Organoiridium chemistry

Organoiridium chemistry is the chemistry of organometallic compounds containing an iridium-carbon chemical bond. Organoiridium compounds are relevant to many important processes including olefin hydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals. Organoiridium compounds share many characteristics with those of rhodium, but less so with cobalt. Iridium can exist in oxidation states of -III to +V, but iridium(I) and iridium(III) are the more common. iridium(I) compounds (d8 configuration) usually occur with square planar or trigonal bipyramidal geometries, whereas iridium(III) compounds (d6 configuration) typically have an octahedral geometry. Iridium(0) complexes are binary carbonyls, the principal member being tetrairidium dodecacarbonyl, Ir4(CO)12. Unlike the related Rh4(CO)12, all CO ligands are terminal in Ir4(CO)12, analogous to the difference between Fe3(CO)12 and Ru3(CO)12. A well known example is Vaska's complex, bis(triphenylphosphine)iridium carbonyl chloride. Although iridium(I) complexes are often useful homogeneous catalysts, Vaska' complex is not. Rather it is iconic in the diversity of its reactions. Other common complexes include Ir2Cl2(cyclooctadiene)2, chlorobis(cyclooctene)iridium dimer, The analogue of Wilkinson's catalyst, IrCl(PPh3)3), undergoes orthometalation: IrCl(PPh3)3 → HIrCl(PPh3)2(PPh2C6H4) This difference between RhCl(PPh3)3 and IrCl(PPh3)3 reflects the generally greater tendency of iridium to undergo oxidative addition. A similar trend is exhibited by RhCl(CO)(PPh3)2 and IrCl(CO)(PPh3)2, only the latter oxidatively adds O2 and H2. The olefin complexes chlorobis(cyclooctene)iridium dimer and cyclooctadiene iridium chloride dimer are often used as sources of "IrCl", exploiting the lability of the alkene ligands or their susceptibility to removal by hydrogenation. Crabtree's catalyst ([Ir(P(C6H11)3)(pyridine)(cyclooctadiene)]PF6) is a versatile homogeneous catalyst for hydrogenation of alkenes.