Iridium 2 oxidation state

Different from the ruthenium system previously discussed, [Ir(COD)Cl]2 reacted with the electron-rich olefin (217) to afford the homoleptic organoiridium tricycle complex (223) with an increase of the iridium oxidation state from (I) to (III) (equation 41). This spontaneous orthometallation process... 

Rhodium and iridium differ from ruthenium and osmium in not forming oxo anions or volatile oxides. Their chemistry centers mainly around the oxidation states —I, 0, I and III for rhodium and I, III and IV for iridium. Oxidation states exceeding IV are limited to hexafluorides and to salts of the IrFg ion. 

Two fundamental properties influencing the electrochemical behavior of acid-growth hydrous iridium oxide films are (1) the acidity of the hydrated material and the variation of this property with change in the iridium oxidation state,4 and (2) the difference in electrical conductivity between the reduced and oxidized form of the surface film.170 174 According to Burke et al.9M both properties... 

In Figure 14.4, the iridium oxidation state increases from (I) to (111), and the coordination number increases from 4 to 6. The new ligands add in a cis or trans fashion, with their orientation a function of the mechanistic pathway. The expansion of the coordination number of the metal brings the newly added ligands into close proximity to the original ligands this can enable reactions to occur between them. Such reactions are encountered frequently in the mechanisms of catalytic cycles. 

The first step in the Cativa process is the reaction between Mel and c -[Ir(CO)2l2]. However, the catalyst may also react with HI and this step initiates a water gas shift reaction that competes with the main catalytic cycle, (a) What chemical is manufactured in the Cativa process Why is this product of industrial importance (b) Why is HI present in the system (c) Give an equation for the water gas shift reaction, and state conditions typically used in industry, (d) Figure 25.22 shows the competitive catalytic cycle described above. Suggest identities for species A, B, C and D. What type of reaction is the conversion of czj -[Ir(CO)2l2] to A What changes in iridium oxidation state occur on going around the catalytic cycle, and what is the electron count in each iridium complex ... 

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. 

Organometallic Compounds. The predominant oxidation states of indium in organometalUcs are +1 and +3. Iridium forms mononuclear and polynuclear carbonyl complexes including [IrCl(P(C3H3)3)2(CO)2] [14871-41-1], [Ir2014(00)2] [12703-90-1], [Ir4(CO)22] [18827-81 -1], and the conducting, polymeric [IrCl(CO)3] [32594-40-4]. Isonitnle and carbene complexes are also known. 

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...

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]... 

A wide range of iridium complexes are formed in the -1-3 oxidation state, the most important for iridium, with a variety of ligands. The vast majority have octahedral coordination of iridium. 

The chemistry of the iridium +4 oxidation state is limited, owing to the stability of the low-spin d6 Ir3+ complexes, which makes their oxidation difficult. The best known are IrXg- (X = F, Cl, Br) (see section 2.3). 

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). 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Fig. 7.59 Ir (73 keV) isomer shifts as a function of oxidation state of iridium (from [268])...

The compounds containing the hexahalogen complexes [IrCle], [IrBrs] with Ir(lV) formal oxidation state and iridium hexafluoride with hexavalent... 

The coordination chemistry of iridium has continued to flourish since 1985/86. All common donor atoms can be found bound to at least one oxidation state of iridium. The most common oxidation states exhibited by iridium complexes are I and III, although examples of all oxidation states from —I to VI have been synthesized and characterized. Low-oxidation-state iridium species usually contain CO ligands or P donor atoms, whereas high-oxidation-number-containing coordination compounds are predominantly hexahalide ones. 

The most common use of iridium coordination compounds remains in the catalysis field, although interest is developing in the luminescent properties of iridium compounds. The wide range of accessible oxidation states available to iridium (—1) to (VI) is reflected in the diverse nature of its coordination compounds. 

Iridium(IV) complexes are less common when compared to Ir111 or Ir1. There are examples of N, P, As, Sb, O, S, Se, Te, and halide ligands, often prepared by oxidation from an Ir111 salt. Recent developments (in 1991) in the chemistry of the platinum metals in high oxidation states were addressed by Levason, including complexes of IrIV and Irv.25... 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Although the redox reaction mechanisms of iridium oxide are still not clear, most researchers believe that the proton exchange associated with oxidation states of metal oxides is one of the possible pH sensing mechanisms [41, 87, 100, 105], During electrochemical reactions, oxidation state changes in the hydrated iridium oxide layer are... 

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