Infrared Spectroscopy of Si(CO)n+ Complexes: Evidence for

Article pubs.acs.org/JPCA

Infrared Spectroscopy of Si(CO)n+ Complexes: Evidence for Asymmetric Coordination Antonio D. Brathwaite and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States ABSTRACT: Si(CO)n+ and Si(CO)n+Ar complexes are produced via laser vaporization with a pulsed nozzle source and cooled in a supersonic beam. The ions are mass selected in a reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy near the free molecular CO vibration (2143 cm−1). Si(CO)n+ complexes larger than n = 2 fragment by the loss of CO, whereas Si(CO)n+Ar complexes fragment by the loss of argon. All clusters have resonances near the free molecular CO stretch that provide distinctive patterns from which information on their structure and bonding can be obtained. The number of infrared-active bands, their frequency positions, and relative intensities indicate that larger species consist of an asymmetrically coordinated Si(CO)2+ core with additional CO ligands attached via van der Waals interactions. Density functional theory computations are carried out in support of the experimental spectra.

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INTRODUCTION The structure and bonding of metal carbonyls have been well characterized, and their applications are pervasive throughout inorganic and organometallic chemistry.1−5 However, there is great disparity between the volume of data available on transition metal carbonyls versus those of nontransition metals. Transition metal carbonyl ions have been investigated extensively in the gas phase using mass spectrometry.6−12 In addition, reactions involving these species have been probed, and the dissociation energies of many transition metal cation− CO complexes have been measured.9−12 The structures of stable neutral transition metal carbonyls have been investigated using infrared spectroscopy in the gas phase and the condensed phase.13−17 These spectroscopic studies have been extended recently to include ionized transition metal carbonyl complexes.18−20 This has facilitated the investigation of trends among isoelectronic metal−carbonyl analogues.19,20 These systems have also been investigated with theory.18−26 Despite the increasing abundance of data on transition metal carbonyls, research on nonmetal or nontransition metal species is notably limited. In the present work, we employ infrared photodissociation spectroscopy in the C−O stretching region to investigate the coordination of carbonyl ligands to cations of silicon. The C−O stretching frequency provides a sensitive indicator of the type of bonding and chemical environment of many carbonyls.1−5,18−21 The shifts in frequency from that of the free CO molecule and the bonding in transition metal carbonyls can be explained using the Dewar−Chatt−Duncanson complexation model.1−5,18−21,23,25,26 In this paradigm of metal carbonyl bonding, two dominant interactions influence the observed vibrational frequencies. The first is σ donation, in which the carbonyl donates electron density from its HOMO along the metal−CO axis into empty metal d orbitals. Because the HOMO © 2012 American Chemical Society

has partial antibonding character, the removal of electron density increases the bond order as well as the vibrational frequency. The second interaction is π back-bonding, in which partially filled metal d orbitals donate charge into the antibonding LUMO on CO. The addition of electron density to this orbital weakens the CO bond and reduces the vibrational frequency. In so-called “classical” transition metal carbonyls, the effects of π back-bonding tend to outweigh those of σ donation. This results in a C−O stretch that is lower (i.e., red-shifted) in frequency than that of the free CO molecule (fundamental = 2143 cm−1).27 The same concepts used to describe the bonding in transition metal carbonyls are believed to carry over to main group systems. Similar to transition metals, the main group elements should be able to form metal−ligand bonds with carbonyls via σ donation and π back-bonding interactions. Although Si+ does not have occupied d orbitals, either σ donation or π back-bonding should be possible when its p orbitals interact with carbonyl ligands. However, the relative efficiency of these p interactions has not been well characterized. In addition to our spectroscopic studies of transition metal carbonyls,19,20 our research group has previously investigated the spectroscopy of various main group ion−molecule systems.28−33 These complexes have also been investigated using theory.30,32−34 The combined results of theory and experiment have allowed the structure and bonding of these species to be compared, revealing an interesting trend in the structure of multiligand main group complexes. As observed in previous studies, ligands tend to cluster symmetrically around transition metal ions, whereas clustering around main group Received: December 1, 2011 Revised: January 11, 2012 Published: January 12, 2012 1375

dx.doi.org/10.1021/jp211578t | J. Phys. Chem. A 2012, 116, 1375−1382

The Journal of Physical Chemistry A

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ions such as Mg+ and Al+ is notably asymmetric; i.e., the ligands bind together on one side of the cation. This behavior is observed because the valence electrons in these ions reside mainly in the highly polarizable 3s orbital.30,32−34 Electrostatic bonding via charge−dipole (e.g., M+−CO complexes) and charge−quadrupole (e.g., M+−OCO complexes) interactions causes polarization of the 3s electrons, and this leads to electron density being localized spatially opposite to the first ligand. Subsequent ligands tend to avoid this electron density and bind on the same side as the first ligand. This eventually leads to an ion located at the surface of a molecular cluster, as opposed to at its core. Silicon cations have electrons in the same 3s orbital as Mg+ and Al+, with an additional electron in the 3p orbital. Previous investigations of silicon−carbon dioxide clusters of the form Si+(CO2)n have shown that the ligand addition in these complexes is dominated by the presence of the occupied 3p orbital.33 Therefore, it is interesting to investigate the Si+(CO)n species to compare the bonding of various ligands to silicon cations. Previous experiments in our lab have studied the infrared photodissociation spectroscopy of a number of transition metal carbonyl ions.19,20 However, this is our first investigation of a main group cation−carbonyl system. Spectroscopic studies on cationic nontransition metal and nonmetal carbonyls are extremely scarce. Alkali metal−carbonyl cations have been investigated with theory and experiment,35−38 but this work emphasized the bonding energetics and not the spectroscopy of these systems. Neutral Al(CO)n clusters, which are isoelectronic to Si(CO)n+, have been studied experimentally and theoretically.39−41 The most recent spectroscopic study of isoelectronic aluminum carbonyls was conducted by the Douberly group using helium nanodroplet isolation.42 It will be interesting to compare the results of charged and uncharged isovalent nontransition metal carbonyl complexes. Neutral silicon carbonyl clusters have been investigated by experiment43 and theory,44−46 whereas Si(CO)n+ has been investigated only with theory.47 To our knowledge, this is the first experimental investigation of the infrared spectroscopy of silicon carbonyl cations.

absorption and intramolecular vibrational energy relaxation (IVR) take place on a time scale much smaller than the residence time in the reflectron (1−2 μs), leading to dissociation. Infrared spectra are obtained by monitoring the appearance of a specific fragment ion as a function of laser wavelength. In support of the experimental studies, Si(CO)n+ and Si(CO)n+Ar complexes were investigated using density functional theory (DFT) calculations, considering the doublet and quartet states of each cluster size. The calculations were performed using the B3LYP functional48,49 as implemented in the Gaussian03 computational package.50 The DZP basis set51 was employed for silicon, carbon, and oxygen atoms, and the 6-311++G** basis set was used for argon. Harmonic vibrations were scaled by calculating the frequency of molecular CO at the same level of theory and calculating the correction factor (0.982) needed to bring this mode into agreement with the known experimental value (2143 cm−1).27

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RESULTS AND DISCUSSION Figure 1 shows the mass spectrum of the Si(CO)n+ complexes formed via laser vaporization of a silicon rod in a pure CO

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EXPERIMENTAL SECTION Si(CO)n+ and Si(CO)n+Ar clusters are produced in a pulsed nozzle laser vaporization source using the third harmonic of a pulsed Nd:YAG laser (355 nm; Spectra-Physics GCR-150). The laser is focused onto a rotating and translating 1/4 in. diameter silicon rod mounted on the front of a pulsed nozzle (General Valve Series 9) in the so-called cutaway configuration which has been described previously.31 Si(CO)n+ ions are produced using a pure carbon monoxide (National Specialty Gas) expansion, whereas mixed complexes of the form Si(CO)n+Ar are produced using gas mixtures of 10% CO in argon. The expansion is skimmed into a second chamber where positive ions are pulse-extracted into a homemade reflectron time-of-flight mass spectrometer. Ions of a specific mass are selected by their flight time using pulsed deflection plates located at the end of the first flight tube. These ions are excited in the turning region of the reflectron with the tunable output of an infrared Optical Parametric Oscillator/Amplifier system (OPO/OPA; Laser Vision) pumped by the fundamental of a Nd:YAG laser (1064 nm; Spectra Physics Pro 230). This laser system provides infrared light in the region 2000−4000 cm−1 with a line width of about 1 cm−1. When the output of the infrared laser is on resonance with a vibration of the complex,

Figure 1. Mass spectrum of Si(CO)n+ complexes produced by laser ablation of a silicon rod in an expansion of carbon monoxide.

expansion. As indicated, a variety of mass peaks are produced corresponding to ions of the form Si(CO)n+. A series of minor peaks is observed at intermediate masses, which correspond to SiC(CO)n+ complexes. The Si(CO)2+ peak is the most abundant ion in the mass spectrum, indicating that this complex is preferentially formed and suggesting that it may have enhanced stability. Si(CO)n+ complexes out to n = 16 or more are produced. The CO ligands in the larger complexes cannot possibly all be directly coordinated to the silicon ion. Instead, larger clusters are likely to have CO ligands which are not bound strongly to the silicon ion, but attached via weaker electrostatic forces. The formation of these larger complexes is due to the cold conditions that exist in our supersonic expansion. As a result, these complexes are not likely to be stable at room temperature. To investigate the stability of these various complexes, we have done photodissociation experiments on mass-selected clusters with infrared excitation near the C−O stretch, and we have performed computational studies of the structures, spin 1376



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(n = 1, 2) do not fragment when excited with infrared light. This is consistent with the bond energies (1.35, 1.51 eV, respectively, for the elimination of one CO from n = 1, 2) determined computationally (Table 2). The calculated bond energies are much greater than the energy of an IR photon in the CO stretching region (2200 cm−1 is approximately 0.27 eV). It is therefore not surprising that no fragmentation is observed for these complexes. As illustrated in Figure 2, complexes larger

states, and energetics of these complexes. The computational results are presented in Tables 1−3. Small Si(CO)n+ complexes Table 1. Structures, Electronic Ground States, and Relative Energies for Si(CO)n+ Complexes Computed Using DFT complex Si(CO)+ Si(CO)+Ar Si(CO)2+ Si(CO)2+Ar Si(CO)3+ Si(CO)4+ Si(CO)5+ Si(CO)6+ Si(CO)7+

spin state

symmetry

relative energy (kcal/mol)

doublet quartet doublet quartet doublet quartet doublet quartet doublet quartet doublet quartet doublet quartet doublet quartet doublet quartet

C∞v C∞v Cs C∞v C2v C2v Cs C2v Cs C2v C2v C1 Cs Cs C2v Cs C2v C1

0.0 +57.7 0.0 +54.5 0.0 +60.0 0.0 +59.9 0.0 +53.0 0.0 +52.5 0.0 +51.7 0.0 +51.7 0.0 +49.9

Table 2. Computed Binding Energies in kcal/mol for CO in Si(CO)n+ Complexes and the Argon in Si(CO)n+Ar Complexes complex

E[Si(CO)n+−CO]

Si(CO)+ Si(CO)Ar+ Si(CO)2+ Si(CO)2Ar+ Si(CO)3+ Si(CO)3Ar+ Si(CO)4+ Si(CO)5+ Si(CO)6+ Si(CO)7+

31.13

Figure 2. Photofragmentation mass spectra of Si(CO)n+ species for n = 3−5, showing the fragmentation channels resulting from infrared excitation near the carbonyl stretching vibration. The negative peak represents the depletion of the mass-selected parent ion, while the positive peaks represent the resulting smaller mass fragments.

E[Si(CO)n+−Ar] 3.52

34.75 0.50

than n = 2 dissociate efficiently. These photofragmentation data for the n = 3−5 complexes are obtained by adjusting the photodissociation laser to the most intense resonance for each ion and taking the difference between the mass spectra with the laser on versus off. The negative peaks indicate that depletion of the parent has occurred via photodissociation, whereas positive peaks indicate the fragment ions produced. Based on the computed binding energies of CO to these complexes (Table 2), the dissociation of clusters larger than n = 2 is expected to occur via the absorption of a single photon. The fragmentation of all these species terminates at n = 2, consistent with the stability of the Si(CO)2+ species suggested above by the mass spectrum, and indicating that ligands beyond this size are only weakly bound. These photodissociation data agree with the computational data, confirming that Si(CO)2+ is the fully coordinated complex. The vibrational spectroscopy of these systems can be obtained by studying the wavelength dependence of these fragmentation processes. As noted above, the energy of IR photons near the CO stretching frequency is not sufficient to induce dissociation in smaller clusters. As a result, we employ rare gas “tagging” to enhance the dissociation yield and reveal the vibrational frequencies of these smaller species.31,52−55 Mixed complexes of the form Si(CO)n+Ar can fragment by elimination of argon following excitation of the CO vibration. The possible effects of tagging on the spectra are not ignored. Despite previous experiments indicating that tagging has a negligible effect on the spectra,19,20,31 we exercise due caution

6.32 0.43 5.62 4.35 4.13 2.67

Table 3. Vibrational Frequencies Computed (Scaled by 0.982) for Doublet and Quartet States of Si(CO)n+ and Si(CO)nAr+ Complexes, Along with Comparisons to Our Experimental Resultsa complex Si(CO)+Ar Si(CO)+ Si(CO)2+Ar

experimental frequency 2129 2123 2154

Si(CO)2+ Si(CO)3+

Si(CO)4+

2160 2123 2153 2177 2122 2152 2177 2196

calcd freq. (doublet) 2119 2130 2118 2159 2118 (202) 2115 2156 2201 2113 2153 2195 (45)

(325) (287) (203) (568) (570) 2149 (547) (226) (61) (543) (234) (88) 2201

calcd freq. (quartet) 2046 2049 2056 2138 2052 (15) 2012 2019 2070 1986 1990 2053 (66)

(592) (502) (1510) (38) (1453) (1523) (785) (2) (1549) (914) (52)

a

All frequencies are in wavenumbers, and computed IR intensities (km/mol) are listed in parentheses. 1377



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in interpreting the present data. Computations were conducted on both tagged and neat complexes to evaluate the effects of tagging. Figure 3 shows the spectra of the n = 1 and 2 species measured via argon tagging. Efficient photodissociation is

Figure 4. Infrared spectra of the Si(CO)n+ complexes detected in the loss of CO mass channel.

located at higher energy. The positions of the two main bands remain essentially unchanged for each cluster size, whereas the intensity of the band at highest frequency increases with the size of the cluster. The two main bands in the neat spectra occur at virtually the same positions as the two peaks in the Si(CO)2+Ar spectrum. This behavior is expected if the n = 2 species represents the core ion, and subsequent CO ligands are bound weakly external to this. The high frequency band is only observed for complexes larger than n = 2. Its intensity increases with cluster size, and there is slight variation in its position. By analogy to previous work on other metal carbonyls,20 we assign this band to the stretch of the external CO ligands not coordinated strongly to the cation. Theory shows that the bonding in these external ligands is progressively weaker as more ligands are added. The frequencies of these external ligands are all similar to each other, although these values can shift slightly for binding in different positions. This likely explains the width of these bands and the slight changes in their position as cluster size increases. To further interpret these spectra, we consider the results of our DFT calculations on neat Si(CO)n+ for n = 1−7 and tagged Si(CO)n+Ar for n = 1 and 2. Various isomers and spin states were investigated, and the most relevant numerical data are presented in Tables 1 and 2. The relative energies of the doublet and quartet spin states are presented in Table 1. For each of the complexes, the doublet state is found to be lower in energy that the quartet by about 55 kcal/mol. For comparison to this, the doublet 3s23p1 ground state of the isolated Si+ lies about 123 kcal/mol below the excited 3s13p2 quartet state.56 Selected structures for these doublet ground states are shown in Figure 5. The theoretical results presented in Table 2 can aid in the explanation of the photodissociation efficiency of the Si(CO)n+ and Si(CO)n+Ar systems. The first and second CO’s are calculated to be strongly bound (31.12 and 34.17 kcal/mol, respectively), consistent with the low fragmentation yields for these clusters. The binding energy of subsequent CO ligands decreases with cluster size after n = 2. The n = 3 species is calculated to be bound by only about 6.3 kcal/mol (2200 cm−1).

Figure 3. Infrared photodissociation spectra of Si(CO)n+Ar complexes, detected by the elimination of argon. The dashed red line indicates the frequency of gas phase CO.

achieved for both cluster sizes by tagging with a single argon. The position of the free CO stretch at 2143 cm−1 is indicated by the dashed vertical line. The spectrum for the n = 1 complex has one broad band that is slightly red-shifted. The width of this band is greater than those in larger complexes, indicating that perhaps photodissociation of this cluster size is not as efficient. In many other small clusters that we have studied, such broadening has been seen when the argon binding energy is higher and dissociation is possible only through a multiphoton process. In this case, the computed argon binding energy (3.53 kcal/mol; 1230 cm−1) is less than the IR photon energy, and so this should not be a problem. The width seen may be the result of an unresolved rotational contour or because these complexes are not cooled as efficiently as others in the expansion. The spectrum of the n = 2 complex has two peaks of different intensity, centered around the free CO stretch. Based on previous systems that we have studied, these bands are likely the symmetric and asymmetric stretches for a C2v structure. If the ligands were positioned symmetrically to either side of the Si+ in a linear structure, there would be only one IR-active mode. Thus, the two-band spectrum indicates that the structure is not linear and that the ligands are bound on the same side of the ion in a more asymmetric coordination. The photodissociation spectra for Si(CO)n+ (n = 3−7) complexes, obtained by monitoring the loss of CO ligands, are shown in Figure 4. The bands in these neat spectra appear to be sharper than those for the tagged species. This is not surprising because these clusters are significantly more abundant in the mass spectrum and ligands beyond the first two are believed to be more weakly bound, making fragmentation more efficient. In each spectrum, two main peaks centered around the free molecular CO stretch are observed, while a third feature is 1378



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orbital repels these ligands, and so they bind in its nodal plane where repulsion is minimized. Table 3 provides a comparison of the band positions for the carbonyl stretching frequencies measured experimentally to those calculated in this work. The computations consider the doublet and quartet spin states of silicon cation. As shown in the table, argon tagging is predicted to induce a perturbation in the spectra of n = 1 species. The calculated CO stretching frequency in the neat complex is 2130 cm−1, whereas in the tagged complex, it is 2119 cm−1. This is understandable, as the argon is expected to occupy a vacant ligand site. The perturbation of the vibrations caused by argon in the n = 2 complex is predicted to be minimal (neat, 2118 and 2160 cm−1; tagged, 2118 and 2159 cm−1), consistent with its expected weak binding. The frequencies for the untagged n = 2 species computed in this work differ slightly from those calculated by Chen and co-workers at the DFT/B3LYP/6-311+G* level.47 The unscaled asymmetric and symmetric stretches calculated in this work are 2156.7 and 2199.3 cm−1, respectively, whereas those calculated by Chen are 2176.9 and 2220.2 cm−1, respectively. This discrepancy is understandable, as different basis sets were used. Our scaled CO stretching frequency for Si(CO)+ is 2130 cm−1, while the experimental value for Si(CO)+Ar is 2129 cm−1. This indicates that tagging might actually induce a very minor perturbation on the n = 1 species, and it is possible that this effect is overestimated by theory. For each cluster size studied, only the structures and spectra corresponding to the doublet spin state agree with the experiment. Calculations were conducted on the quartet state, which has an s1p2 electronic configuration. However, relative intensities and band positions in these spectra do not coincide with the experimental spectra, as illustrated in Table 3. The spectra predicted for the quartet states all have band positions red-shifted to the edge of the range under investigation (2000− 2300 cm−1). In the case of the n = 4 species, two of the band positions are outside this range. The strong frequency shifts exhibited by complexes in the quartet state and the large relative energetics favoring the doublets lead us to conclude that all the complexes have an s2p1 doublet ground state configuration. Our group has previously investigated the difference in vibrational frequencies of cationic metal carbonyls and their neutral counterparts.19,20 Though these isoelectronic analogues have been found to have identical structures, their vibrational frequencies are strikingly different. Bands corresponding to the same CO vibration differ by over 100 cm−1 in position. In all the cases observed before, the neutral complex has bands that are more red-shifted than the corresponding cation. This is thought to occur because the charge on the metal ions contracts the valence electrons, limiting their ability to undergo back-donation, resulting in the red shift being less for cations. It is interesting to see if main group ion carbonyls and their neutral counterparts follow this trend. The experimental CO stretching frequencies obtained here for Si(CO)2+ can be compared to those reported by Douberly and co-workers for the isovalent complex Al(CO)2.42 Both Si(CO)2+ and Al(CO)2 have a V-shaped structure with C2v symmetry and a doublet ground state, but their CO stretching frequencies are quite different. The charged Si(CO)2+ complex has an asymmetric stretch at 2123 cm−1 and a symmetric stretch at 2154 cm−1, which are only slightly shifted to either side of the free-CO frequency (2143 cm−1). However, the bands of neutral Al(CO)2 are significantly red-shifted from this, lying at

Figure 5. Calculated doublet ground state structures for Si(CO)n+ complexes, where n = 1−4. The n = 3 and 4 structures are planar.

Although DFT is not expected to handle weak electrostatic bonding interactions accurately, the calculated values support our results that complexes larger than n = 2 contain weakly bound CO ligands and undergo efficient fragmentation. The argon binding energy of the n = 1 complex is higher than that of the n = 2 complex. This is not surprising, as the argon occupies a vacant ligand binding site in the n = 1 complex, thus creating a stronger bond. Argon binding to the n = 2 species is extremely weak, consistent with the observed efficiency of argon photoelimination. The computed lowest energy doublet structures are shown in Figure 5 for Si(CO)n+, where n = 1−4. The n = 1 complex is linear, the n = 2 complex is bent with C2v symmetry, the n = 3 complex has one CO ligand coordinated in the same plane as the first two, but in a remote position further away from the Si+, and the n = 4 complex has two such remote CO’s, with all ligands in the same plane and C2v symmetry. Complexes larger than n = 2 have the same C2v core ion with additional weakly bound CO molecules solvating this. The remote ligands in the n = 3 and 4 complexes are much further away from the silicon ion than the first two, and their binding energies are much lower. Thus we refer to these as “external” ligands even though they are coordinated directly to the Si+. Previous computational studies on Si(CO)2+ were reported by Chen and co-workers.47 The most stable configuration of Si(CO)2+ found by that group is in agreement with the structure found here. Both studies find an n = 2 complex with C2v symmetry and a doublet spin state for the lowest energy structure. The bent structure found here for Si+(CO)2 is completely analogous to the structure we found previously for the Si+(CO2)2 complex.33 Likewise, the CO2 complexes with Si+ had the same sharp drop in binding energy after the n = 2 complex.33 The bent structure of the n = 2 core ion and the planar positions of the n = 3 and 4 ligands are all likely the result of the singly occupied p orbital in the position perpendicular to the plane of these ligands (seen in molecular orbital occupation calculations). The electron density in this 1379



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1919.6 (asymmetric) and 1959.7 cm−1 (symmetric), respectively. Though greater in magnitude, the difference in vibrational frequencies here is qualitatively consistent with the trend previously noted for transition metal carbonyls. The significant red-shift of the Al(CO)2 vibrations arises from its strong π back-donation, but in this case it is the valence p electron causing this. Because the p orbital in both of these complexes lies out of the plane of the ligands, it is lined up spatially to allow π back-donation. However, in the case of Si(CO)2+, the valence p electron is contracted because of the charge, and this back-donation is greatly inhibited. One final issue of interest is the magnitude of the CO stretching frequencies observed for core and external ligands. Table 4 lists the stretching frequencies of the core and external

externally to the core ligands via weak van der Waals forces. These CO ligands progressively solvate the fully coordinated metal−carbonyl cluster and are called “second-sphere” or “external” carbonyls. The magnitude and direction of the shift in frequency observed for these second-sphere ligands can be understood using arguments outlined by Goldman and KroghJespersen.25 Their study showed that the polarization of a CO molecule by a nearby charge leads to a blue-shift in the CO stretching frequency. As shown in Table 4, the surface CO stretching frequency of silicon carbonyl is approximately 10 cm−1 higher than that of most transition metal carbonyls. This can be explained by the difference in the coordination model of the two types of systems. In transition metal ions, the fully coordinated complex is usually solvated symmetrical by strongly bound CO ligands. This first sphere of CO ligands shields the second-sphere ligands from the metal cation, limiting their polarization. In the present system, the coordination is asymmetric and localized to one side of the cation, so that the cation is not enclosed. External CO ligands here can still interact directly with the central cation for the n = 3 and 4 complexes, resulting in more efficient electrostatic polarization and an increased blue shift in the frequency. For complexes with n > 4, presumably the additional CO ligands are more remote from the central cation, and consequently less polarized by it. This should lead to a smaller CO blue shift, more like those seen before for transition metal ions. Indeed, this is evident in the spectra. The higher frequency bands for the n = 5−7 species are broader, suggesting an additional component on the lower frequency side. A trace of a doublet is present here for the n = 6 species, and the n = 7 band is noticeably peaked more to lower frequency.

Table 4. Comparison of the Band Positions of Core and External CO Ligands for Various Metal−Carbonyl Cation Complexes Measured by Our Group Previously to Those of Si(CO)n+ complex

core CO frequency (cm−1)

external CO frequency (cm−1)

Si(CO)n+ Ti(CO)n+ V(CO)n+ Co(CO)n+ Mn(CO)n+ Cu(CO)n+ Au(CO)n+

2122, 2152 2114 2103 2141, 2150 2122 2196 2215f

2177 2164a 2165b 2165c 2174d 2162e

a

Reference 20g. bReference 20d. cReference 20c. dReference 20e. Reference 20f. fReference 20b.

e

CO ligands for various transition metal ion carbonyls measured by our group, compared to those for Si(CO)n+. As explained previously, the CO stretching frequencies observed for core ligands in transition metal carbonyls deviate from the free-CO value, depending on the dominant bonding interactions at work. For the core CO ligands in many transition metal ion carbonyls, π back-donation from partially filled d orbitals is the dominant interaction, and a red-shifted CO frequency is usually observed for these species. Cation species generally have smaller red shifts than corresponding neutral species because of the reduced efficiency of the back-donation. The importance of the π back-donation is highlighted even more in the case of metal−carbonyl ions of gold and copper.20b,f These “nonclassical” carbonyl systems, in which the metal cation has a filled d orbital, have extremely limted back-donation, and this results in blue-shifted CO stretches. Unlike transition metals, silicon cation does not have occupied d orbitals. However, the magnitude and direction of the shift in stretching frequencies for the Si(CO)n+ species are in line with those observed for transition metal carbonyls. This indicates that partially filled p orbitals have the ability to undergo π back-donation and their role in ion carbonyl bonding is comparable to that of d orbitals in transition metal−carbonyl bonding. The stretching frequency of the weakly bound CO ligands is also worth discussion. As noted above, all clusters larger than the fully coordinated Si(CO)2+ species exhibit a feature that is significantly blue-shifted from the free molecular CO stretch and grows in intensity with increasing cluster size. These observations are consistent with the results of previous investigations on transition metal carbonyls.19,20 In transition metal systems, the blue-shifted band is attributed to CO ligands not coordinated directly to the central metal ion, but bound

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CONCLUSION

Silicon carbonyl cations of the form Si(CO)n+ (n = 1−7), and their rare gas tagged analogues Si(CO)n+Ar, were produced in a laser vaporization source with a pulsed nozzle and studied with infrared photodissociation spectroscopy and density functional theory. Each complex exhibited CO stretching frequencies within the region of the infrared spectrum under investigation (2000−2300 cm−1). The number of infrared active bands, frequency positions, and relative intensities provide distinctive patterns which elucidate the geometries and electronic states of these complexes. These results were confirmed by comparisons to the predictions of theory. All Si(CO)n+ complexes were found to have an s2p1 doublet ground state configuration on the silicon ion. The n = 2 complex, Si(CO)2+, has a filled coordination and a V-shaped structure with localized asymmetric carbonyl coordination and C2v symmetry, similar to that of the isoelectronic Al(CO)2 complex. The two infrared active bands present in Si(CO)2+ correspond to the asymmetric and symmetric CO stretching vibrations (2123 and 2154 cm−1, respectively). These bands are significantly less red-shifted than those measured for Al(CO)2 (1919.6 and 1959.7 cm−1). Complexes larger than n = 2 have weakly bound CO ligands, with blue-shifted carbonyl stretching frequencies due to electrostatic polarization from the silicon cation. The weakly bound CO ligands in n = 3 and 4 clusters were found to be more blue-shifted than those measured previously by our group for transition metal carbonyls due to the localized asymmetric coordination of Si(CO)2+. 1380



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the generous support for this work from the U.S. Department of Energy (grant no. DE-FG02-96ER14658) and the Air Force Office of Scientific Research (grant no. FA95509-1-0166).

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REFERENCES

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