Compounds of Cobalt, Rhodium and Iridium

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

R. S. Dickson, Organometallic Chemistry of Rhodium and Iridium, Academic Press, New York, 1983, 432 pp. C. White, Organometallic Compounds of Cobalt, Rhodium and Iridium, Chapman Hall, London 1985, 296 pp. 

C. White, Organometallic Compounds of Cobalt, Rhodium, and Iridium , Chapman and Hall, New York, 1985. 

The reactions of fluoroalkyl acetylenes with complexes of cobalt, rhodium, and iridium give a wide range of compounds, some of which have been indicated above. Other examples for which l9F NMR... 

There are no results to consider for palladium, osmium, and iridium and few for iron, nickel, and ruthenium. Salient features of the results for the last three metals are compared with those for related compounds of cobalt, rhodium, and platinum in Figure 2. The overall behavior is closely similar for all six metals, although there are variations between them just as there are in their chemical behavior. Thus we shall give a general discussion using the most suitable results from among those for the six metals. 

Many metal clusters are air-stable compounds. However those of the first transition metal series are in general more sensitive to air. Thus, binary carbonyl of ruthenium, rhodium, and iridium are rather air-stable species while those of iron, Fe3(CO)i2, and cobalt, Co4(CO)i2, rapidly decompose apparently via formation of the carbonates. 

Listing of organotin derivatives containing cobalt, rhodium and iridium concludes in Table 260. Other bimetallic and polymetallic organotin-cobalt compounds are summarized in Ohapters. 5 6.1 and 6.6, respectively. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

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. 

Double sulphides of iron and nickel are present in nickel matte, and are hence of commercial importance. Double sulphides with potassium, K2S.3NiS, and barium, BaS.4NiS, may be obtained by fusing nickel, sulphur, and an alkali at a high temperature.8 They are crystalline compounds. Cobalt yields only the sesquisulphide, Co2S3, in like circumstances. Nickel thus resembles palladium and platinum, whilst cobalt resembles rhodium and iridium in these respects. The position of nickel after cobalt in the Periodic Table thus receives further justification. 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

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. 81 (1984) 1918-1921. 

Therefore 4d and 5d electron metals interact with ligands in a more effective manner and thus form more covalent compounds. Because of valence orbital energy and orbital sizes, compounds of these elements in their lower oxidation states, particularly organometallic ones, are more stable than analogous complexes of M electron metals. The increased stability of olefin and acetylene compounds with increasing atomic number in a given group may serve as an example. Olefin complexes of cobalt are few and very unstable, while rhodium and iridium olefin compounds are quite common and usually air-stable. 

Cobalt and its heavier homologs form paramagnetic metallocenes Mcp2 (Tabele 9.14). Biscyclopentadienylcobalt(II) is relatively stable, while compounds of rhodium and iridium are stable only at low (liquid nitrogen) or high (above 393 K) temperatures in gaseous state. At room temperature, they dimerize to afford the diene-dienyl complexes (9.91). 

Interest in metal complexes containing polyfluoroalkyl- and polyfluoro-aryl-acetylenes as ligands has continued to be high, and has included compounds of platinum, palladium, gold, iridium, rhodium, - ruthenium, cobalt, - - nickel, molybdenum, and iron. These are reviewed in detail elsewhere in the Report (see Chapter 5). Such complexes may acquire usefulness for organic synthesis in due course thus significant amounts of hexakis(trifiuoromethyl)benzene are formed when perfluorobut-2-yne is incorporated into certain cobalt and nickel complexes. Similarly, the interesting compound hexakis(pentafluorophenyl)benzene was isolated in 40—70% yield hy trimerization of perfluorodiphenylacetylene over 7C-cyclopentadienylrhodium dicarbonyl in toluene. ... 

The chemistry of pincer complexes of the group 9 has been dominated by the study of the derivatives with rhodium and iridium. To date, the reactivity of cobalt pincer compounds with carbon monoxide has not been reported, mainly due to the fact that the number of pincer complexes derived from cobalt is reduced [17]. Thus, the interest in the Rh and Ir PCP pincer complexes is mainly owing to the fact that they have been proved efficient in the catalytic dehydrogenation of aliphatic C—H bonds this is particularly true in the case of the iridium PCP pincer complexes [3j. 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

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