Rhodium elements

The rhodium element has only one stable isotope, Rh, with spin-1/2. Accordingly, in all cases, a Rh resonance can be observed. The only problem is its low receptivity which may lead to problems in direct detection. Consequently indirect detection is often used. Despite these restrictions,... 

EINECS 231-125-0 HSDB 2534 Rh Rhodium Rhodium, elemental Rhodium fume Rhodium, metal. Metallic element in the platinum group alloy with platinum for high temperature thermocouples, furnace windings, laborabory crucibles, spinerets for rayon, electrical contacts, jewelry, catalyst, mp = 1966° d = 12.41. Atomergic Chemetals Degussa AG Noah Cham. Sigma-Aldrich Fine Cham. 

The metal is silvery white and at red heat slowly changes in air to the resquioxide. At higher temperatures it converts back to the element. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. 

Phosphorus and rhodium are unusual among the elements in that they consist of atoms that naturally contain only one ratio of protons to neutrons and therefore have only one mass 31 (15 protons plus 16 neutrons) for phosphorus and 103 (45 protons and 58 neutrons) for rhodium. Such elements are called monoisotopic — each of their atoms has one (and only one) mass in each case. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Subsequentiy, the PGM solution is oxidized and acidified to reconvert Ir(III) to Ir(IV). Tri- -octylamine is again used as the extractant, this time to extract iridium. The iridium ia the organic phase is reduced to Ir(III) and recovered. The remaining element is rhodium, which is recovered from impurities in the original solution by conventional precipitation or ion exchange (qv). 

The replacement of rhodium from a wide range of rhodacycles to form condensed furans, thiophenes, selenophenes, tellurophenes and pyrroles has been widely explored and a range of examples is shown in Scheme 97. The rhodacycles are readily generated from the appropriate dialkyne and tris(triphenylphosphine)rhodium chloride. Replacement of the rhodium by sulfur, selenium or tellurium is effected by direct treatment with the element, replacement by oxygen using m-chloroperbenzoic acid and by nitrogen using nitrosobenzene. 

Rhodium and iridium are exceedingly rare elements, comprising only 0.0001 and 0.001 ppm of the earth s crust respectively, and even... 

The metals are lustrous and silvery with, in the case of cobalt, a bluish tinge. Rhodium and iridium are both hard, cobalt less so but still appreciably harder than iron. Rhodium and Ir have fee structures, the first elements in the transition series to do so this is in keeping... 

Table 26.1 Some properties of the elements cobalt, rhodium and iridium...

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Rhodium was discovered in 1803 by the eminent Norfolk scientist W.H. Wollaston he dissolved platinum metal concentrates in aqua regia and found that on removing platinum and palladium he was left with a red solution. From this he obtained the salt Na3RhCl6, which yielded the metal on reduction with hydrogen. The rose-red colour (Greek rhodon) of many rhodium salts gave the element its name. 

Both rhodium (m.p. 1976°C, b.p. 3730°C) and iridium (m.p. 2410°C, b.p. 4130°C) are unreactive silvery metals, iridium being considerably more dense (22.65gem-3) than rhodium (12.41 gem-3), the densest element known apart from osmium. Both form fee (ccp) lattices and, like the other platinum metals, are ductile and malleable. Neither is affected by aqua regia and they only react with oxygen and the halogens at red heat. 

This index is divided by element into eight parts. Each part is subdivided into sections devoted to each oxidation state, preceded by a general section. Thus if you want to fmd out about phosphine complexes of Rhodium, there is a general entry to phosphine complexes as well as separate references to phosphine complexes trader the headings of Rhodium(0), (I), (II) and (III). 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

A technologically important effect of the lanthanide contraction is the high density of the Period 6 elements (Fig. 16.5). The atomic radii of these elements are comparable to those of the Period 5 elements, but their atomic masses are about twice as large so more mass is packed into the same volume. A block of iridium, for example, contains about as many atoms as a block of rhodium of the same volume. However, each iridium atom is nearly twice as heavy as a rhodium atom, and so the density of the sample is nearly twice as great. In fact, iridium is one of the two densest elements its neighbor osmium is the other. Another effect of the contraction is the low reactivity—the nobility —of gold and platinum. Because their valence electrons are relatively close to the nucleus, they are tightly bound and not readily available for chemical reactions. 

Identify the element with the larger atomic radius in each of the following pairs (a) vanadium and titanium (b) silver and gold (c) vanadium and tantalum (d) rhodium and iridium. 

Pauling, L. Evidence from Bond Lengths and Bond Angles for Enneacovalence of Cobalt, Rhodium, Iridium, Iron, Ruthenium, and Osmium in Compounds with Elements of Medium Electronegativity Proc. Natl. Acad. Sci. (USA) 1984, 81, 1918-1921. 

Fig. 2.—-Calculated values of p for the elements yttrium to rhodium and for binary alloys between adjacent elements. 0.2 / ...

In the Li-Rh system LiRh is prepared from rhodium metal foil and liq Li in a 25 at% excess of the 1 1 molar ratio. The mixture is heated in an iron crucible to 750-880°C in Ar. The direct reaction of the elements in a molybdenum crucible at 800°C for 7 d produces LiRh. Identical methods produce Lilr and Lilrj with which the rhodium compounds are isostructural . The reaction of Rh metal with LiH at 600°C gives the ternary hydrides Li4RhH4 and Li4RhH5. 

The addition of metal hydrides to C—C or C—O multiple bonds is a fundamental step in the transition metal catalyzed reactions of many substrates. Both kinetic and thermodynamic effects are important in the success of these reactions, and the rhodium porphyrin chemistry has been important in understanding the thermochemical aspects of these processes, particularly in terms of bond energies. For example, for first-row elements. M—C bond energies arc typically in the range of 2, i-. i() kcal mol. M—H bond energies are usually 25-30 kcal mol. stronger, and as a result, addition of M—CH bonds to CO or simple hydrocarbons is thermodynamically unfavorable. 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

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