Distinctive Coordination of CO vs N2 to Rhodium Cations: An

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Distinctive Coordination of CO vs N to Rhodium Cations:An Infrared and Computational Study Antonio D. Brathwaite, Heather Lynn Abbott-Lyon, and Michael A. Duncan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07749 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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The Journal of Physical Chemistry

J. Phys. Chem. A

Distinctive Coordination of CO vs N2 to Rhodium Cations: An Infrared and Computational Study A. D. Brathwaite,1 H. L. Abbott-Lyon,2 and M. A. Duncan3* 1

College of Science and Mathematics, University of the Virgin Islands, St. Thomas, United States Virgin Islands 00802 2 Department of Chemistry & Biochemistry, Kennesaw State University, Kennesaw, GA 30144 3 Department of Chemistry, University of Georgia, Athens, GA 30602 *Corresponding author. Email: [email protected]; Phone: 706-542-1998

ABSTRACT Carbonyl and nitrogen complexes with Rh+ are produced in a molecular beam using laser ablation and a pulsed-nozzle source. Mass-selected ions of the form Rh(CO)n+ and Rh(N2)n+ are investigated via infrared laser photodissociation spectroscopy. The fragmentation patterns and infrared spectra provide information on the coordination and geometries of these complexes. The shifts in vibrational frequencies relative to the uncoordinated ligands give insight into the nature of the bonding interactions involved. Experimental band positions and intensities are compared to those predicted by Density Functional Theory (DFT). Rh+ coordinates only four nitrogen molecules, whereas it can accommodate five carbonyl ligands. The fifth CO ligand resides in an axial site with bonding intermediate between coordination and solvation. The carbonyl stretch in Rh(CO)4+ (2160 cm-1) is blue shifted with respect to the molecular CO vibration (2143 cm-1). Conversely, the N−N stretch in Rh(N2)4+ (2297 cm-1) is red shifted with respect to the free N2 vibration (2330 cm-1). The opposite directions of these frequency shifts is explained by a combination of σ donation and electrostatic ligand polarization. 1 ACS Paragon Plus Environment


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INTRODUCTION The widespread importance of metal carbonyls has led to numerous experimental and theoretical studies of their structure, bonding, and reactivity over the past several decades.1-11 Insight into the bonding and reactivity can be obtained by comparing the C−O vibrational frequency in different chemical situations to that of the isolated CO molecule (2143 cm-1).12 Metal-nitrogen complexes, which are isoelectronic to metal carbonyls, have also been investigated for their catalytic and biological significance.13-18 Activation of the N−N bond is a crucial step in the conversion of nitrogen to useful products such as ammonia or its incorporation into the DNA, RNA, or proteins necessary for life. Although the N−N vibration is not infrared active in the free molecule, coordination to a metal ion causes the IR activity to "switch on." Vibrational studies of metal-nitrogen complexes therefore provide insight into the degree of activation of N2.14 Because N2 and CO are isoelectronic, there has been longstanding interest in comparing their metal binding.19-22 In the present report, we investigate the structures and bonding interactions of gas-phase Rh(CO)n+ and Rh(N2)n+ complexes by probing their C−O and N−N vibrations with infrared spectroscopy. Metal carbonyl ions have been produced in the gas phase and studied extensively with mass spectrometry,23-28 theory5,6,29-35 and vibrational spectroscopy.6,36-59 Recently, mass-selected ion infrared photodissociation spectroscopy has been employed to study the structures, coordination and bonding interactions of these systems.39-59 Rh(CO)n+ ions have also been studied in the condensed phase as salts stabilized with counterions because of their catalytic activity with olefins and alcohols.10,11,60-66 Additionally, the structures of Rh(CO)n+ complexes were explored by comparing matrix isolation infrared spectra with the predictions of Density

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Functional Theory (DFT) calculations.6,36-37 Gas-phase studies on these systems can provide a perturbation-free environment to probe their intrinsic vibrational behavior. Although metal-carbonyl and metal-nitrogen complexes are isoelectronic, there is significantly less information on the spectroscopy of the latter. Metal-nitrogen cations have been produced in the gas phase and studied with infrared laser photodissociation spectroscopy.67-69 Duncan and coworkers investigated the structure and bonding in V(N2)n+ and Nb(N2)n+ complexes,67,68 while Tang et al. studied the coordination and bonding of the M(N2)n+ (M = Y, La, Ce; n = 7−8) species.69 Neutral and anionic Rh(N2)n complexes were studied with matrix isolation.70-72 However, to our knowledge, these cations have not been studied in the gas phase. We explore the Rh(N2)n+ complexes here to compare to their carbonyl counterparts. The bonding in metal carbonyl and metal nitrogen complexes can be explained using the Dewar-Chatt-Duncanson (DCD) complexation model. This paradigm involves a combination of σ-type donation from the ligand into empty d orbitals on the metal, and π−type back donation from the metal d orbitals into antibonding π* orbitals on the ligand.1-7 The effects of these bonding interactions on the vibrational frequencies of CO ligands are well documented.5,6,50 σ donation leads to a blue shift in the CO stretch, whereas π back-bonding shifts these frequencies to the red. The dominant interaction in these systems is π back-bonding, and the frequencies observed are generally red shifted compared to the isolated CO stretch vibration. In addition to these familiar interactions, electrostatic ligand polarization has been recently discussed as an additional bonding consideration in these systems.31-33 As shown in Figure 1, the 5σ orbital of CO is polarized toward oxygen, and is thought to possess partial antibonding character. Consequently, cation-induced polarization strengthens the C−O bond by evenly redistributing the electron density, thereby making it more "N2-like." Consistent with this, when CO possesses a 3 ACS Paragon Plus Environment


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full unit of charge, its cation has a higher vibrational frequency (2184 cm-1) than the neutral (2143 cm-1), whereas N2+ has a lower frequency (2175 cm-1) than N2 (2330 cm-1).12 Goldman and Krogh-Jesperson showed that although both σ donation and electrostatic polarization increase the C−O vibrational frequency, electrostatic polarization is the dominant interaction.31 These ideas conveniently explain the blue-shifted C−O frequencies observed for so-called "nonclassical" carbonyls containing late transition metals that cannot undergo efficient π back bonding.45,46,50,51 The DCD model has also been invoked to explain the bonding in metalnitrogen cations.67-69 However, unlike the case of CO, in nitrogen systems both σ donation and π back-bonding produce red-shifted vibrational frequencies. Furthermore, electrostatic polarization of the already symmetric 5σ orbital of N2 weakens the bond and induces a red shift in its vibrational frequency. Until now, nitrogen complexes for metals with inefficient π backbonding have not been investigated. The electron configurations in metal carbonyls often follow the 18-electron rule,1-7 which predicts stability for complexes with a noble gas configuration on the metal. Recent work has shown that this concept developed in conventional inorganic chemistry is also valid for gasphase ions.47-59 While d8 metals like Rh+ are known to form 16-electron square-planar complexes,4 the 18-electron rule suggests that a penta-coordinate Rh(CO)5+ complex analogous to Fe(CO)5 and Co(CO)5+ could be stable. Rh(CO)4+ has been synthesized and characterized in acidic media62-66 and produced in rare gas matrices.37 These studies concluded that it has a square-planar geometry and one infrared active band, but the frequency of this band was the subject of some contention. Experiments in superacid media reported bands at 2138 and 2139 cm-1,62,65,66 whereas those in neon matrices found this feature at 2162 cm-1.37 Our mass-selected experiments in the gas phase provide unambiguous species identification and a perturbation-free 4 ACS Paragon Plus Environment

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environment, ideal to investigate intrinsic molecular properties. The present study employs these methods to investigate Rh(CO)n+ and Rh(N2)n+ complexes. Their infrared spectra provide new insight into the distinctive bonding interactions of the isoelectronic CO and N2 ligands.

EXPERIMENTAL SECTION Rh(CO)n+ and Rh(N2)n+ ions are produced by laser vaporization of a rhodium target in a pulsed-nozzle expansion of pure CO or N2 gas.73 Low ion densities preclude the use of conventional infrared absorption, so we employ IR-laser photodissociation spectroscopy and rare gas atom tagging74-80 to record infrared spectra. Mixed complexes of the form Rh(CO)nAr+ and Rh(N2)nAr+ are produced using argon-CO or argon-N2 mixtures. These complexes are detected and mass selected in a time-of-flight mass spectrometer.81 Mass-selected ions are excited in the turning region of the reflectron with the tunable output of an infrared OPO laser system (LaserVision) pumped by a Nd:YAG laser (Spectra Physics Pro 230). This OPO provides tunable infrared light in the region of 2000 – 4000 cm-1 with a line width of about 1 cm-1. Infrared spectra are recorded by monitoring the appearance of one or more fragment ions as a function of the laser wavelength. DFT calculations were carried out to determine the structure and bonding of these complexes using the B3LYP functional,82,83 as implemented in the Gaussian09 computational package.84 The LANL2DZ ECP basis set85-87 was used for rhodium, the DZP basis set88 for carbon, oxygen and nitrogen, and the 6-311+G* basis set for argon. Relative energies are presented throughout the paper with zero point corrections. The computed carbonyl frequencies were scaled by a factor of 0.966,89 whereas, the N−N vibrations were scaled by 0.972.69



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Theoretical spectra were given a 10 cm-1 FWHM Lorentzian line shape for comparison to the experimental spectra. Full computational details are provided in the Supporting Information.

RESULTS AND DISCUSSION Figure 2 shows a mass spectrum of the Rh(CO)n+ ions formed in the experiment. The peaks corresponding to Rh(CO)4+ and Rh(CO)5+ are relatively more intense, suggesting that these species are formed preferentially and may have enhanced stability. Rh(CO)n+ complexes up to about n = 17 are produced. It is unlikely that the ligands in the larger complexes are all coordinated directly to the central rhodium cation. Instead, these complexes are expected to consist of a strongly-bound metal-ligand core ion solvated by additional "external" or "secondsphere" carbonyls, bound via weak electrostatic and/or van der Waals interactions. The formation of these larger complexes is possible because of the cold supersonic expansion used in the ion source.73 Efficient elimination of these external ligands is expected upon IR absorption. The mass spectrum of the corresponding Rh(N2)n+ complexes is provided in the Supporting Information (Figure S2). It has slightly enhanced peaks for the n = 4 and 6 ions. Again, these enhanced intensities suggest that these ions may be more stable than others in the distribution. However, mass spectral intensities are not a reliable measure of stability, as ion abundances can vary with source conditions and spectrometer focusing.50,73 We therefore employ infrared laser photodissociation spectroscopy to further investigate these systems. Smaller Rh(CO)n+ complexes are not expected to photodissociate efficiently following infrared excitation in the C–O stretching region because of their stronger metal-ligand bonding. However, larger species should fragment efficiently by eliminating external CO molecules. The resulting patterns can be used to identify coordination numbers. Figure 3 shows the


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photodissociation mass spectra for Rh(CO)n+ (n = 5−8) ions when the laser is tuned to the most intense C–O stretching frequency for each complex. The negative peak indicates the depletion of the parent ion and the positive peaks indicate the resulting photofragments. Because of the different mass spectrometer focusing, the parent ion intensity does not match the integrated intensities of the fragments. As shown, these complexes undergo sequential ligand elimination terminating at the Rh(CO)4+ ion. The corresponding photodissociation spectra for Rh(N2)n+ complexes (n = 5–8) are presented as Figure S3 in the Supporting Information, and a similar pattern is observed, also terminating at the n = 4 ion. The photodissociation of these ions is expected to depend on the ligand binding energy, the photon energy employed, and the number of photons absorbed. Fragmentation behavior in which one particular ion is resistant to further dissociation has been seen in the past for numerous metal ion-CO and N2 complexes.45-53,67,68 The results here suggest that four ligands of either CO or N2 complete the coordination around rhodium cation. These ideas are investigated further with DFT computations and infrared spectra. Infrared photodissociation spectra were recorded by monitoring the wavelength dependence of these photodissociation processes. As noted above, complexes with four or fewer ligands do not undergo efficient photodissociation upon irradiation with infrared light. We therefore employ rare gas atom tagging74-80 to obtain the spectra of the small Rh(CO)n+ and Rh(N2)n+ complexes. In this method, Rh(CO)n+Ar and Rh(N2)n+Ar ions are produced and infrared excitation eliminates the argon atom. The tagging method was successful for all of these small systems except the n = 1 and 2 complexes of N2, where we were unable to produce enough stable signal for the desired parent ions.



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Figure 4 shows the spectra of the Rh(CO)+Ar2 and Rh(CO)n+Ar (n = 2−4) complexes. Argon apparently binds too strongly to the n = 1 complex, so that Rh(CO)+Ar does not photodissociate, but this species does dissociate by argon elimination when two argons are attached. The dashed vertical line indicates the position of the free CO stretch at 2143 cm-1. As shown, these spectra contain a single band for the n = 1 and 4 species, and two bands for the n = 2 and 3 species. The bands for the n = 1 and 4 complexes occur at frequencies higher than that of the free CO molecule, whereas those for the n = 2 and 3 species straddle this line, with one red-shifted and one blue-shifted resonance. Blue-shifted bands have been seen before for the non-classical carbonyls of Pt+, Au+ and Cu+.45,46,51 The bands measured for Rh(CO)2,3+Ar are somewhat broader than those for the other complexes, suggesting that their dissociation is not as efficient. This makes sense because the binding energies of the argons to these clusters are probably higher than the photon energy (see discussion below and Table 1), and this weaker signal may also be the result of some two-photon absorption. The two bands seen for these clusters also suggest that they have lower symmetry; a linear n = 2 or a trigonal planar n = 3 structure would each have only one IR-active C−O stretch. The spectrum for Rh(CO)4+Ar has a single band at 2160 cm-1 suggesting high-symmetry, consistent with the square-planar structure mentioned earlier. Figure 5 compares the spectrum for the n = 4 complex to those of larger clusters. The n = 4 and 5 spectra were measured with tagging, while those for the n = 6 and 7 clusters were measured by the loss of CO. All of the resonances for these clusters appear at frequencies higher than that of isolated CO. Because the photodissociation experiments suggested that the n = 4 complex has the completed coordination sphere, we expected larger complexes to have basically the same spectrum as this core ion, but with slight shifts induced by solvation of external CO


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molecules. This behavior has been seen for many other metal carbonyl ion clusters.47-53 Surprisingly, the spectra for Rh(CO)5+Ar, Rh(CO)6+ and Rh(CO)7+ are quite different from that of Rh(CO)4+Ar, but are all similar to each other. They do not have a resonance at 2160 cm-1, but instead have two main bands near 2148 and 2175 cm-1 and a weak feature near 2206 cm-1. This change in the spectral pattern suggests that Rh(CO)5-7+ do not have the same core ion structure as Rh(CO)4+. Instead, the similarity between their spectra suggests that their core ion structures are the same, i.e., that Rh(CO)6,7+ represent a Rh(CO)5+ core ion solvated with one or two external CO molecules. It is not yet clear why there was no evidence for this n = 5 ion in the fragmentation mass spectra. The computational work on these ion structures, as described below, makes it possible to test these ideas and to explain these data further. The IR spectra for Rh(N2)n+ (n = 3−6) are shown in Figure 6. Spectra for the n = 5 and 6 species are measured via N2 elimination, whereas the n = 3 and 4 complexes require tagging. The fundamental frequency of molecular N2 is represented by the dashed line at 2330 cm-1. As expected from previous work, binding to a metal cation induces IR intensity in these N−N stretches.67-69 As shown, each spectrum has a single band, and all of these fall at frequencies lower than the vibration of isolated N2. Rh(N2)3+Ar has a broad resonance at 2286 cm-1. The spectrum of Rh(N2)4+Ar has a single peak at 2297 cm-1, consistent with a highly symmetric structure. The larger Rh(N2)5+ and Rh(N2)6+ ions also have similar bands in virtually the same position. This behavior, taken together with the fragmentation data, suggests that the n = 4 species is the fully coordinated ion, and that larger complexes have this same core, with additional molecules acting as weakly bound external or solvating species. Consistent with previous work, external N2 molecules would have unshifted frequencies, but little or no IR



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intensity, and are not detected here. The red shifts seen here for Rh(N2)n+ complexes are noticeably smaller than those reported for other M(N2)n+ systems studied previously.67-69 To investigate the structures and binding energies for these complexes, DFT/B3LYP calculations were performed on various isomers for the singlet and triplet spin states of Rh(CO)n+ and Rh(N2)n+ ions. Full details of these calculations are presented in the Supporting Information. Table 1 reports the structures and relative energies obtained for these complexes, as well as their binding energies for the elimination of CO, N2 or Ar. As shown in the table, the binding energies for all the carbonyl complexes are significantly greater than those of the corresponding nitrogen complexes. In their most stable configurations, the smallest carbonyl complexes have ligand binding energies of 40−50 kcal/mol, whereas those of the nitrogen systems are 20−30 kcal/mol. The external ligands or argon tag atoms are bound much more weakly, as expected. None of these binding energies from DFT calculations are expected to be highly accurate, but their trends should be reasonably reliable. Although DFT calculations are not highly quantitative for the energetics of transition metal complexes, the harmonic frequencies produced by these calculations (with proper scaling) provide good representations of the vibrational patterns for metal carbonyls.50,89 We have therefore computed the vibrational frequencies and intensities for these complexes in different structures and spin states so that we can compare their vibrational patterns to the measured spectra. For ions whose spectra were measured with tagging, we examine both the naked complex and that with attached argon. These computational results are presented in Table 2. The experiment versus theory comparisons for the n = 1 and 2 complexes are presented in the Supporting Information (Figures S12 and S17). The n = 1 complex could only be measured when tagged with two argons. The spectrum predicted for the complex with two argons binding


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opposite the CO ligand matches the experiment for a rhodium cation in its triplet state. This spin state is not surprising, as the ground state of the rhodium cation is the d8 3F.90 The n = 2 spectrum, measured with a single argon tag, matches that predicted by theory for a bent OC-Rh+CO complex with an angle of 87.5° between the CO ligands. Beginning with this complex and continuing for all the larger species, the ground electronic state that matches the experiment best is the singlet. The first excited d8 1D state in the isolated rhodium cation lies at an energy 11,643.7 cm-1 higher than the ground state.90 However, the low spin state is well known for Rh+ complexes in inorganic chemistry, as this configuration makes it possible to minimize ligandelectron repulsion.4 The spectrum of the Rh(CO)3+Ar complex, as well as those predicted by theory for its singlet and triplet states, are shown in Figure 7. As shown in the inset of the figure, the singlet state has the ligands arranged at right angles to each other, building toward the square-planar configuration, with the argon occupying a site where the fourth ligand would go. As indicated in Table 1, the argon binding energy in this complex is low and efficient photodissociation is detected in the 2000–2300 cm-1 region. The experimental spectrum contains two intense bands at 2134 and 2158 cm-1, and a very weak feature at 2206 cm-1. The singlet species with a C2v structure is predicted to be lower in energy than the triplet (+19.7 kcal/mol), which has a Cs structure. The spectrum predicted for the singlet reproduces the pattern of two intense bands, along with the barely noticeable feature at higher frequency. Therefore, we assign our spectrum to the singlet ground state. A matrix isolation study by Andrews and coworkers reported a single broad band at 2168 cm-1 as the C–O stretch of Rh(CO)3+.37 However, the ions in that experiment were not mass selected and the broad spectrum could have come from overlapping bands of different cluster sizes.



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Figure 8 shows the experimental spectrum of Rh(CO)4+Ar along with those predicted by theory for the singlet and triplet species. The singlet has a square-planar configuration, with argon binding to the metal ion above the plane of the CO ligands. The triplet has a similar structure, with the argon and one CO ligand switching places. These complexes are predicted to have extremely low argon binding energies (

Distinctive Coordination of CO vs N2 to Rhodium Cations: An

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