Theoretical studies in palladium and platinum molecular chemistry.

Publisher Summary Mossbauer spectroscopy can be likened to ultraviolet spectroscopy. Both techniques employ a source of radiation, an absorber and a detector. In Mossbauer spectroscopy, we consider transitions between nuclear energy levels with the emission and absorption of y rays; whereas, in ultraviolet spectroscopy, we consider the transitions between electronic energy levels with the emission and absorption of ultraviolet radiation. To observe resonance, a range of source photon energies is scanned: in ultraviolet by the use of a prism or grating, and in Mossbauer by employing the Doppler effect. The energy of the y ray (E,) is varied by the well-known Doppler formula d E = (V/C)E, where AE = change in energy of y photon, 11 = velocity of source relative to the absorber, and c = velocity of light. As in ultraviolet, absorption is plotted versus the energy of source photon (usually in velocity units for Mossbauer). Different compounds of one isotope give different spectra— that is, the nuclear energy levels are sensitive to the extranuclear environment. These differences in spectra can be attributed to the hyperfine interactions; the interactions between the nuclear charge distribution, and the extranuclear electric and magnetic fields. These hyperfine interactions give rise to the isomer shift (IS.) , the quadrupole splitting (Q.S.), and the magnetic Zeeman splitting.

Mössbauer Spectra of Inorganic Compounds: Bonding and Structure

The mechanisms for Ni(dppe)Cl2-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the monomer concentration. 31P NMR spectroscopic studies of the reactive organometallic intermediates suggest that the resting states are unsymmetrical NiII−biaryl and NiII−bithiophene complexes. In combination, the data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was found to have no effect on the rate-determining step or molecular weight distribution in the arene polymerization.

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Mechanistic studies on Ni(dppe)Cl2-catalyzed chain-growth polymerizations: Evidence for rate-determining reductive elimination

A general discussion of electronegativity and electronegativity equalization is presented as background for the concept of charge capacity; the latter is the ability of an atom to absorb or yield electronic charge. We have discussed charge capacity in relation to a variety of atomic and molecular properties, e.g. atomic polarizabilities and outer shell properties, electron affinities, dipole moments, electronic density distributions, dissociation energies, electrostatic potentials, spectroscopic measurements, acidities, etc. Significant increases in charge capacity and decreases in electronegativity in going from the first to subsequent rows of the periodic table are important in explaining (especially for columns IV–VII) the discontinuities in many physical and chemical properties that occur between these rows. It is demonstrated that both intrinsic electronegativities and charge capacities must be considered in order to understand the redistribution that occurs when atoms interact to form molecules. We discuss many seemingly anomalous observations of chemical behavior that are explained quite readily when both of these factors are taken into account.

Electronegativity and the concept of charge capacity

The reaction of bis(acetylacetonato)palladium(II), [Pd(acac)2] with AlR2(OEt) in the presence of tertiary phosphine, L, in diethyl ether yields a series of dialkylpalladium(II) complexes, [PdR2L2], where R=Me, Et, and Prn; L=PEt3, PPh2Me, and 1/2Ph2PCH2CH2PPh2 (dpe). These complexes were characterized by means of elemental analysis, IR and 1H-NMR spectra and of some decomposition reaction such as thermolysis and acidolysis. The trans- and cis-configurations of [PdMe2L2] (L=PEt3 and PPh2Me, respectively) in solution were established on the basis of the 1H-NMR spectra. The examination of 1H-NMR spectra in the presence of tertiary phosphine suggests the ligand exchange processes promoted by the added phosphine. [PdR2L2] (L=PEt3 and PPh2Me) reacted with CO in toluene to yield the quantitative amounts of the corresponding dialkyl ketones. The reactions of the complexes [PdR2L2] with iodine afforded [PdI2L2] and RI, and of [PdR2(dpe)] with MeI gave CH3H, CH3CH3, R(–H), RH, R·CH3, R·R, and RI together with [PdI2...

S. C. Nyburg

Papers

Molecular structures and proton magnetic resonance spectra of ethylene complexes of nickel and platinum