Palladium cluster compounds

Abstract

Over the past 20 years, the field of metal cluster chemistry has been one of the most rapidly developing in inorganic chemistry, and the current interest in this area can be illustrated by the recent publication of two books II'z). Clusters with up to 70 metal atoms <3) have been synthesized and characterized crystallographically, and the structures of many of these clusters have been justified theoretically in terms of molecular orbital analyses and electron counting rules <4's). Metal cluster compounds can be regarded as intermediate between coordination complexes and bulk metal surfaces. As such, the study of their chemistries can provide a model for reactions that occur on the surface of metals during heterogeneous catalytic processes the so-called 'cluster-surface analogy'% Cluster compounds themselves may prove to be of use as homogeneous catalysts, and research into the possibility of synthesizing highly selective cluster catalysts is rapidly increasing~ 7-9). Palladium, in the form of both the metal and mononuclear compounds, is one of the best catalysts for a whole host of reactions r176 so it is somewhat surprising that palladium cluster chemistry is less well developed than that of other metals, such as gold (12), ruthenium ~13) and platinum tl4). There are no obvious reasons for this lack of development, especially as palladium displays an extensive organometallic mononuclear chemistry ~ls). This review aims to collate the research that has been completed on palladium cluster compounds; the only previous reviews were limited to carbonyl phosphine clusters ~a6'7). This review will examine the syntheses of palladium cluster compounds, the methods of characterization available, structural and bonding aspects, and finally their reactivities. In cases where research has not been done with palladium examples are drawn from platinum chemistry. The review is limited to compounds containing Pd -Pd bonds, therefore, aggregates such a s [ P d 3 ( / A O A c ) 6 ] (17) and [Pd 8(/t-SPr)16] (18), in which there are no significant Pd -Pd interactions, have not been included.