The problem of σ- and π-bonding in the tin substituted tetracarbonyl cobalt compounds RlXmSn {Co(CO)4n}, R = alkyl,phenyl, X = halogen, l + m + n = 4

The results of each type of spectroscopic research on organometallics, such as r_ix_m Sn- [Co(CO)₄]_n (R = Alkyl, Phenyl, X = Halogen, l + m + n = 4) provide information clarifying the bonding in these heteronuclear metal—metal bonded molecules. This report is a concise survey comparing the conclusions drawn from nuclear magnetic, nuclear quadrupole and nuclear -γ resonance spectroscopy, together with the polar characteristics investigated by the dipole moments method and the vibrational data obtained from the far infrared region. The experimental data are critically discussed in terms of (1) metal-metal σ bonding changes, (2) d_π-d_π contributions which may occur between a group IV-B metal and a transition metal and (3) p_π-d_π overlap resulting from a back-donation of the unshared halogen p-electrons into symmetry-proper vacant d-orbitals of the tin atom.     

Influence of alkyl substituents on the π* negative ion states of benzene and its derivatives : Electron transmission spectroscopy of p-alkylanilines

Electron affinities and electron attachment energies associated with the π^* orbitals of several p-alkylanilines (alkyl = Me, Et, i-Pr, and t-Bu) have been determined in the gas phase by electron transmission spectroscopy and have been analyzed by ab initio molecular orbital calculations at the STO-3G and 6-31G levels of basis set. The results lead to the conclusions that (a) the ²A₂ negative ion state in toluene lies below the ²B₁ state, (b) the ²A₂ state is stabilized in toluene relative to benzene by bonding overlap between the ortho carbon (C₂(2p_π)) and the Me H(1s) orbitals in the LUMO, (c) the ²B₁ state is stabilized on adding Me groups to toluene to form t-butylbenzene through reduced antibonding interactions between C₁(2p_π) and Me C(2p_π) orbitals, and (d) the lowest resonance in t-butylbenzene is extensively overlapped with the second resonance and may involve strong mixing of the ²A₂ and ²B₁ states.     

Organoboron compounds: XXIX. A comparison of pπ-pπ, bonding in alkoxy and alkylthiopiperidinophenylboranes

The results of a VT ¹³C NMR study of alkoxy and alkylthio derivatives of 2- and 3-methylpiperidinophenylboranes demonstrate that the p_π-p_π bonding between boron and oxygen is some 3 kcal mol⁻¹ stronger than that between sulphur and boron.     

Studies on the circular dichroism of penam and penam sulfoxides

Optical rotatory properties of models of 6-aminopenicillanic acid (6-AP A) as obtained from earlier molecular orbital and rotatory strength calculations are corroborated by new CD data for 6-AP A. The new spectra extend down to 180 nm, revealing a positive Cotton effect near 185 nm due to carbonyl π→π^* transitions. The pH sensitivity of the optical activity of the β-lactam amide π→π^* and the 203-nm n→π^* transitions is explainable in terms of electrostatic effects and orbital interactions present in α-aminoketones and 4-membered rings which tend to flatten out the NH₂ group. These same interactions are responsible for the unusually low pK_a of 4.75 for the amino group; (the carboxyl group of 6-AP A has a normal pK_a of 2.5). The previous calculational procedures are extended to the sulfoxides of 6-AP A, and the predicted CD spectra are compared to new experimental curves and also to available data for penicillin sulfoxides. The MO's associated with the principal bands are illustrated with the aid of electron density maps. The positive Cotton effect observed and calculated near 230 nm is due to two charge transfer transitions within the asymmetric penam nucleus. Similar to the situation with the unoxidized penams, the sulfoxide of 6-AP A displays a very low pK_a for the amino group (3.8) and a change toward a less positive CD band at 202 nm upon protonation of this group.     

Infrared spectroscopy of oxygen bond systems

The vibrational spectra of various types of oxygen-bonded systems with other elements are interpreted in terms of factor group analysis and normal vibration analysis. The stereochemical structures of pseudo-octahedral (MO_nL_6-n) and pseudo-tetrahedral (MO_nL_4-n) compounds with terminal oxygen atoms and of M₂OL_n compounds with oxygen bridge bonding are discussed on the basis of our own data and those from the literature.The present work also contains certain general statements concerning p_π-d_π interactions in oxy compounds of metals having d and p electrons.     

Preparation and properties of the formamidino complexes [M(π-C5H5){HC(NR)2}(CO)2] (M = molybdenum or tungsten; R = aryl or alkyl)

The preparation and properties of the complexes [M(π-C₅H₅){HC(NR)₂}CO)₂] (M = Mo, W; R = aryl or alkyl) are reported. The complex [Mo(π-C₅H₅){HC(N-p-tolyl)₂}(CO)₂] could be prepared by (a) reaction of MoCp(CO)₃Cl with M′{HC(N-p-tolyl)₂} (M′ = K, Ag or Cu); (b) irradiation of MoCp(CO)₃Cl with HC(HN-p-tolyl)N-p-tolyl); and (c) reaction of [MoCp(CO)₃]₂ with M′{HC(N-p-tolyl)₂} (M′ = Ag or Cu). The several routes to this complex give indications of the mechanisms of formation. The structure of these complexes and the bonding nature of the metal with the formamidino group is discussed on basis of the ¹H and ¹³C NMR and IR spectra.Reaction of N,N′-dimethyl formamidine with MCp(CO)₃Cl gave the complex [M(π-C₅H₅){HC(NMe)N(CO)Me}(CO)₂], containing a carbonyl inserted between the metal and the formamidino group. Irradiation of this carbamoyl complex caused decarbonylation, yielding the complex [M(π-C₅H₅){HC(NMe)₂}CO)₂].     

The crystal structure of triphenylallyltin

The crystal structure of (C₆H₅)₃SnCH₂CHCH₂ has been determined. The asymmetric unit consists of two molecules of triphenylallyltin having virtually the same conformation. Their geometrical parameters exclude p_π-d_π bonding between the metal and the allyl double bond, and seem to be in a better agreement with a σ-π conjugation effect.     

Boron-sulphur compounds—II: A proton magnetic resonance study of complexes of pyridine and γ-picoline with trialkylthioboranes

Proton magnetic resonance studies of the pyridine and γ-picoline complexes of trimethyl-and triethylthioboranes provide strong evidence that the p_π—d_π bonding in the boron-sulphur bond is much weaker than the p_π? p_π bonding in the boron-oxygen bond.     

The chemical bonding in phosphonic acids

The P—O stretching force constants of the phosphonic acids of the type CH_xCl_3-xPO₃H₂, in which x varies from 0 to 3, and their anions are interpreted in terms of (d_π—p_π)—bonding. The behaviour of the P—C stretching force constants in these compounds is also discussed.     

An ab initio evaluation of the role of p,π interaction: I. Ethylene derivatives

The RHF/6-311G(d) and MP2/6-311G(d) calculations with full geometry optimization were performed for XCH=CH₂ molecules (X = F, Cl, Br, CH₃, CH₂CH₃, CH₂F, CHO). The p _y electron density distribution in these molecules and the bonding molecular orbitals formed by the p _y orbitals of atoms of the planar fragment of these molecule (atomic orbitals whose symmetry axes are perpendicular to this plane) are not determined by the p,π conjugation between the lone electron pair of the heteroatom in substituent X and π electrons of the C=C bond. Changes in the population of the p _y orbitals of the halogen and carbon atoms in going from X = F to X = Cl and Br are not associated with changes in the extent of this p,π interaction. Taking into account the electon correlation in the MP2 method does not noticeably alter the features of the electron density distribution in these molecules estimated by restricted Hartree-Fock calculations.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

Electronic emission and absorption spectra and the effect of hydrogen bonding on the ground and excited electronic states of some anilines

From electronic absorption and emission spectra in solutions it appears that intramolecular hydrogen bonding, strong enough to resist rupture by dioxane, exists in o-chloroaniline in the excited state only. Fluorescence quenching behaviour in the presence of dioxane indicates that intermolecular hydrogen bonding significantly increases intersystem crossing rate in m-chloroaniline only. This and other emission spectral characteristics in this hydrogen bonding solvent at 77 K show that the first excited singlet electronic state S₁ of m-chloroaniline is ππ*, whereas the states S₁ of aniline, toluidines and p-chloroanilines have some nπ* character. On formation of intermolecular hydrogen bond in dioxane, the corresponding triplet states of the molecules acquire pronounced nπ* character. An examination of phosphorescence decay curves reveals triplet complex formation in m- and p-chloroaniline but there is no evidence of triplet complex in the other aromatic amines studied.     

DFT study of electronic and magnetic structure of perovskite and post-perovskite CaRhO3

Within density functional theory the equations of state for perovskite (PV) and post-perovskite (PPV) forms of CaRhO₃ are obtained with equilibrium values of volume in agreement with experiment. Energy magnitudes point to a stabilization of PPV versus PV. This is interpreted by analyses of the charge density and the chemical bonding plots, showing that the Rh–O interactions within two oxygen sublattices are selectively differentiated and reinforced for one of the two oxygen sublattices within PPV variety with respect to PV one. Investigation of the magnetic properties shows no magnetic order and a metallic character for PV while ferromagnetism occurs in PPV with a tendency to insulating behavior. This long range order is favored by direct t_2g–t_2g interactions through edge sharing octahedra in PPV CaRhO₃, stronger than indirect t_2g–p_π–t_2g ones in PV variety.     

Infrared study of some specificities of the dπpπ interaction in organogermanium compounds

The analysis is presented for the frequencies of stretching modes ν(GeH) in the IR spectra of organogermanium compounds R₂XGeH, RX₂GeH, RXYGeH, X₂YGeH and XYGeH₂ (where R is a substituent which does not make a d_πp_π bond with germanium, and X and Y are groups capable of d_πp_π interaction with germanium). It is shown that ν(GeH) in these compounds is dependent on both the I effect of R, X and Y, and the d_πp_π interaction in GeX and GeY bonds. If only one substituent capable of d_πp_π interaction with germanium is present, the value of such an effect is determined by itsσo_Rconstant. However, when germanium is bound to several substituents capable of d_πp_π interaction its magnitude depends on the effective charge at germanium which is determined by the inductive and mesomeric effects of X and Y. The data obtained are compared to the dependences observed in the IR spectra of similar organosilicon compounds.     

A momentum-space view of the chemical bond. I. The first-row Homonuclear diatomics

The electronic probability distribution in momentum space or electron momentum density (EMD) is studied in detail for the first-row homonuclear diatomics. The total density difference (molecule minus constituting atoms)is analyzed in terms of the separate orbital contributions. The nodal structure shown by the orbital EMD is characteristic for the various types of orbital (σ,σ*,=,=*), and is affected, by the amount of s-p hybridization. Directional and isotropic Compton profiles are used to study the bond-oscillation and bond-directional principles. The bond- directional principle does not hold for pe bonding. Spherically averaged EMD differences (SA Δ EMDs) are related to the changes in kinetic energy (ΔT) upon bond formation. The SA ΔEMDs and ΔT are rationalized by considering the different ranges of internuclear distance that are optimal for 2s-2s, 2p_o-2p_o and 2p_o-2p_o interaction. This leads to a reassessment of the role of the various orbitals in bonding complementing the picture based on orbital Hellmann- Feynman forces.     

High resolution NMR studies of allylic derivatives of group IVB elements

The high resolution NMR spectra of certain Group IVB allyl compounds, FC₃M(CH₂)_nCHCH₂R′ (where R = Me, Et, Ph; R′ = H, Ph; M = Sn, Si, Ge; n = 1 or 2), have been studied in an attempt to detect manifestations of ground state p_πd_π overlap. Analyses, using sub-spectral techniques for ABMX₂ and ABX₂ systems, were confirmed using computer iteration methods. The emphasis has been placed here on the sensitivity of the coupling constants of the allyl group towards anticipated steric and electronic perturbation or towards the interaction between the Group IVB atom d orbitals and the olefinic p_π orbitals. Some ¹¹⁹Sn chemical shifts have also been recorded. The conclusions reached do not support the existence of ground state d_πp_π overlap in these compounds.     

Derives α-carbonyles et “aromaticite” des phospholes trivalents

A two-step synthesis of 2-acetyl and 2-carbethoxy-phospholes involves the metalation of monomeric 3,4-dimethyl phosphole sulfides by t-butyl-lithium followed by reaction with ethyl acetate and ethyl carbonate. The phosphole sulfides thus obtained are reduced by PBu₃ or P(CH₂CH₂CN)₃ to the phospholes 20–24. In the 2-acetyl-phosphole series, variable electron transfer from the dienic system and from the phosphorus atom to the carbonyl group is monitored by ¹H and ³¹P NMR. The main decomposition path of 2-carboxy phosphole 25 in the mass spectrometer involves a PC intracyclic bond cleavage with loss of CO, in sharp contrast with pyrrole and thiophene derivatives 26 and 27. These facts militate against strong 2p_π–3p_π electronic delocalisation in the phosphole nucleus.     

Analogies between photolysis and mass spectrometry of organocyclopolysilanes

Analogies between photolysis and mass spectrometry of organocyclopentasilane and organocyclohexasilane derivatives are shown and the importance of phenyl substituent is noted in the formation of ions by electron bombardment. The stability of these may be explained by p_π-d_π