Distortion of cyclopentadienyl rings in cyclopentadienyl-palladium complexes: crystal structures of [Pd(C5H5)Cl(PMe2Ph)] and [Pd(C5H5)(Ph2PCH2CH2PPh2)][PF6]

The structures of [Pd(C5H5)Cl(PMe2Ph)] and [Pd(C5H5(dppe)]PF6(dppe = Ph2PCH2CH2PPh2) have been determined by X-ray analysis. Both are μ5-cyclopentadienyl compounds, with the Cp ring nearly perpendicular to the PdPCl or PdP2 plane. Comparison of their molecular geometries with those of other cyclopentadienyl-palladium complexes of the types PdCpLL′ and PdCpL2 reveals the existence of a complete range of ring-PdLL′ or ring-PdL2 orientations and suggests that the usual assignment of structures of these types as either staggered or eclipsed is not realistic. Likewise, the patterns of long and short C---C bonds of the Cp rings do not in general follow the sequences expected for the idealized geometries, although large librational distortions of the C5H5 rings in most of the compounds limit the accuracy of these determinations. The lengths of the Pd---C(Cp) bonds appear to be much more dependent on the relative trans influences of L and L′ than on the ring orientations

Nucleosynthesis in Supernovae

We present the status and open problems of nucleosynthesis in supernova explosions of both types, responsible for the production of the intermediate mass, Fe-group and heavier elements (with the exception of the main s-process). Constraints from observations can be provided through individual supernovae (SNe) or their remnants (e.g. via spectra and gamma-rays of decaying unstable isotopes) and through surface abundances of stars which witness the composition of the interstellar gas at their formation. With a changing fraction of elements heavier than He in these stars (known as metallicity) the evolution of the nucleosynthesis in galaxies over time can be determined. A complementary way, related to gamma-rays from radioactive decays, is the observation of positrons released in beta(+)-decays, as e.g. from Al-26, Ti-44, Ni-56,Ni-57 and possibly further isotopes of their decay chains (in competition with the production of e(+)e(-) pairs in acceleration shocks from SN remnants, pulsars, magnetars or even of particle physics origin). We discuss (a) the role of the core-collapse supernova explosion mechanism for the composition of intermediate mass, Fe-group (and heavier?) ejecta, (b) the transition from neutron stars to black holes as the final result of the collapse of massive stars, and the relation of the latter to supernovae, faint supernovae, and gamma-ray bursts/hypernovae, (c) Type Ia supernovae and their nucleosynthesis (e.g. addressing the Mn-55 puzzle), plus (d) further constraints from galactic evolution, gamma-ray and positron observations. This is complemented by the role of rare magneto-rotational supernovae (related to magnetars) in comparison with the nucleosynthesis of compact binary mergers, especially with respect to forming the heaviest r-process elements in galactic evolution

Observational Signatures of Particle Acceleration in Supernova Remnants

We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe what these obser- vations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.Comment: Accepted for publication by Space Science Reviews, 81 page