Withdrawal Of Electron Density Research Articles

Molybdenum and tungsten complexes with metallo-ligands having sulfur donor atoms

The reaction of Mo(CO) 6 with 2,3-pentanedione- bis(β-mercaptoethylimino)nickel(II), Nipe, under mild conditions, results in the formation of NipeMo(CO) 4, while more forcing conditions give a different product, [NipeMo(CO) 3] 2. From an analogous reaction, a tungsten compound of the latter formula was also isolated. The chromium analogue was prepared in a similar manner but lacked the purity necessary to give good analyses. The molybdenum and tungsten complexes were characterized by chemical analyses and solution molecular weight. The magnetic moments of these complexes are anomalous, 2.3–2.4 BM, and suggest that the coordination number of the nickel is greater than four. Electronic spectra support this hypothesis. Cyclic voltammetry data indicate that the M(CO) 3 moiety withdraws electron density from the nickel ion in the metallo-ligand. Reactions of Mo(CO) 6 and W(CO) 6 with bis(2-mercaptoethylamine)nickel(II) (Ni(MEA) 2) gave the complexes Ni(MEA) 2M 2(CO) 10 (M = Mo, W) which were characterized by analyses and infrared and electronic spectra.

Adsorption of Para‐substituted Benzoates on Iron Oxides

AbstractIn order to understand the binding mechanism and adsorptivity of benzoic acids on Fe oxides, adsorption of parasubstituted amino, nitro, methyl, methoxy, and chlorobenzoic acids onto noncrystalline Fe hydroxide and goethite from aqueous solution and vapor state was studied. Adsorption on noncrystalline Fe hydroxide from solution phase was quantified by the determination of adsorption isotherms using UV spectrometry to measure the concentration of unadsorbed organic. Adsorption of the benzoic acids from the vapor phase was further studied, using Fourier‐transform infrared spectroscopy (FTIR) of self‐supported goethite films to identify the binding mechanism and relative bond strength of the organic anion‐oxide complex. Results showed that the higher the pKa of the benzoic acid (i.e., the smaller the Hammett Constant of the substituent) the more readily the organic was adsorbed from solution. From FTIR study, an inner‐sphere coordination of the carboxylate anion to surface Fe involving a ligand exchange process was identified. Quantity of adsorption from solution was correlated to surface Fe‐benzoate bond strength as deduced from FTIR of dry oxide films, with bond strengths of para‐substituted benzoate following the order: amino > methoxy > methyl > chloro > nitro. Bidentate coordination was deduced to be the binding mechanism for chemisorption on air‐dry surfaces; however, physical adsorption was also identified in the absence of competing anions. It is concluded that electron‐with‐drawing groups on the aromatic ring weaken the carboxylate bond to surface Fe atoms by withdrawing electron density from the carboxylate group. Conversely, electron‐donating groups increase adsorption by increasing the electron density of the carboxylate, strengthening its Lewis basicity.

Normal coordinate analysis of molecules adsorbed on zeolite surfaces: Part II. Propene adsorbed in zeolites A

Vibrational analysis has been performed for propene in the gas phase and in zeolites A adsorbed near Li +, Na +, Mg 2+, Ca 2+ amd Zn 2+ ions, using a valence force model. Withdrawal of electron density from the π bond to the unoccupied s orbitals of the cations represents the main contribution to the interaction, giving rise to a monotonous decrease of the CC force constant in the sequence of growing polarizing power of the cations. From this, an approach of the cations by the double bond must be assumed in accord with the weakening of the adjacent CH bonds. From further analysis there are indications for the hindrance of the deformation motion due to adsorption, while the methyl group remains rather unperturbed. Normal coordinate analysis of adsorbed species supports well the predictions of the geometrical arrangement and electronic structure from quantum chemical calculations

ChemInform Abstract: STRUCTURES OF THE GAUCHE CONFORMERS OF SOME SUBSTITUTED DIMETHYL ETHERS. EFFECT OF ADJACENT ATOM LONE PAIRS ON METHYL GROUP ASYMMETRY

Abstract The complete equilibrium structures of CH 3 OCH 3 and of the gauche conformers of CH 3 0CH 2 F, HOCH,F, CH,OCH,Cl and CH,OCH,CN have been determined by ab initio gradient computation at the Hartree-Fock, double zeta-plus-polarization level. The very large asymmetries in CH bond distances previously reported from microwave substitution structures are shown to be non-existent in the equilibrium structures and are presumably artifacts. Small differences, different in direction from those reported from the experiments and nearly an order of magnitude smaller in size, do exist. They reflect three factors: (1) a lengthening of a CH bond which is trans to a lone pair on an adjacent atom, (2) a general shortening of CH bonds originating at a carbon atom bearing a highly electronegative substituent, and (3) a specific interaction in which a CX substituent shortens the nearly parallel CH bond on the other methyl group. The last interaction, not previously reported, is mediated by withdrawal of electron density from the oxygen lone pair which is trans to both groups. Other structural features derived from the microwave studies are supported by the new results. Inclusion of polarization functions in the basis set for oxygen is essential for correct determination of the COC angle and the dihedral angles. The dihedral angles of CH 3 OCH 2 F and HOCH 2 F are not correctly determined by the computation even at this level, although the computed values are improved when d functions are used for oxygen and still more by use of two sets of oxygen d functions. Polarization functions on carbon or on fluorine have no effect on the computed torsional angles. There is no problem in computing the correct dihedral angles with the CI or CN derivatives.

Übergangsmetall-carben-komplexe: CXXXIII. π-cyclopentadienyl(dicarbonyl)[aryl(halogen)-carben]-komplexe des mangans und rheniums

Fluoro-, chloro-, bromo-, and iodo-carbene complexes of manganese and rhenium were obtained on nucleophilic addition of X − (X = F, Cl, Br, I) to various cationic dicarbonyl(cyclopentadienyl)arylcarbyne complexes [C 5H 5(CO) 2MCR] + Y − (M = Mn, Re). This reaction is influenced by the kind of anion Y − as well as solvent effects and depends on the ability of the substituent R to withdraw electron density from the carbyne carbon atom. The synthesis of the new compounds is described and their properties and spectroscopic data are reported.

Carbonyl and trifluorophosphine adducts of bis(2,4-dimethylpentadienyl)titanium and bis(2,4-dimethylpentadienyl)vanadium

The syntheses of mono(carbonyl) and mono(trifluorophosphine) adducts of bis(2,4-dimethylpentadienyl)titanium and bis(2,4-dimethylpentadienyl)vanadium are described. Characterization has been achieved by routine spectral and analytical procedures. Detailed 1H and 13C NMR studies, as well as electron paramagnetic resonance and comparative infrared studies have been carried out in order to gain further understanding of the electronic nature of pentadienylmetal compounds, particularly compared to cyclopentadienylmetal compounds. Thus, infrared and EPR studies of V(C 5H 5) 2(CO) and V(2,4-C 7H 11) 2(CO) suggest that the withdrawal of electron density by the 2,4-dimethylpentadienyl ligand is considerably greater than that by the cyclopentadienyl ligand

Structures of the gauche conformers of some substituted dimethyl ethers. Effect of adjacent atom lone pairs on methyl group asymmetry

The complete equilibrium structures of CH 3OCH 3 and of the gauche conformers of CH 3OCH 2F, HOCH 2F, CH 3OCH 2Cl and H 3OCH 2CN have been determined by ab initio gradient computation at the Hartree—Fock, double zeta-plus-polarization level. The very large asymmetries in CH bond distances previously reported from microwave substitution structures are shown to be non-existent in the equilibrium structures and are presumably artifacts. Small differences, different in direction from those reported from the experiments and nearly an order of magnitude smaller in size, do exist. They reflect three factors: (1) a lengthening of a CH bond which is trans to a lone pair on an adjacent atom, (2) a general shortening of CH bonds originating at a carbon atom bearing a highly electronegative substituent, and (3) a specific interaction in which a CX substituent shortens the nearly parallel CH bond on the other methyl group. The last interaction, not previously reported, is mediated by withdrawal of electron density from the oxygen lone pair which is trans to both groups. Other structural features derived from the microwave studies are supported by the new results. p ]Inclusion of polarization functions in the basis set for oxygen is essential for correct determination of the COC angle and the dihedral angles. The dihedral angles of CH 3OCH 2F and HOCH 2F are not correctly determined by the computation even at this level, although the computed values are improved when d functions are used for oxygen and still more by use of two sets of oxygen d functions. Polarization functions on carbon or on fluorine have no effect on the computed torsional angles. There is no problem in computing the correct dihedral angles with the Cl or CN derivatives.

Structures of the gauche conformers of somE substituted dimethyl ethers. Effect of adjacent atom lone pairs on methyl group asymmetry

The complete equilibrium structures of CH 3OCH 3 and of the gauche conformers of CH 30CH 2F, HOCH,F, CH,OCH,Cl and CH,OCH,CN have been determined by ab initio gradient computation at the Hartree-Fock, double zeta-plus-polarization level. The very large asymmetries in CH bond distances previously reported from microwave substitution structures are shown to be non-existent in the equilibrium structures and are presumably artifacts. Small differences, different in direction from those reported from the experiments and nearly an order of magnitude smaller in size, do exist. They reflect three factors: (1) a lengthening of a CH bond which is trans to a lone pair on an adjacent atom, (2) a general shortening of CH bonds originating at a carbon atom bearing a highly electronegative substituent, and (3) a specific interaction in which a CX substituent shortens the nearly parallel CH bond on the other methyl group. The last interaction, not previously reported, is mediated by withdrawal of electron density from the oxygen lone pair which is trans to both groups. Other structural features derived from the microwave studies are supported by the new results. Inclusion of polarization functions in the basis set for oxygen is essential for correct determination of the COC angle and the dihedral angles. The dihedral angles of CH 3OCH 2F and HOCH 2F are not correctly determined by the computation even at this level, although the computed values are improved when d functions are used for oxygen and still more by use of two sets of oxygen d functions. Polarization functions on carbon or on fluorine have no effect on the computed torsional angles. There is no problem in computing the correct dihedral angles with the CI or CN derivatives.

Preparation and crystal and molecular structure of (η3-octamethylcyclotetraphosphazane-P,P,P)tricarbonylmolybdenum(0), [η3-(CH3PNCH3)4]Mo(CO)3

The synthesis and physical properties of the phosphazane complex [η3-(CH3PNCH3)4]Mo(CO)3 are reported. Crystals of (η3-octamethylcyclotetraphosphazane-P,P,P)tricarbonylmolybdenum(0) are monoclinic, a = 8.6898(9), b = 16.0987(8), c = 14.1156(17) Å, β = 98.321(6)°, Z = 4, space group Cc. The structure was solved by Patterson and Fourier syntheses and was refined by full-matrix least-squares procedures to R = 0.020 and Rw = 0.024 for 2286 reflections with I ≥ 3σ(I). The molybdenum atom is coordinated to three phosphorus atoms of the (MePNMe)4 ligand and to three carbonyl ligands in a distorted octahedral geometry. The molecule has approximate mirror symmetry in the solid state. Important bond lengths (corrected for libration) are: Mo—P = 2.537(1), 2.467(1), and 2.513(1), Mo—C = 1.971(4), 2.002(4), and 1.978(4), mean P—N = 1.704(8) for coordinated P, 1.735(7) Å for uncoordinated P. The structural evidence indicates that the phosphazane is a weaker donor to Mo(CO)3 than to three sulphur atoms, and that coordination withdraws electron density from the fourth phosphorus atom. The complex is fluxional in solution.

An electrochemical study of chromium(0) complexes of arenes which have electronegative substituent groups

Using cyclic voltammetry, E 1 2 values were determined for the process Cr(arene) 2 ⇄ [Cr(arene) 2] + + e (arene = C 6H 5R, for R = H, CF 3, Cl, F, OCH 3; p- and m-C 6H 4(CF 3) 2, o- and p-C 6H 4(CF 3)Cl; 1,4(CH 3) 2-2-ClC 6H 3) in acetonitrile. Compounds with electron withdrawing groups on the arene ligands have higher E 1 2 values indicating that they are more difficult to oxidize. This is in accord with a simple model in which withdrawal of electron density from the metal to ligands stabilizes the HOMO, a primarily metal orbital, in these systems. Possible correlations between E 1 2 and the Hammett substituent constants, σ m and σ p for the substituent groups on the ligands, are investigated.

Extraction of silver(I) in the system tetrabutyl S-dithiopyrophosphate (TBDTPP)—electrolyte

The extraction of Ag+ ions was studied in the systems tetra-n-butyl S-dithiopyrophosphate [(BuO)2P(S)(O)P(S)(BuO)2]—TBDTPP in CCl4/HNO3 and TBDTPP/LiNO3. The extraction coefficients were determined as a function of the electrolyte and TBDTPP concentration. Infrared absorption studies have been undertaken to obtain information on the changes in the organic phase during the extraction of Ag(I). Analysis of the spectra has shown a shift of the P=S stretching band in the direction of lower frequencies. This is indicative of bond weakening and a decrease in the double bond character of P=S, due to withdrawal of electron density by the metal ion, and constitutes evidence for the coordination of Ag+ by thiophosphoryl groups of TBDTPP.

CNDO molecular orbital calculations on porphyrins-I. Ground and excited states of porphyrin, divinylporphyrin and tetraphenylporphyrin

Semi-empirical CNDO/2 MO calculations are reported for the ground states of porphyrin, 2,4-divinylporphyrin (DVP), and α,β,γ,δ-tetraphenylporphyrin (TPP). Results for TPP refer to the conformation with all phenyl groups perpendicular to the porphyrin ring, calculated to be 108 kJ mol −1 more stable than the coplanar conformation. The substituents withdraw electron density where they are attached to the porphyrin ring, increasing selected orbital energies. The vinyl groups also modify the electron population at nitrogen. CNDO/S calculations with extensive configuration interaction are reported for excited states. The lowest singlet states are closely similar in energy and composition for all three molecules, except for an extra state and more complex compositions in DVP above 3 eV. The lowest triplet states of porphyrin and DVP are very similar, while those of TPP are comparable in energy or composition but not both. Experimental data on the excited states are broadly consistent with the calculations, although comparisons for the excited triplets are tentative.

The microwave spectrum and structure of chloryl fluoride

The microwave spectra of three isotopic species of chloryl fluoride, FClO 2, in its ground vibrational state, have been measured in the frequency region 8–37 GHz. The spectra have yielded values for the rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants, as well as the molecular dipole moment, 1.722 ± 0.03 D. The molecule has been shown to have C s symmetry, and a pyramidal configuration, with the chlorine atom at the apex of the pyramid. The following internuclear parameters were obtained: r(Cl−F)1.697±0.003 A r(Cl−F)=1.418±0.002AThe structural parameters, quadrupole coupling constants, dipole moment and force field are explained in terms of a bonding scheme in which a fluorine 2 p atomic orbital overlaps with the highest occupied orbital of ClO 2; there is considerable evidence for withdrawal of electron density from this singly occupied antibonding orbital of ClO 2 toward the fluorine atom.

The electronic structure of aryl thiol esters

A combination of experimental data and the CNDO/2-SCF-MO method are used to evaluate the importance of d-orbitals to the electronic structure of aryl thiol esters. The thioester group is calculated to withdraw electron density through σ and 2p π-3d π bonding and donate by p π-p π bonding. Electron donating substituents para to the thiol ester group cause the latter group to accept electron density regardless of d-orbital participation.

Studies in boron hydrides—VI: Oxidation of carborane methylsulfides

Peracetic acid in ethyl acetate effects smooth oxidation of the methylsulfides of o- and m-carboranes to afford sulfoxides and sulfones. The sulfoxides derived from o-carborane were far more susceptible to nucleophilic attack to give substituted B 9C 2H 12 − than was carborane itself. This is attributed to withdrawal of electron density from B(3,6) by the CH 3SO group(s) attached to carbon.