ARTICLE pubs.acs.org/JPCA
Matrix Infrared Spectroscopic and Computational Studies on the Reactions of Osmium and Iron Atoms with Carbon Monoxide and Dinitrogen Mixtures Zhang-Hui Lu†,‡ and Qiang Xu*,†,‡ † ‡
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan Graduate School of Engineering, Kobe University, Nada Ku, Kobe, Hyogo 657-8501, Japan ABSTRACT: Reactions of laser-ablated osmium and iron atoms with CO and N2 mixtures in excess neon have been investigated using matrix isolation infrared spectroscopy. The (NN)xMCO (M = Os, Fe; x = 1, 2) complexes are formed as reaction products during sample deposition and on annealing. These reaction products are characterized on the basis of the results of isotopic substitution, mixed isotopic splitting patterns, stepwise annealing, broad-band irradiation, and change of reagent concentration and laser energy. Density functional theory calculations have been performed on these products. Overall agreement between the experimental and calculated results supports the identification of these species from the matrix infrared spectra. The bonding characteristics and reaction mechanisms have been discussed. The MC bonds are stronger than the MN bonds in the same molecules. The formation of metal carbonyl dinitrogen complexes from the addition of CO to metal dinitrogen complexes is found to be more energetically favorable than that from the reactions of N2 with metal carbonyls.
’ INTRODUCTION Dinitrogen (N2) is 78% of the Earth’s atmosphere; it has a strong triple bond energy of 226 kcal/mol, which makes the cleavage of the NN bond very difficult.1 The bonding of N2 with transition metal centers is of great interest from an academic or an industrial viewpoint because binding of N2 to transition metal centers may lead to the cleavage of the strong NN bond as in the catalytic synthesis of ammonia.13 The longstanding goal of elucidating mechanisms of the reactions involving N2 has motivated numerous experimental and theoretical investigations of the interactions between metals and N2.1,4 For example, the reactions of laser-ablated group 8 metals (Fe, Ru, Os) with N2 have been extensively studied, and various metal nitrides and dinitrogen complexes have been characterized.5,6 CO, like N2, has a strong triple bond of 257 kcal/mol. Many industrial processes employ CO as reagent and metal compounds as heterogeneous catalysts and involve the intermediate of metal carbonyls. Various metal carbonyls have been prepared in low-temperature matrix samples by codeposition of laser-ablated transition metal atoms with CO.7 Recently, it was reported that an organometallic hafnium compound induces N2 cleavage on the addition of CO.8 Yet, we found that CO preadsorption enables N2 coadsorption to Cu atoms in argon matrices, while N2 itself is inert toward Cu atoms.9 Group 8 metals (Fe, Ru, Os) are very important metals in many fields such as materials, chemistry, and biology. The interactions of these metal atoms with separate CO or N2 molecules have been extensively investigated from both theoretical and experimental points of view during the past decade.5,6,10,11 Recently, the reactions of Ru atoms with CO and N2 mixtures r 2011 American Chemical Society
have been investigated in this laboratory, and the reactivity of Ru toward CO was found to be prior to N2.12 Time-resolved infrared absorption spectroscopy has been used to study the gas-phase reactions of Fe(CO)3 and Fe(CO)4 with N2, and the Fe(CO)3(N2)2 complex was characterized.13,14 In this paper, we report a combined matrix isolation infrared spectroscopic and theoretical study of the reactions of laser-ablated Os and Fe atoms with CO/N2 mixtures in excess neon. IR spectroscopy coupled with theoretical calculation provides evidence for the formation of the metal carbonyl dinitrogen complexes, (NN)xMCO (M = Os, Fe; x = 1, 2). The formation of metal carbonyl dinitrogen complexes from the addition of CO to metal dinitrogen complexes is found to be more energetically favorable than that from the reactions of N2 with metal carbonyls or primary metal carbonyl dinitrogen complexes.
’ EXPERIMENTAL AND THEORETICAL METHODS The experiments for laser ablation and matrix isolation infrared spectroscopy are similar to those previously reported.15 Briefly, the Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused on the rotating Os and Fe targets. The laser energy was varied from 5 to 25 mJ/pulse. The laser-ablated Os and Fe atoms were codeposited with CO and N2 mixtures in excess neon onto a CsI window cooled normally to 4 K by means of a closed-cycle helium refrigerator. CO (99.95%, Japan Fine Products), 13C16O (99%, 18O < 1%, ICON), 12 18 C O (99%, ICON), N2 (99.95%, SUZUKI SHOKAN Co., Received: July 12, 2011 Revised: August 27, 2011 Published: August 30, 2011 10783
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The Journal of Physical Chemistry A Ltd.), 15N2 (99.8%, SHOKO Co., Ltd.), and mixed isotopic samples were used to prepare the CO/N2/Ne mixtures. In general, matrix samples were deposited for 3060 min with a typical rate of 24 mmol/h. After sample deposition, IR spectra were recorded on a BIO-RAD FTS-6000e spectrometer at 0.5 cm1 resolution using a liquid nitrogen cooled HgCdTe (MCT) detector for the spectral range of 5000400 cm1. Samples were annealed at different temperatures and subjected to broad-band irradiation (λ > 250 nm) using a high-pressure mercury arc lamp (Ushio, 100 W). Density functional theory calculations were performed to predict the structures and vibrational frequencies of the observed reaction products using the Gaussian 03 program.16 The B3LYP density functional method was utilized.17 The 6-311+G(d) basis set was used for the C, O, and N atoms,18 the Los Alamos effectivecore-potential plus double-ζ (LANL2DZ) basis set was used for the Os atom, and the DZVP basis set was used for the Fe atom.19 All geometrical parameters were fully optimized, and the harmonic vibrational frequencies were calculated with analytical second derivatives. A natural bond orbital (NBO) approach was employed to elucidate the bonding characteristics.20 Trial calculations and recent investigations have shown that such computational methods can provide reliable information for metal complexes, such as infrared frequencies, relative absorption intensities, and isotopic shifts.2124
’ RESULTS AND DISCUSSION Experiments have been done for laser-ablated Os and Fe atoms reactions with CO and N2 mixtures in excess neon using low laser energy with different CO and N2 concentrations. Typical infrared spectra for the products in the selected regions are illustrated in Figures 14 and the absorption bands in different isotopic experiments are listed in Table 1. Absorptions common to these experiments such as metal dinitrogen complexes5,6 and metal carbonyls10,11 have been reported previously and are not listed here. The stepwise annealing and photolysis behavior of the product absorptions is also shown in the figures and will be discussed below. Meanwhile, experiments have also been done for the codeposition of laser-ablated Os and Fe atoms with separated CO and N2 samples to confirm the new absorptions. While the Os(CO)x, Fe(CO)x, and Fe(NN)x (x = 15) complexes have been characterized in solid neon,5,10,11 the Os(NN)x (x = 15) complexes have only been reported in argon and pure nitrogen matrices.6 In our Os + N2/Ne experiments, absorptions of neutral osmium dinitrogen complexes have been observed for Os(NN) (2045.5 cm1), Os(NN)2 (2096.4 cm1), Os(NN)3 (2072.4 cm1), Os(NN)4 (2119.0 cm1), and Os(NN)5 (2145.6 cm1), respectively, which about 1, 13, 2, 13, and 7 cm1 blue-shifted from the argon matrix counterparts.6 Density functional theory calculations have been carried out for the possible isomers and electronic states of the potential product molecules. Figure 5 shows the ground state structures, electronic states, and point groups of the new products. Table 2 reports a comparison of the observed and calculated IR frequencies and isotopic frequency ratios for the NN and CO stretching modes of the new products. The summary of electronic configurations and MC, MN, CO, and NN bond orders for metal carbonyl dinitrogen complexes are listed in Table 3. Energetic analysis for possible reactions of Os and Fe atoms with CO and N2 is given in Table 4. Molecular orbital depictions of the highest occupied molecular orbitals (HOMOs)
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Figure 1. Infrared spectra in the 220011925 cm1 region from codeposition of laser-ablated Os atoms with 0.1% CO + 0.3% N2 in Ne: (a) 40 min of sample deposition at 4 K, (b) after annealing to 8 K, (c) after 10 min of broad-band irradiation, and (d) after annealing to 10 K.
Figure 2. Infrared spectra in the 22001900 cm1 region for laserablated Os atoms codeposited with isotopic CO/N2 mixtures in Ne after annealing to 8 K: (a) 0.1% CO + 0.3% N2, (b) 0.1% 13CO + 0.3% N2, (c) 0.08% 12CO + 0.08% 13CO + 0.3% N2, (d) 0.1% C18O + 0.3% N2, (e) 0.08% C16O + 0.08% C18O + 0.3% N2, (f) 0.1% CO + 0.3% 15N2, and (g) 0.1% CO + 0.3% 14N2 + 0.3% 15N2.
and HOMO1s of these products are representatively illustrated in Figure 6. NNMCO
ðM ¼ Os, FeÞ
In the Os + CO + N2 experiments, the new absorptions at 2130.4 and 2025.7 cm1 appear together during sample deposition and increase after broad-band irradiation and annealing (Table 1 and Figure 1). The 2025.7 cm1 band shows a 46 cm1 shift with 13C16O and a 42 cm1 shift with 12C18O, but only a 6 cm1 15 N2 isotopic shift. The isotopic frequency ratios (12C16O/ 13 16 C O, 1.0233; 12C16O/12C18O, 1.0209; 14N2/15N2, 1.0031) suggest that this band is due to a terminal CO stretching 10784
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vibration with very small coupling to nitrogen. The mixed 12C16O + 13 16 C O + N2 (Figure 2, trace c) and 12C16O + 12C18O + N2 (Figure 2, trace e) isotopic spectra only provide the sum of pure isotopic bands, implying that one CO unit is involved in this mode.25 The upper band at 2130.4 cm1 shows very small shifts with 13C16O and 12C18O (Table 1 and Figure 2) but a large shift with 15N2, indicating that this mode is mainly due to a NN stretching vibration. The mixed CO + 14N2 + 15N2 isotopic spectra (Figure 2, trace g) only provide the sum of pure isotopic bands, suggesting that only one N2 unit is involved in the complex.25 Accordingly, the absorptions at 2130.4 and 2025.7 cm1 are assigned to the NN and CO stretching modes of NNOsCO, respectively. In the Fe + CO + N2 experiments, the similar new absorptions at 2204.3 and 1998.1 cm1 can be assigned to the NNFeCO complex. Briefly, the two bands appear together upon broadband irradiation and slightly increase after sample annealing to 10 K (Table 1 and Figure 3). The 1998.1 band shows large shifts (41 and 45 cm1) with 13C16O and 12C18O, but only a small shift (6 cm1) with 15N2, indicating that this band is due to a terminal CO stretching vibration with little involvement of nitrogen. This band is about 64 cm1 higher than the band for FeCO in a neon matrix.5 The 2204.3 cm1 band shifts to 2130.8 cm1 with 15 N2 (Table 1) and shows a very small shift with 12C18O. Only one CO as well as one N2 subunit participate in these modes according to the doublet isotopic structures observed in mixed 12 16 C O + 13C16O + N2 (Figure 4, trace c), 12C16O + 12C18O + N2
(Figure 4, trace e), and CO + 14N2 + 15N2 (Figure 4, trace g) isotopic experiments.25 Density functional calculations have been performed on the NNOsCO and NNFeCO complexes to support the above assignments. The NNOsCO complex is predicted to have a 3Σ ground state with linear structure, but NNFeCO to have a 3A00 ground state with Cs symmetry (Figure 5) and the linear structure was calculated to be 1.7 kcal/mol higher in energy than the Cs symmetry structure. For NNOsCO, the NN and CO stretching vibrational frequencies are calculated to be 2294.9 and 2066.9 cm1, respectively, in accord with the corresponding experimental values (Table 2). For the CO stretching mode of the NNOsCO complex, the calculated 12C16O/13C16O, 12C16O/12C18O, and 14 N/15N isotopic frequency ratios of 1.0233, 1.0209, and 1.0031 (Table 2) are consistent with the experimental observations, 1.0238, 1.0217, and 1.0016, respectively. The calculated 12C16O/13C16O, 12 16 C O/12C18O, and 14N/15N isotopic frequency ratios of the NN stretching mode of the NNOsCO complex also agree with the experimental values (Table 2). Similar results have also been obtained for the NNFeCO complex (Table 2 and Figure 5). ðNNÞ2 MCO
ðM ¼ Os, FeÞ
In the Os + CO + N2 experiments, the two bands at 2097.8 and 1993.7 cm1 appear together during sample deposition, slightly
Figure 4. Infrared spectra in the 22501800 cm1 region for laserablated Fe atoms codeposited with isotopic CO/N2 mixtures in Ne after annealing to 10 K: (a) 0.15% CO + 0.3% N2, (b) 0.15% 13CO + 0.3% N2, (c) 0.1% 12CO + 0.1% 13CO + 0.3% N2, (d) 0.15% C18O + 0.3% N2, (e) 0.1% C16O + 0.1% C18O + 0.3% N2, (f) 0.15% CO + 0.3% 15N2, and (g) 0.15% CO + 0.3% 14N2 + 0.3% 15N2.
Figure 3. Infrared spectra in the 22501800 cm1 region from codeposition of laser-ablated Fe atoms with 0.15% CO + 0.3% N2 in Ne: (a) 30 min of sample deposition at 4 K, (b) after annealing to 8 K, (c) after 10 min of broad-band irradiation, and (d) after annealing to 10 K.
Table 1. IR Absorptions (in cm1) Observed from Reaction of Laser-Ablated Os or Fe Atoms with CO and N2 Mixtures in Excess Neon CO + N2
13
CO + N2
C18O + N2
CO + 15N2
R(12CO/13CO)
R(C16O/C18O)
R(14N/15N)
assignment
2130.4
2130.3
2130.3
2059.3
1.0001
1.0001
1.0345
NNOsCO NN str.
2097.8
2097.8
2097.8
2027.9
1.0000
1.0000
1.0345
(NN)2OsCO NN str.
2025.7
1979.6
1984.2
2019.5
1.0233
1.0209
1.0031
NNOsCO CO str.
1993.7
1948.3
1951.9
1989.6
1.0233
1.0214
1.0021
(NN)2OsCO CO str.
2204.3 2175.1
2204.3 2175.1
2203.4 2174.5
2130.8 2102.0
1.0000 1.0000
1.0004 1.0003
1.0345 1.0348
NNFeCO NN str. (NN)2FeCO NN str.
1998.1
1957.5
1953.1
1992.4
1.0207
1.0230
1.0029
NNFeCO CO str.
1961.3
1917.5
1915.8
1961.2
1.0228
1.0238
1.0001
(NN)2FeCO CO str.
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C16O, to 1915.8 cm1 with 12C18O, and to 1961.2 cm1 with N2, giving the 12C16O/13C16O, 12C16O/12C18O, and 14N2/15N2 frequency ratios of 1.0228, 1.0238, and 1.0001 (Table 1), respectively. The 2175.1 cm1 bands shift to 2102.0 cm1 with 15N2 (Table 1 and Figure 4), implying that this mode is mainly due to the NN stretching vibration. One CO and two N2 subunits participate in these modes according to the doublet isotopic structures observed in mixed 12C16O + 13C16O + N2 (Figure 4, trace c) and 12C16O + 12C18O + N2 (Figure 4, trace e) experiments, and the triplet isotopic structures observed in mixed CO + 14N2 + 15N2 (Figure 4, trace g) isotopic experiments.25 The assignments are confirmed by DFT calculations summarized in Table 2. The (NN)2OsCO and (NN)2FeCO complexes are predicted to have 1A1 and 3A1 ground states with C2v symmetry (Figure 5), respectively. The symmetric and asymmetric NN and CO stretching frequencies of the (NN)2OsCO species are calculated to be 2275.3, 2226.2, and 2032.4 cm1 (Table 2), respectively. The band 2275.3 cm1 with very low intensity (2 km/mol) is not readily observed, which is in line with the absence of the symmetric NN stretching vibration of the (NN)2OsCO complex from the present experiment. The calculated 12C16O/13C16O, 12C16O/12C18O, and 14N/15N isotopic frequency ratios of the asymmetric NN and CO stretching vibrations are consistent with the experimental values (Table 2), respectively. Similarly, agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of the (NN)2FeCO complex (Table 2 and Figure 5). The intensity of the symmetric 13
increase upon sample annealing, and almost disappear after broad-band irradiation (Table 1 and Figure 1). The lower band at 1993.7 cm1 shifts to 1948.3 cm1 with 13C 16O and to 1951.9 cm1 with 12C18O, exhibiting isotopic frequency ratios (12C16O/13C16O, 1.0233; 12C16O/12C18O, 1.0214) characteristic of CO stretching vibrations. The mixed 12C16O + 13C16O + N2 (Figure 2, trace c) and 12C16O + 12C18O + N2 (Figure 2, trace e) isotopic spectra only provide the sum of pure isotopic bands, implying that one CO unit is involved in this mode.25 Furthermore, the 1993.7 cm1 band also shows a small shift with 15N2 (Table 1 and Figure 2), suggesting that the N2 unit is involved in this complex. The upper band at 2097.8 cm1 shows a large shift with 15N2 (14N2/15N2, 1.0345), indicating that this mode is mainly due to a NN stretching vibration. A triplet isotopic pattern is observed in the mixed CO + 14N2 + 15N2 isotopic spectra (Figure 2, trace g), which indicates that two N2 units are involved in the complex.25 Accordingly, the absorptions at 2097.8 and 1993.7 cm1 are assigned to the asymmetric NN and CO stretching modes of (NN)2OsCO, respectively. In the Fe + CO + N2 experiments, the 2175.1, and 1961.3 cm1 bands are assigned to (NN)2FeCO. These bands exhibit the same behavior of deposition, annealing, and broadband irradiation experiments, suggesting that they are due to different vibrational modes of the same molecule (Table 1 and Figure 3). The 1961.3 cm1 band shifts to 1917.5 cm1 with
15
Table 3. Electronic Configuration and Bond Order for (NN)xMCO (M = Os, Fe; x = 1, 2) Computed Using the B3LYP Method bond order Fe/Os electronic configuration
MC
MN1
CO
N1N2a
NNOsCO
6s1.255d6.66
1.209
0.517
2.074
2.756
(NN)2OsCO
6s0.835d7.26
1.563
0.665
2.022
2.704
NNFeCO
4s0.443d7.17
0.953
0.487
2.072
2.713
(NN)2FeCO
4s0.323d7.11
0.781
0.435
2.091
2.741
species
Figure 5. Optimized structures (bond lengths in angstrom, bond angles in degree), electronic ground states, and point groups of (NN)xMCO (M = Os, Fe; x = 1, 2) calculated using the B3LYP method.
a
N1 is the atom bonded to metal atoms, and N2 is the atom bonded to N1.
Table 2. Comparison of Observed and Calculated IR Frequency and Frequency Ratios for the New Products freq (cm1) mode
obsd
NNOsCO
νNN
2130.4
2294.9 (272, σ)
νCO
2025.7
2066.9 (1721, σ)
(NN)2OsCO
NNFeCO (NN)2FeCO
a
calcda
species
νNN
R(12CO/13CO)
R(C16O/C18O)
R(14N/15N)
obsd
calcd
obsd
calcd
obsd
calcd
1.0001
1.0007
1.0001
1.0005
1.0345
1.0334
1.0238
1.0209
1.0217
1.0031
10.233
2275.3 (2, A1)
1.0003
1.0002
1.0016 1.0342
νNN
2097.8
2226.2 (1275, B2)
1.0000
1.0000
1.0000
1.0000
1.0345
νCO
1993.7
2032.4 (1087, A1)
1.0233
1.0249
1.0214
1.0207
1.0021
1.0007
νNN
2204.3
2255.3 (436, A0 )
1.0000
1.0005
1.0004
1.0003
1.0345
1.0337
νCO
1.0350
1998.1
2052.3 (1907, A0 )
1.0207
1.0235
1.0230
1.0225
1.0029
1.0012
νNN νNN
2175.1
2282.6 (111, A1) 2234.2 (1420, B2)
1.0000
1.0005 1.0000
1.0003
1.0004 1.0000
1.0348
1.0337 1.0350
νCO
1961.3
2033.8 (1285, A1)
1.0228
1.0229
1.0238
1.0231
1.0001
1.0013
Intensities (in parentheses) are given in kilometers per mole. 10786
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Table 4. Energetics for Possible Reactions of Metal Atoms (M = Os, Fe) with CO and N2 Calculated Using the B3LYP Method reaction energya no.
reaction
Os
Fe 18.1
1
M + CO f MCO
52.0
2
M + N2 f MNN
12.6
2.2
3
MNN + N2 f M(NN)2
37.3
22.3
4
MNN + CO f NNMCO
63.5
39.2
5 6
MCO + N2 f NNMCO M(NN)2 + CO f (NN)2MCO
24.2 39.8
23.3 29.8
7
NNMCO + N2 f (NN)2MCO
13.6
12.9
a
A negative value of energy denotes that the reaction is exothermic. Reaction energy is given in kilocalories per mole (kcal/mol).
Figure 6. Molecular orbital depictions of the highest occupied molecular orbitals (HOMOs) and HOMO1s of (NN)xMCO (M = Os, Fe; x = 1, 2).
NN stretching vibration (2282.6 cm1) in (NN)2FeCO is predicted to be 1/13 that of asymmetric NN stretching vibration, which is too weak to be observed in the present experiments (Figure 3). Bonding Considerations. It is interesting to understand the bond strength and the frontier molecular orbitals (FMOs) of (NN)xMCO (M = Os, Fe; x = 1, 2). The NBO analyses reveal that there some electrons transferred from metal s to d orbital, implying the strong hybridization between the two orbitals (Table 3). As can be seen from the bond orders for (NN)xMCO in Table 3, the CO and NN bonds of NNOsCO are stronger than those of (NN)2OsCO, respectively. In contrast to osmium carbonyl dinitrogen complexes, the CO and NN bonds of NNFeCO are weaker than those of (NN)2FeCO. The OsC and OsN bonds are enhanced whereas the FeC and FeN are weakened upon the increase of one NN ligand. Furthermore, the MC bonds are stronger than the MN bonds in the same metal carbonyl dinitrogen complexes. As illustrated in Figure 6, both HOMO and HOMO1 of NNOsCO, NNFeCO, and (NN)2FeCO are singly occupied molecular orbital (SOMO), while those of (NN)2OsCO are doubly occupied since the former three compounds are triplet and the latter one is singlet. The highest occupied molecular orbital (HOMO) in NNOsCO is of σ-type, whereas the HOMO in
(NN)2OsCO is largely Os 5d in character and is nonbonding. As for NNFeCO, the HOMO is of δ-type with the contribution mainly from the Fe 3d atomic orbitals. The HOMO in (NN)2FeCO and the HOMO1 in (NN)xMCO (M = Os, Fe; x = 1, 2) are of π-type, in which the contribution is mainly from the interaction between the atomic orbitals of metal atoms and molecular orbitals of CO and/or N2 (Figure 6). Reaction Mechanisms. Binary metal carbonyls and dinitrogen complexes have been prepared by reactions of Os and Fe atoms with CO and N2 separately and characterized by matrix isolation infrared spectroscopy.5,6,10,11 Under the present experimental conditions, laser-ablated Os and Fe atoms react with CO and N2 mixtures to produce the metal carbonyl dinitrogen complexes, as well as metal dinitrogen complexes5,6 and metal carbonyls.10,11 M(CO)15 and M(NN)15 are formed via CO and N2 addition, respectively.5,6,10,11 Both MCO and MNN can be formed by direct reaction during matrix deposition, but CO is more reactive than N2 to metal atoms (reactions 1 vs 2). Similar reaction characteristic for CO and N2 with MNN is also found in the reactions 3 and 4. The metal carbonyl dinitrogen complexes may be formed from the reactions of metal carbonyls with N2 or metal dinitrogen complexes with CO (Table 4, reactions 47). The absorptions of the reaction products increase on annealing, suggesting that the reactions to form these species are spontaneous. As can be seen in Table 4, all association reactions are predicted to be exothermic, in agreement with the experimental observations. Furthermore, the reactions of MNN + CO are calculated to be exothermic by 63.5 kcal/mol for Os and 39.2 kcal/mol for Fe (reaction 4), and the reactions of MCO + N2 are calculated to be exothermic by 24.2 kcal/mol for Os and 23.3 kcal/mol for Fe (reaction 5), respectively, implying that the formation of the primary NNMCO products from the reactions of MNN + CO is more energetically favorable than that from the reactions of MCO + N2. Similarly, the formation of (NN)2MCO complexes from the addition of CO to M(NN)2 (reaction 6) is predicted to be more energetically favorable than that from the reactions of NNMCO + N2 (reaction 7). It is interesting to note that adding CO is more exothermic than adding N2 to form the metal carbonyl dinitrogen complexes with the same number of ligands (i.e., reactions 4 vs 5, 6 vs 7) as the single metal atom reaction with CO is more exothermic than with N2 (reaction 1 vs 2), which may be due to fact that CO is a stronger σ donor and π acceptor than the isoelectronic N2 molecules.26
’ CONCLUSIONS Laser-ablated osmium and iron atoms react with CO and N2 mixtures in excess neon to produce metal carbonyl dinitrogen complexes, (NN)xMCO (M = Os, Fe; x = 1, 2), as well as metal carbonyls and dinitrogen complexes. These metal carbonyl dinitrogen complexes are characterized using infrared spectroscopy on the basis of the results of the isotopic substitution and mixed isotopic splitting patterns. Density functional theory calculations have been performed to understand the structures, ground electronic states, and bonding characteristics of the osmium and iron carbonyl dinitrogen complexes. The identifications of these metal complexes are confirmed by the overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts. In addition, the bonding characteristics and reaction mechanisms have been discussed. 10787
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
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