Rhodium trivalent

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. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

This protonation may, besides the desired CO insertion, also form inactive trivalent rhodium iodides. In the Monsanto acetic acid process the addition of the reducing agent H2 is not required for two reasons ... 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

The discovery and use of fluorophosphites and chlorophosphites as trivalent phosphorus ligands in the rhodium catalyzed, low-pressure hydroformylation reaction are described. The hydroformylation reaction with halophosphite ligands has been demonstrated with terminal and internal olefins. For the hydroformylation of propylene, the linear to branched ratio of the butyraldehyde product shows a strong dependency on the ligand to rhodium molar ratios, the reaction temperature, and the carbon monoxide partial pressure. 

In these the metal is divalent, tetravalent, and trivalent respectively. The ammino-iridous and the ammino-iridic salts correspond to the ammino-derivatives of palladium and platinum, whilst those of the sesqui-salts are analogous to the ammino-derivatives of cobalt, chromium, and rhodium. 

Rhodium, incorporated in the silver halide grains, decreases sensitivity and increases contrast. This action has been attributed to depression of latent image formation because of deep electron trapping by the trivalent rhodium ion (183-185). Eachus and Graves (184) showed that rhodium, probably as a complex, acts as a deep trap for electrons at room temperature. Weiss and associates (186) concluded that the rhodium salts introduce deep traps for both electrons and holes. Monte Carlo simulation showed that the photographic properties could be accounted for in this way over a wide range of exposure times. 

The same synthetic method was used in preparation (3.136) of cyclo-octene complexes of trivalent rhodium [11] ... 

The metallic properties observed for the d7 compounds listed in Table I are also consistent with the Goodenough model. The rhodium-selenium system is of particular interest and demonstrates clearly the important relationships between structure and transport properties. Cations may be removed from the superconducting compound (Tc = 6°K), RhSe2. The pyrite structure is maintained as the eg band is gradually emptied, and at the composition Rh2/3Se2 (RhSe3), all cations are trivalent—i.e., have the configuration 4d6. It is not known yet if the ideal... 

An interesting scries of salts is afforded by the alums of trivalent iron, cobalt, rhodium, and iridium. These have the general formula... 

The more important simple derivatives of cobalt are divalent, the metal only yielding stable trivalent salts in conjunction with other metallic derivatives, as, for example, the cobalti-nitrites and eobalii-cyanides, or in the complex ammino derivatives. Rhodium and iridium function almost exclusively as trivalent metals in their salts. 

Equilibration with carbon monoxide at room temperature and low pressure (a few torr ) yielded the rhodium(I)-dicarbonyl compound (13) in addition to the Rh(I)(C0) paramagnetic complexe (11). The structure of this complex was elucidated by ESCA and UV measurements (13) which showed that the trivalent rhodium was indeed reduced to the monovalent state and by infrared spectroscopy which provided evidence for a gem dicarbonyl (14). Use of 1 1 C0 ... 

Trivalent metallocorrole complexes containing metals other than cobalt have also been prepared from hydrobromide salts of dideoxybiladienes-ac. For instance, in 1988, Boschi, et al. reported two approaches to the formation of in-plane trivalent rhodium complexes. The first involved reacting octamethylbiladiene-ac 2.106 with... 

To date, only one example of a monovalent metallocorrole has been reported. It was reported in 1976 by Grigg, et al. and involves a rhodium corrole, which was obtained in 36% yield as the result of reacting free-base diethyl-hexamethyl corrole 2.6 with Rh2(CO)4Cl2. Unlike the trivalent corrole complex 2.134, obtained earlier by the treatment of a dideoxybiladiene-ac with this same metal salt, the complex isolated in this instance analyzed as being the monovalent Rh(CO)2Corrole, 2.157 (Scheme 2.1.42). This complex was later prepared in 72% yield,although it was... 

It is presumed that hemiporphycene will exhibit a rich metal coordination chemistry. Indeed, preliminary reports have indicated that complexes of divalent magnesium, zinc, nickel, and copper, trivalent iron, cobalt and rhodium, and tetra-valent tin may readily be prepared. Of particular interest in such metalation studies is the fact that metal complexes of hemiporphycene containing an axial substituent (e.g., 3.153 Figure 3.3.3) bear metal-centered chirality because of the dissymmetric nature of the ligand. Unfortunately, further details with regard to this point and other aspects of hemiporphycene coordination chemistry are still lacking at this time. 

Rhodium is almost wholly basic in character, its salts being generally trivalent. The trichloride forms double salts with alkali chlorides which are called hexachlorrhodites, M jRhCls, and pentachlorrhodites, MiRHClj, respectively. These may be considered as double chlorides rather than as salts of the respective chloro-acids, since the existence of the latter is doubtful. The trichloride is insoluble in water and acids, but its hydrate is soluble. 

In its trivalent form, iridium forms the sesquisulfate, Ir2(S04)j, which like the corresponding salts of cobalt and rhodium forms a series of alums. 

Dithiocarbazic acid (40d) forms 2 1 complexes with bivalent nickel, platinum, zinc, cadmium and lead, with trivalent rhodium, and a 3 1 complex with chromium(III). Electronic spectral details are consistent with N—S rather than S—S coordination. Similar donor behaviour is exhibited by a-N-substituted derivatives, although -N substituents may change the mode of coordination to S—S. S-Methyl derivatives are known to form stable complexes in both neutral and deprotonated forms, with bidentate N—S coordination being indicated." Recent X-ray analyses of Pd", Ni", Co" and Pt" S-methyl derivatives of dithiocarbazic acid verify this coordination mode. The Pd" chelate is cis planar" with a smaller tetrahedral distortion than was observed in a previously reported Ni" analogue. Three closely related tris chelates of Ni" were all reported to be distorted octahedral. 

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