Peter M. Maitlis

THE CHEMISTRY OF SOME NOVEL RHODIUM AND IRIDIUM COMPLEXES

The chemistry of the group VIII transition metals has been extensively investigated over the past few years, owing to the importance of many of these elements in catalytic processes. The reactions of organic complexes of rhodium and its neighbors have, in this connection, received much attention as model systems. Our interest in this field stems from our discovery of a simple route to pentamethylcyclopentadienyl-rhodium and -iridium complexes. The properties of these complexes facilitate detailed mechanistic studies of both catalytic and stoichiometric reactions with unsaturated organic molecules. Many rhodium and iridium halide complexes are catalytically active; the simple chloride complexes suffer from the disadvantage that the anhydrous trichlorides are exceedingly insoluble while the hydrated chlorides [MCl, xH20] are not clearly defined compounds whose composition varies. Furthermore, reduction to the metal, with consequent deactivation of the catalyst, occurs during many reactions. The most common means for avoiding this has been to add tertiary phosphines or similar ligands to suppress reduction to the metal. Ctrnsiderable work on complexes of this type [e.g. (Ph,P).,RhCl] has been carried out by Wilkinson and his co-workers.1.2 Other reactions promoted by rhodium have been discussed by Cramer,, and more recent references are included in a 1969 review.*

REDISTRIBUTION REACTIONS INVOLVING TRANSITION‐METAL π‐COMPLEXES

Redistribution reactions of transition-metal a-complexes have not been very widely studied, or even used synthetically, up to the present. The reason is, in part, that more straightforward methods of synthesis are usually available and that the study of reactions in this field is usually very complicated. Stable a-complexes are only known for the transition metals which possess partially filled d-orbitals. The stability of these complexes must therefore be explained in terms of some type of bonding which involves the metal d-electrons. A simple example of a a-complex is the ethylene complex, Zeises salt, K[CzH4PtCIz]. Ethylene is a very weak Lewis base but has vacant a*-orbitals of appropriate symmetry and energy to interact with filled platinum d-orbitals. The bonding can thus be described as arising by the interaction of an ethylene a-orbital (filled) with a metal dsp2-hybrid orbital (vacant) and by overlap of an ethylene **-orbital (vacant) with a metal dp-hybrid orbital (filled). The latter interaction implies a back-bonding from the metal to the olefin and accounts for the stability of the olefin-metal bond. This type of description is applicable, with modification, to other a-complexes. There are broadly three main methods for the synthesis of a-complexes: (A) Reaction of the free ligand with a metal complex, (9) Reaction of a derivative of the free ligand with a metal complex, (C) The interconversion of one type of organotransition-metal complex to