Platinum Metals Review - Volume 37, Issue 1, 1993

The organometallic chemistry of palladium is dominated by the +II oxidation state, and the chemistry of complexes containing simple organic groups bonded to palladium in the +IV oxidation state has developed only recently. Organic synthesis and catalytic reactions that may involve undetected palladium(IV) intermediates have been suggested frequently, and the new oxidation state +IV chemistry provides some support for these proposals, and gives encouragement for the development of new systems involving palladium(IV). The chemistry of organopalladium(IV) is reviewed here, and possible catalytic roles forpalladium(IV) are discussed. The synthesis and decomposition reactions of palladium(IV) complexes provide “models” for catalytic proposals. The palladium(IV) complexes are formed by oxidative addition of organohalides to palladium(II) complexes, and most complexes decompose under mild conditions by carbon-carbon bond formation in reductive elimination reactions, for example, for methyl(pheny1) (2,2’-bipyridyl)palladium(II) as a substrate, oxidative addition of benzyl bromide gives Pd^IV BrMePh(CH2Ph) (bpy), which reductively eliminates toluene to form the complex Pd^IIBr(CH2Ph)(bpy).

The reduction of nitric oxide by ammonia on platinum catalysts has been investigated in the temperature range 300 to 400°C at reactant partial pressures between 0.05 and 5 mbar. During a combined potentiometric and kinetic study, a discontinuous change in both the reaction rates and the surface state has been observed at a partial pressure ratio for nitric oxide:ammonia of 1.5. If ammonia is in excess in the gas phase then nitric oxide is directly reduced to nitrogen. However, an excess of nitric oxide leads to a strong formation of nitrous oxide, and the nitrogen formation thus proceeds via nitrous oxide.

It is well known that the performance of platinum electroplating baths can deteriorate suddenly during operation. This review suggests that 195-platinum nuclear magnetic resonance spectroscopy is a powerful technique for investigating the changes in the composition of the solution that lead to a loss in current efficiency and/or poor deposit quality and, indeed, may be used for the routine monitoring of the state of health of operational baths. This technique can also be used to gain an understanding of the complex chemistry of modern platinum electroplating baths and so assist in the continuing search for commercial baths with superior performance.