Infrared spectra and density functional calculations of the M←NCCCH3, η2-M(NC)-CH3, CH 3-MNC, CH2M(H)NC, and CHM(H)2NC complexes produced by reactions of Group 6 metal atoms with acetonitrile

Article abstract of DOI:10.1016/j.jorganchem.2011.12.007

The M←NCCH₃, η²-M(NC)-CH₃, CH₃-MNC, CH₂M(H)NC, and HCM(H)₂NC complexes are produced by reactions of laser-ablated Group 6 metal atoms with acetonitrile and identified from the matrix IR spectra through isotopic substitution and comparison to frequencies calculated by density functional theory in the harmonic approximation (B3LYP/6-311++(3df,3pd) and BPW91/6-311++G(3df,3pd), this all-electron set for Cr, and SDD for Mo and W). Good general agreement is shown in tables for all observed frequencies and these calculations. The products follow the reaction path proposed earlier for Group 4 and 5 metal reactions, which are supported by density functional energy calculations. The M←NCCH₃ complex strengthens the assumption that the lone electron pair on the N-end first attracts the metal atom. The π-complexes show that the back-donation of the Group 6 metal is weaker than those of Zr and Group 5 metals. The observed products clearly show the increasing preference for the higher oxidation-state complex with increasing atomic number of the Group 6 metal, and subsequent photolysis increases the yields for the higher oxidation-state complexes. The methylidyne isocyanide HCW(H)₂NC has a C_s structure similar to the HCW(H)₂X complexes prepared earlier.

Full text of DOI:10.1016/j.jorganchem.2011.12.007

Journal of Organometallic Chemistry 703 (2012) 25e33
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Infrared spectra and density functional calculations of the M)NCCCH3,
h2eM(NC)eCH3, CH3eMNC, CH2]M(H)NC, and CH^M(H)2NC complexes
produced by reactions of Group 6 metal atoms with acetonitrile
,
,b,
Han-Gook Cho a b, Lester Andrews a
*
a Department of Chemistry, University of Incheon, 119 Academy-ro, Yonsu-gu, Incheon 406-772, South Korea
b Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, VA 22904-4319, USA
a r t i c l e i n f o
a b s t r a c t
The M)NCCH3, h2eM(NC)eCH3, CH3eMNC, CH2]M(H)NC, and HC^M(H)2NC complexes are produced
by reactions of laser-ablated Group 6 metal atoms with acetonitrile and identified from the matrix IR
spectra through isotopic substitution and comparison to frequencies calculated by density functional
theory in the harmonic approximation (B3LYP/6-311þþ(3df,3pd) and BPW91/6-311þþG(3df,3pd), this
all-electron set for Cr, and SDD for Mo and W). Good general agreement is shown in tables for all
observed frequencies and these calculations. The products follow the reaction path proposed earlier for
Group 4 and 5 metal reactions, which are supported by density functional energy calculations. The
M)NCCH3 complex strengthens the assumption that the lone electron pair on the N-end first attracts
Article history:
Received 10 November 2011
Received in revised form
5 December 2011
Accepted 6 December 2011
Keywords:
Matrix infrared spectra
DFT calculations of frequencies and
structures
the metal atom. The
p-complexes show that the back-donation of the Group 6 metal is weaker than
those of Zr and Group 5 metals. The observed products clearly show the increasing preference for the
higher oxidation-state complex with increasing atomic number of the Group 6 metal, and subsequent
photolysis increases the yields for the higher oxidation-state complexes. The methylidyne isocyanide
HC^W(H)2NC has a Cs structure similar to the HC^W(H)2X complexes prepared earlier.
Ó 2012 Elsevier B.V. All rights reserved.
Cr, Mo, W
Acetonitrile reaction products
1. Introduction
A series of studies have shown that reactions of transition-
metals with small hydrocarbons and their halides are efficient
Acetonitrile has been a model compound in the development of
vibrational spectroscopy due to its relatively simple spectra from
the C3v structure and coincidence with the CO2 laser lines [1]. Its
CeN stretching band, which is located in a relatively clean area, has
also been the subject of many spectroscopic studies, particularly
because it is a useful probe for examining the coordination strength
with its N-end to electron-deficient species [2]. The lone electron
pair on the N-end provides substantial anti-bonding character in
the molecular axis, particularly in the CN bond, which often leads to
blue shifts of the stretching bands upon coordination to an acceptor
particularly in solution [3]. The photochemical reactions of aceto-
nitrile, such as conversions between CH3CN and CH3NC, have also
drawn much attention [4]. Acetonitrile isomers and fragments,
such as H2CCNH, HCNCH2, and HCNC, have also been prepared from
photo-dissociation of larger precursors [5].
routes to produce new breed of small characteristic complexes,
including carbenes, carbynes, -complexes, and cyclic products via
p
CeH(X) bond insertion and H(X) migration from C to M [6e11].
Because of their relatively small number of atoms, they are also
more amenable to high level theoretical approaches to provide
helpful information for spectral assignments and understanding
the reaction mechanism [12]. They also allow opportunities to
closely monitor molecular processes involved in metal coordina-
tion, bond insertion, H(X) migration, and their photochemical
behaviors. These results show that CeH(X) bond insertion by
transition-metals and subsequent reactions are in fact a general
chemical phenomenon and can also be extended to other organic
species [6e12].
Recently reactions of Zr and Group 5 metals with CH3CN have
been investigated in this laboratory [13,14]. The nitrile h2
ep-
complex and isocyanide products (CH3eZrNC and CH2]Zr(H)NC)
result from reactions of Zr with acetonitrile without a trace of the
cyanide complexes, leading to the proposal of reaction path (1)
[13]. This path was later corroborated by the products formed in
Group 5 metal reactions with acetonitrile, namely h2eV(NC)eCH3
* Corresponding author. Department of Chemistry, University of Virginia, P.O. Box
400319, Charlottesville, VA 22904-4319, USA.
E-mail address: lsa@virginia.edu (L. Andrews).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.12.007

26
H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
produced from V reactions and CH3eVNC in subsequent photol-
ysis, CH3eNbNC and h2eNb(NC)eCH3 from Nb reactions and
CH2]Nb(H)NC during photolysis, and CH2]Ta(H)NC from Ta
reactions [14].
M þ CH3CN / M)NCCH3 / h2eM(NC)eCH3
/ CH3eMNC / CH2]M(H)NC / HC^M(H)2NC
(1)
In metal atom reactions with CH3CN, the electron-rich N-end of
acetonitrile is expected to initially attract the electron-deficient
metal atom, as it does on solvation or on forming acetonitrile
adducts [2,3]. However, the coordination complex (M)NCCH3)
was not identified in the matrix spectra probably due to its rela-
tively higher energy and rapid conversion to the nitrile h2
ep-
complex.
In this investigation, reactions of Group 6 metals with acetoni-
trile isotopomers are carried out and the results are compared with
the previous Zr and Group 5 metal results [13,14]. The primary
products are identified through isotopic substitution and with
helpful information from DFTcomputations. The observed products
are the five constituents of reaction path (1) while showing
increasing preference for the higher oxidation-state product with
increasing atomic number of the Group 6 metal.
Fig. 1. Infrared spectra from reaction products of laser-ablated chromium with CH3CN
in excess argon at 10 K. (a) Cr and CH3CN (0.50% in argon) co-deposited for 1 h, (b) as
(a) after visible (
arc ( > 220 nm) photolysis. Relatively small intensity changes are observed on visible
irradiation and the product absorptions gradually decrease on annealing (see text). n,
, and i designate the product absorption groups while P and c stand for the precursor
l > 420 nm) and UV (240e380 nm) irradiation and (c) as (b) after full
l
p
and common absorptions in the CH3CN matrix spectra. H2CCNH, HCNCH2, and CH3NC
absorptions are also indicated.
2. Experimental and computational methods
Laser-ablated Cr, Mo, and W atoms were reacted with acetoni-
trile isotopomers (CH3CN, CD3CN, and 13CH313CN) in excess argon
during condensation at 10 K using a closed-cycle refrigerator (Air
Products Displex). These methods have been described in detail in
previous publications [6a,15]. Reagent gas mixtures are typically
0.5% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz
repetition rate, 10 ns pulse width) was focused onto a rotating
metal target (Johnson-Matthey) using 5e10 mJ/pulse. After initial
reaction, infrared spectra were recorded at 0.5 cmꢀ1 resolution
using a Nicolet 550 spectrometer with a liquid nitrogen cooled
HgeCdeTe range B detector. Then samples were irradiated for
20 min periods by a mercury arc street lamp (Sylvania, 175 W) with
the globe removed using a combination of optical filters or
annealed to allow further reagent diffusion.
In order to provide support for the assignment of new experi-
mental frequencies and to correlate with previous related works
[6e11], density functional theory (DFT) calculations were per-
formed in the harmonic approximation using the Gaussian 09
package [16a], the B3LYP density functional [17], the 6-
311þþG(3df,3pd) basis sets for H, C, N, and Cr [18] and the SDD
pseudopotential and basis sets [19] for Mo and W to provide
vibrational frequencies for prospective reaction products. We find
the all-electron DZVP set [16b,c] to give higher frequencies than the
above sets for Mo as a test case, which are thus further above the
observed frequencies. Geometries were fully relaxed during opti-
mization, and the optimized geometries were confirmed by
vibrational analysis. The BPW91 and M06 functionals [20] were
also employed to complement the B3LYP results. The vibrational
frequencies were calculated analytically, and zero-point energy is
included in the calculation of binding and reaction energies.
Previous investigations have shown that DFT calculated harmonic
frequencies are usually slightly higher than observed frequencies
[6e11,21], but they correlate well and provide useful predictions
and support for assigning the infrared spectra of new molecules.
Tables 1e3) and density functional frequency calculations of the
products (Tables S1eS15) and their relative energies (Figs. 7e9) and
structures (Figs. 10e12) will be presented in turn.
3.1. Cr þ acetonitrile
The first figures illustrate product spectra from reactions of
laser-ablated Cr atoms with acetonitrile isotopic modifications and
show their variation on subsequent sample photolysis and
annealing. The isomerization products of acetonitrile (CH2CNH,
CH2NCH, and CH3NC) [4,5], due to the laser-plume radiation during
ablation [6a,15] are also observed, but these absorptions are weak
relative to those in previous studies [4,5], and the Mo and W cases
described below. Here three sets of new product absorptions are
marked n,
p, and i (for N-coordination, p-, and insertion
Fig. 2. Infrared spectra from reaction products of laser-ablated chromium with CD3CN
in excess argon at 10 K. (a) Cr and CD3CN (0.50% in argon) co-deposited for 1 h, (b) as
(a) after visible (
arc ( > 220 nm) photolysis. Relatively small intensity changes are observed on visible
irradiation and the product absorptions gradually decrease on annealing (see text). n,
, and i stand for the product absorption groups while P and c designate the precursor
l > 420 nm) and UV (240e380 nm) irradiation, and (c) as (b) after full
l
3. Results and discussion
p
and common absorptions in the CD3CN matrix spectra. D2CCND, DCNCD2, and CD3NC
absorptions are also indicated. Bands labeled c are common for this precursor and
different metals.
Reactions of Group 6 metal atoms with acetonitrile were
investigated and infrared spectra (Figs. 1e6 and S1eS3, and

H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
27
Fig. 3. Infrared spectra from reaction products of laser-ablated molybdenum with CH3CN in excess argon at 10 K. (a) Mo and CH3CN (0.50% in argon) co-deposited for 1 h, (b) as (a)
after visible ( > 420 nm) irradiation, (c) as (b) after UV (240e380 nm) irradiation, (d) as (c) after visible irradiation, and (e) as (d) after UV irradiation. The product absorptions
gradually decrease on annealing (see text). , i, and m stand for the product absorption groups while P and c designate the precursor and common absorptions in the CH3CN matrix
spectra. H2CCNH, HCNCH2, CH3NC, and HCNC absorptions are also indicated. Bands labeled c are common for this precursor and different metals.
l
p
complexes) on the basis of intensity variation during photolysis and
annealing of the deposited samples.
CrCN, would not show NeC and CeN stretching bands in this high
frequency region. In fact, the observed frequency is close to the CeN
stretching frequency of 2258 cmꢀ1 for CH3CN [13,14,22]. We attri-
bute this photosensitive band to the CeN stretching mode of the N-
coordinated complex [2,3], Cr)NCCH3, on the basis of good
correlation with the computed values (B3LYP, Table S1, 2297.0 cmꢀ1
and 13C shift of 56.1 cmꢀ1).
The n absorption decreases slightly on visible (
irradiation, and almost disappears on the following UV
(240e380 nm) irradiation. The absorptions decrease to about
l > 420 nm)
p
a third of their original intensity on visible irradiation and almost
disappear on UV irradiation. On the other hand, the i absorptions
remain unchanged on visible irradiation, more than triple on UV
The observed CeN stretching band is the strongest one we
calculated for the N-coordinated complex with a bent CreNeCeC
backbone, and all other bands are too weak to observe, as shown in
Table S1. Unfortunately, the B3LYP calculation gave a small 31 cmꢀ1
imaginary CrNC bending frequency with B3LYP/6-311þþ(3df,3pd),
which was even larger (ꢀ38 cmꢀ1) with B3LYP/DZVP [16]. Many
attempts with different initial structures failed to remove this small
imaginary frequency, which arises with the B3LYP functional.
Although BPW91/6-311þþG(3df,3pd) gave a real 24 cmꢀ1 CrNC
bending frequency, the calculated CeN stretch was 88 cmꢀ1 lower
than the observed value. However, the ab initio MP2 method [16c]
irradiation, further increase on full arc (
decrease on annealing. The observed frequencies are listed in
Table and compared with the DFT computed values in
l > 220 nm) photolysis, and
1
Tables S1eS5 for plausible reaction products.
The n absorption observed at 2240.8 cmꢀ1 has a D counterpart
2040 cmꢀ1 shoulder band and a 13C counterpart at 2188.9 cmꢀ1
(carbon 12/13 frequency ratio of 1.024). It most probably originates
from a CeN stretching mode on the basis of its frequency position
and large 13C shift. However, its frequency is substantially higher
than those of the previously studied cyanide and isocyanide
complexes, which are expected in the 2150e2050 cmꢀ1 region.
Other possible cyanide or isocyanide products, such as CrNC and
computed
a linear complex, a
real 47 cmꢀ1 CrNC bending
frequency, and a much higher value (2431 cmꢀ1) for the observed
CeN stretch. This latter value shows the usual relationship between
Fig. 4. Infrared spectra from reaction products of laser-ablated molybdenum with
CD3CN in excess argon at 10 K. (a) Mo and CD3CN (0.50% in argon) co-deposited for 1 h,
Fig. 5. Infrared spectra from reaction products of laser-ablated tungsten with CH3CN in
(b) as (a) after visible (
l
> 420 nm) and UV (240e380 nm) irradiation, (c) as (b) after
excess argon at 10 K. (a) W and CH3CN (0.50% in argon) co-deposited for 1 h and (b) as
visible irradiation, and (d) as (c) after UV irradiation. Relatively small intensity changes
(a) after visible (l > 420 nm) and UV (240e380 nm) irradiation. Relatively small
are observed on the first visible irradiation and the product absorptions gradually
intensity changes are observed on visible irradiation and the product absorptions
gradually decrease on annealing (see text). y stands for the product absorption while P
and c designate the precursor and common absorptions in the CH3CN matrix spectra.
HCNCH2 and HWO2 absorptions are also indicated. Bands labeled c are common for
this precursor and different metals.
decrease on annealing (see text).
p, i, and m stand for the product absorption groups
while P and c designate the precursor and common absorptions in the CD3CN matrix
spectra. D2CCND, DCNCD2, and CD3NC absorptions are also indicated. Bands labeled c
are common for this precursor and different metals.

28
H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
Table 2
Vibrational frequencies of product absorptions observed from reactions of Mo with
acetonitrile isotopomers in excess argon.a
CH3CN
CD3CN
13C133CN
Description
A1 CeN str.
N
2220.0
Covered
2167.5
928.5
2919.1
1684.5
1436.0
1071.5, 1068.2
574.1, 571.7
2010.9
2029.2, 2026.7
1823.6, 1821.7
823.9
906.4
1915.4
1644.5
1432.8
1048
570.0, 567.1
1971.4
1986.3
1823.2, 1821.7
A1 CeC str.
A0 CH3 s. str.
A0 CCN as. str.
A0 CH3 bend
A0 CH3 rock
A0 MoeN str.
A0 NeC str.
P
2107.5
1697.7, 1682.7
Covered
945.9, 944.1
548.5
2010.2
i
m
2029.3, 2026.6
1311.7, 1310.5
A0 NeC str.
A0 MoeN str.
A0 CeMo str.
a
All frequencies are in cmꢀ1. Stronger matrix site absorptions in a set are bold.
Fig. 6. Infrared spectra from reaction products of the laser-ablated tungsten with
Description gives major coordinate.
13CH313CN in excess argon at 10 K. (a) W and 13CH313CN (0.50% in argon) co-deposited
for 1 h and (b) as (a) after visible (l > 420 nm) and UV (240e380 nm) irradiation.
Relatively small intensity changes are observed on visible irradiation and the product
absorptions gradually decrease on annealing (see text). y stands for the product
absorptions while p and c designate the precursor and common absorptions in the
13CH313CN matrix spectra. H123C13CNH and H13CN13CH2 absorptions are also indicated.
Bands labeled c are common for this precursor and different metals.
1683.5 and 1647.0 cmꢀ1 on D and 13C substitution and is assigned to
the CCN anti-symmetric stretching mode on the basis of the small D
and large 13C shifts. In the high frequency region, the
p absorption
at 2912.7 cmꢀ1 along with its 13C counterpart at 2909.1 cmꢀ1 is
designated to the CH3 symmetric stretching mode. The observed
five absorptions substantiate generation of the nitrile
[23].
p-complex
MP2 and B3LYP frequencies [21] and suggests that the small
31 cmꢀ1 imaginary CrNC bending frequency with B3LYP does not
invalidate the higher calculated frequencies (Table S1). We also give
the computed Cartesian coordinates using B3LYP for the structures
in each table.
The strꢀo1ngest product absorption (marked “i”) is observed at
2061.6 cm
with D
and 13C counterparts at 2061.4 and
2021.0 cmꢀ1. We assign the uniquely strong i absorption to the NeC
stretching mode of CH3eCrNC on the basis of its frequency and
good correlation with the DFT values (Table S3), parallel to the Zr
and Group 5 metal cases [13,14]. The observed frequencies and D
and 13C isotopic shifts are compared with the B3LYP values of
2114.0 and 0.1 and 42.2 cmꢀ1 (Table S3). A weak i absorption
assigned to the CeCr stretching mode is observed at 550.6 cmꢀ1 in
the low frequency region along with its D and 13C counterparts at
483.9 and 546.3 cmꢀ1. Another i absorption at 491.0 cmꢀ1 in the
CD3CN spectra is designated to the CreN stretching mode with the
13C counterpart at 511.9 cmꢀ1. The observed absorptions substan-
tiate formation of the insertion complex. The insertion complex is
also the most stable among the plausible Cr products in the
proposed reaction. The Cr)NCCH3, h2eCr(NC)eCH3, CH3eCrNC,
CH2]Cr(H)NC, and HC^Cr(H)2NC reaction products in their septet,
We believe that this is the first observation of an acetonitrile N-
coordination complex with a metal atom in matrix isolation
spectra. The N-coordinated complexes with metal cations are
commonly discovered in acetonitrile adducts, which explain the
solution spectra of various salts [2,3]. Coordination with its N-end
alleviates the anti-bonding character supplied from the lone elec-
tron pair and thus, leads to only a small red shift and sometimes
even a blue shift particularly in solution.
A strong
and 13C counterparts at 554.1 and 568.0 cmꢀ1. Among the plausible
products, only the
-complex (h2eCr(NC)eCH3) has such a strong
p
absorption is observed at 571.4 cmꢀ1 along with its D
p
absorption (the NeCr stretching band) in the low frequency region
[13,14]. The observed frequencies are between the B3LYP values of
567.8, 550.9, and 565.2 cmꢀ1 and the BPW91 values of 581.8, 568.4,
and 579.2 cmꢀ1 (Table S2). The other
p
absorptions also support
absorption at
quintet,
quintet,
triplet,
and
singlet
ground
states
generation of h2eCr(NC)eCH3. The strong
p
are ꢀ22, ꢀ4, ꢀ62, þ125, and þ302 kJ/mol (B3LYP values) with
respect to the reactants (Cr(7S) þ CH3CN), respectively. Thus, we
only observe the products with energies lower than reactants.
1050.6 cmꢀ1 on the shoulder of a precursor band shifts to
922.0 cmꢀ1 on deuteration. While the 13C counterpart is believed to
be covered by precursor absorption, it is designated to the CH3
rocking mode. The
p
absorption at 1434.9 cmꢀ1 [shoulder, requires
3.2. Mo þ acetonitrile
expanded spectra for clear observation] is accompanied with D and
13C counterparts at 1090.7 and 1432.1 cmꢀ1, and assigned to the
The product absorptions from molybdenum atom reactions
with CH3CN isotopomers are displayed in Figs. 3 and 4, and S2.
Unlike the Cr case, the Mo ablation plume [15] yields strong
acetonitrile isomerization product absorptions (H2CCNH, HCNCH2
[5], CH3NC [4], and HCNC [24]). Four groups of product absorptions
CH3 bending mode. Another
p
absorption at 1685.9 cmꢀ1 shifts to
Table 1
Vibrational frequencies of product absorptions observed from reactions of Cr with
acetonitrile isotopomers in excess argon.a
Table 3
CH3CN
CD3CN
13CH313CN
Description
A1 CeN str.
Vibrational frequencies of product absorptions observed from reactions of W with
N
P
2240.8
2912.7
1685.9
1434.9
1050.6
571.4
2240 sh
Covered
1683.5
1090.7
922.0
554.1
2061.4
483.9
2188.9
2909.1
1647.0
1432.1
Covered
568.0
2021.0
546.3
511.9
acetonitrile isotopomers in excess argon.a
A0 CH3 s. str.
A0 CCN as. str.
A0 CH3 bend
A0 CH3 rock
A0 CreN str.
A0 NeC str.
A0 CeCr str.
A0 CreN str.
CH3CN
CD3CN
13CH313CN
Description
A0 NeC str.
A0 WH2 s. str.
A00 WH2 as str.
A0 WH2 wag
A00 WH2 twist
y
2023.0
1927.5
1898.7
649.5
2023.1
Covered
1354.9
1984.4
1927.4
1898.3
647.4
i
2061.6
550.6
611.7, 607.4
607.0
491.0
a
All frequencies are in cmꢀ1. Stronger absorptions in a set are bold. Description
All frequencies are in cmꢀ1. Description gives major vibrational coordinate.
gives major vibrational coordinate.
a

H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
29
Fig. 7. Electronic energies (ZPE correction included) for the plausible Cr products in
the proposed reaction path (1) relative to the reactants (Cr(7S) þ CH3CN). Black lines
denote B3LYP energies for several electronic states of each product, but only the
ground state for the methylidene: Red lines denote BPW91 and green lines indicate
M06 energies for the ground state structure of each product. Only Cr)NCCH3,
h2eCr(NC)eCH3, CH3eCrNC are observed in the matrix IR spectra. In the process of
photolysis, the Cr)NCCH3 and Creh2e(NC)eCH3, absorptions decrease while those of
CH3eCrNC increase, corroborating the reaction path. CH2]Cr(H)NC is not observed
probably due to its high energy (see text). T indicates triplet, Qi denotes quintet, and Sp
indicates septet states. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Fig. 9. Electronic energies (ZPE correction included) for the plausible W products in
the proposed reaction path (1) relative to the reactants (W(5D) þ CH3CN). Black lines
denote B3LYP energies for several electronic states of each product: Red lines denote
BPW91 and green lines indicate M06 energies for the ground state structure of each
product. Energies of the plausible W products in the proposed reaction path (1)
relative to the reactants (W(5D) þ CH3CN). Only HC^W(H)2NC, the most stable
product, is observed. The methylidyne product absorptions increase on UV irradiation.
T indicates triplet, Qi denotes quintet, and Sp indicates septet states. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
marked “n,
p, i, and m” (m for methylidene) are observed and
grouped depending on intensity changes upon photolysis and
annealing. The n absorptions decrease to less than a fifth of their
original intensity on the first visible irradiation, remain unchanged
on the next UV irradiation, and completely disappear on the second
visible photolysis. Similar to the Cr case, the
p absorptions decrease
w50% on visible photolysis and almost disappear on UV irradiation.
The i and m absorptions show reversible intensity changes during
photolysis. They decrease on visible irradiation but increase on UV
photolysis repeatedly as shown in Figs. 3 and 4, and S2. The i
absorptions, however, show more dramatic intensity variation, for
example, becoming 8 times as strong on the first UV irradiation and
decreasing to a third in intensity on the second visible irradiation.
A stronger n absorption is observed at 2220.0 cmꢀ1, and its 13C
counterpart at 2167.5 cmꢀ1. Following the Cr case, where the cor-
responding n absorption is observed at 2240.8 cmꢀ1, this band is
assigned to the CeN stretching mode of the N-coordinated complex
(Mo)NCCH3), while the D counterpart is not observed in the
congested precursor band area. The observed frequency and 13C
shift correlate well with the B3LYP values of 2290.4 and 56.2 cmꢀ1
.
Another n absorption and its 13C counterpart are observed at 928.5
and 906 cmꢀ1 and are appropriate for the CeC stretching mode
(Table S6). The N-coordinated complexes in both the Cr and Mo
systems provide evidence for the initial coordination of acetonitrile
with its N-end to the metal atom.
The strongest
p
absorption is observed at 571.7 cmꢀ1 (with a site
Fig. 8. Electronic energies (ZPE correction included) for the plausible Mo products in
the proposed reaction path (1) relative to the reactants (Mo(7S) þ CH3CN). Black lines
denote B3LYP energies for several electronic states of each product, but only the
ground state for the methylidyne: Red lines denote BPW91 and green lines indicate
M06 energies for the ground state structure of each product. Energies of the plausible
Mo products in the proposed reaction path (1) relative to the reactants (Mo(7S) þ
CH3CN). Mo)NCCH3, h2eMo(NC)eCH3, CH3eMoNC and CH2]Mo(H)NC are observed
in the original matrix IR spectra. The insertion and methylidene product absorptions
increase on UV irradiation but decrease on visible increase in concert. HC^Mo(H)2NC
is not observed probably due to its high energy. T indicates triplet, Qi denotes quintet,
and Sp indicates septet states. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
absorption at 574.1 cmꢀ1) and its D and 13C counterparts at 548.5
and 567.1 cmꢀ1 (H/D and 12/13 ratios of 1.042 and 1.008). Parallel to
the Cr case, only the
its strongest band in the low frequency region, and it is assigned to
the MoeN stretching mode of h2eMo(NC)eCH3. Other
absorp-
-complex as well. The
p-complex among the plausible products has
p
tions all support formation of the
p
p
absorptions at 1436.0 and 1071.5 cmꢀ1 are assigned to the CH3
bending and rocking modes on the basis of their frequencies and
isotopic (large D and small 13C) shifts (Tables 2 and S7). The
p
absorption at 1684.5 cmꢀ1 holds its D and 13C counterpart at 1682.7

30
H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
Fig. 10. The B3LYP structures of several Cr products using the 6-311þþG(3df,3pd) basis sets for H, C, N, and Cr. Bond distances and angles are in Å and deg. The Cr)NCCH3 complex
has a septet ground state, the Cr methylidene has a triplet ground state, whereas the others have quintet ground state. CH2]Cr(H)NC has a C1 structure.
and 1644.5 cmꢀ1 (H/D and 12/13 ratios of 1.001 and 1.024) and is
designated to the CCN anti-symmetric stretching mode. In the high
observed frequency and isotopic shifts are consistent with the
predicted frequency of 2077.6 cmꢀ1 and the isotopic shifts of 0.1
and 41.8 cmꢀ1 (Table S8). The insertion product has a uniquely
strong NeC stretching band and other much weaker bands, and it is
also the most stable among the plausible products. The
Mo)NCCH3, h2eMo(NC)eCH3, CH3eMoNC, CH2]Mo(H)NC, and
HC^Mo(H)2NC reaction products in the septet, quintet, quintet,
triplet, and singlet ground states are 14, 55, 95, 54, and 36 kJ/mol
lower in energy (B3LYP values) than the reactants (Mo(7S) þ
CH3CN), respectively.
frequency region, another
p
absorption at 2919.1 cmꢀ1 is assigned
to the CH3 symmetric stretching mode with its D and 13C coun-
terparts at 2107.5 and 2915.4 cmꢀ1 (H/D and 12/13 ratio of 1.385
and 1.001). The observed five absorptions and their isotopic
counterparts substantiate production of the
p-complex in reaction
of Mo with acetonitrile, parallel to the Cr case.
A strong i absorption is observed at 2010.9 cmꢀ1 and its D and
13C counterparts at 2010.2 and 1971.4 cmꢀ1 (H/D and 12/13 ratios of
1.000 and 1.020) with dramatic intensity variation during photol-
ysis. We designate the i absorption to the NeC stretching mode of
CH3eMoNC while other i absorptions are too weak to observe. The
A
strong m absorption pair are observed at 1823.6 and
1821.7 cmꢀ1 overlapped each other with similar intensities, and the
D counterparts at 1311.7 and 1310.5 cmꢀ1 (H/D ratio of both 1.390),
Fig. 11. The B3LYP structures of several Mo products using the 6-311þþG(3df,3pd) basis sets for H, C, and N and SDD pseudo potential and basis for Mo. Bond distances and angles
are in Å and deg. The Mo)NCCH3 complex has a septet ground state, the Mo methylidene and methylidyne isocyanides have triplet and singlet ground states while the others have
quintet grounds states. Notice the agostic distortion of the methylidene complex.

H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
31
Fig. 12. The B3LYP structures of several W products using the 6-311þþG(3df,3pd) basis sets for H, C, and N and SDD pseudo potential and basis for W. Bond distances and angles are
in Å and deg. Parallel to the Cr and Mo cases, the W methylidene and methylidyne isocyanides have triplet and singlet ground states and the other complexes have quintet ground
states.
and 13C counterparts at 1823.2 and 1821.7 cmꢀ1 (essentially no 13
C
1888.3 cmꢀ1 for HC^WH3, and 1905.6 cmꢀ1 for CH2]W(H)F, and
1936.4 and 1930.4 cmꢀ1 for HC^W(H)2F [6].
shifts). The large H/D ratio and negligible 13C ratio reveal that
a reaction product with an MoeH bond is generated in the reaction
of Mo with acetonitrile. The MoeH stretching frequency is also
compared with 1761.9 and 1718.7 cmꢀ1 for MoH2, 1812.1 cmꢀ1 for
MoH4, 1857.6 cmꢀ1 for MoH6 [25], and 1791.6 and 1781.5 cmꢀ1 for
CH2]MoH2, 1839.7, 1836.4, 1830.0 cmꢀ1 for HC^MoH3,
1797.7 cmꢀ1 for CH2]Mo(H)F, and 1849.7 and 1844.8 cmꢀ1 for
HC^Mo(H)2F [6]. Another strong m absorption pair are observed at
2029.2 and 2026.7 cmꢀ1, and the D counterparts at 2029.3 and
2026.6 cmꢀ1 and 13C counterpart at 1986.3 cmꢀ1. We assign the
observed m absorptions to the MoeH and NeC stretching modes of
CH2]Mo(H)NC, a small metal containing conjugated system. A
weak m absorption at 823.9 cmꢀ1 is designated to the CeMo
stretching mode without observation of its D and 13C counterparts.
The Mo methylidene complex is one of the most stable plausible
products in the proposed reaction path (1). In the previously
studied Mo þ CH4 system [6a], where both the methylidene and
methylidyne products are observed, the CH2]MoH2 and
HC^MoH3 in their triplet and singlet ground states are 50 and
46 kJ/mol higher than CH3eMoH in the quintet ground state. The
Mo methylidyne (HC^Mo(H)2NC) is not observed in this study
probably due to its higher energy. The identified complexes are the
first four products of reaction path (1).
The two WeH stretching absorptions suggest a primary product
with a WH2 group. Another y absorption at 2023.0 cmꢀ1 on the side
of a 2024.2 cmꢀ1 common peak most likely originates from an
isocyanide stretching motion as it shifts to 1984.4 cmꢀ1 on 13C
substitution. We, therefore, tentatively assign these three absorp-
tions to the NeC and WH2 stretching modes of HC^W(H)2NC. The
y absorptions at 649.5 and 607.4 cmꢀ1 with 13C counterparts at
647.4 and 607.0 cmꢀ1 are assigned to the WH2 wagging and
twisting modes. The observed five y absorptions support the
probable formation the tungsten methylidyne product (Table S15).
No other possible product absorptions are observed in the W
spectra.
The HC^W(H)2NC product tentatively identified here would be
the first transition-metal methylidyne prepared from acetonitrile,
and unlike the Cr and Mo cases, the methylidyne for W, the last step
in reaction path (1), is the most stable among the plausible prod-
ucts. W)NCCH3, h2eW(NC)eCH3, CH3eWNC, CH2]W(H)NC, and
HC^W(H)2NC in their quintet, quintet, quintet, triplet, and singlet
ground states are 99, 132, 199, 219, and 240 kJ/mol lower in energy
(B3LYP values) than the reactants (W(5D) þ CH3CN), respectively.
3.4. Reactions
3.3. W þ CH3CN
The new reaction products identified here (N-coordination,
p-,
and insertion complexes for Cr, the N-coordination, -, insertion,
p
Relatively weak product absorptions (all marked “y” for meth-
ylidyne) are observed from reactions of W with acetonitrile and the
13C-enriched reagent as shown in Figs. 5 and 6. They remain
unchanged on visible irradiation, but double on UV irradiation.
However, the acetonitrile isomerization product absorptions are as
strong as in the Mo spectra. The y absorptions at 1927.5 and
1898.7 cmꢀ1 show negligible 13C shifts, but these absorptions are
weak, and we do not observe the deuterium counterparts as deu-
teride transfer counterparts tend to be even weaker in these
experiments [6a]. The WeH stretching frequency is compared with
1852.6 cmꢀ1 for WH, 1866.2 cmꢀ1 for WH4, 1923.7 cmꢀ1 for WH6
[26], 1864.5 and 1817.2 cmꢀ1 for CH2]WH2, 1896.3, 1893.1, and
and methylidene complexes for Mo, and the methylidyne complex
for W) are the species that constitute reaction path (1). The
tendency that the higher oxidation-state complex is more favored
with moving down the group column is apparently the driving
force to produce the final product (methylidyne complex) in the
reaction of W. The trapped initial species (N-coordination and
p-
complexes) in the matrix from reaction of Cr and Mo also provide
strong evidence for reaction path (1).
The energies of the products in reaction path (1) (M)NCCH3,
h2eM(NC)eCH3, CH3eMNC, CH2]M(H)NC, and HC^M(H)2NC)
relative to the reactants in the Cr, Mo, and W systems, respectively,
are illustrated in Figs. 7e9. The generation of M)NCCH3 and the

32
H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
isocyanide products reflects preferential initial binding of N to the
metal atom, leading to the conversion of h2e(NC)eCH3 to
CH3eMNC rather than to the CeC bond insertion product
(CH3eMCN). The energies are basically consistent with the identi-
fied products (each system produces the most stable products). The
observed products also reveal fast conversions between the species
including H migration from C to M to reach the energy minima in
the reaction path. The conversions to the higher oxidation-state
complexes are evidently accompanied by system crossing to the
lower multiplicity ground states.
Nevertheless, the absence of the cyanide products (CH3eMCN,
CH2]M(H)CN, and HC^M(H)2CN) is a lasting question [27]. They
are energetically comparable with the isocyanide counterparts, for
example, CH3eMoCN, CH2]Mo(H)CN, and HC^Mo(H)2CN are 112,
54, and 45 kJ/mol more stable than the reactants. The initial reac-
The CeN bonds of the h2eM(NC)eCH3 complexes are shorter
than those of the Group 4 and 5 analogs. For example, the CeN
bond length 1.242 Å for h2eMo(NC)eCH3 is compared with 1.280 Å
for the Zr analog and 1.269 Å for the Nb analog [13,14]. The binding
energy also decreases in the order of Zr, Nb, and Mo (189, 148, 55 kJ/
mol). This reconfirms the previous conclusion that the extent of
back-donation from the metal atom to the p*-orbital of the coor-
dinated double bond decreases with moving right from the
exceptionally strong Group 4 case [11a,13].
The Mo and W isocyanide methylidenes are apparently more
agostic than the Group 4 and 5 analogs [13,14]. The smallest metal
containing conjugate system has a highly distorted CH2 group, and
the distortion increases with moving right in the same row. The
HeCeM bond angles of Zr, Nb, and Mo are 91.3, 90.0, and 86.4ꢁ in
line with the increasing electronegativities of 1.4, 1.6, and 1.8 for the
transition-metals [28]. Similar to the case of Group 4 and 5 metals,
the extent of distortion is, however, comparable to those of CH2]
MHX (X ¼ halogen) [6], for example, the agostic angles of 84.6 and
85.3ꢁ for CH2]Mo(H)F and CH2]Mo(H)Cl. Figs. 10e12 also show
that the Group 6 metal methylidynes have Cs structures, which
resemble those of HC^MH2X investigated in this laboratory [6a].
The contribution of conjugation effects in C]MeN^C: to the
molecular structures are apparently minor, parallel to the cases of
Group 4 and 5 metals [13,14].
tion steps (N-coordination to M and formation of the
p-complex)
are expected to be the same as shown in reaction (2). The strongest
CH3 rocking bands of the cyanide insertion complexes (CH3eMCN)
would appear in the 580e550 cmꢀ1 region, where no such
absorptions are observed in this study. Their CeN stretching bands
expected in the 2120e2170 cmꢀ1 region are not observed as well.
M þ CH3CN / M)NCCH3 / h2eM(NC)eCH3
/ CH3eMCN / CH2]M(H)CN / HC^M(H)2CN
(2)
However, the possibility for formation of the cyanide products
still cannot be ruled out due to the relatively lower intensities of
their CeN stretching and CH3 rocking absorptions. Particularly the
CeN stretching band, which is normally strong in many other
cyanide compounds, is remarkably weak in these cyanide
complexes. For example, the CeN stretching band of CH3eMoCN is
fifty times weaker than the NeC stretching band of CH3eMoNC. In
these complexes, the CeN and NeC stretching modes are incor-
porated with the neighboring MeC and MeN stretching modes,
essentially making them to the MeCeN and MeNeC anti-
symmetric stretching modes. Due to the electron-rich N: center,
the polarizations of the MeC and CN bonds of CH3eMCN are in the
same direction whereas those of the MeN and NeC bond of
CH3eMNC in the opposite direction. Therefore, the dipole moment
changes of the MeC and CN bonds in CH3eMCN cancel each other,
leading to a weak absorption, whereas those of the MeN and NeC
bonds in CH3eMNC are added resulting in a strong absorption.
4. Conclusions
Several products from reactions of laser-ablated Group 6 metal
atoms with acetonitrile are observed in the matrix IR spectra.
Cr)NCCH3, h2eCr(NC)eCH3, and CH3eCrNC are identified in the
Cr spectra, Mo)NCCH3, h2eMo(NC)eCH3, CH3eMoNC, and
CH2]Mo(H)NC in the Mo spectra, and HC^W(H)2NC provision-
ally in the W spectra. These products are steps in the reaction
path proposed for reactions of Group 4 and 5 metals with
acetonitrile [13,14]. They also reconfirm the general tendency that
higher oxidation-state complexes are favored with increasing
atomic number of the Group 6 metal. The observed Cr and Mo N-
coordination complexes suggest that the acetonitrile reaction
initiates with coordination with the N-end to the metal atom,
similar to the previous observations in acetonitrile solutions and
adducts [2,3].
The identified products including the isocyanide complexes
indicate that the preferential binding between N and M remains in
the reaction course. The observed Cr and Mo N-coordination
complexes are bent and carry a weak MeN bond in the septet
3.5. Molecular structures
Figs. 10e12 illustrate calculated structures for the M)NCCH3,
h2eM(NC)eCH3, CH3eMNC, CH2]M(H)NC, and HC^M(H)2NC
products (M ¼ Cr, Mo, and W). While the Cr, Mo, and W N-coordi-
nation complexes (M)NCCH3) have near Cs, Cs, and C3v structures
in their ground septet, septet, and quintet states, DFT computations
started with symmetry constraints overcome the restriction in the
first step of geometry optimization. The N-coordinated complex
carries a linear MeNeCeC backbone in the quintet state with the
shortest NeM bond among the plausible complexes, whereas it is
bent in the septet state with the longest NeM bonds, reflecting
weak coordination. It is also notable that the multiplicities of these
Cr and Mo N-coordination complexes in the ground states are high.
We compare computed structures for the weak Cr)NCCH3
complex in Fig. S3 first using B3LYP and the all-electron basis sets 6-
311þþ(3df,3pd) and DZVP, which find 31 and 38 cmꢀ1 imaginary
CrNC bending frequencies involving the weak CreN interaction and
second using BPW91 and MP2 with the 6-311þþ(3df,3pd) set,
which find all real frequencies. The problem here is in describing
the weak CreN interaction, as the NCCH3 subunit dimensions
change little in these four calculations.
ground state. The relatively short CN bond in the Group 6
p-
complex reveals that the back-donation of the metal atom to the
CeN p* orbital is weaker than in the Group 4 and 5 metal analogs.
The Mo and W methylidenes are more agostic than the Group 4 and
5 analogs, parallel to the previously studied methylidene halides
[6], and the extents of distortion are also comparable. The meth-
ylidyne isocyanide (HC^W(H)2NC) has a Cs structure similar to
HC^W(H)2X [6a].
This combined computational and experimental investigation of
Group 6 metal atom reactions with acetonitrile clearly benefits
from the close working relationship between these two comple-
mentary scientific methods.
Acknowledgments
We gratefully acknowledge financial support from National
Science Foundation (U.S.) Grant CHE 03-52487 to L.A., and support
from the Korea Research Foundation (KRF) grant funded by the
Korean government (MEST) (No 2010-0016527).

H.-G. Cho, L. Andrews / Journal of Organometallic Chemistry 703 (2012) 25e33
33
(e) H.-G. Cho, L. Andrews, J. Phys. Chem. A 111 (2007) 5201 (
complexes).
p
- and cyclic
Appendix. Supplementary data
[12] (a) G. von Frantzius, R. Streubel, K. Brandhorst, J. Grunenberg, Organometallics
25 (2006) 118 and references therein;
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.12.007.
(b) N. Berkaine, P. Reinhardt, M.E. Alikhani, Chem. Phys. 343 (2008) 241;
(c) B.O. Roos, R. Lindh, H.-G. Cho, L. Andrews, J. Phys. Chem. A 111 (2007)
6420;
(d) E. Clot, O. Eisenstein, in: N. Kaltzoyannis, J.E. McGrady (Eds.), Computa-
tional Inorganic Chemistry, Structure and Bonding, Springer, Heidelberg,
2004, pp. 1e36;
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