Khider Aljibury, A. J.; Redington, R. L. J. Chem. Phys. 1970, 52,
453. (b) Brum, J. L.; Hudgens, J. W. J. Chem. Phys. 1997, 106, 485.

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6299
Table 1. Observed and Calculated Fundamental Frequencies of the N÷MF3 Nitrene Complexes in the Ground 3A1 Electronic States with C3v Structuresa
N÷TiF3
N÷ZrF3
N÷HfF3
N÷HfF3 (BPW91)
approx. mode description
obs
calc
int
obs
calc
int
obs
calc
int
calc
int
M-F str, a1
M-F str, e
M-F str, a1
N÷M str, a1
MF3 def, e
MF3 def, a1
N-M-F def, e
666.7
643.9
645
639
34
156 ꢀ 2
630
626
30
142 ꢀ 2
782.1b
705.1b
596.7
796
719
629
203
190
176
255 ꢀ 2
102
29
667.4c
658.2
553.1
669
647
565
168
152
155
208 ꢀ 2
61
58
548.1
550
162
141
152
49
3 ꢀ 2
537
162
138
150
39
4 ꢀ 2
15 ꢀ 2
22 ꢀ 2
21
26
23
19
1 ꢀ 2
6 ꢀ 2
24 ꢀ 2
16 ꢀ 2
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311+G(2d)
in the harmonic approximation using the SDD core potential and basis set for metal atoms. Calculations using the BPW91 functional gave similar
frequencies as shown for the hafnium product. b The mode symmetry notations are based on the C3v structure. c Band probably over some ZrF4
absorption.
Figure 2. Infrared spectra for group 4 metal atom reaction products
with PF3 in excess argon in the 800-600 cm-1 region. (a) Spectrum after
co-deposition of laser-ablated Ti and PF3 at 0.5% in argon at 8 K for 60
min, (b) after 290 nm irradiation for 20 min, and (c) after >220 nm
irradiation for 20 min. (d) Spectrum after co-deposition of laser-ablated
Zr and PF3 at 0.13% in argon at 8 K for 60 min, (e) after >290 nm
irradiation, and (f) after >220 nm irradiation. (g) Spectrum after co-
deposition of laser-ablated Hf and PF3 at 0.5% in argon at 8 K for 60 min,
(h) after >290 nm irradiation, and (i) after >220 nm irradiation.
Figure 3. Infrared spectra for group 4 metal atom reaction products
with AsF3 in excess argon in the 800-610 cm-1 region. (a) Spectrum after
co-deposition of laser-ablated Ti and AsF3 at 0.5% in argon at 5 K for 60
min, (b) after 290 nm irradiation for 20 min, and (c) after >220 nm
irradiation for 20 min. (d) Spectrum after co-deposition of laser-ablated
Zr and AsF3 at 0.13% in argon at 5 K for 60 min, (e) after >290 nm
irradiation, and (f) after >220 nm irradiation. (g) Spectrum after co-
deposition of laser-ablated Hf and AsF3 at 0.5% in argon at 5 K for 60
min, (h) after >290 nm irradiation, and (i) after >220 nm irradiation.
bands at 664.2 and 651.4 cm-1 were very intense (absor-
bance 0.2 and 0.1, respectively) using 0.5% precursor in
argon, so the sample was diluted to 0.13%, and these
bands with the same relative intensity are shown in
Figure 2. The 664.2 and 651.4 cm-1 bands were not
changed on the initial >290 nm irradiation but increased
together 50% on the next >220 nm irradiation, spectra
(d,e,f). Using Hf the product band relative intensities
reverse at 662.6 and 647.4 cm-1, and these bands increase
10% on >290 nm irradiation and another 20% on >220
nm irradiation, scans (g,h,i).
are compared in Table 1 with frequencies calculated
for the lowest energy product molecule. The metal
atom reaction with NF3 most likely proceeds through
an insertion step like the halomethane reactions investi-
gated extensively in this laboratory,32 and here the
reaction proceeds from the difluoroamine inserted species
to the fluoroimine and then to the nitrene species. These
steps in the reaction are 216, 235, and 274 kcal/mol
lower in energy than the initial reactants Ti and NF3,
respectively. The zirconium fluoroimine and nitrene
species are 266 and 313 kcal/mol lower energy than
the combined reagents, and the analogous hafnium
products are 261 and 319 kcal/mol lower than the starting
materials.
Spectra for group 4 metal reactions with AsF3
are shown in Figure 3. The bands at 791.8, 740.2, and
674.2 cm-1 in the Ti reaction for TiF4,3,2 are weaker
than with NF3, and a new absorption was observed at
765.9 cm-1. The corresponding band for Zr is 663.6 cm-1
,
3
Mð FÞ þ NF3fF2N-MFfFNdMF2fN ÷MF3 ð1Þ
and ZrF4 absorption underneath increases the AsF2
peak at 668.6 cm-1. Using Hf the HfF4 band is
observed at 651.0 cm-1 and the new product lower at
Our experience with Ti-F, Zr-F, and Hf-F stretching
frequencies is that the B3LYP calculated values slightly
exceed the argon matrix observed values by about 5% for
646.9 cm-1
.
Identification of the N÷MF3 molecules. Reactions of Ti,
Zr, and Hf atoms with NF3 gave new strong, medium,
and weak product absorptions with each metal, which
(32) Lyon, J. T.; Andrews, L. Inorg. Chem. 2007, 46, 568. (Gr. 4 +
fluoromethanes).

6300 Inorganic Chemistry, Vol. 48, No. 13, 2009
Wang et al.
Table 2. Observed and Calculated Fundamental Frequencies of the P÷MF3 Complexes in the Ground 3A1 Electronic States with C3v Structuresa
P÷TiF3
P÷ZrF3
P÷HfF3
calc int
P÷HfF3 (BPW91)
approx. mode description
obs
calc
int
obs
calc
int
obs
calc
int
M-F str, a1
M-F str, e
M-F str, a1
P÷M str, a1
MF3 def, e
MF3 def, a1
P-M-F def, e
662.6
647.4
639
635
10
138 ꢀ 2
625
623
67
124 ꢀ 2
769.0
688.2
785
699
369
193
172
133
219 ꢀ 2
138
28
664.2
651.4
665
640
330
167
146
115
181 ꢀ 2
98
32
308
158
138
111
20
9 ꢀ 2
304
155
132
107
17
7 ꢀ 2
8 ꢀ 2
11 ꢀ 2
11
15
16
13
0 ꢀ 2
3 ꢀ 2
3 ꢀ 2
4 ꢀ 2
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311+G(2d)
in the harmonic approximation using the SDD core potential and basis set for metal atoms. Calculations using the BPW91 functional gave similar
frequencies as shown for the hafnium product. The mode symmetry notations are based on the C3v structure.
Table 3. Observed and Calculated Fundamental Frequencies of the As÷MF3 Complexes in the Ground 3A1 Electronic States with C3v Structuresa
As÷TiF3
As÷ZrF3
As÷HfF3
As÷HfF3 (BPW91)
approx. mode description
obs
calc
int
obs
calc
int
obs
calc
int
calc
int
M-F str, a1
M-F str, e
M-F str, a1
As÷M str, a1
MF3 def, e
MF3 def, a1
As-M-F def, e
n o.
646.9
639
635
78
133 ꢀ 2
625
622
74
120 ꢀ 2
765.9
n.o.
783
697
295
192
148
117
209 ꢀ 2
147
24
663.6
651.1
664
640
246
167
132
100
173 ꢀ 2
108
26
214
162
133
100
16
10 ꢀ 2
210
155
125
92
13
7 ꢀ 2
7 ꢀ 2
11 ꢀ 2
6
9
10
8
0 ꢀ 2
2 ꢀ 2
2 ꢀ 2
2 ꢀ 2
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with B3LYP/6-311+G(2d)
in the harmonic approximation using the SDD core potential and basis set for metal atoms. Calculations using the BPW91 functional gave similar
frequencies as shown for the hafnium product. The mode symmetry notations are based on the C3v structure.
Table 4. Structural Parameters and Physical Constants for Triplet State Pnictinidenes E÷MF3 (E = N, P, As, M = Ti, Zr, Hf) in C3v Symmetrya
parameter
N÷TiF3
P÷TiF3
As÷TiF3
N÷ZrF3
P÷ZrF3
As÷ZrF3
N÷HfF3
P÷HfF3
As÷HfF3
2.715
1.922
107.0
r(E÷M) 1.959
r(M-F) 1.756
<(EMF) 105.8
2.426
1.759
105.4
2.530
1.759
105.2
2.143
1.925
106.8
2.606
1.926
106.8
2.715
1.925
107.0
2.146
1.922
106.5
2.609
1.923
106.8
q(E)b
q(M)b
q(F)b
-0.19/-0.19 0.06/0.14
1.14/1.60 0.80/1.27
0.003/0.16
0.82/1.25
-0.44/-0.42 -0.22/-0.08 -0.27/-0.05 -0.42/-0.45 -0.08/-0.13 -0.18/-0.09
1.62/2.27 1.39/192 1.43/1.99 1.50/2.38 1.10/2.04 1.16/2.01
-0.32/-0.47 -0.28/-0.47 -0.27/-0.47 -0.39/-0.62 -0.39/-0.61 -0.39/-0.61 -0.36/-0.64 -0.34/-0.64 -0.33/-0.64
E(s)c
1.89
3.30
0.16
1.97
0.73
1.90
5.56
1.90
2.95
0.28
2.10
0.31
1.90
5.57
1.92
2.91
0.29
2.10
0.32
1.90
5.57
1.91
3.49
0.15
1.38
0.18
1.93
5.68
1.91
3.15
0.27
1.55
0.25
1.92
5.68
1.93
3.11
0.28
1.55
0.26
1.92
5.68
1.91
3.53
0.21
1.22
0.19
1.93
5.71
1.91
3.20
0.33
1.35
0.26
1.92
5.71
1.92
3.15
0.35
1.35
0.27
1.92
5.71
E(p)c
M(s)c
M(d)c
M(p)c
F(s)c
F(p)c
s(E)d
s(M)d
s(F)d
1.925
0.008
0.023
1.882
0.077
0.014
2.010
-0.071
0.021
1.885
0.082
0.011
1.802
0.175
0.008
1.924
0.039
0.012
1.894
0.080
0.009
1.810
0.195
0.002
1.936
0.044
0.007
μe
0.15
274
1.05
313
1.28
319
0.39
122
0.59
138
0.79
169
0.54
150
0.49
191
0.70
197
ΔE f
a
Bond lengths and angles are in A and degrees. All calculations performed at the B3LYP//6-311+G(2d)/SDD level. b Mulliken atomic/Natural
charges. c Natural electron configuration for valence orbitals. d Mulliken atomic spin densities: value given for only one fluorine atom. e Molecular dipole
moment in D. f Binding energy in kcal/mol relative to M + EF3.
˚
Ti-F, and within 2% on either side for Zr-F and Hf-F
modes.18,32 The observed and calculated frequencies in
Table 1 fit even better than this relationship, which
substantiates our identification of these new triplet state
nitrene molecules. The observed and calculated infrared
intensities are also qualitatively correct. Notice the re-
versal in the order of antisymmetric and symmetric M-F
stretching frequencies with increasing metal mass be-
tween Zr and Hf, and this is also matched by our
calculations.
Notice that the M÷N stretching frequencies for the
group 4 nitrene series at 596.7, 553.1, and 548.1 cm-1 are
significantly lower than the MtN stretching frequencies
for the group 6 nitride series at 1015, 1075, and 1091 cm-1
,
respectively, although the analogous M-F stretching
frequencies differ by less than 50 cm-1 15
.
Observed
frequencies for the N÷ZrF3 and N÷HfF3 molecules
may be compared with those for the isoelectronic
HC÷ZrF3 [659.7, antisym; 651.7, sym; 627.6 cm-1, C-Zr str]
and HC÷HfF3 [648.6, sym; 642.9, antisym; 621.7 cm-1
;

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6301
Table 5. Description of the E÷M Bonding Molecular Orbitals for Triplet State Terminal Pnictinidenes E÷MF3 (E = N, P, As, M = Ti, Zr, Hf)a
m. o.b
N÷TiF3
P÷TiF3
As÷TiF3
N÷ZrF3
P÷ZrF3
As÷ZrF3
N÷HfF3
P÷HfF3
As÷HfF3
σR
σβ
πR
πR
77%N,23%Ti 62%P,38%Ti 60%As,40%Ti 82%N,18%Zr 68%P,32%Zr 67%As,33%Zr 83%N,17%Hf 69%P,31%Hf 68%As,32%Hf
67%N,33%Ti 54%P,46%Ti 51%As,49%Ti 77%N,23%Zr 64%P,36%Zr 62%As,38%Zr 78%N,22%Hf 66%P,34%Hf 64%As,36%Hf
89%N,11%Ti 86%P,14%Ti 87%As,13%Ti 100%N,0%Zr 88%P,12%Zr 88%As,12%Zr 100%N,0%Hf 89%P,11%Hf 89%As,11%Hf
89%N,11%Ti 86%P,14%Ti 87%As,13%Ti 100%N,0%Zr 88%P,12%Zr 88%As,12%Zr 100%N,0%Hf 89%P,11%Hf 89%As,11%Hf
a All calculations performed at the B3LYP//6-311+G(2d)/SDD level. Natural bond orbital analysis employed. b Bonding molecular orbital
occupancies greater than 0.98 electron. Electron spins are indicated R and β. The π molecular orbitals are degenerate and are dominated by pnictide
p (100%) and metal d (>79%) atomic orbitals.
Figure 4. Structures calculated for nine group 4 transition metal trifluoride pnictinidenes using B3LYP/6-311+G(2d)/SDD methods.
C-Hf str] triplet state species.32 The four observed
M-F stretching frequencies for the N÷M nitrenes are
1-18 cm-1 higher than for the C÷M species, but the
C-Zr stretching frequency is 74.5 cm-1 higher than the
N-Zr stretching mode and the C-Hf frequency is
73.6 cm-1 above the N-Hf value. These consistent
frequency trends for closely related isoelectronic mole-
cules substantiate our assignments.
less exothermic: the final products are 122, 138,
and 169 kcal/mol lower in energy that the reagents.
3
Mð FÞ þ PF3fF2P-MFfFPdMF2fP ÷MF3 ð2Þ
For the Ti reaction, new Ti-F stretching absorptions
at 769.0 and 688.2 cm-1 fall 13-17 cm-1 lower in
frequency than the nitrene product, but the strong pro-
duct bands with Zr are only 3-7 cm-1 lower than the
nitrene values as the heavier metal mediates the group 15
mass change. Notice the reversal in mode intensities for
the Hf product at 662.6 and 647.4 cm-1 as the symmetric
and antisymmetric M-F3 stretching mode positions re-
verse. This relationship is in accord with our vibrational
frequency calculations (Table 2).
Identification of the As÷MF3 molecules. Arsenic tri-
fluoride is intermediate in reactivity, as attested by the
intermediate yield of TiF2,3,4 and HfF4, and the overall
reaction energetics. Reaction (3) is 150, 191, and 197 kcal/
mol exothermic for the three metals, respectively. The
spectra in Figure 3 reveal one new metal dependent
product band for each metal, at 765.9 cm-1 for Ti, at
A major contribution to the energy differences between
the last steps of reaction 1 is the strong M-F bonds
compared to weak N-F bonds. An interesting compar-
ison can be made between the isoelectronic HNdCH2 and
N÷CH3 reactive species. The latter is computed at the
B3LYP level to be 50 kcal/mol higher in energy, and a
major factor here is the single N-C bond compared to the
NdC bond. However, in contrast, as mentioned above
the trifluorotitanium nitrene is more stable (39 kcal/mol)
than the FNdTiF2 imine isomer owing to the stronger
Ti-F bond.
Identification of the P÷MF3 molecules. Phosphorus
trifluoride is less reactive, and the infrared spectra
contain fewer absorptions. The overall reaction 2 is

6302 Inorganic Chemistry, Vol. 48, No. 13, 2009
Wang et al.
0.175 and 0.195, respectively. This combination repre-
sents the most favorable (p-d) π orbital overlap. Even so
the (p-d) π molecular orbitals are dominated by the P
atom with 88 and 89% contributions from P 3p (Table 5).
Both the smaller N 2p and the larger As 4p valence
orbitals are less effective than the P 3p orbitals. Although
(p-d) π bonding is more important for N than P, this is
based largely on interaction with first row elements.33,34
When the partner is an early transition metal nd orbital, P
3p dominates the (p-d) π bonding scene.
Another comparison of interest is with the isoelectronic
HC÷MF3 molecules, which have been prepared by the
analogous reactions with fluoroform,32 where the com-
puted HC÷MF3 and N÷MF3 bond lengths are almost the
˚
same (within 0.02 A). The analogous calculation for
HC÷TiF3 finds 1.837 and 0.123 spin densities on C and
Ti, whichmay be comparedwiththe 1.925 and 0.008 values
given for N÷TiF3 in Table 4. In addition, natural bond
orbital calculations find that the more diffuse C 2p orbital
overlaps more effectively than N 2p with Ti 3d, and the πR
molecular orbitals are 79% C and 21% Ti, which shows
moreinteraction thanthe 89% N and 11% Ti πR molecular
orbitals for N÷TiF3 (Table 5). Although the spin densities
for N (1.885) and Zr (0.082) indicate electron transfer, the
computed molecular orbitals are 100% N, but the bonding
situation is more favorable for C with Zr as the C (1.894)
and Zr (0.115) spin densities and the πR MO 85% C and
15% Zr indicate. Similarly, the C (1.921) and Hf (0.094)
spin densities for HC÷HfF3 are near the nitrene values
(Table 4), but the πR molecular orbitals are 86% C as
compared to 100% N (Table 5, Figures 4, 5).
Conclusions
The NF3, PF3, or AsF3 molecules react with laser-ablated
Ti, Zr, and Hf atoms to produce triplet state terminal
pnictinidene N÷MF3, P÷MF3, or As÷MF3 molecules, which
are trapped in an argon matrix and identified by their infrared
spectra and comparison to computed vibrational frequencies.
Density functional theory calculations converge to C3v sym-
metry triplet state structures with relatively long terminal
group 15-group 4 bonds for these lowest energy reaction
products. The two unpaired electrons in nitrogen 2p, phos-
phorus 3p, or arsenic 4p orbitals are shared to various degrees
with empty metal nd orbitals leading to very weak degenerate
π molecular orbitals based on DFT bonding orbital analysis
and spin density calculations. This weak π bonding interac-
tion with early transition metal group 4 nd orbitals appears to
work best with Zr and Hf with phosphorus 3p orbitals, and
almost as well for arsenic 4p orbitals. Although there are more
terminal nitride than phosphide complexes, and even fewer
arsenide complexes,35 our work suggests that phosphinidenes
and even arsinidenes can be stabilized by early transition
metal complexes in macroscopic reactions.
Figure 5. One of the weak degenerate π molecular orbitals calculated for
the pnictinidene series N÷ZrF3, P÷ZrF3, and As÷ZrF3 using B3LYP/6-311
+G(2d)/SDD methods and plotted withan iso-electron density of 0.03 e/au3.
663.6 cm-1 for Zr, and at 646.9 cm-1 for Hf. These bands
fall stepwise in line below the analogous NMF3 and
PMF3 product antisymmetric M-F stretching frequen-
cies (Tables 1 and 2) as the heavier transition metal
dampens the effect of pnictide substitution. Our DFT
predictions for the strongest infrared absorption correlate
very closely24,25 with the observed absorptions (Table 3).
Bonding in the Terminal Pnictinidenes. Natural bond
orbital analysis26 was performed for the group 4 metal
terminal pnictinidene series, and the results are outlined
in Tables 4 and 5. The first point to notice is that the
Mulliken atomic electron spin densities on the N, P, and
As centers are 1.8 or greater indicating clearly that there is
only a small amount of (p-d) π bonding interaction in
these triplet ground-state molecules. Perhaps the most
interesting finding is that the lowest of these group 15 spin
densities, 1.802 and 1.810, are on P in the Zr and Hf
phosphinidines, and the metals have spin densities of
Acknowledgment. We gratefully acknowledge financial
support from NSF Grant CHE 03-52487 and NCSA
Grant CHE07-0004N to L.A.
(33) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999.
(34) Massey, A. G. Main Group Chemistry, 2nd ed.; John Wiley and Sons:
New York, 2000.
(35) Balazs, G.; Gregoriades, L. J.; Scheer, M. Organometallics 2007, 26,
3058; and references therein.