Transition Metal Complexes In Solution Research Articles

Dynamic frequency shift in the E.S.R. spectra of transition metal ions

The factors which determine the electron spin resonance lineshapes and shifts of transition metal complexes in solution are discussed. It is shown that for ions with S > ½ the dynamic frequency shift may have pronounced effects on the E.S.R. lineshapes, and experimental evidence for this effect in complexes of Cr3+, Fe3+ and Gd3+ is presented. A quantitative interpretation of these spectra in terms of a relaxation mechanism due to fluctuation of the quadratic zero field splitting interaction is given.

The activation of saturated hydrocarbons by transition-metal complexes in solution. Part III. Use of photoelectron spectra of halogen-substituted hydrocarbons and hydrocarbons containing quaternary carbon atoms to interpret the nature of platinum(II)-catalysed hydrogen–deuterium exchange

Representative fluoro-, chloro-, bromo-, and iodo-substituted alkanes and aromatic compounds, and two branched-chain hydrocarbons that contain an ethyl group adjacent to a quaternary carbon atom, have been studied both by photoelectron spectroscopy and in hydrogen–deuterium exchange reactions catalysed by PtCl42– in 50 mol % acetic [2H1]acid–deuterium oxide. For the halogen-substituted compounds, H–D exchange occurs in those molecules for which the first ionisation potential in the photoelectron spectrum is associated with a molecular orbital centered on carbon–hydrogen bonds. For these compounds, an exponential relationship exists between the rate constant for exchange and this potential. When the first ionisation potential is associated with electrons of the halogen atom, then displacement of the halogen occurs. For the branched-chain hydrocarbons, the rate constant for exchange in the ethyl group correlates with the rate constants for the linear alkanes, provided an ionisation potential attributable to electrons of the ethyl group is used.

The activation of saturated hydrocarbons by transition-metal complexes in solution. Part II. Hydrogen–deuterium exchange in saturated and unsaturated hydrocarbons catalysed by solvated platinum(II) chloro-complexes

Exchange of hydrogen for deuterium in alkanes and in aromatic hydrocarbons is catalysed by platinum(II) chloro-complexes in acetic [2H1]acid–deuterium oxide mixtures at ca. 100 °C. Solutions of chloro-complexes having Cl:Pt ratios in the range 4·0–2·3 have been used. The effects on the exchange process of hydrocarbon concentration, solvent composition, and catalyst composition are reported. The rate constant for exchange varies exponentially with the adiabatic ionisation potential of the hydrocarbon. This correlation breaks down for certain aromatic hydrocarbons when Cl:Pt < 4 owing to extensive complex formation between reactant and catalyst; values for the formation constants of such complexes are reported.

The activation of saturated hydrocarbons by transition-metal complexes in solution. Part I. Hydrogen–deuterium exchange in alkanes catalysed by potassium tetrachloroplatinate(II) in acetic acid

All isomeric alkanes from methane to the hexanes, cyclopentane, and cyclohexane, undergo hydrogen–deuterium exchange with a deuterium oxide–acetic [2H1]acid solvent catalysed by potassium tetrachloroplatinate(II). The catalyst is stabilised by the acidity of the solution and the presence of a small concentration of a weak complexing agent such as pyrene. Exchange is preceded by dissociation of chloride ligands from the added tetrachloroplatinate(II) to form an uncharged platinum(II) complex which may be monomeric or dimeric. The relative reactivity for the hydrogen-deuterium exchange is in the order primary > secondary > tertiary which is in the reverse order to the reactivity expected from bond energy data. Exchange rates in the series of n-alkanes have been related to their ionisation potentials. Branched chain alkanes are less reactive than their respective n-isomers, whereas the cycloalkanes are more reactive. The results indicate that the initial interaction between the alkanes and the platinum involves a reversible electron transfer from the delocalised molecular orbitals in the alkanes to the un-charged platinum(II) complex. Subsequent steps in the proposed exchange mechanism involve a reversible oxidative-addition of the alkane to the platinum(II) complex to form a hydrido-platinum(IV) complex which can lose HCl to, and gain DCl from, the solvent.

Electron spin resonance line widths of transition metal ions in solution. Relaxation through zero-field splitting

The factors which determine the electron resonance line widths of transition metal complexes in solution are considered. For complex ions possessing two or more unpaired electrons the dominant relaxation mechanism arises from coupling of the zero-field splitting of the spin multiplet with the random tumbling of the molecules in fluid solution. The theory of this effect is re-examined using the time-dependent density matrix methods of Redfield.