Comment on “Maximum Interaction Model of Force Constant Calculation of the MLx(CO)6-x Type Molecule”

Since the introduction of non-mechanical coupling model proposed by Cotton and Kraihanzel^1-3) for the calculation of nonrigorous carbonyl force constants of metal carbonyls, several criticisms^4-3) and modifications^6-13) on this simplified CO-factored force field has been reported. Recently, a different approach of so-called “maximum interaction model” was proposed by Chen and Hsiang¹⁴⁾. However, we found that basic assumptions of this model are very shaky and questionable since three serious abnormal results could be noted as a result of maximum interaction between carbonyl groups. Here we would like to make some comments on this model.     

An analytical approach to obtaining a valid solution to the secular equations for cis-disubstituted octahedral metal carbonyls

Using an analytical approach we have been able to obtain a valid solution to the secular equations for cis-ML₂(CO)₄ molecules. The solution has led to the relations by which carbonyl stretching force constants and interaction constants can be calculated directly from ¹²C¹⁶O stretching frequencies. It has been found that calculated force constants are in excellent agreement with those obtained from isotopic enrichment studies and exactly equal to those calculated through the iterative procedure employed by Jernigan et al.     

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%.     

Maximum Interaction Model Force Constant Calculation of the Mlx (Co)6-x Type Molecule

C—O stretching frequency of inorganic carbonyl molecules is quite characteristic in the vibrational spectroscopy (the IR and Raman). Since such stretching modes are quite different from the skeletal modes, a non-mechanical coupling model has been proposed by Cotton¹⁾. The C—O stretching force constants and their interaction constants may be calculated semiquantitatively from their characteristic frequencies. The trouble is that the number of vibrational frequencies is always less than the number of force constants, whenever a symmetry irreducible representation contains more than one frequency. Cotton suggest that the interaction constants for octahedral type molecule from the dpπ bonding point of view^1-2). Instead of this, a model of maximum interaction between M—C—O bonds is proposed in this paper. Such model not only gives better remedy to the problem of octahedral complexes, but also is useful to solve the normal modes in molecules of trigonal bipyramidal and many other symmetries.     

Synthesis and characterization of tungsten carbonyl complexes containing N-methyl substituted urea and thiourea ligands

Six new mixed-ligand tungsten carbonyl complexes containing N-methyl substituted urea and thiourea of the type W(CO)₄[RCH₂N-(C=X)NH₂] where X?=?O or S and R?=?morpholine, piperidine and diphenylamine are reported. These have been prepared by refluxing hexacarbonyl tungsten(0) with corresponding ligands in THF to produce cis-disubstituted products, [(L-L)W(CO)₄] where L-L?=?a chelating bidentate ligand, morpholinomethyl urea (MMU), morpholinomethyl thiourea (MMTU), piperidinomethyl urea (PMU), piperidinomethyl thiourea (PMTU), diphenylaminomethyl urea (DAMU) and diphenylaminomethyl thiourea (DAMTU). The compounds have been characterized by elemental analysis, IR, electronic and ¹³C NMR spectra, magnetic moments and conductivity measurements. The IR spectra suggests that in all the complexes, the ligands are bidentate chelating, coordinating the metal through carbonyl oxygen or thiocarbonyl sulphur and the ring nitrogen or tert-nitrogen of diphenylamine. The CO force constants and CO–CO interaction constants for these derivatives have also been calculated using Cotton–Kraihanzel secular equations, which indicate poor π-bonding ability of the ligands. ¹³C NMR and electronic spectra reveal loss of cis-carbonyl ligands to produce cis-disubstituted tetracarbonyl derivatives. Molecular modeling studies have been carried out using Hyperchem release 7.52 which suggest a distorted octahedral geometry for these complexes.     

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.     

Superparameterization of low-temperature vibrational data for lanthanide hexahalide anions

A complete dataset is presented for the modes of vibration of LnCl³⁻₆ in Cs₂NaLnCl₆, from low temperature Raman and vibronic spectroscopy. Conventional force constant calculations with this dataset are unsatisfactory since, although the datafits are quite good, the physical significance of the derived force constant parameters is not clear in all cases. A parameterization of 30 vibrational wavenumbers at a time, comprising five datasets of six modes of vibration, enables the relative importance of the GVFF interaction force constants to be ascertained. The trans stretch—stretch force constant is about 2.5 times larger than the cis interaction constant, and the angle—angle interaction constants all have comparable magnitudes. However, the same ratios for the magnitudes of the interaction force constants fail to provide a satisfactory datafit for PrBr³⁻₆ in Cs₂NaPrBr₆. This is attributed to the stronger trans directing influence of the bromide ligand than the chloride ligand, and an increased ratio for the trans/cis stretching interaction force constant provides a more satisfactory fit. Datasets are also presented for LnF³⁻₆ and Cs₂LiLnCl₆. The nature of the cation A in Cs₂ALnCl₆ appreciably modifies the moiety mode vibrational wavenumbers.     

An ab initio study of the geometry,harmonic and anharmonic force fields,and fundamental vibrational frequencies of cis- and trans- thiolformic acid

The geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies of cis- and trans-thiolformic acid are studied ab initio in the 4-31G basis set. An extensive comparison is made between changes in diagonal and off-diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between S? H and O?C, compared with O–H and O?C, in keeping with the weaker hydrogenbonding property of the S? H group in general.     

Intramolecular donor-acceptor interaction in stilbene and styrene π-complexes with tricarbonylchromium. Observation in the IR spectra

The IR spectra of complexes derived from conjugated arylalkenes and tricarbonylchromium, namely (stilbene)tricarbonylchromium and (styrene)tricarbonylchromium, displayed three absorption bands instead of two expected in the region of carbonyl stretching vibrations (1800–2000 cm⁻¹). Additional absorption bands also appeared in the region corresponding to metal-π-fragment stretching vibrations (250–400 cm⁻¹). These findings indicated additional interaction involving the central metal atom, carbonyl ligands, and aromatic π-system. Such interaction increases mobility of the tricarbonylchromium fragment which may become capable of readily migrating from one π-fragment to another under certain conditions.     

Octahedral metal carbonyls-XXIX: Mass spectra of cis-disubstituted derivatives of the group VI-B metal carbonyls

The mass spectra of a number of cis-disubstituted derivatives of the Group VI-B metal carbonyls (M = Cr, Mo, W), in which the substituents are bidentate ligands bonding to the metal through As, S or N donor atoms, have been obtained. The spectra for derivatives of o-phenylenebis-(dimethylarsine) (diars) have been examined in greatest detail. For these latter complexes, competitive loss of carbonyls and diars methyl groups was observed. The proposed use of such competition in the evaluation of metal-carbonyl bond strengths is examined. For complexes containing ethylene-bridged bidentate ligands, ions indicative of the loss of five fragments of 28 mass units from the parent ion, corresponding to loss of four carbonyls and the ethylene bridge, are observed. The question of whether ethylene loss occurs prior to, competitively with or after carbonyl loss is considered.     

Influence of Substituents on Kinetics of the Reaction of Carbonyl Oxides with Ethanol

The reactivity toward EtOH of nine carbonyl oxides (ROO) obtained by the thermolysis of the corresponding diazo compounds (RN₂) is studied by the chemiluminescence method. The reactivity is characterized by the ratio of constants k ^EtOH ₃₃/k ₃₁, where k ^EtOH ₃₃and k ₃₁are the rate constants of the reactions of ROO with EtOH and RN₂, respectively. The negative slope of the Taft correlations ( < 0) indicates the electrophilic character of the reaction of ROO with EtOH. The substituents, electron density acceptors, increase the relative reactivity of the carbonyl oxides.     

Development of a highly sensitive method for determining atmospheric carbonyl compounds by passive sampling and application of the method to a survey of indoor air

A simple, highly sensitive analytical method for measuring many kinds of carbonyls in air using a passive sampler containing a sorbent (silica gel) coated with 2,4-dinitrophenylhydrazine has been developed. The carbonyls collected by the sampler were extracted with a solvent, and the extracts were subjected to high-performance liquid chromatography (HPLC; UV detection) without first being concentrated. In this method, the volume injection is examined, and is found to have a sensitivity at least 20 times that of ordinary HPLC methods. The air concentrations of nine carbonyls collected by passive sampling over a period of 24?h were estimated by means of conversion equations derived from the results of active sampling;c?=?^{10[log ( y} )^{??? b ] a} , where c is the carbonyl concentration in air (µg/m³); y is the amount of carbonyl collected by the passive sampler (µg); and a and b are constants for each carbonyl compound. The calculated air concentrations were consistent with the concentrations measured by active sampling. This method may be useful in determining personal exposure to ambient carbonyls.     

Bond Angles in Lactones and Lactams

The carbonyl group in lactones and, to a lesser extent, in lactams tends to show a C? C?O angle that is larger than the O? C?O or N? C?O angle, the difference increasing in magnitude as the ring size decreases. The observed trend provides information on ratios of force constants characterising the flexibility of the s-cis-ester group and may be interpreted in terms of incipient chemical reaction to ^?O? (CH₂)_n? C?O⁺. Molecular orbital calculations (MINDO/3, MNDO, EH) for model compounds provide an electronic interpretation of this angle difference in terms of an anomeric interaction between the p-type lone pair on the carbonyl O-atom and the antibonding C-O or C-N orbital.     

REACTION AND QUENCHING OF SINGLET MOLECULAR OXYGEN WITH ESTERS OF POLYUNSATURATED FATTY ACIDS

Abstract The rate constants for the reactive (k_R) and unreactive (k_Q) interaction of singlet molecular oxygen with three esters of polyunsaturated fatty acids (PUFA: cis-methyl oleate, MO; cis-methyl linoleate, MLA and cis-ethyl linolenate, ELN) are determined. The values of the ratio k_Q/k_R are 0.51, 0.26 and 0.20 for MO, MLA and ELN, respectively. This variation results principally from that of k_R because the values of k_Q are only slightly different (1.24 × 10⁴M^?1 s^?1 for MO and ～1.0 × 10⁴M^?1 s^?1 for MLA and ELN). It is shown that the rate constant k_Q characterizes mainly an interaction with the unreactive part of the molecule rather than with the double bonds (solventlike quenching). Contrary to the already reported case of 1,5-polyenes for which k_Q <<k_R, the present results and those obtained from a number of literature data show that for PUFA and their esters, neither k_R+ k_Q nor k_R are proportional to the total number of double bonds or of methylene groups adjacent to the double bonds. Instead, a linear correlation is observed by plotting k_Rvs the number of methylene groups adjacent to two double bonds. It is deduced that contrary to a common assumption, biallylic hydrogens have a reactivity higher than that of singly allylic hydrogens (reactivity ratio 1.19). The consequence of this result on the estimation of relative contributions of singlet oxygen and radical mechanisms to oxidation processes is discussed. Moreover, the whole of these results allows prediction of the values of k_R and k_Q for all unsaturated fatty acids (and their esters) of similar structure.     

Kinetics of electrophilic brominations. Mechanistic significance of the third-order term

The rates of electrophilic bromination of various donors follow complex kinetics which include both first-order and second-order dependences on bromine, especially in the less polar solvents. The second-order rate constant k_s and the third-order rate constant k_t are evaluated for alkene bromination in carbon tetrachloride, and they are compared to those already listed for the electrophilic brominations of substituted styrenes, arenes, and metal carbonyls in the extant literature. Despite the varying magnitudes of the second– and third-order rate constants for these diverse donors (and in different solvents), the ratio log(k_s/k_t) is remarkably invariant. The mechanistic implication of this unique observation is discussed in the context of charge transfer interactions which are common to the activated complexes in the electrophilic brominations of various donors.     

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

Force constants in molecular crystals of methyl-and perdeuteriomethylmercury(II) halides

Force constants for the internal vibrations involving the metal and for the lattice vibrations of Hg(CH₃)X and Hg(CD₃)X (X = Cl, Br or I) are calculated on the basis of a D_4h⁷ layer structure. The internal HgX stretching force constants are much lower than for these molecules in solution, but HgC stretching force constants are slightly higher. The HgX and longitudinal translatory force constants within the lattice layer are close in value to the strong and weak HgX bond stretching force constants respectively in the unsymmetrical [Hg(CH₃)X₂]^? complex ions.     

Physical and chemical properties of hydroxyflavones. IV. Infrared absorption spectra of dihydroxyflavones containing the 5-hydroxyl group

Solid state infrared curves (O-H and C-H stretching region) are given for 5, n-dihydroxyflavones, where n is 2′, 3′, 4′, 6, 7 and 8. In chloroform solution spectra of 3,5-dihydroxyflavone and 3-hydroxy-5-methoxyflavone, the 3-OH stretching band appears at 3400 and 3334 cm^?1, respectively, indicative of a stronger hydrogen bond in the latter substance. Solid state and solution carbonyl bands are presented for twenty-six flavone derivatives which contain a hydroxyl, methoxyl or acetoxyl group at the 5-position. The solution spectra (dioxane or carbon tetrachloride) of fourteen flavone derivatives containing a free 5-hydroxyl group show carbonyl bands at 1655±2 cm^?1. Eleven flavones in which the 5-hydroxyl is blocked (carbon tetrachloride solution) give spectra with flavone carbonyl bands at 1653±3 cm^?1. The high resolution chloroform solution spectrum of 3, 5-dihydroxyflavone possesses a multi-peaked carbonyl band with midpoint at 1641 cm^?1. The chloroform solution spectrum of 3-hydroxy-5-methoxyflavone has a very strong band at 1616 cm^?1, with shoulder at 1646 cm^?1. Spectral data of this and a previous paper support the postulate that in 4′-hydroxyflavone the flavone carbonyl oxygen is the donor atom in an intermolecular hydrogen bond. Certain details of synthesis, and analytical data, are given for 3, 5-dihydroxyflavone.     

Homoleptic Metal Carbonyl Cations of the Electron-Rich Metals: Their Generation in Superacid Media Together with Their Spectroscopic and Structural Characterization

Homoleptic carbonyl cations of the electron-rich metals in Groups 8 through 12 are the newest members of the large family of transition metal carbonyls. They can be distinguished from typical metal carbonyl complexes in several respects. Their synthesis entails carbonylation of metal salts in such superacids as fluorosulfuric acid and “magic acid” HSO₃F? SbF₅. Thermally stable salts with [Sb₂F₁₁]^? as counterion are obtained with antimony pentafluoride as reaction medium. Both the [Sb₂F₁₁]^? anion and superacid reaction media have previously found little application in the organometallic chemistry of d-block elements. Also unprecedented in metal carbonyl chemistry are the coordination geometries with coordination numbers 4 (square-planar coordination) and 2 (linear coordination) for the cation. Formal oxidation states of the metals, and the charges of the complex cations, extend from + 1 to +3: thus CO is largely σ-bonded to the metal, and the CO bond is strongly polarized. Minimal metal → CO π-backbonding and a positive partial charge on carbon are manifested in long M? C bonds, short C? O bonds, high frequencies for C? O stretching vibrations (up to 2300 cm^?1), and small ¹³C NMR chemical shifts (up to δ_c, = 121). Prominent examples of these unusual homoleptic carbonyl cations, which were recently the subject of a Highlight in this journal, include the first carbonyl cation of a p-block metal [Hg(CO)₂]²⁺, the first trivalent carbonyl cation [Ir(CO)₆]³⁺, and the first multiply charged carbonyl cation of a 3d metal [Fe(CO)₆]²⁺. In this overview we propose to (a) outline the historical origins of cationic metal carbonyls and their methods of synthesis; (b) present a summary of the general field of carbonyl cations, which has developed over a yery short period of time; (c) discuss the structural and spectroscopic characteritics of metal–CO bonding; (d) discuss the special significance associated with reaction media and the [Sb₂F₁₁]^? anion; and (e) point to the most recent results and anticipated future developments.     

Correlations of substituent parameters with the carbonyl stretching force constant in arenetricarbonylchromium complexes

Correlations are established between selected substituent parameters (σ_I,σ_P, σ_R^P,σ_P^O,σ_R^O) and the carbonyl stretching force constant, k(CO), for 28 mono and poly-substituted tricarbonylchromium-complexed arene compounds. On the basis of the statistical results it is concluded that the overall electronic substituent effect transmitted to the carbonyl groups involves both mesomeric and inductive mechanisms. Within the restricted domain, including substituent group and benzene ring, transmission proceeds largely by resonance, with a minor inductive (through-bond and field) effect operative in the same domain. Further transmission from the substituted arene ring to the chromium atom predominantly involves an inductive mechanism. This result, in support of existing literature data, suggests appreciable participation of the ring carbon σ framework in the metal—ring bond formation.