Ruthenium and Its Organic Compounds

As the 74th most abundant element in Earth's crust, ruthenium is comparatively rare, found in about 100 parts per trillion. This element is mostly found in ores with the other PGMs in the Ural Mountains and in North and South America. Small but economically significant amounts are also found in pentlandite extracted from Sudbury, Ontario, Canada, and in pyroxenite deposits in South Africa. Native metallic ruthenium is a very rare mineral (Ir commonly replaces part of Ru in its structure). Around 12 tons of ruthenium are mined annually with world reserves estimated at about 5,000 tons.

Organoruthenium Compounds

The chemistry of organoruthenium compounds did not develop until the discovery of ferrocene in 1951. Ferrocene consists of an iron ion sandwiched in between two cyclopentadienyl rings. After its discovery, similar compounds were developed and ruthenocene was reported in 1952. Ferrocene and ruthenocene exhibit similar chemical properties but differ in their redox properties. Ruthenocene is more stable toward oxidation compared with ferrocene. Sandwich structures with arenes are also common. The bis(arene) Ru complex show interesting chemistry through its ability to undergo two-electron redox change that converts one arene from a η6- into a bent η4-ligand. In addition to the many variations in ruthenocene, bis(arene), and the (arene)(cyclopentadienyl) complexes, Ru also forms complexes with carbonyls (CO) and phosphines especially the triphenylphosphines (PPh3). A phosphine complex of ruthenium, RuCl2(PPh3)3, has been used as starting material for the synthesis of another important class of organoruthenium complexes now called Grubbs first-generation catalysts. These catalysts are useful in olefin metathesis and prepared by onepot synthesis using RuCl2(PPh3)3, phenyldiazomethane, and tricyclohexylphosphine. This first-generation catalyst is an important precursor for the succeeding catalysts that were developed.

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