Spectroscopic and Theoretical Investigations of Vibrational Frequencies in Binary Unsaturated Transition-Metal Carbonyl Cations, Neutrals, and Anions

Abstract

Figure 18 presents the C-O stretching vibrational frequencies of the first-row transition-metal monocarbonyl cations, neutrals, and anions in solid neon; similar diagrams have been reported for neutral MCO species in solid argon, but three of the early assignments have been changed by recent work and one new assignment added. The laser-ablation method produces mostly neutral atoms with a few percent cations and electrons for capture to make anions; in contrast, thermal evaporation gives only neutral species. Hence, the very recent neon matrix investigations in our laboratory provide carbonyl cations and anions for comparison to neutrals on a level playing field. Several trends are very interesting. First, for all metals, the C-O stretching frequencies follow the order cations > neutrals > anions with large diagnostic 100-200 cm-1 separations, which is consistent with the magnitude of the metal d to CO pi * donation. Second, for a given charge, there is a general increase in C-O stretching vibrational frequencies with increasing metal atomic number, which demonstrates the expected decrease in the metal to CO pi * donation with increasing metal ionization potential. Some of the structure in this plot arises from the extra stability of the filled and half-filled d shell and from the electron pairing that occurs at the middle of the TM row; the plot resembles the "double-humped" graph found for the variation in properties across a row of transition metals. For the anions, the variation with metal atom is the smallest since all of the metals can easily donate charge to the CO ligand. Third, for the early transition-metal Ti, V, and Cr families, the C-O stretching frequencies decrease when going down the family, but the reverse relationship is observed for the late transition-metal Fe, Co, and Ni families. In most of the present discussion, we have referred to neon matrix frequencies; however, the argon matrix frequencies are complementary, and useful information can be obtained from comparison of the two matrix hosts. In most cases, the neon-to-argon red shift for neutral carbonyls is from 11 to 26 cm-1, but a few (CrCO) lie outside of this range. In the case of FeCO and Fe(CO)2, it appears that neon and argon trap different low-lying electronic states. In general, the carbonyl neutrals and anions have similar shifts but carbonyl cations have larger matrix shifts. For example, the FeCO+ fundamental is at 2123.0 cm-1 in neon and 2081.5 cm-1 in argon, a 42.5 cm-1 shift, which is larger than those found for FeCO- (11.7 cm-1) and FeCO (11.7 cm-1). It is unusual for different low-lying electronic states to be trapped in different matrices, but CUO provides another example. The linear singlet state (1047.3, 872.2 cm-1) is trapped in solid neon, and a calculated 1.2 kcal/mol higher triplet state is trapped in solid argon (852.5, 804.3 cm-1) and stabilized by a specific interaction with argon. The bonding trends are well described by theoretical calculations of vibrational frequencies. Table 5 compares the scale factors (obse...

Influence of Carbon Support on Electronic Structure and Catalytic Activity of Pt Catalysts: Binding to the CO Molecule

“…[67][68][69][70][71] Bauschlicher et al pointed out some of the subtleties in σ donation versus electrostatic effects, and discussed how their contributions change for different systems. 69 They discuss that metal-carbonyl bonding involves a complex synergism between σ donation, electrostatic effects, and π back-bonding, but the relative importance of their contributions changes from system to system. For example, in the metal-carbonyl cations, the π back-bonding is, in general, much smaller than for the neutral species.…”

IR Photodissociation Spectroscopy and Theory of Au+(CO)n Complexes: Nonclassical Carbonyls in the Gas Phase

“…4 Gas-phase metal carbonyl ions have been studied extensively in mass spectrometry, [5][6][7] and the structures and bonding of these ions have been investigated with theory. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The reactions of carbonyl ions have been well characterized, and bond energies have been determined, [21][22][23][24][25][26][27][28][29][30][31] but there is much less data on their spectroscopy. However, recent experiments have employed photoelectron spectroscopy to study a variety of metal carbonyl anions, providing vibrational information for the corresponding neutral ground states.…”

“…However, recent experiments have employed photoelectron spectroscopy to study a variety of metal carbonyl anions, providing vibrational information for the corresponding neutral ground states. [32][33][34][35][36] Infrared spectroscopy of unsaturated neutral metal carbonyls isolated in rare gas matrices have been reported, 7,[37][38][39] and infrared photodissociation measurements have been described for metal cluster carbonyl cations using free electron lasers. [40][41][42][43][44][45] In the present study, we employ new optical parametric oscillator infrared lasers to investigate the unusual carbonyl complexes of atomic gold cations.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][48][49][50][51] Numerous theoretical studies have discussed the origins of this vibrational blue shift, which appears to be found for only a few systems, including the noble metals. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Several groups have collected infrared spectra for cationic, neutral and anionic gold-carbonyl clusters frozen in rare-gas matrices. [37][38][39] Infrared spectra were recorded for CO bound to cationic gold clusters, Au n + (n ) 3-20) in the gas-phase by Meijer and co-workers.…”

Competing effects of rare gas atoms in matrix isolation spectroscopy: A case study of vibrational shift of BeO in Xe and Ar matrices

“…1 Since these spectra are usually obtained at very low temperatures, they are almost free from perturbations due to thermal fluctuations, thus enabling one to perform highly precise measurements. Rare gas atoms are almost exclusively used as a matrix medium, and since they are chemically inert, it is naturally assumed that the effect of surrounding rare gas atoms is marginal and in fact spectral shifts due to the presence of rare gas atoms are typically less than 0.5%.…”