Iridium occurrence

Iridium s Link to Dinosaur Extinction Iridium occurrences may hold the key to solving the mystery of why the dinosaurs went extinct. The question of what happened to the dinosaurs has long been one of the most interesting and puzzling issues in science. What happened to make these huge reptiles disappear in such a short period of geologic time ... 

For organometailic compounds, the situation becomes even more complicated because the presence of elements such as platinum, iron, and copper introduces more complex isotopic patterns. In a very general sense, for inorganic chemistry, as atomic number increases, the number of isotopes occurring naturally for any one element can increase considerably. An element of small atomic number, lithium, has only two natural isotopes, but tin has ten, xenon has nine, and mercury has seven isotopes. This general phenomenon should be approached with caution because, for example, yttrium of atomic mass 89 is monoisotopic, and iridium has just two natural isotopes at masses 191 and 193. Nevertheless, the occurrence and variation in patterns of multi-isotopic elements often make their mass spectrometric identification easy, as depicted for the cases of dimethylmercury and dimethylplatinum in Figure 47.4. 

Electrochemical reduction of iridium solutions in the presence azodye (acid chrome dark blue [ACDB]) on slowly dropping mercury electrode is accompanied by occurrence of additional peaks on background acetic-ammonium buffer solutions except for waves of reduction azodye. Potentials of these peaks are displaced to cathode region of the potential compared to the respective peaks of reduction of the azodye. The nature of reduction current in iridium solutions in the presence ACDB is diffusive with considerable adsorptive limitations. The method of voltamiuetric determination of iridium with ACDB has been developed (C 1-2 x 10 mol/L). 

Structural, thermodynamic, and kinetic studies have shown that hydroxo-bridged polynuclear complexes of (diromium(III), cobalt(III), rhodium(III), and iridium(III) have many general features in common. Structurally, the four metal ions exhibit an almost identical pattern, and in particular the occurrence of many well-characterized oligomers... 

Occurrence and History.—Osmium occurs in nature alloyed with iridium as iridosmine or osmiridium, which is found as irregular flattened grains and, more rarely, as hexagonal prisms. It has a metallic lustre, tin-white to light steel-grey in appearance hardness 6-7 density 19-3 to 21-12.1 It occurs in Choco, South America in the Urals in auriferous and other drifts in New South Wales in auriferous beach-sands of North California and also in gold washings of certain Canadian rivers.1... 

Occurrence and History.—Iridium is found in nature alloyed with platinum or other allied metals, particularly in platiniridium and osmiridium (see analyses, pp. 208, 258). Nevyanskite, a variety of osmiridium, contains over 40 per cent, of iridium, whilst siserskite, another variety, contains up to 80 per cent, of the same metal.1... 

It has also been found that this general method, which involves reductive carbonylation of an iridium salt under CO in an alcoholic solvent followed by addition of phosphine ligand, is not always reliable. On occasion the carbonylation step is unsuccessful, leading to formation of a dark suspension instead of the expected clear yellow solution addition of phosphine to the suspension affords no product. Although the reason for this erratic behavior is not clear, its frequency of occurrence appears to vary from one sample of starting iridium salt to another, suggesting that small amounts of impurities in some samples may be responsible. 

Occurrence in earth s crust about 0.001 ppm. Discovered in 1804 by Tennant. Occurs in nature in the metallic state, usually as a natural alloy with osmium (osmiridium) found in small quantities alloyed with native platinum (platinum mineral) or with native gold. Recovery and purification from osmiridium Deville, De-bray, Ann, Chim. phys. 61, 84 (1861) from the platinum mineral Wichers, J. Res. Nat. Bur. Stand. 10, 819 (1933). Reviews of prepn, properties and chemistry of iridium and other platinum metals Gilchrist, Chem. Rev. 32, 277-372 (1943) W p. Griffith, The Chemistry of the Rare Platinum Metals (John Wiley, New York, 1967) pp 1-41, 227-312 Livingstone in Comprehensive Inorganic Chemistry vol. 3, J. C. Bailar Jr. et at, Eds. (Pergamoti Press, Oxford, 1973) pp 1163-1189, 1254-1274. 

Electron-richer dM compounds can also be considered as H2-activating alternatives to compounds with the unfavorable dM configuration. In the case of the bis-dppm bridged Rh(I)Ir(-I) complex 14, the d d configuration has been found to result in a metal-metal bonded species in which the coordination around the rhodium center is similar to that in planar homovalent d compounds. [47] The kinetic product of dihydrogen addition to 14 is consistent with the occurrence of a single-metal oxidative addition to the Rh(I) (Scheme 12). This kinetic product is thermally unstable and reductively eliminates methane from the iridium center. The overall reaction constitutes a clear example of bimetallic cooperation, since the oxidative addition to one center provokes a reductive elimination in the other metal. 

Natural occurrence. Osmium is an extremely rare element in the Earth s crust with an average abundance of 1 pg/kg (i.e., 1 ppb wt.). It naturally occurs as a native element alloyed with iridium and other PGMs in minerals such as osmiridium and in all platinum ores. 

Natural occurrence. Platinum metal occurs free in nature as a native metal contaminated with small amounts of all the PGMs such as iridium, osmium, palladium, ruthenium, and rhodium. These native minerals are found in placer ore deposits. 

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