CO Stretching Force Constants Research Articles

New Approach to Tolman’s Electronic Parameter Based on Local Vibrational Modes

Tolman's electronic parameter (TEP) derived from the A1-symmetrical CO stretching frequency of nickel-phosphine-tricarbonyl complexes, R3PNi(CO)3, is brought to a new, improved level by replacing normal with local vibrational frequencies. CO normal vibrational frequencies are always flawed by mode-mode coupling especially with metal-carbon stretching modes, which leads to coupling frequencies as large as 100 cm(-1) and can become even larger when the transition metal and the number of ligands is changed. Local TEP (LTEP) values, being based on local CO stretching force constants rather than normal mode frequencies, no longer suffer from mode coupling and mass effects. For 42 nickel complexes of the type LNi(CO)3, it is shown that LTEP values provide a different ordering of ligand electronic effects as previously suggested by TEP and CEP values. The general applicability of the LTEP concept is demonstrated.

Polarizability effect in transition metal carbonyl complexes

The literature date on substituent influence on the carbonyl stretching frequencies (ν), CO stretching force constants (k), as well as 13C NMR carbonyl chemical shifts (δ) have been analyzed for 19 series of the transition metal carbonyl complexes. It was established for the first time that the ν, k and δ values depend not only on the inductive and resonance effects but also on the polarizability of substituents. The polarizability contribution ranges up to 37%.

Infrared Emission and Theoretical Study of Carbon Monoxide Adsorbed on Alumina-Supported Rh, Ir, and Pt Catalysts

The infrared emission spectra of CO adsorbed on alumina-supported 1, 3, and 5 wt % Rh, Ir, and Pt metal-containing catalysts were studied at 423 and 473 K. While CO is adsorbed in dicarbonyl (dimer), linearly (on-top) bonded and bridged carbonyl forms on rhodium and platinum, the dimer form is dominant on iridium. The relative intensity of Rh-CO and Ir-CO linear bands decrease with increasing temperature compared to the intensity of the dicarbonyl bands; the corresponding bands on Pt behave the opposite way. Two dicarbonyl and two linear Pt-CO bands were identified in the infrared spectra of Pt/Al(2)O(3) catalysts. The surface structure (kinked or planar Pt atoms), the dispersity of the metal, the temperature, and the quantity of adsorbed CO on the surfaces all have an effect on the fine structure of the Pt-CO stretching bands. The metal-carbon and CO stretching force constants were calculated for surface dicarbonyl, linearly bonded CO, and bridged carbonyl species. The metal-carbon stretching wavenumbers and force constants were predicted and compared between surface species and metal carbonyl complexes. The iridium-carbon bonds were found always stronger than the Rh-C and Pt-C ones in all surface species. The observed stretching wavenumbers and force constants seem to support the idea that CO and metal-carbon bonds are always stronger in metal carbonyl complexes than in adsorbed surface species. The distribution and mode of CO adsorption on surface metal sites can be effectively studied by means of infrared emission spectroscopy.

Syntheses, molecular structures, and vibrational spectra of chloropentacarbonylrhodium(III) and -iridium(III) undecafluorodiantimonate(V), [Rh(CO)5Cl][Sb2F11]2 and [Ir(CO)5Cl][Sb2F11]2: an experimental and density functional study.

The reactions of either bis(mu-chloro)tetracarbonyldirhodium(I), [Rh(CO)2(mu-Cl)]2, or chlorotricarbonyliridium(I), [Ir(CO)3Cl]n, in the conjugate Brønsted-Lewis superacid HF-SbF5 and in a CO atmosphere, produce [Rh(CO)5Cl][Sb2F11]2 or [Ir(CO)5Cl][Sb2F11]2, respectively. In these oxidative carbonylation reactions, antimony(V) fluoride functions as an oxidizing agent. The reduced product is identified as 6SbF3.5SbF5. [Rh(CO)5Cl][Sb2F11]2 is obtained in the form of single crystals. Crystal data: monoclinic, space group P2(1) (No. 4); a = 9.721(1), b = 12.602(1), c = 10.538(1) A; beta = 106.51(1) degrees; V = 1237.7(2) A3; Z = 2; T = 300 K; R1 [I > 3 sigma (I)] = 0.0367, wR2 = 0.0739. Single crystals of [Ir(CO)5Cl][Sb2F11]2 are produced in small amounts from a solution of mer-Ir(CO)3(SO3F)3 in magic acid, HSO3F-SbF5. The possible source of chlorine will be discussed. Crystal data for [Ir-(CO)5Cl][Sb2F11]2: monoclinic, space group P2(1) (No. 4); a = 9.686(2), b = 12.585(2), c = 10.499(2) A; beta = 106.59(2) degrees; V = 1226.5(4) A3; Z = 2; T = 294 K; R1[I > 3 sigma (I)] = 0.032, Rw = 0.031. The bond lengths and bond angles are nearly identical in the two isostructural salts; however, the cell volume of [Ir(CO)5Cl][Sb2F11]2 is slightly smaller than that of [Rh(CO)5Cl][Sb2F11]2. The cations (point group C4v) feature unusually long M-C bonds (M = Rh, Ir) and correspondingly short CO bonds, as well as high CO stretching wavenumbers and high CO stretching force constants. The [Sb2F11]- anions are not symmetry related, and their dihedral and bridge angles differ slightly in both salts. There are significant interionic contacts in [Ir(CO)5Cl][Sb2F11]2 exclusively of the C-F type (about 2 for each C atom of the five carbonyl groups) resulting in extended structures. The vibrational spectra for both [M(CO)5Cl]2+ cations (M = Rh, Ir) are assigned with the help of density functional calculations, which also provide intensities for IR and Raman bands. While [Rh(CO)5Cl]2+ is the first cationic carbonyl derivative of Rh(III), the vibrational and structural parameters for [Ir(CO)5Cl]2+ are compared to data for [Ir(CO)6]3+ and mer-Ir(CO)3(SO3F)3.

Carbon-13 Kinetic Isotope Effects in CO Oxidation by Ag

In the catalytic oxidation of carbon monoxide over silver wool the 13C kinetic isotope effects in the 343−453 K temperature range were experimentally determined and the following temperature dependence was found: 100 ln(k12/k13) = (3.398 − 630/T) ± 0.083. A reaction CO/O2 gas mixture of 1:2 ratio was used in a static system with initial pressures ranging from 20 to 40 kPa. Under these conditions the reaction is of order 1 with respect to CO and order 0 with respect to O2 and CO2 pressure. The apparent activation energy is 59.3 ± 1.7 kJ mol-1. In our theoretical interpretation of the experimental data various geometries of (CO2)⧧ and (CO3)⧧ transition states were applied, and only a (CO2)⧧ with an interbond angle of 110° and CO stretching force constants of 1700 and 1000−1400 N m-1, respectively, with an asymmetric reaction coordinate was found to be acceptable.

Structural and electronic changes accompanying reduction of Cr(CO)4(bpy) to its radical anion: a quantum chemical interpretation of spectroelectrochemical experiments

Optimised molecular structures and charge distributions within Cr(CO)4(bpy) and its radical anion were calculated using density functional theory (DFT). It was found that, although reduction predominantly concerns the bpy ligand, its structural and electronic effects extend to the Cr(CO)4 fragment. Each equatorial and axial CO ligand was calculated to accept 7.1 and 4.8%, respectively, of the extra electron density in Cr(CO)4(bpy)˙–. This is in accordance with the IR spectroelectrochemical results which show that the corresponding CO stretching force constants decrease by 68 and 21 N m–1, respectively. The calculated spin density in Cr(CO)4(bpy)˙– resides predominantly on the bpy ligand which behaves spectroscopically as bpy˙–. The spin density is delocalised to both axial and equatorial pairs of CO ligands by mixing of π*(CO) orbitals with the, predominantly π*(bpy), SOMO. In addition, part of the spin density is delocalised selectively to the axial CO ligands by an admixture of their σ orbitals into the SOMO. This σ–π* contribution is responsible for isotropic EPR hyperfine splitting which was observed from the axial 13C(CO) atoms only. Accordingly, the isotropic hyperfine splitting constants correlate with calculated Fermi contact terms instead of total spin densities. Complete active space self-consistent field (CASSCF)-calculated changes in charge distribution upon a Cr→bpy MLCT excitation show that the electron density localised on the bpy ligand increases by about the same amount upon reduction or MLCT-excitation of Cr(CO)4(bpy). The axial CO ligands are depopulated by MLCT excitation ca. 1.6 times more than the equatorial ones. These conclusions can be generalised and applied to other coordination and organometallic complexes of low-valent metals which contain a reducible or radical-anionic ligand.

Cr(CO) 5(phen) as an intermediate of photochemical CO substitution in Cr(CO) 6 with 1,10-phenanthroline

Cr(CO) 5(phen) generated as an intermediate of photochemical carbonyl substitution in Cr(CO) 6 with 1,10-phenanthroline (phen) has been investigated by laser flash photolysis with infrared detection. The Cr(CO) 5 moiety was found to have a square-pyramidal structure by analyzing with the Cotton-Kraihanzel force field. Simulation, based on the correlation of CO-stretching force constants with the bond indices calculated by ZINDO-MO, revealed that phen coordinated unusually in a monodentate fashion and the interaction between Cr and another nitrogen was antibonding.

The vibrational spectra of molecular ions isolated in solid neon. VII. CO+, C2O+2, and C2O−2

When a Ne:CO=100 mixture is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, several new infrared absorptions appear in the resulting sample deposit. Detailed isotopic substitution experiments support the assignment of these absorptions to CO+, trans-OCCO+, and trans-OCCO−. Analysis of the infrared data has permitted determination of the CO-stretching force constants of the dimer cation and anion and has yielded a preliminary estimate of the valence angles for trans-OCCO−. The role of electron capture in the ‘‘photobleaching’’ of trans-OCCO+ by visible radiation is considered.

ChemInform Abstract: Hydride Transfer to Transition‐Metal Carbonyls in the Gas Phase. Formation and Relative Stabilities of Anionic Formyl Complexes.

Abstract The hydride binding energies of Fe(CO) 5 , Cr(CO) 6 , Mo(CO) 6 and W(CO) 6 ( HA (M(CO) n ) = D[(CO) n − MCOH − ]) have been estimated from bimolecular hydride transfer reactions in the gas phase. Hydride transfer to the metal carbonyls yields the corresponding anionic formyl complexes and the hydride affinity bracketing experiments establish HA (Fe(CO) 5 ) = 56±4 kcal mol −1 and HA (Cr(CO) 6 ) = HA (Mo(CO) 6 ) = HA (W(CO) 6 ) = 44±4 kcal mol −1 . Gas-phase heats of formation and metal-carbon bond strengths for the metal formyl anions are derived from the hydride binding energies. The hydride affinity ordering for the metal carbonyls parallels their relative reactivities towards nucleophilic addition in solution and correlates with the relative magnitudes of their CO-stretching force constants ( k co ). The new metal formyl data are combined with known or estimated heats of formation for metal hydrides to show that CO-insertion into the metal-hydrogen bond is thermodynamically unfavourable for these systems.

Hydride transfer to transition-metal carbonyls in the gas phase. Formation and relative stabilities of anionic formyl complexes

The hydride binding energies of Fe(CO) 5, Cr(CO) 6, Mo(CO) 6 and W(CO) 6 ( HA(M(CO) n ) = D[(CO) n− MCOH −]) have been estimated from bimolecular hydride transfer reactions in the gas phase. Hydride transfer to the metal carbonyls yields the corresponding anionic formyl complexes and the hydride affinity bracketing experiments establish HA(Fe(CO) 5) = 56±4 kcal mol −1 and HA(Cr(CO) 6) = HA(Mo(CO) 6) = HA(W(CO) 6) = 44±4 kcal mol −1. Gas-phase heats of formation and metal-carbon bond strengths for the metal formyl anions are derived from the hydride binding energies. The hydride affinity ordering for the metal carbonyls parallels their relative reactivities towards nucleophilic addition in solution and correlates with the relative magnitudes of their CO-stretching force constants ( k co). The new metal formyl data are combined with known or estimated heats of formation for metal hydrides to show that CO-insertion into the metal-hydrogen bond is thermodynamically unfavourable for these systems.

Relaxation in zinc oxide (10.lovin.10), (0001), and (000.lovin.1) surfaces and the adsorption of carbon monoxide

An atom superposition and electron delocalization molecular orbital study using cluster models shows surface zinc and oxygen ions relax into the (0001), (0001), and (1010) surfaces. The unsaturated zinc ions present surface states consisting of empty 4s, 4p hybridized orbitals with energy levels beneath the empty 4s, 4p band. The filled bands are O 2p nonbonding, Zn 4s + O 2p bonding, and Zn 3d, in order of increasing ionization potential. A study of CO adsorption to an unsaturated zinc ion on the (1010) surface produces an adsorption energy of 12 kcal/mol, a CO bond length decrease of 0.02 A, and an increase in the CO stretching force constant. The zinc ion unrelaxes to a position above the bulk-like position when CO is coordinated to it. The bonding of CO to zinc consists in closed shell sigma donation interactions of the CO 5sigma orbital with the filled Zn 3d and Zn 4s + O 2p bands with the antibonding counterpart stabilized by the Zn 4s, 4p surface state orbital. This 5sigma donation strengthens the CO bond and transfers charge to the surface. These results support a number of experimental structural, vibrational, and electronic studies in the literature.

In-plane force constants of the peptide group: Least-squares adjustment starting from ab initio values of N-methylacetamide

Least-squares adjustment of the force constants of N-methylacetamide belonging to the A′ symmetry block was carried out starting from the force constant values obtained by an ab initio SCF MO calculation with the 4–31G basis set. Fourteen diagonal force constants were adjusted by the use of 122 vibrational frequencies of 9 isotope derivatives in the liquid state observed in the region 2000-100 cm −1. Here, all the off-diagonal elements were fixed at the initial values calculated with the 4–31G basis set. The greatest difference between the observed and calculated frequencies was 30 cm −1, and the differences for 94 modes were less than 10 cm −1. Most of the force constatns of bond stretching modes were reduced by 5–10%, and those of angle deformation modes by 20–30% in the course of the least-squares adjustment. The CO stretching force constant was lowered by 30% in the adjustment, and this is understandable because the adjustment involves the effect of bringing the peptide group from a free to a hydrogen-bonded state.

Thio-Schiff Base Derivatives of Chromium and Molybdenum Hexacarbonyls

Abstract The reactions of chromium and molybdenum hexacarbonyls with bidentate thio-Schiff bases (TSB), derived by the condensation of S-methyldithiocarbazate with benzaldehyde, salicylaldehyde, acetophenone and furfuraldehyde, have been studied in anhydrous benzene under refluxing condition. The isolated products of the type [M(CO)4 (TSB)] (M = Cr or Mo) have been characterized by micro analyses, electrical conductance and magnetic susceptibility measurements and infrared spectral data. CO stretching force constants as well as CO-CO interaction force constants have also been deduced.

Molecular orbital study of CO chemisorption on a Pt(111) surface in the presence of potassium

A molecular orbital study was mode, using an atom superposition and electron delocalization (ASED) technique, to examine the chemisorptive properties of CO on a Pt(111) surface in the presence of coadsorbed potassium atoms. To understand the effect of preadsorbed K on CO chemisorption, we have studied the adsorption of K on a cluster model of a Pt(111) surface. K. Is predicted to be preferentially adsorbed at a three-fold position and the calculated binding energy is in reasonable agreement with other estimates. K adsorption is found to lead to a significant charge transfer which in turn increases the electron density of all surface Pt atoms. The presence and increasing coverage of K on the Pt(111) surface has been simulated in our present theoretical study by decreasing the valence state ionization potential of Pt atoms in our cluster models. With the decreasing values (i.e. with increasing K coverage), the CO molecule is predicted to change its site of occupation from a one-fold position tot he two-fold or three-fold positions. The CO stretching force constant constant is predicted to decrease for all the sites with decreasing VSIP values of Pt. We find the decrease in CO force constant on going from the one-fold to two-fold site is smaller than the decrease on going from the one-fold to three-fold site. Our theoretical results corroborate with the recent experimental findings of Garfunkel et al. for this system.

Correlative studies of some spectroscopic and bonding parameters in octahedrally coordinated metal carbonyl complexes

Empirical correlations between infrared absolute intensity of CO-stretching modes and ionization energy and MC and CO bond polarizability derivatives and 13C-chemical shifts and charge-transfer spectra CT, and calculated 2π-bond population on CO-group in octahedrally coordinated metal carbonyls are discussed and rationalizations of bonding sequences are suggested using data in the literature and new data obtained in this laboratory. The results support the suggestion that the intensity should be a more sensitive measure of the importance of π-bonding in complexes of this type than the CO stretching force constant. The results obtained are discussed in terms of orbital energetics and electronic distribution. Additional considerations are the mass spectral features of several monosubstituted group VIB metal carbonyls and the relative strength of the L-M + bond from the relative abundances of ions.

Metal ion interactions with Mn(CO) −5 in ether solutions

Infrared spectra of LiMn(CO) 5 and NaMn(CO) 5 in the CO stretching region have been examined in ether solvents. These spectra are attributed to an ion-paired species of C 3ν symmetry in which alkali metal ion is bonded to carbonyl oxygen. Simple force constant calculations yield CO stretching force constants which are reasonable in terms of the structure hypothesized. In THF the NaMn(CO) 5 spectra show the presence of an equilibrium between solvent-separated and contact ion-pairs. Treatment of NaMn(CO) 5 in THF with Mg 2+ gives a species whose IR spectrum is also consistent with a MgOC interaction. Mixtures of LiMn(CO) 5 or LiCo(CO) 4 with CH 3 Li in ethyl ether were examined at −50° using 7 Li NMR. Only resonances due to the pure components were observed, indicating that no mixed tetrameric species of the form Li 4Me n - X 4− n are formed.

Vibrational Spectra of the Hexacarbonyl Manganese(I) Cation

The infrared and Raman spectra of the hexacarbonylmanganese cation as Mn(CO)6+ PF6− were assigned by comparison with other hexacarbonyl species. The CO stretching force constants for Mn(CO)6+ and Re(CO)6+ as indicated by the CO stretching vibrational frequencies are not significantly different. However, the Mn—C force constant in Mn(CO)6+ is likely to be considerably less than the Cr—C force constant in Cr(CO)6, whereas Re(CO)6+ and W(CO)6 have similar values. It is postulated that, although Mn(CO)6+ has equal metal–carbon σ bonding to Re(CO)6+, the metal-carbon σ bonding is less for Mn(CO)6+.

The vibrational spectrum of the molecule Os 4O 4(CO) 12

The i.r. and Raman spectra of the molecule Os 4O 4(CO) 12 have been obtained in the range 2200–2267 cm −1. The solution data in the CO stretching region indicate that the molecule has T d symmetry in solution. On the basis of a partial normal coordinate analysis, approximate CO stretching force constants have been calculated.

Transition metal chemistry : VII. Calculation of CO stretching force constants and separation of mesomeric and inductive effects for LM(CO) 5 complexes

Refined values of the CO stretching force constants for monosubstituted metal pentacarbonyl complexes have been obtained by rigorous direct methods of calculation. The following compounds have been treated by this method: manganese carbonyl complexes, halogenometal pentacarbonyls, some recently prepared aminocarbene complexes [RHN(Me)C]Cr(CO) 5, carbene pentacarbonyl compounds [R(OC 2H 5)C]M(CO) 5 (R = CH 3, n-C 4H 9 and M = Cr, W), phosphacarborane metal carbonyls of general formula (CH 3) 4N[B 9H 10CHE M(CO) 5] with E = P, As and M = Mo, W, Cr and arsine pentacarbonyl complexes of Cr, Mo and W. Using the more exact carbonyl stretching force constants obtainable from such calculations, relative σ and π Graham parameters for a series of LMn(CO) 5 compounds have been derived. The relative importance of σ and π bonding in some organotin derivatives is discussed.

Transition metal chemistry VI. Calculation of force constants in the CO stretching region of cis-disubstituted group VI metal carbonyls and iron and carbonyl complexes with C2v symmetry

For complexes of the type cis-L 2M(CO) 4 (M = Mo, W, Cr; L = a phosphine, arsine or amine), (RHg) 2Fe(CO) 4 (R = Me, Bu) and (R 3M) 2Fe(CO) 4 (M = Pb, Sn, Si or Ge) all the CO stretching force constants are calculated using a rigorous algebraic procedure based on the Cotton and Kraihanzel force field.