Rhodium 1 oxidation states

Similarly, for RhH(CO)(PPh3)3, structure 2.59, the rhodium oxidation state is 1+ because the hydrogen atom is assumed to carry, with some justification, a formal negative charge. The five ligands, H", CO, and three PPhj, each donate two electrons, and the electron count therefore is 8 + 5 x 2 = 18. If we do not assign an oxidation state then the hydrogen atom donates one electron, and rhodium is in the zero oxidation state. The electron count is 1+9 + 4x2 = 18. 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Ca.ta.lysis, The readily accessible +1 and +3 oxidation states of rhodium make it a useful catalyst. There are several reviews of the catalytic properties of rhodium available (130—132). Rhodium-catalyzed methanol carbonylation (Monsanto process) accounted for 81% of worldwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°0 and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likely reduced in a water... 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

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... 

Table 26.2 Oxidation states and stereochemistries of some compounds of cobalt, rhodium and iridium...

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

A simplified reaction scheme is shown in Fig. 26.5 Again, the ability of rhodium to change its coordination number and oxidation state is crucial, and this catalyst has the great advantage over the conventional cobalt carbonyl catalyst that it operates efficiently at much lower temperatures and pressures and produces straight-chain as opposed to branched-chain products. 

The pattern of iridium halides resembles rhodium, with the higher oxidation states only represented by fluorides. The instability of iridium(IV) halides, compared with stable complexes IrCl4L2 and the ions IrX (X = Cl, Br, I), though unexpected, finds parallels with other metals, such as plutonium. Preparations of the halides include [19]... 

Until recently, well-authenticated cases of the rhodium(II) oxidation state were rare, with the exception of the dinuclear carboxylates. They fall into two main classes, although there are other rhodium(II) complexes ... 

ESCA data support a rhodium(II) oxidation state in these compounds. Therefore, the Rh 3d5//2 binding energy is c. 309.2 eV in simple car-boxylates, midway between those in typical rhodium(I) complexes (c. 308.5 eV) and rhodium(III) complexes (c. 310.7 eV) [72],... 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

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 chemistry of ruthenium, osmium, rhodium, iridium, palladium and platinum in the higher oxidation states. D. J. Gulliver and W. Levason, Coord. Chem. Rev., 1982,46,1-127 (1131). 

For rhodium the oxidation state + 3 for the metal is normal, although complexes with the metal in the +A and even the -l- 5 state are reported. 

Dujardin, C., Mamede, A.-S., Payen, E. et al. (2004) Influence of the oxidation state of rhodium in three-way catalysts on their catalytic performances An in situ FTIR and MS study, Top. Catal. 30-31, 347. 

---