Platinum Metals Review - Volume 45, Issue 4, 2001

This paper presents an up-to-date investigation relating to the design and synthesis of the recently disclosed single-site ruthenium carbene metathesis catalysts. Created as a convenient counterpart of the earlier tungsten and molybdenum carbene catalysts, these novel ruthenium carbene complexes bear specific heterocyclic ligands and display comparable activity and selectivity in metathesis reactions, as well as good tolerance toward organic functionalities, air and moisture. Due to their unique properties, they can be successfully applied in numerous organic and polymer syntheses involving cross-metathesis, ring-opening and ring-closing metatheses, as well as ring-opening metathesis polymerisation. This paper updates our previous review on metathesis reactions published in this Journal last year.

The first part of this paper, published in the July issue of this Journal, described the combination of physical metallurgical techniques and hydrogen solubilities used to help in characterising defects in palladium (Pd) and Pd alloys. In this second part, the solubilities of hydrogen (H2) in internally oxidised Pd alloys are discussed. Internal oxidation, for example of a palladium-aluminium alloy, results in the formation of small alumina precipitates within the Pd matrix. Dissolved H in the alloy is strongly trapped at the metal/oxide interface. This can be detected by deviations in H solubility from that expected for Pd. Hydrogen in Pd and its alloys has been modelled mathematically as the occupation by H atoms of interstitial sites within fixed metal sublattices. However, recently it has been realised that at moderately high temperatures and H2 pressures some alloy lattices are not fixed, as the dissolved H promotes metal atom diffusion. This results in phase separation in some alloys, for example (Pd + Pt + H), according to a ternary equilibrium. The dissolved H can be removed from such alloys at low temperatures allowing the metastable, phase-separated alloy lattices to be characterised via measurements of H2 solubilities and suitable physical metallurgical techniques.

Refractory iridium has a unique position among metals due to its resistance to corrosion and its inclination to brittle fracture, the latter only occurring under tension. Since 1960 iridium has been the subject of much research, but this has not lead to an understanding of its inclination to brittle fracture under load, although the problem of working iridium has been successfully solved. Grain boundary brittleness during mechanical treatment is the main problem with iridium, and is a continuing and important topic for academic discussion about iridium. Discussion of this problem, begun forty years ago, is continued here.